Merge pull request #532 from MichaelMure/dag-entity

Work towards a reusable entity datastructure + commit signature

17 files changed, 2141 insertions, 49 deletions

diff --git a/entity/dag/clock.go b/entity/dag/clock.go
new file mode 100644
index 00000000..dc9bb72d
--- /dev/null
+++ b/entity/dag/clock.go
@@ -0,0 +1,37 @@
+package dag
+
+import (
+	"fmt"
+
+	"github.com/MichaelMure/git-bug/identity"
+	"github.com/MichaelMure/git-bug/repository"
+)
+
+// ClockLoader is the repository.ClockLoader for Entity
+func ClockLoader(defs ...Definition) repository.ClockLoader {
+	clocks := make([]string, len(defs)*2)
+	for _, def := range defs {
+		clocks = append(clocks, fmt.Sprintf(creationClockPattern, def.Namespace))
+		clocks = append(clocks, fmt.Sprintf(editClockPattern, def.Namespace))
+	}
+
+	return repository.ClockLoader{
+		Clocks: clocks,
+		Witnesser: func(repo repository.ClockedRepo) error {
+			// We don't care about the actual identity so an IdentityStub will do
+			resolver := identity.NewStubResolver()
+
+			for _, def := range defs {
+				// we actually just need to read all entities,
+				// as that will create and update the clocks
+				// TODO: concurrent loading to be faster?
+				for b := range ReadAll(def, repo, resolver) {
+					if b.Err != nil {
+						return b.Err
+					}
+				}
+			}
+			return nil
+		},
+	}
+}
diff --git a/entity/dag/common_test.go b/entity/dag/common_test.go
new file mode 100644
index 00000000..25289b76
--- /dev/null
+++ b/entity/dag/common_test.go
@@ -0,0 +1,173 @@
+package dag
+
+import (
+	"encoding/json"
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/MichaelMure/git-bug/entity"
+	"github.com/MichaelMure/git-bug/identity"
+	"github.com/MichaelMure/git-bug/repository"
+)
+
+// This file contains an example dummy entity to be used in the tests
+
+/*
+ Operations
+*/
+
+type op1 struct {
+	author identity.Interface
+
+	OperationType int               `json:"type"`
+	Field1        string            `json:"field_1"`
+	Files         []repository.Hash `json:"files"`
+}
+
+func newOp1(author identity.Interface, field1 string, files ...repository.Hash) *op1 {
+	return &op1{author: author, OperationType: 1, Field1: field1, Files: files}
+}
+
+func (o *op1) Id() entity.Id {
+	data, _ := json.Marshal(o)
+	return entity.DeriveId(data)
+}
+
+func (o *op1) Validate() error { return nil }
+
+func (o *op1) Author() identity.Interface {
+	return o.author
+}
+
+func (o *op1) GetFiles() []repository.Hash {
+	return o.Files
+}
+
+type op2 struct {
+	author identity.Interface
+
+	OperationType int    `json:"type"`
+	Field2        string `json:"field_2"`
+}
+
+func newOp2(author identity.Interface, field2 string) *op2 {
+	return &op2{author: author, OperationType: 2, Field2: field2}
+}
+
+func (o *op2) Id() entity.Id {
+	data, _ := json.Marshal(o)
+	return entity.DeriveId(data)
+}
+
+func (o *op2) Validate() error { return nil }
+
+func (o *op2) Author() identity.Interface {
+	return o.author
+}
+
+func unmarshaler(author identity.Interface, raw json.RawMessage) (Operation, error) {
+	var t struct {
+		OperationType int `json:"type"`
+	}
+
+	if err := json.Unmarshal(raw, &t); err != nil {
+		return nil, err
+	}
+
+	switch t.OperationType {
+	case 1:
+		op := &op1{}
+		err := json.Unmarshal(raw, &op)
+		op.author = author
+		return op, err
+	case 2:
+		op := &op2{}
+		err := json.Unmarshal(raw, &op)
+		op.author = author
+		return op, err
+	default:
+		return nil, fmt.Errorf("unknown operation type %v", t.OperationType)
+	}
+}
+
+/*
+  Identities + repo + definition
+*/
+
+func makeTestContext() (repository.ClockedRepo, identity.Interface, identity.Interface, identity.Resolver, Definition) {
+	repo := repository.NewMockRepo()
+	id1, id2, resolver, def := makeTestContextInternal(repo)
+	return repo, id1, id2, resolver, def
+}
+
+func makeTestContextRemote(t *testing.T) (repository.ClockedRepo, repository.ClockedRepo, repository.ClockedRepo, identity.Interface, identity.Interface, identity.Resolver, Definition) {
+	repoA := repository.CreateGoGitTestRepo(false)
+	repoB := repository.CreateGoGitTestRepo(false)
+	remote := repository.CreateGoGitTestRepo(true)
+
+	err := repoA.AddRemote("remote", remote.GetLocalRemote())
+	require.NoError(t, err)
+	err = repoA.AddRemote("repoB", repoB.GetLocalRemote())
+	require.NoError(t, err)
+	err = repoB.AddRemote("remote", remote.GetLocalRemote())
+	require.NoError(t, err)
+	err = repoB.AddRemote("repoA", repoA.GetLocalRemote())
+	require.NoError(t, err)
+
+	id1, id2, resolver, def := makeTestContextInternal(repoA)
+
+	// distribute the identities
+	_, err = identity.Push(repoA, "remote")
+	require.NoError(t, err)
+	err = identity.Pull(repoB, "remote")
+	require.NoError(t, err)
+
+	return repoA, repoB, remote, id1, id2, resolver, def
+}
+
+func makeTestContextInternal(repo repository.ClockedRepo) (identity.Interface, identity.Interface, identity.Resolver, Definition) {
+	id1, err := identity.NewIdentity(repo, "name1", "email1")
+	if err != nil {
+		panic(err)
+	}
+	err = id1.Commit(repo)
+	if err != nil {
+		panic(err)
+	}
+	id2, err := identity.NewIdentity(repo, "name2", "email2")
+	if err != nil {
+		panic(err)
+	}
+	err = id2.Commit(repo)
+	if err != nil {
+		panic(err)
+	}
+
+	resolver := identityResolverFunc(func(id entity.Id) (identity.Interface, error) {
+		switch id {
+		case id1.Id():
+			return id1, nil
+		case id2.Id():
+			return id2, nil
+		default:
+			return nil, identity.ErrIdentityNotExist
+		}
+	})
+
+	def := Definition{
+		Typename:             "foo",
+		Namespace:            "foos",
+		OperationUnmarshaler: unmarshaler,
+		FormatVersion:        1,
+	}
+
+	return id1, id2, resolver, def
+}
+
+type identityResolverFunc func(id entity.Id) (identity.Interface, error)
+
+func (fn identityResolverFunc) ResolveIdentity(id entity.Id) (identity.Interface, error) {
+	return fn(id)
+}
diff --git a/entity/dag/entity.go b/entity/dag/entity.go
new file mode 100644
index 00000000..c4368514
--- /dev/null
+++ b/entity/dag/entity.go
@@ -0,0 +1,439 @@
+// Package dag contains the base common code to define an entity stored
+// in a chain of git objects, supporting actions like Push, Pull and Merge.
+package dag
+
+import (
+	"encoding/json"
+	"fmt"
+	"sort"
+
+	"github.com/pkg/errors"
+
+	"github.com/MichaelMure/git-bug/entity"
+	"github.com/MichaelMure/git-bug/identity"
+	"github.com/MichaelMure/git-bug/repository"
+	"github.com/MichaelMure/git-bug/util/lamport"
+)
+
+const refsPattern = "refs/%s/%s"
+const creationClockPattern = "%s-create"
+const editClockPattern = "%s-edit"
+
+// Definition hold the details defining one specialization of an Entity.
+type Definition struct {
+	// the name of the entity (bug, pull-request, ...)
+	Typename string
+	// the Namespace in git (bugs, prs, ...)
+	Namespace string
+	// a function decoding a JSON message into an Operation
+	OperationUnmarshaler func(author identity.Interface, raw json.RawMessage) (Operation, error)
+	// the expected format version number, that can be used for data migration/upgrade
+	FormatVersion uint
+}
+
+// Entity is a data structure stored in a chain of git objects, supporting actions like Push, Pull and Merge.
+type Entity struct {
+	// A Lamport clock is a logical clock that allow to order event
+	// inside a distributed system.
+	// It must be the first field in this struct due to https://github.com/golang/go/issues/36606
+	createTime lamport.Time
+	editTime   lamport.Time
+
+	Definition
+
+	// operations that are already stored in the repository
+	ops []Operation
+	// operations not yet stored in the repository
+	staging []Operation
+
+	lastCommit repository.Hash
+}
+
+// New create an empty Entity
+func New(definition Definition) *Entity {
+	return &Entity{
+		Definition: definition,
+	}
+}
+
+// Read will read and decode a stored local Entity from a repository
+func Read(def Definition, repo repository.ClockedRepo, resolver identity.Resolver, id entity.Id) (*Entity, error) {
+	if err := id.Validate(); err != nil {
+		return nil, errors.Wrap(err, "invalid id")
+	}
+
+	ref := fmt.Sprintf("refs/%s/%s", def.Namespace, id.String())
+
+	return read(def, repo, resolver, ref)
+}
+
+// readRemote will read and decode a stored remote Entity from a repository
+func readRemote(def Definition, repo repository.ClockedRepo, resolver identity.Resolver, remote string, id entity.Id) (*Entity, error) {
+	if err := id.Validate(); err != nil {
+		return nil, errors.Wrap(err, "invalid id")
+	}
+
+	ref := fmt.Sprintf("refs/remotes/%s/%s/%s", def.Namespace, remote, id.String())
+
+	return read(def, repo, resolver, ref)
+}
+
+// read fetch from git and decode an Entity at an arbitrary git reference.
+func read(def Definition, repo repository.ClockedRepo, resolver identity.Resolver, ref string) (*Entity, error) {
+	rootHash, err := repo.ResolveRef(ref)
+	if err != nil {
+		return nil, err
+	}
+
+	// Perform a breadth-first search to get a topological order of the DAG where we discover the
+	// parents commit and go back in time up to the chronological root
+
+	queue := make([]repository.Hash, 0, 32)
+	visited := make(map[repository.Hash]struct{})
+	BFSOrder := make([]repository.Commit, 0, 32)
+
+	queue = append(queue, rootHash)
+	visited[rootHash] = struct{}{}
+
+	for len(queue) > 0 {
+		// pop
+		hash := queue[0]
+		queue = queue[1:]
+
+		commit, err := repo.ReadCommit(hash)
+		if err != nil {
+			return nil, err
+		}
+
+		BFSOrder = append(BFSOrder, commit)
+
+		for _, parent := range commit.Parents {
+			if _, ok := visited[parent]; !ok {
+				queue = append(queue, parent)
+				// mark as visited
+				visited[parent] = struct{}{}
+			}
+		}
+	}
+
+	// Now, we can reverse this topological order and read the commits in an order where
+	// we are sure to have read all the chronological ancestors when we read a commit.
+
+	// Next step is to:
+	// 1) read the operationPacks
+	// 2) make sure that the clocks causality respect the DAG topology.
+
+	oppMap := make(map[repository.Hash]*operationPack)
+	var opsCount int
+
+	for i := len(BFSOrder) - 1; i >= 0; i-- {
+		commit := BFSOrder[i]
+		isFirstCommit := i == len(BFSOrder)-1
+		isMerge := len(commit.Parents) > 1
+
+		// Verify DAG structure: single chronological root, so only the root
+		// can have no parents. Said otherwise, the DAG need to have exactly
+		// one leaf.
+		if !isFirstCommit && len(commit.Parents) == 0 {
+			return nil, fmt.Errorf("multiple leafs in the entity DAG")
+		}
+
+		opp, err := readOperationPack(def, repo, resolver, commit)
+		if err != nil {
+			return nil, err
+		}
+
+		err = opp.Validate()
+		if err != nil {
+			return nil, err
+		}
+
+		if isMerge && len(opp.Operations) > 0 {
+			return nil, fmt.Errorf("merge commit cannot have operations")
+		}
+
+		// Check that the create lamport clock is set (not checked in Validate() as it's optional)
+		if isFirstCommit && opp.CreateTime <= 0 {
+			return nil, fmt.Errorf("creation lamport time not set")
+		}
+
+		// make sure that the lamport clocks causality match the DAG topology
+		for _, parentHash := range commit.Parents {
+			parentPack, ok := oppMap[parentHash]
+			if !ok {
+				panic("DFS failed")
+			}
+
+			if parentPack.EditTime >= opp.EditTime {
+				return nil, fmt.Errorf("lamport clock ordering doesn't match the DAG")
+			}
+
+			// to avoid an attack where clocks are pushed toward the uint64 rollover, make sure
+			// that the clocks don't jump too far in the future
+			// we ignore merge commits here to allow merging after a loooong time without breaking anything,
+			// as long as there is one valid chain of small hops, it's fine.
+			if !isMerge && opp.EditTime-parentPack.EditTime > 1_000_000 {
+				return nil, fmt.Errorf("lamport clock jumping too far in the future, likely an attack")
+			}
+		}
+
+		oppMap[commit.Hash] = opp
+		opsCount += len(opp.Operations)
+	}
+
+	// The clocks are fine, we witness them
+	for _, opp := range oppMap {
+		err = repo.Witness(fmt.Sprintf(creationClockPattern, def.Namespace), opp.CreateTime)
+		if err != nil {
+			return nil, err
+		}
+		err = repo.Witness(fmt.Sprintf(editClockPattern, def.Namespace), opp.EditTime)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	// Now that we know that the topological order and clocks are fine, we order the operationPacks
+	// based on the logical clocks, entirely ignoring the DAG topology
+
+	oppSlice := make([]*operationPack, 0, len(oppMap))
+	for _, pack := range oppMap {
+		oppSlice = append(oppSlice, pack)
+	}
+	sort.Slice(oppSlice, func(i, j int) bool {
+		// Primary ordering with the EditTime.
+		if oppSlice[i].EditTime != oppSlice[j].EditTime {
+			return oppSlice[i].EditTime < oppSlice[j].EditTime
+		}
+		// We have equal EditTime, which means we have concurrent edition over different machines and we
+		// can't tell which one came first. So, what now? We still need a total ordering and the most stable possible.
+		// As a secondary ordering, we can order based on a hash of the serialized Operations in the
+		// operationPack. It doesn't carry much meaning but it's unbiased and hard to abuse.
+		// This is a lexicographic ordering on the stringified ID.
+		return oppSlice[i].Id() < oppSlice[j].Id()
+	})
+
+	// Now that we ordered the operationPacks, we have the order of the Operations
+
+	ops := make([]Operation, 0, opsCount)
+	var createTime lamport.Time
+	var editTime lamport.Time
+	for _, pack := range oppSlice {
+		for _, operation := range pack.Operations {
+			ops = append(ops, operation)
+		}
+		if pack.CreateTime > createTime {
+			createTime = pack.CreateTime
+		}
+		if pack.EditTime > editTime {
+			editTime = pack.EditTime
+		}
+	}
+
+	return &Entity{
+		Definition: def,
+		ops:        ops,
+		lastCommit: rootHash,
+		createTime: createTime,
+		editTime:   editTime,
+	}, nil
+}
+
+type StreamedEntity struct {
+	Entity *Entity
+	Err    error
+}
+
+// ReadAll read and parse all local Entity
+func ReadAll(def Definition, repo repository.ClockedRepo, resolver identity.Resolver) <-chan StreamedEntity {
+	out := make(chan StreamedEntity)
+
+	go func() {
+		defer close(out)
+
+		refPrefix := fmt.Sprintf("refs/%s/", def.Namespace)
+
+		refs, err := repo.ListRefs(refPrefix)
+		if err != nil {
+			out <- StreamedEntity{Err: err}
+			return
+		}
+
+		for _, ref := range refs {
+			e, err := read(def, repo, resolver, ref)
+
+			if err != nil {
+				out <- StreamedEntity{Err: err}
+				return
+			}
+
+			out <- StreamedEntity{Entity: e}
+		}
+	}()
+
+	return out
+}
+
+// Id return the Entity identifier
+func (e *Entity) Id() entity.Id {
+	// id is the id of the first operation
+	return e.FirstOp().Id()
+}
+
+// Validate check if the Entity data is valid
+func (e *Entity) Validate() error {
+	// non-empty
+	if len(e.ops) == 0 && len(e.staging) == 0 {
+		return fmt.Errorf("entity has no operations")
+	}
+
+	// check if each operations are valid
+	for _, op := range e.ops {
+		if err := op.Validate(); err != nil {
+			return err
+		}
+	}
+
+	// check if staging is valid if needed
+	for _, op := range e.staging {
+		if err := op.Validate(); err != nil {
+			return err
+		}
+	}
+
+	// Check that there is no colliding operation's ID
+	ids := make(map[entity.Id]struct{})
+	for _, op := range e.Operations() {
+		if _, ok := ids[op.Id()]; ok {
+			return fmt.Errorf("id collision: %s", op.Id())
+		}
+		ids[op.Id()] = struct{}{}
+	}
+
+	return nil
+}
+
+// Operations return the ordered operations
+func (e *Entity) Operations() []Operation {
+	return append(e.ops, e.staging...)
+}
+
+// FirstOp lookup for the very first operation of the Entity
+func (e *Entity) FirstOp() Operation {
+	for _, op := range e.ops {
+		return op
+	}
+	for _, op := range e.staging {
+		return op
+	}
+	return nil
+}
+
+// LastOp lookup for the very last operation of the Entity
+func (e *Entity) LastOp() Operation {
+	if len(e.staging) > 0 {
+		return e.staging[len(e.staging)-1]
+	}
+	if len(e.ops) > 0 {
+		return e.ops[len(e.ops)-1]
+	}
+	return nil
+}
+
+// Append add a new Operation to the Entity
+func (e *Entity) Append(op Operation) {
+	e.staging = append(e.staging, op)
+}
+
+// NeedCommit indicate if the in-memory state changed and need to be commit in the repository
+func (e *Entity) NeedCommit() bool {
+	return len(e.staging) > 0
+}
+
+// CommitAsNeeded execute a Commit only if necessary. This function is useful to avoid getting an error if the Entity
+// is already in sync with the repository.
+func (e *Entity) CommitAsNeeded(repo repository.ClockedRepo) error {
+	if e.NeedCommit() {
+		return e.Commit(repo)
+	}
+	return nil
+}
+
+// Commit write the appended operations in the repository
+func (e *Entity) Commit(repo repository.ClockedRepo) error {
+	if !e.NeedCommit() {
+		return fmt.Errorf("can't commit an entity with no pending operation")
+	}
+
+	err := e.Validate()
+	if err != nil {
+		return errors.Wrapf(err, "can't commit a %s with invalid data", e.Definition.Typename)
+	}
+
+	for len(e.staging) > 0 {
+		var author identity.Interface
+		var toCommit []Operation
+
+		// Split into chunks with the same author
+		for len(e.staging) > 0 {
+			op := e.staging[0]
+			if author != nil && op.Author().Id() != author.Id() {
+				break
+			}
+			author = e.staging[0].Author()
+			toCommit = append(toCommit, op)
+			e.staging = e.staging[1:]
+		}
+
+		e.editTime, err = repo.Increment(fmt.Sprintf(editClockPattern, e.Namespace))
+		if err != nil {
+			return err
+		}
+
+		opp := &operationPack{
+			Author:     author,
+			Operations: toCommit,
+			EditTime:   e.editTime,
+		}
+
+		if e.lastCommit == "" {
+			e.createTime, err = repo.Increment(fmt.Sprintf(creationClockPattern, e.Namespace))
+			if err != nil {
+				return err
+			}
+			opp.CreateTime = e.createTime
+		}
+
+		var parentCommit []repository.Hash
+		if e.lastCommit != "" {
+			parentCommit = []repository.Hash{e.lastCommit}
+		}
+
+		commitHash, err := opp.Write(e.Definition, repo, parentCommit...)
+		if err != nil {
+			return err
+		}
+
+		e.lastCommit = commitHash
+		e.ops = append(e.ops, toCommit...)
+	}
+
+	// not strictly necessary but make equality testing easier in tests
+	e.staging = nil
+
+	// Create or update the Git reference for this entity
+	// When pushing later, the remote will ensure that this ref update
+	// is fast-forward, that is no data has been overwritten.
+	ref := fmt.Sprintf(refsPattern, e.Namespace, e.Id().String())
+	return repo.UpdateRef(ref, e.lastCommit)
+}
+
+// CreateLamportTime return the Lamport time of creation
+func (e *Entity) CreateLamportTime() lamport.Time {
+	return e.createTime
+}
+
+// EditLamportTime return the Lamport time of the last edition
+func (e *Entity) EditLamportTime() lamport.Time {
+	return e.editTime
+}
diff --git a/entity/dag/entity_actions.go b/entity/dag/entity_actions.go
new file mode 100644
index 00000000..2926e992
--- /dev/null
+++ b/entity/dag/entity_actions.go
@@ -0,0 +1,260 @@
+package dag
+
+import (
+	"fmt"
+
+	"github.com/pkg/errors"
+
+	"github.com/MichaelMure/git-bug/entity"
+	"github.com/MichaelMure/git-bug/identity"
+	"github.com/MichaelMure/git-bug/repository"
+)
+
+// ListLocalIds list all the available local Entity's Id
+func ListLocalIds(def Definition, repo repository.RepoData) ([]entity.Id, error) {
+	refs, err := repo.ListRefs(fmt.Sprintf("refs/%s/", def.Namespace))
+	if err != nil {
+		return nil, err
+	}
+	return entity.RefsToIds(refs), nil
+}
+
+// Fetch retrieve updates from a remote
+// This does not change the local entity state
+func Fetch(def Definition, repo repository.Repo, remote string) (string, error) {
+	return repo.FetchRefs(remote, def.Namespace)
+}
+
+// Push update a remote with the local changes
+func Push(def Definition, repo repository.Repo, remote string) (string, error) {
+	return repo.PushRefs(remote, def.Namespace)
+}
+
+// Pull will do a Fetch + MergeAll
+// Contrary to MergeAll, this function will return an error if a merge fail.
+func Pull(def Definition, repo repository.ClockedRepo, resolver identity.Resolver, remote string, author identity.Interface) error {
+	_, err := Fetch(def, repo, remote)
+	if err != nil {
+		return err
+	}
+
+	for merge := range MergeAll(def, repo, resolver, remote, author) {
+		if merge.Err != nil {
+			return merge.Err
+		}
+		if merge.Status == entity.MergeStatusInvalid {
+			return errors.Errorf("merge failure: %s", merge.Reason)
+		}
+	}
+
+	return nil
+}
+
+// MergeAll will merge all the available remote Entity:
+//
+// Multiple scenario exist:
+// 1. if the remote Entity doesn't exist locally, it's created
+//    --> emit entity.MergeStatusNew
+// 2. if the remote and local Entity have the same state, nothing is changed
+//    --> emit entity.MergeStatusNothing
+// 3. if the local Entity has new commits but the remote don't, nothing is changed
+//    --> emit entity.MergeStatusNothing
+// 4. if the remote has new commit, the local bug is updated to match the same history
+//    (fast-forward update)
+//    --> emit entity.MergeStatusUpdated
+// 5. if both local and remote Entity have new commits (that is, we have a concurrent edition),
+//    a merge commit with an empty operationPack is created to join both branch and form a DAG.
+//    --> emit entity.MergeStatusUpdated
+//
+// Note: an author is necessary for the case where a merge commit is created, as this commit will
+// have an author and may be signed if a signing key is available.
+func MergeAll(def Definition, repo repository.ClockedRepo, resolver identity.Resolver, remote string, author identity.Interface) <-chan entity.MergeResult {
+	out := make(chan entity.MergeResult)
+
+	go func() {
+		defer close(out)
+
+		remoteRefSpec := fmt.Sprintf("refs/remotes/%s/%s/", remote, def.Namespace)
+		remoteRefs, err := repo.ListRefs(remoteRefSpec)
+		if err != nil {
+			out <- entity.MergeResult{Err: err}
+			return
+		}
+
+		for _, remoteRef := range remoteRefs {
+			out <- merge(def, repo, resolver, remoteRef, author)
+		}
+	}()
+
+	return out
+}
+
+// merge perform a merge to make sure a local Entity is up to date.
+// See MergeAll for more details.
+func merge(def Definition, repo repository.ClockedRepo, resolver identity.Resolver, remoteRef string, author identity.Interface) entity.MergeResult {
+	id := entity.RefToId(remoteRef)
+
+	if err := id.Validate(); err != nil {
+		return entity.NewMergeInvalidStatus(id, errors.Wrap(err, "invalid ref").Error())
+	}
+
+	remoteEntity, err := read(def, repo, resolver, remoteRef)
+	if err != nil {
+		return entity.NewMergeInvalidStatus(id,
+			errors.Wrapf(err, "remote %s is not readable", def.Typename).Error())
+	}
+
+	// Check for error in remote data
+	if err := remoteEntity.Validate(); err != nil {
+		return entity.NewMergeInvalidStatus(id,
+			errors.Wrapf(err, "remote %s data is invalid", def.Typename).Error())
+	}
+
+	localRef := fmt.Sprintf("refs/%s/%s", def.Namespace, id.String())
+
+	// SCENARIO 1
+	// if the remote Entity doesn't exist locally, it's created
+
+	localExist, err := repo.RefExist(localRef)
+	if err != nil {
+		return entity.NewMergeError(err, id)
+	}
+
+	if !localExist {
+		// the bug is not local yet, simply create the reference
+		err := repo.CopyRef(remoteRef, localRef)
+		if err != nil {
+			return entity.NewMergeError(err, id)
+		}
+
+		return entity.NewMergeNewStatus(id, remoteEntity)
+	}
+
+	localCommit, err := repo.ResolveRef(localRef)
+	if err != nil {
+		return entity.NewMergeError(err, id)
+	}
+
+	remoteCommit, err := repo.ResolveRef(remoteRef)
+	if err != nil {
+		return entity.NewMergeError(err, id)
+	}
+
+	// SCENARIO 2
+	// if the remote and local Entity have the same state, nothing is changed
+
+	if localCommit == remoteCommit {
+		// nothing to merge
+		return entity.NewMergeNothingStatus(id)
+	}
+
+	// SCENARIO 3
+	// if the local Entity has new commits but the remote don't, nothing is changed
+
+	localCommits, err := repo.ListCommits(localRef)
+	if err != nil {
+		return entity.NewMergeError(err, id)
+	}
+
+	for _, hash := range localCommits {
+		if hash == remoteCommit {
+			return entity.NewMergeNothingStatus(id)
+		}
+	}
+
+	// SCENARIO 4
+	// if the remote has new commit, the local bug is updated to match the same history
+	// (fast-forward update)
+
+	remoteCommits, err := repo.ListCommits(remoteRef)
+	if err != nil {
+		return entity.NewMergeError(err, id)
+	}
+
+	// fast-forward is possible if otherRef include ref
+	fastForwardPossible := false
+	for _, hash := range remoteCommits {
+		if hash == localCommit {
+			fastForwardPossible = true
+			break
+		}
+	}
+
+	if fastForwardPossible {
+		err = repo.UpdateRef(localRef, remoteCommit)
+		if err != nil {
+			return entity.NewMergeError(err, id)
+		}
+		return entity.NewMergeUpdatedStatus(id, remoteEntity)
+	}
+
+	// SCENARIO 5
+	// if both local and remote Entity have new commits (that is, we have a concurrent edition),
+	// a merge commit with an empty operationPack is created to join both branch and form a DAG.
+
+	// fast-forward is not possible, we need to create a merge commit
+	// For simplicity when reading and to have clocks that record this change, we store
+	// an empty operationPack.
+	// First step is to collect those clocks.
+
+	localEntity, err := read(def, repo, resolver, localRef)
+	if err != nil {
+		return entity.NewMergeError(err, id)
+	}
+
+	editTime, err := repo.Increment(fmt.Sprintf(editClockPattern, def.Namespace))
+	if err != nil {
+		return entity.NewMergeError(err, id)
+	}
+
+	opp := &operationPack{
+		Author:     author,
+		Operations: nil,
+		CreateTime: 0,
+		EditTime:   editTime,
+	}
+
+	commitHash, err := opp.Write(def, repo, localCommit, remoteCommit)
+	if err != nil {
+		return entity.NewMergeError(err, id)
+	}
+
+	// finally update the ref
+	err = repo.UpdateRef(localRef, commitHash)
+	if err != nil {
+		return entity.NewMergeError(err, id)
+	}
+
+	// Note: we don't need to update localEntity state (lastCommit, operations...) as we
+	// discard it entirely anyway.
+
+	return entity.NewMergeUpdatedStatus(id, localEntity)
+}
+
+// Remove delete an Entity.
+// Remove is idempotent.
+func Remove(def Definition, repo repository.ClockedRepo, id entity.Id) error {
+	var matches []string
+
+	ref := fmt.Sprintf("refs/%s/%s", def.Namespace, id.String())
+	matches = append(matches, ref)
+
+	remotes, err := repo.GetRemotes()
+	if err != nil {
+		return err
+	}
+
+	for remote := range remotes {
+		ref = fmt.Sprintf("refs/remotes/%s/%s/%s", remote, def.Namespace, id.String())
+		matches = append(matches, ref)
+	}
+
+	for _, ref = range matches {
+		err = repo.RemoveRef(ref)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
diff --git a/entity/dag/entity_actions_test.go b/entity/dag/entity_actions_test.go
new file mode 100644
index 00000000..45e69c7d
--- /dev/null
+++ b/entity/dag/entity_actions_test.go
@@ -0,0 +1,412 @@
+package dag
+
+import (
+	"sort"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/MichaelMure/git-bug/entity"
+	"github.com/MichaelMure/git-bug/repository"
+)
+
+func allEntities(t testing.TB, bugs <-chan StreamedEntity) []*Entity {
+	t.Helper()
+
+	var result []*Entity
+	for streamed := range bugs {
+		require.NoError(t, streamed.Err)
+
+		result = append(result, streamed.Entity)
+	}
+	return result
+}
+
+func TestEntityPushPull(t *testing.T) {
+	repoA, repoB, remote, id1, id2, resolver, def := makeTestContextRemote(t)
+	defer repository.CleanupTestRepos(repoA, repoB, remote)
+
+	// A --> remote --> B
+	e := New(def)
+	e.Append(newOp1(id1, "foo"))
+
+	err := e.Commit(repoA)
+	require.NoError(t, err)
+
+	_, err = Push(def, repoA, "remote")
+	require.NoError(t, err)
+
+	err = Pull(def, repoB, resolver, "remote", id1)
+	require.NoError(t, err)
+
+	entities := allEntities(t, ReadAll(def, repoB, resolver))
+	require.Len(t, entities, 1)
+
+	// B --> remote --> A
+	e = New(def)
+	e.Append(newOp2(id2, "bar"))
+
+	err = e.Commit(repoB)
+	require.NoError(t, err)
+
+	_, err = Push(def, repoB, "remote")
+	require.NoError(t, err)
+
+	err = Pull(def, repoA, resolver, "remote", id1)
+	require.NoError(t, err)
+
+	entities = allEntities(t, ReadAll(def, repoB, resolver))
+	require.Len(t, entities, 2)
+}
+
+func TestListLocalIds(t *testing.T) {
+	repoA, repoB, remote, id1, id2, resolver, def := makeTestContextRemote(t)
+	defer repository.CleanupTestRepos(repoA, repoB, remote)
+
+	// A --> remote --> B
+	e := New(def)
+	e.Append(newOp1(id1, "foo"))
+	err := e.Commit(repoA)
+	require.NoError(t, err)
+
+	e = New(def)
+	e.Append(newOp2(id2, "bar"))
+	err = e.Commit(repoA)
+	require.NoError(t, err)
+
+	listLocalIds(t, def, repoA, 2)
+	listLocalIds(t, def, repoB, 0)
+
+	_, err = Push(def, repoA, "remote")
+	require.NoError(t, err)
+
+	_, err = Fetch(def, repoB, "remote")
+	require.NoError(t, err)
+
+	listLocalIds(t, def, repoA, 2)
+	listLocalIds(t, def, repoB, 0)
+
+	err = Pull(def, repoB, resolver, "remote", id1)
+	require.NoError(t, err)
+
+	listLocalIds(t, def, repoA, 2)
+	listLocalIds(t, def, repoB, 2)
+}
+
+func listLocalIds(t *testing.T, def Definition, repo repository.RepoData, expectedCount int) {
+	ids, err := ListLocalIds(def, repo)
+	require.NoError(t, err)
+	require.Len(t, ids, expectedCount)
+}
+
+func assertMergeResults(t *testing.T, expected []entity.MergeResult, results <-chan entity.MergeResult) {
+	t.Helper()
+
+	var allResults []entity.MergeResult
+	for result := range results {
+		allResults = append(allResults, result)
+	}
+
+	require.Equal(t, len(expected), len(allResults))
+
+	sort.Slice(allResults, func(i, j int) bool {
+		return allResults[i].Id < allResults[j].Id
+	})
+	sort.Slice(expected, func(i, j int) bool {
+		return expected[i].Id < expected[j].Id
+	})
+
+	for i, result := range allResults {
+		require.NoError(t, result.Err)
+
+		require.Equal(t, expected[i].Id, result.Id)
+		require.Equal(t, expected[i].Status, result.Status)
+
+		switch result.Status {
+		case entity.MergeStatusNew, entity.MergeStatusUpdated:
+			require.NotNil(t, result.Entity)
+			require.Equal(t, expected[i].Id, result.Entity.Id())
+		}
+
+		i++
+	}
+}
+
+func assertEqualRefs(t *testing.T, repoA, repoB repository.RepoData, prefix string) {
+	t.Helper()
+
+	refsA, err := repoA.ListRefs("")
+	require.NoError(t, err)
+
+	var refsAFiltered []string
+	for _, ref := range refsA {
+		if strings.HasPrefix(ref, prefix) {
+			refsAFiltered = append(refsAFiltered, ref)
+		}
+	}
+
+	refsB, err := repoB.ListRefs("")
+	require.NoError(t, err)
+
+	var refsBFiltered []string
+	for _, ref := range refsB {
+		if strings.HasPrefix(ref, prefix) {
+			refsBFiltered = append(refsBFiltered, ref)
+		}
+	}
+
+	require.NotEmpty(t, refsAFiltered)
+	require.Equal(t, refsAFiltered, refsBFiltered)
+
+	for _, ref := range refsAFiltered {
+		commitA, err := repoA.ResolveRef(ref)
+		require.NoError(t, err)
+		commitB, err := repoB.ResolveRef(ref)
+		require.NoError(t, err)
+
+		require.Equal(t, commitA, commitB)
+	}
+}
+
+func assertNotEqualRefs(t *testing.T, repoA, repoB repository.RepoData, prefix string) {
+	t.Helper()
+
+	refsA, err := repoA.ListRefs("")
+	require.NoError(t, err)
+
+	var refsAFiltered []string
+	for _, ref := range refsA {
+		if strings.HasPrefix(ref, prefix) {
+			refsAFiltered = append(refsAFiltered, ref)
+		}
+	}
+
+	refsB, err := repoB.ListRefs("")
+	require.NoError(t, err)
+
+	var refsBFiltered []string
+	for _, ref := range refsB {
+		if strings.HasPrefix(ref, prefix) {
+			refsBFiltered = append(refsBFiltered, ref)
+		}
+	}
+
+	require.NotEmpty(t, refsAFiltered)
+	require.Equal(t, refsAFiltered, refsBFiltered)
+
+	for _, ref := range refsAFiltered {
+		commitA, err := repoA.ResolveRef(ref)
+		require.NoError(t, err)
+		commitB, err := repoB.ResolveRef(ref)
+		require.NoError(t, err)
+
+		require.NotEqual(t, commitA, commitB)
+	}
+}
+
+func TestMerge(t *testing.T) {
+	repoA, repoB, remote, id1, id2, resolver, def := makeTestContextRemote(t)
+	defer repository.CleanupTestRepos(repoA, repoB, remote)
+
+	// SCENARIO 1
+	// if the remote Entity doesn't exist locally, it's created
+
+	// 2 entities in repoA + push to remote
+	e1A := New(def)
+	e1A.Append(newOp1(id1, "foo"))
+	err := e1A.Commit(repoA)
+	require.NoError(t, err)
+
+	e2A := New(def)
+	e2A.Append(newOp2(id2, "bar"))
+	err = e2A.Commit(repoA)
+	require.NoError(t, err)
+
+	_, err = Push(def, repoA, "remote")
+	require.NoError(t, err)
+
+	// repoB: fetch + merge from remote
+
+	_, err = Fetch(def, repoB, "remote")
+	require.NoError(t, err)
+
+	results := MergeAll(def, repoB, resolver, "remote", id1)
+
+	assertMergeResults(t, []entity.MergeResult{
+		{
+			Id:     e1A.Id(),
+			Status: entity.MergeStatusNew,
+		},
+		{
+			Id:     e2A.Id(),
+			Status: entity.MergeStatusNew,
+		},
+	}, results)
+
+	assertEqualRefs(t, repoA, repoB, "refs/"+def.Namespace)
+
+	// SCENARIO 2
+	// if the remote and local Entity have the same state, nothing is changed
+
+	results = MergeAll(def, repoB, resolver, "remote", id1)
+
+	assertMergeResults(t, []entity.MergeResult{
+		{
+			Id:     e1A.Id(),
+			Status: entity.MergeStatusNothing,
+		},
+		{
+			Id:     e2A.Id(),
+			Status: entity.MergeStatusNothing,
+		},
+	}, results)
+
+	assertEqualRefs(t, repoA, repoB, "refs/"+def.Namespace)
+
+	// SCENARIO 3
+	// if the local Entity has new commits but the remote don't, nothing is changed
+
+	e1A.Append(newOp1(id1, "barbar"))
+	err = e1A.Commit(repoA)
+	require.NoError(t, err)
+
+	e2A.Append(newOp2(id2, "barbarbar"))
+	err = e2A.Commit(repoA)
+	require.NoError(t, err)
+
+	results = MergeAll(def, repoA, resolver, "remote", id1)
+
+	assertMergeResults(t, []entity.MergeResult{
+		{
+			Id:     e1A.Id(),
+			Status: entity.MergeStatusNothing,
+		},
+		{
+			Id:     e2A.Id(),
+			Status: entity.MergeStatusNothing,
+		},
+	}, results)
+
+	assertNotEqualRefs(t, repoA, repoB, "refs/"+def.Namespace)
+
+	// SCENARIO 4
+	// if the remote has new commit, the local bug is updated to match the same history
+	// (fast-forward update)
+
+	_, err = Push(def, repoA, "remote")
+	require.NoError(t, err)
+
+	_, err = Fetch(def, repoB, "remote")
+	require.NoError(t, err)
+
+	results = MergeAll(def, repoB, resolver, "remote", id1)
+
+	assertMergeResults(t, []entity.MergeResult{
+		{
+			Id:     e1A.Id(),
+			Status: entity.MergeStatusUpdated,
+		},
+		{
+			Id:     e2A.Id(),
+			Status: entity.MergeStatusUpdated,
+		},
+	}, results)
+
+	assertEqualRefs(t, repoA, repoB, "refs/"+def.Namespace)
+
+	// SCENARIO 5
+	// if both local and remote Entity have new commits (that is, we have a concurrent edition),
+	// a merge commit with an empty operationPack is created to join both branch and form a DAG.
+
+	e1A.Append(newOp1(id1, "barbarfoo"))
+	err = e1A.Commit(repoA)
+	require.NoError(t, err)
+
+	e2A.Append(newOp2(id2, "barbarbarfoo"))
+	err = e2A.Commit(repoA)
+	require.NoError(t, err)
+
+	e1B, err := Read(def, repoB, resolver, e1A.Id())
+	require.NoError(t, err)
+
+	e2B, err := Read(def, repoB, resolver, e2A.Id())
+	require.NoError(t, err)
+
+	e1B.Append(newOp1(id1, "barbarfoofoo"))
+	err = e1B.Commit(repoB)
+	require.NoError(t, err)
+
+	e2B.Append(newOp2(id2, "barbarbarfoofoo"))
+	err = e2B.Commit(repoB)
+	require.NoError(t, err)
+
+	_, err = Push(def, repoA, "remote")
+	require.NoError(t, err)
+
+	_, err = Fetch(def, repoB, "remote")
+	require.NoError(t, err)
+
+	results = MergeAll(def, repoB, resolver, "remote", id1)
+
+	assertMergeResults(t, []entity.MergeResult{
+		{
+			Id:     e1A.Id(),
+			Status: entity.MergeStatusUpdated,
+		},
+		{
+			Id:     e2A.Id(),
+			Status: entity.MergeStatusUpdated,
+		},
+	}, results)
+
+	assertNotEqualRefs(t, repoA, repoB, "refs/"+def.Namespace)
+
+	_, err = Push(def, repoB, "remote")
+	require.NoError(t, err)
+
+	_, err = Fetch(def, repoA, "remote")
+	require.NoError(t, err)
+
+	results = MergeAll(def, repoA, resolver, "remote", id1)
+
+	assertMergeResults(t, []entity.MergeResult{
+		{
+			Id:     e1A.Id(),
+			Status: entity.MergeStatusUpdated,
+		},
+		{
+			Id:     e2A.Id(),
+			Status: entity.MergeStatusUpdated,
+		},
+	}, results)
+
+	// make sure that the graphs become stable over multiple repo, due to the
+	// fast-forward
+	assertEqualRefs(t, repoA, repoB, "refs/"+def.Namespace)
+}
+
+func TestRemove(t *testing.T) {
+	repoA, repoB, remote, id1, _, resolver, def := makeTestContextRemote(t)
+	defer repository.CleanupTestRepos(repoA, repoB, remote)
+
+	e := New(def)
+	e.Append(newOp1(id1, "foo"))
+	require.NoError(t, e.Commit(repoA))
+
+	_, err := Push(def, repoA, "remote")
+	require.NoError(t, err)
+
+	err = Remove(def, repoA, e.Id())
+	require.NoError(t, err)
+
+	_, err = Read(def, repoA, resolver, e.Id())
+	require.Error(t, err)
+
+	_, err = readRemote(def, repoA, resolver, "remote", e.Id())
+	require.Error(t, err)
+
+	// Remove is idempotent
+	err = Remove(def, repoA, e.Id())
+	require.NoError(t, err)
+}
diff --git a/entity/dag/entity_test.go b/entity/dag/entity_test.go
new file mode 100644
index 00000000..6d621bbe
--- /dev/null
+++ b/entity/dag/entity_test.go
@@ -0,0 +1,68 @@
+package dag
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestWriteRead(t *testing.T) {
+	repo, id1, id2, resolver, def := makeTestContext()
+
+	entity := New(def)
+	require.False(t, entity.NeedCommit())
+
+	entity.Append(newOp1(id1, "foo"))
+	entity.Append(newOp2(id1, "bar"))
+
+	require.True(t, entity.NeedCommit())
+	require.NoError(t, entity.CommitAsNeeded(repo))
+	require.False(t, entity.NeedCommit())
+
+	entity.Append(newOp2(id2, "foobar"))
+	require.True(t, entity.NeedCommit())
+	require.NoError(t, entity.CommitAsNeeded(repo))
+	require.False(t, entity.NeedCommit())
+
+	read, err := Read(def, repo, resolver, entity.Id())
+	require.NoError(t, err)
+
+	assertEqualEntities(t, entity, read)
+}
+
+func TestWriteReadMultipleAuthor(t *testing.T) {
+	repo, id1, id2, resolver, def := makeTestContext()
+
+	entity := New(def)
+
+	entity.Append(newOp1(id1, "foo"))
+	entity.Append(newOp2(id2, "bar"))
+
+	require.NoError(t, entity.CommitAsNeeded(repo))
+
+	entity.Append(newOp2(id1, "foobar"))
+	require.NoError(t, entity.CommitAsNeeded(repo))
+
+	read, err := Read(def, repo, resolver, entity.Id())
+	require.NoError(t, err)
+
+	assertEqualEntities(t, entity, read)
+}
+
+func assertEqualEntities(t *testing.T, a, b *Entity) {
+	// testify doesn't support comparing functions and systematically fail if they are not nil
+	// so we have to set them to nil temporarily
+
+	backOpUnA := a.Definition.OperationUnmarshaler
+	backOpUnB := b.Definition.OperationUnmarshaler
+
+	a.Definition.OperationUnmarshaler = nil
+	b.Definition.OperationUnmarshaler = nil
+
+	defer func() {
+		a.Definition.OperationUnmarshaler = backOpUnA
+		b.Definition.OperationUnmarshaler = backOpUnB
+	}()
+
+	require.Equal(t, a, b)
+}
diff --git a/entity/dag/operation.go b/entity/dag/operation.go
new file mode 100644
index 00000000..a320859f
--- /dev/null
+++ b/entity/dag/operation.go
@@ -0,0 +1,48 @@
+package dag
+
+import (
+	"github.com/MichaelMure/git-bug/entity"
+	"github.com/MichaelMure/git-bug/identity"
+	"github.com/MichaelMure/git-bug/repository"
+)
+
+// Operation is a piece of data defining a change to reflect on the state of an Entity.
+// What this Operation or Entity's state looks like is not of the resort of this package as it only deals with the
+// data structure and storage.
+type Operation interface {
+	// Id return the Operation identifier
+	//
+	// Some care need to be taken to define a correct Id derivation and enough entropy in the data used to avoid
+	// collisions. Notably:
+	// - the Id of the first Operation will be used as the Id of the Entity. Collision need to be avoided across entities
+	//   of the same type (example: no collision within the "bug" namespace).
+	// - collisions can also happen within the set of Operations of an Entity. Simple Operation might not have enough
+	//   entropy to yield unique Ids (example: two "close" operation within the same second, same author).
+	//   If this is a concern, it is recommended to include a piece of random data in the operation's data, to guarantee
+	//   a minimal amount of entropy and avoid collision.
+	//
+	//   Author's note: I tried to find a clever way around that inelegance (stuffing random useless data into the stored
+	//   structure is not exactly elegant) but I failed to find a proper way. Essentially, anything that would reuse some
+	//   other data (parent operation's Id, lamport clock) or the graph structure (depth) impose that the Id would only
+	//   make sense in the context of the graph and yield some deep coupling between Entity and Operation. This in turn
+	//   make the whole thing even less elegant.
+	//
+	// A common way to derive an Id will be to use the entity.DeriveId() function on the serialized operation data.
+	Id() entity.Id
+	// Validate check if the Operation data is valid
+	Validate() error
+	// Author returns the author of this operation
+	Author() identity.Interface
+}
+
+// OperationWithFiles is an extended Operation that has files dependency, stored in git.
+type OperationWithFiles interface {
+	Operation
+
+	// GetFiles return the files needed by this operation
+	// This implies that the Operation maintain and store internally the references to those files. This is how
+	// this information is read later, when loading from storage.
+	// For example, an operation that has a text value referencing some files would maintain a mapping (text ref -->
+	// hash).
+	GetFiles() []repository.Hash
+}
diff --git a/entity/dag/operation_pack.go b/entity/dag/operation_pack.go
new file mode 100644
index 00000000..72063c60
--- /dev/null
+++ b/entity/dag/operation_pack.go
@@ -0,0 +1,358 @@
+package dag
+
+import (
+	"encoding/json"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/pkg/errors"
+	"golang.org/x/crypto/openpgp"
+
+	"github.com/MichaelMure/git-bug/entity"
+	"github.com/MichaelMure/git-bug/identity"
+	"github.com/MichaelMure/git-bug/repository"
+	"github.com/MichaelMure/git-bug/util/lamport"
+)
+
+const opsEntryName = "ops"
+const extraEntryName = "extra"
+const versionEntryPrefix = "version-"
+const createClockEntryPrefix = "create-clock-"
+const editClockEntryPrefix = "edit-clock-"
+
+// operationPack is a wrapper structure to store multiple operations in a single git blob.
+// Additionally, it holds and store the metadata for those operations.
+type operationPack struct {
+	// An identifier, taken from a hash of the serialized Operations.
+	id entity.Id
+
+	// The author of the Operations. Must be the same author for all the Operations.
+	Author identity.Interface
+	// The list of Operation stored in the operationPack
+	Operations []Operation
+	// Encode the entity's logical time of creation across all entities of the same type.
+	// Only exist on the root operationPack
+	CreateTime lamport.Time
+	// Encode the entity's logical time of last edition across all entities of the same type.
+	// Exist on all operationPack
+	EditTime lamport.Time
+}
+
+func (opp *operationPack) Id() entity.Id {
+	if opp.id == "" || opp.id == entity.UnsetId {
+		// This means we are trying to get the opp's Id *before* it has been stored.
+		// As the Id is computed based on the actual bytes written on the disk, we are going to predict
+		// those and then get the Id. This is safe as it will be the exact same code writing on disk later.
+
+		data, err := json.Marshal(opp)
+		if err != nil {
+			panic(err)
+		}
+		opp.id = entity.DeriveId(data)
+	}
+
+	return opp.id
+}
+
+func (opp *operationPack) MarshalJSON() ([]byte, error) {
+	return json.Marshal(struct {
+		Author     identity.Interface `json:"author"`
+		Operations []Operation        `json:"ops"`
+	}{
+		Author:     opp.Author,
+		Operations: opp.Operations,
+	})
+}
+
+func (opp *operationPack) Validate() error {
+	if opp.Author == nil {
+		return fmt.Errorf("missing author")
+	}
+	for _, op := range opp.Operations {
+		if op.Author().Id() != opp.Author.Id() {
+			return fmt.Errorf("operation has different author than the operationPack's")
+		}
+	}
+	if opp.EditTime == 0 {
+		return fmt.Errorf("lamport edit time is zero")
+	}
+	return nil
+}
+
+// Write write the OperationPack in git, with zero, one or more parent commits.
+// If the repository has a keypair able to sign (that is, with a private key), the resulting commit is signed with that key.
+// Return the hash of the created commit.
+func (opp *operationPack) Write(def Definition, repo repository.Repo, parentCommit ...repository.Hash) (repository.Hash, error) {
+	if err := opp.Validate(); err != nil {
+		return "", err
+	}
+
+	// For different reason, we store the clocks and format version directly in the git tree.
+	// Version has to be accessible before any attempt to decode to return early with a unique error.
+	// Clocks could possibly be stored in the git blob but it's nice to separate data and metadata, and
+	// we are storing something directly in the tree already so why not.
+	//
+	// To have a valid Tree, we point the "fake" entries to always the same value, the empty blob.
+	emptyBlobHash, err := repo.StoreData([]byte{})
+	if err != nil {
+		return "", err
+	}
+
+	// Write the Ops as a Git blob containing the serialized array of operations
+	data, err := json.Marshal(opp)
+	if err != nil {
+		return "", err
+	}
+
+	// compute the Id while we have the serialized data
+	opp.id = entity.DeriveId(data)
+
+	hash, err := repo.StoreData(data)
+	if err != nil {
+		return "", err
+	}
+
+	// Make a Git tree referencing this blob and encoding the other values:
+	// - format version
+	// - clocks
+	// - extra data
+	tree := []repository.TreeEntry{
+		{ObjectType: repository.Blob, Hash: emptyBlobHash,
+			Name: fmt.Sprintf(versionEntryPrefix+"%d", def.FormatVersion)},
+		{ObjectType: repository.Blob, Hash: hash,
+			Name: opsEntryName},
+		{ObjectType: repository.Blob, Hash: emptyBlobHash,
+			Name: fmt.Sprintf(editClockEntryPrefix+"%d", opp.EditTime)},
+	}
+	if opp.CreateTime > 0 {
+		tree = append(tree, repository.TreeEntry{
+			ObjectType: repository.Blob,
+			Hash:       emptyBlobHash,
+			Name:       fmt.Sprintf(createClockEntryPrefix+"%d", opp.CreateTime),
+		})
+	}
+	if extraTree := opp.makeExtraTree(); len(extraTree) > 0 {
+		extraTreeHash, err := repo.StoreTree(extraTree)
+		if err != nil {
+			return "", err
+		}
+		tree = append(tree, repository.TreeEntry{
+			ObjectType: repository.Tree,
+			Hash:       extraTreeHash,
+			Name:       extraEntryName,
+		})
+	}
+
+	// Store the tree
+	treeHash, err := repo.StoreTree(tree)
+	if err != nil {
+		return "", err
+	}
+
+	// Write a Git commit referencing the tree, with the previous commit as parent
+	// If we have keys, sign.
+	var commitHash repository.Hash
+
+	// Sign the commit if we have a key
+	signingKey, err := opp.Author.SigningKey(repo)
+	if err != nil {
+		return "", err
+	}
+
+	if signingKey != nil {
+		commitHash, err = repo.StoreSignedCommit(treeHash, signingKey.PGPEntity(), parentCommit...)
+	} else {
+		commitHash, err = repo.StoreCommit(treeHash, parentCommit...)
+	}
+
+	if err != nil {
+		return "", err
+	}
+
+	return commitHash, nil
+}
+
+func (opp *operationPack) makeExtraTree() []repository.TreeEntry {
+	var tree []repository.TreeEntry
+	counter := 0
+	added := make(map[repository.Hash]interface{})
+
+	for _, ops := range opp.Operations {
+		ops, ok := ops.(OperationWithFiles)
+		if !ok {
+			continue
+		}
+
+		for _, file := range ops.GetFiles() {
+			if _, has := added[file]; !has {
+				tree = append(tree, repository.TreeEntry{
+					ObjectType: repository.Blob,
+					Hash:       file,
+					// The name is not important here, we only need to
+					// reference the blob.
+					Name: fmt.Sprintf("file%d", counter),
+				})
+				counter++
+				added[file] = struct{}{}
+			}
+		}
+	}
+
+	return tree
+}
+
+// readOperationPack read the operationPack encoded in git at the given Tree hash.
+//
+// Validity of the Lamport clocks is left for the caller to decide.
+func readOperationPack(def Definition, repo repository.RepoData, resolver identity.Resolver, commit repository.Commit) (*operationPack, error) {
+	entries, err := repo.ReadTree(commit.TreeHash)
+	if err != nil {
+		return nil, err
+	}
+
+	// check the format version first, fail early instead of trying to read something
+	var version uint
+	for _, entry := range entries {
+		if strings.HasPrefix(entry.Name, versionEntryPrefix) {
+			v, err := strconv.ParseUint(strings.TrimPrefix(entry.Name, versionEntryPrefix), 10, 64)
+			if err != nil {
+				return nil, errors.Wrap(err, "can't read format version")
+			}
+			if v > 1<<12 {
+				return nil, fmt.Errorf("format version too big")
+			}
+			version = uint(v)
+			break
+		}
+	}
+	if version == 0 {
+		return nil, entity.NewErrUnknownFormat(def.FormatVersion)
+	}
+	if version != def.FormatVersion {
+		return nil, entity.NewErrInvalidFormat(version, def.FormatVersion)
+	}
+
+	var id entity.Id
+	var author identity.Interface
+	var ops []Operation
+	var createTime lamport.Time
+	var editTime lamport.Time
+
+	for _, entry := range entries {
+		switch {
+		case entry.Name == opsEntryName:
+			data, err := repo.ReadData(entry.Hash)
+			if err != nil {
+				return nil, errors.Wrap(err, "failed to read git blob data")
+			}
+			ops, author, err = unmarshallPack(def, resolver, data)
+			if err != nil {
+				return nil, err
+			}
+			id = entity.DeriveId(data)
+
+		case strings.HasPrefix(entry.Name, createClockEntryPrefix):
+			v, err := strconv.ParseUint(strings.TrimPrefix(entry.Name, createClockEntryPrefix), 10, 64)
+			if err != nil {
+				return nil, errors.Wrap(err, "can't read creation lamport time")
+			}
+			createTime = lamport.Time(v)
+
+		case strings.HasPrefix(entry.Name, editClockEntryPrefix):
+			v, err := strconv.ParseUint(strings.TrimPrefix(entry.Name, editClockEntryPrefix), 10, 64)
+			if err != nil {
+				return nil, errors.Wrap(err, "can't read edit lamport time")
+			}
+			editTime = lamport.Time(v)
+		}
+	}
+
+	// Verify signature if we expect one
+	keys := author.ValidKeysAtTime(fmt.Sprintf(editClockPattern, def.Namespace), editTime)
+	if len(keys) > 0 {
+		keyring := PGPKeyring(keys)
+		_, err = openpgp.CheckDetachedSignature(keyring, commit.SignedData, commit.Signature)
+		if err != nil {
+			return nil, fmt.Errorf("signature failure: %v", err)
+		}
+	}
+
+	return &operationPack{
+		id:         id,
+		Author:     author,
+		Operations: ops,
+		CreateTime: createTime,
+		EditTime:   editTime,
+	}, nil
+}
+
+// unmarshallPack delegate the unmarshalling of the Operation's JSON to the decoding
+// function provided by the concrete entity. This gives access to the concrete type of each
+// Operation.
+func unmarshallPack(def Definition, resolver identity.Resolver, data []byte) ([]Operation, identity.Interface, error) {
+	aux := struct {
+		Author     identity.IdentityStub `json:"author"`
+		Operations []json.RawMessage     `json:"ops"`
+	}{}
+
+	if err := json.Unmarshal(data, &aux); err != nil {
+		return nil, nil, err
+	}
+
+	if aux.Author.Id() == "" || aux.Author.Id() == entity.UnsetId {
+		return nil, nil, fmt.Errorf("missing author")
+	}
+
+	author, err := resolver.ResolveIdentity(aux.Author.Id())
+	if err != nil {
+		return nil, nil, err
+	}
+
+	ops := make([]Operation, 0, len(aux.Operations))
+
+	for _, raw := range aux.Operations {
+		// delegate to specialized unmarshal function
+		op, err := def.OperationUnmarshaler(author, raw)
+		if err != nil {
+			return nil, nil, err
+		}
+		ops = append(ops, op)
+	}
+
+	return ops, author, nil
+}
+
+var _ openpgp.KeyRing = &PGPKeyring{}
+
+// PGPKeyring implement a openpgp.KeyRing from an slice of Key
+type PGPKeyring []*identity.Key
+
+func (pk PGPKeyring) KeysById(id uint64) []openpgp.Key {
+	var result []openpgp.Key
+	for _, key := range pk {
+		if key.Public().KeyId == id {
+			result = append(result, openpgp.Key{
+				PublicKey:  key.Public(),
+				PrivateKey: key.Private(),
+			})
+		}
+	}
+	return result
+}
+
+func (pk PGPKeyring) KeysByIdUsage(id uint64, requiredUsage byte) []openpgp.Key {
+	// the only usage we care about is the ability to sign, which all keys should already be capable of
+	return pk.KeysById(id)
+}
+
+func (pk PGPKeyring) DecryptionKeys() []openpgp.Key {
+	result := make([]openpgp.Key, len(pk))
+	for i, key := range pk {
+		result[i] = openpgp.Key{
+			PublicKey:  key.Public(),
+			PrivateKey: key.Private(),
+		}
+	}
+	return result
+}
diff --git a/entity/dag/operation_pack_test.go b/entity/dag/operation_pack_test.go
new file mode 100644
index 00000000..73960800
--- /dev/null
+++ b/entity/dag/operation_pack_test.go
@@ -0,0 +1,159 @@
+package dag
+
+import (
+	"math/rand"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/MichaelMure/git-bug/identity"
+	"github.com/MichaelMure/git-bug/repository"
+)
+
+func TestOperationPackReadWrite(t *testing.T) {
+	repo, id1, _, resolver, def := makeTestContext()
+
+	opp := &operationPack{
+		Author: id1,
+		Operations: []Operation{
+			newOp1(id1, "foo"),
+			newOp2(id1, "bar"),
+		},
+		CreateTime: 123,
+		EditTime:   456,
+	}
+
+	commitHash, err := opp.Write(def, repo)
+	require.NoError(t, err)
+
+	commit, err := repo.ReadCommit(commitHash)
+	require.NoError(t, err)
+
+	opp2, err := readOperationPack(def, repo, resolver, commit)
+	require.NoError(t, err)
+
+	require.Equal(t, opp, opp2)
+
+	// make sure we get the same Id with the same data
+	opp3 := &operationPack{
+		Author: id1,
+		Operations: []Operation{
+			newOp1(id1, "foo"),
+			newOp2(id1, "bar"),
+		},
+		CreateTime: 123,
+		EditTime:   456,
+	}
+	require.Equal(t, opp.Id(), opp3.Id())
+}
+
+func TestOperationPackSignedReadWrite(t *testing.T) {
+	repo, id1, _, resolver, def := makeTestContext()
+
+	err := id1.(*identity.Identity).Mutate(repo, func(orig *identity.Mutator) {
+		orig.Keys = append(orig.Keys, identity.GenerateKey())
+	})
+	require.NoError(t, err)
+
+	opp := &operationPack{
+		Author: id1,
+		Operations: []Operation{
+			newOp1(id1, "foo"),
+			newOp2(id1, "bar"),
+		},
+		CreateTime: 123,
+		EditTime:   456,
+	}
+
+	commitHash, err := opp.Write(def, repo)
+	require.NoError(t, err)
+
+	commit, err := repo.ReadCommit(commitHash)
+	require.NoError(t, err)
+
+	opp2, err := readOperationPack(def, repo, resolver, commit)
+	require.NoError(t, err)
+
+	require.Equal(t, opp, opp2)
+
+	// make sure we get the same Id with the same data
+	opp3 := &operationPack{
+		Author: id1,
+		Operations: []Operation{
+			newOp1(id1, "foo"),
+			newOp2(id1, "bar"),
+		},
+		CreateTime: 123,
+		EditTime:   456,
+	}
+	require.Equal(t, opp.Id(), opp3.Id())
+}
+
+func TestOperationPackFiles(t *testing.T) {
+	repo, id1, _, resolver, def := makeTestContext()
+
+	blobHash1, err := repo.StoreData(randomData())
+	require.NoError(t, err)
+
+	blobHash2, err := repo.StoreData(randomData())
+	require.NoError(t, err)
+
+	opp := &operationPack{
+		Author: id1,
+		Operations: []Operation{
+			newOp1(id1, "foo", blobHash1, blobHash2),
+			newOp1(id1, "foo", blobHash2),
+		},
+		CreateTime: 123,
+		EditTime:   456,
+	}
+
+	commitHash, err := opp.Write(def, repo)
+	require.NoError(t, err)
+
+	commit, err := repo.ReadCommit(commitHash)
+	require.NoError(t, err)
+
+	opp2, err := readOperationPack(def, repo, resolver, commit)
+	require.NoError(t, err)
+
+	require.Equal(t, opp, opp2)
+
+	require.ElementsMatch(t, opp2.Operations[0].(OperationWithFiles).GetFiles(), []repository.Hash{
+		blobHash1,
+		blobHash2,
+	})
+	require.ElementsMatch(t, opp2.Operations[1].(OperationWithFiles).GetFiles(), []repository.Hash{
+		blobHash2,
+	})
+
+	tree, err := repo.ReadTree(commit.TreeHash)
+	require.NoError(t, err)
+
+	extraTreeHash, ok := repository.SearchTreeEntry(tree, extraEntryName)
+	require.True(t, ok)
+
+	extraTree, err := repo.ReadTree(extraTreeHash.Hash)
+	require.NoError(t, err)
+	require.ElementsMatch(t, extraTree, []repository.TreeEntry{
+		{
+			ObjectType: repository.Blob,
+			Hash:       blobHash1,
+			Name:       "file0",
+		},
+		{
+			ObjectType: repository.Blob,
+			Hash:       blobHash2,
+			Name:       "file1",
+		},
+	})
+}
+
+func randomData() []byte {
+	var letterRunes = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+	b := make([]byte, 32)
+	for i := range b {
+		b[i] = letterRunes[rand.Intn(len(letterRunes))]
+	}
+	return b
+}
diff --git a/entity/doc.go b/entity/doc.go
deleted file mode 100644
index 4682d545..00000000
--- a/entity/doc.go
+++ /dev/null
@@ -1,8 +0,0 @@
-// Package entity contains the base common code to define an entity stored
-// in a chain of git objects, supporting actions like Push, Pull and Merge.
-package entity
-
-// TODO: Bug and Identity are very similar, right ? I expect that this package
-// will eventually hold the common code to define an entity and the related
-// helpers, errors and so on. When this work is done, it will become easier
-// to add new entities, for example to support pull requests.
diff --git a/entity/err.go b/entity/err.go
index 90304d03..408e27b4 100644
--- a/entity/err.go
+++ b/entity/err.go
@@ -31,28 +31,31 @@ func IsErrMultipleMatch(err error) bool {
 	return ok
 }
 
-// ErrOldFormatVersion indicate that the read data has a too old format.
-type ErrOldFormatVersion struct {
-	formatVersion uint
+type ErrInvalidFormat struct {
+	version  uint
+	expected uint
 }
 
-func NewErrOldFormatVersion(formatVersion uint) *ErrOldFormatVersion {
-	return &ErrOldFormatVersion{formatVersion: formatVersion}
-}
-
-func (e ErrOldFormatVersion) Error() string {
-	return fmt.Sprintf("outdated repository format %v, please use https://github.com/MichaelMure/git-bug-migration to upgrade", e.formatVersion)
-}
-
-// ErrNewFormatVersion indicate that the read data is too new for this software.
-type ErrNewFormatVersion struct {
-	formatVersion uint
+func NewErrInvalidFormat(version uint, expected uint) *ErrInvalidFormat {
+	return &ErrInvalidFormat{
+		version:  version,
+		expected: expected,
+	}
 }
 
-func NewErrNewFormatVersion(formatVersion uint) *ErrNewFormatVersion {
-	return &ErrNewFormatVersion{formatVersion: formatVersion}
+func NewErrUnknownFormat(expected uint) *ErrInvalidFormat {
+	return &ErrInvalidFormat{
+		version:  0,
+		expected: expected,
+	}
 }
 
-func (e ErrNewFormatVersion) Error() string {
-	return fmt.Sprintf("your version of git-bug is too old for this repository (version %v), please upgrade to the latest version", e.formatVersion)
+func (e ErrInvalidFormat) Error() string {
+	if e.version == 0 {
+		return fmt.Sprintf("unreadable data, you likely have an outdated repository format, please use https://github.com/MichaelMure/git-bug-migration to upgrade to format version %v", e.expected)
+	}
+	if e.version < e.expected {
+		return fmt.Sprintf("outdated repository format %v, please use https://github.com/MichaelMure/git-bug-migration to upgrade to format version %v", e.version, e.expected)
+	}
+	return fmt.Sprintf("your version of git-bug is too old for this repository (format version %v, expected %v), please upgrade to the latest version", e.version, e.expected)
 }
diff --git a/entity/id.go b/entity/id.go
index 1b78aacd..c8dbdb94 100644
--- a/entity/id.go
+++ b/entity/id.go
@@ -1,6 +1,7 @@
 package entity
 
 import (
+	"crypto/sha256"
 	"fmt"
 	"io"
 	"strings"
@@ -8,8 +9,8 @@ import (
 	"github.com/pkg/errors"
 )
 
-const IdLengthSHA1 = 40
-const IdLengthSHA256 = 64
+// sha-256
+const idLength = 64
 const humanIdLength = 7
 
 const UnsetId = Id("unset")
@@ -17,6 +18,15 @@ const UnsetId = Id("unset")
 // Id is an identifier for an entity or part of an entity
 type Id string
 
+// DeriveId generate an Id from the serialization of the object or part of the object.
+func DeriveId(data []byte) Id {
+	// My understanding is that sha256 is enough to prevent collision (git use that, so ...?)
+	// If you read this code, I'd be happy to be schooled.
+
+	sum := sha256.Sum256(data)
+	return Id(fmt.Sprintf("%x", sum))
+}
+
 // String return the identifier as a string
 func (i Id) String() string {
 	return string(i)
@@ -55,7 +65,11 @@ func (i Id) MarshalGQL(w io.Writer) {
 
 // IsValid tell if the Id is valid
 func (i Id) Validate() error {
-	if len(i) != IdLengthSHA1 && len(i) != IdLengthSHA256 {
+	// Special case to detect outdated repo
+	if len(i) == 40 {
+		return fmt.Errorf("outdated repository format, please use https://github.com/MichaelMure/git-bug-migration to upgrade")
+	}
+	if len(i) != idLength {
 		return fmt.Errorf("invalid length")
 	}
 	for _, r := range i {
diff --git a/entity/id_interleaved.go b/entity/id_interleaved.go
new file mode 100644
index 00000000..5423afee
--- /dev/null
+++ b/entity/id_interleaved.go
@@ -0,0 +1,68 @@
+package entity
+
+import (
+	"strings"
+)
+
+// CombineIds compute a merged Id holding information from both the primary Id
+// and the secondary Id.
+//
+// This allow to later find efficiently a secondary element because we can access
+// the primary one directly instead of searching for a primary that has a
+// secondary matching the Id.
+//
+// An example usage is Comment in a Bug. The interleaved Id will hold part of the
+// Bug Id and part of the Comment Id.
+//
+// To allow the use of an arbitrary length prefix of this Id, Ids from primary
+// and secondary are interleaved with this irregular pattern to give the
+// best chance to find the secondary even with a 7 character prefix.
+//
+// Format is: PSPSPSPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPP
+//
+// A complete interleaved Id hold 50 characters for the primary and 14 for the
+// secondary, which give a key space of 36^50 for the primary (~6 * 10^77) and
+// 36^14 for the secondary (~6 * 10^21). This asymmetry assume a reasonable number
+// of secondary within a primary Entity, while still allowing for a vast key space
+// for the primary (that is, a globally merged database) with a low risk of collision.
+//
+// Here is the breakdown of several common prefix length:
+//
+// 5:    3P, 2S
+// 7:    4P, 3S
+// 10:   6P, 4S
+// 16:  11P, 5S
+func CombineIds(primary Id, secondary Id) Id {
+	var id strings.Builder
+
+	for i := 0; i < idLength; i++ {
+		switch {
+		default:
+			id.WriteByte(primary[0])
+			primary = primary[1:]
+		case i == 1, i == 3, i == 5, i == 9, i >= 10 && i%5 == 4:
+			id.WriteByte(secondary[0])
+			secondary = secondary[1:]
+		}
+	}
+
+	return Id(id.String())
+}
+
+// SeparateIds extract primary and secondary prefix from an arbitrary length prefix
+// of an Id created with CombineIds.
+func SeparateIds(prefix string) (primaryPrefix string, secondaryPrefix string) {
+	var primary strings.Builder
+	var secondary strings.Builder
+
+	for i, r := range prefix {
+		switch {
+		default:
+			primary.WriteRune(r)
+		case i == 1, i == 3, i == 5, i == 9, i >= 10 && i%5 == 4:
+			secondary.WriteRune(r)
+		}
+	}
+
+	return primary.String(), secondary.String()
+}
diff --git a/entity/id_interleaved_test.go b/entity/id_interleaved_test.go
new file mode 100644
index 00000000..ef9218c9
--- /dev/null
+++ b/entity/id_interleaved_test.go
@@ -0,0 +1,36 @@
+package entity
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestInterleaved(t *testing.T) {
+	primary := Id("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWX______________")
+	secondary := Id("YZ0123456789+/________________________________________________")
+	expectedId := Id("aYbZc0def1ghij2klmn3opqr4stuv5wxyz6ABCD7EFGH8IJKL9MNOP+QRST/UVWX")
+
+	interleaved := CombineIds(primary, secondary)
+	require.Equal(t, expectedId, interleaved)
+
+	// full length
+	splitPrimary, splitSecondary := SeparateIds(interleaved.String())
+	require.Equal(t, string(primary[:50]), splitPrimary)
+	require.Equal(t, string(secondary[:14]), splitSecondary)
+
+	// partial
+	splitPrimary, splitSecondary = SeparateIds(string(expectedId[:7]))
+	require.Equal(t, string(primary[:4]), splitPrimary)
+	require.Equal(t, string(secondary[:3]), splitSecondary)
+
+	// partial
+	splitPrimary, splitSecondary = SeparateIds(string(expectedId[:10]))
+	require.Equal(t, string(primary[:6]), splitPrimary)
+	require.Equal(t, string(secondary[:4]), splitSecondary)
+
+	// partial
+	splitPrimary, splitSecondary = SeparateIds(string(expectedId[:16]))
+	require.Equal(t, string(primary[:11]), splitPrimary)
+	require.Equal(t, string(secondary[:5]), splitSecondary)
+}
diff --git a/entity/interface.go b/entity/interface.go
index dd5d69b1..fb4735e4 100644
--- a/entity/interface.go
+++ b/entity/interface.go
@@ -2,5 +2,11 @@ package entity
 
 type Interface interface {
 	// Id return the Entity identifier
+	//
+	// This Id need to be immutable without having to store the entity somewhere (ie, an entity only in memory
+	// should have a valid Id, and it should not change if further edit are done on this entity).
+	// How to achieve that is up to the entity itself. A common way would be to take a hash of an immutable data at
+	// the root of the entity.
+	// It is acceptable to use such a hash and keep mutating that data as long as Id() is not called.
 	Id() Id
 }
diff --git a/entity/merge.go b/entity/merge.go
index 3ce8edac..0661b7fc 100644
--- a/entity/merge.go
+++ b/entity/merge.go
@@ -8,14 +8,15 @@ import (
 type MergeStatus int
 
 const (
-	_ MergeStatus = iota
-	MergeStatusNew
-	MergeStatusInvalid
-	MergeStatusUpdated
-	MergeStatusNothing
-	MergeStatusError
+	_                  MergeStatus = iota
+	MergeStatusNew                 // a new Entity was created locally
+	MergeStatusInvalid             // the remote data is invalid
+	MergeStatusUpdated             // a local Entity has been updated
+	MergeStatusNothing             // no changes were made to a local Entity (already up to date)
+	MergeStatusError               // a terminal error happened
 )
 
+// MergeResult hold the result of a merge operation on an Entity.
 type MergeResult struct {
 	// Err is set when a terminal error occur in the process
 	Err error
@@ -23,10 +24,10 @@ type MergeResult struct {
 	Id     Id
 	Status MergeStatus
 
-	// Only set for invalid status
+	// Only set for Invalid status
 	Reason string
 
-	// Not set for invalid status
+	// Only set for New or Updated status
 	Entity Interface
 }
 
@@ -41,34 +42,50 @@ func (mr MergeResult) String() string {
 	case MergeStatusNothing:
 		return "nothing to do"
 	case MergeStatusError:
-		return fmt.Sprintf("merge error on %s: %s", mr.Id, mr.Err.Error())
+		if mr.Id != "" {
+			return fmt.Sprintf("merge error on %s: %s", mr.Id, mr.Err.Error())
+		}
+		return fmt.Sprintf("merge error: %s", mr.Err.Error())
 	default:
 		panic("unknown merge status")
 	}
 }
 
-func NewMergeError(err error, id Id) MergeResult {
+func NewMergeNewStatus(id Id, entity Interface) MergeResult {
 	return MergeResult{
-		Err:    err,
 		Id:     id,
-		Status: MergeStatusError,
+		Status: MergeStatusNew,
+		Entity: entity,
 	}
 }
 
-func NewMergeStatus(status MergeStatus, id Id, entity Interface) MergeResult {
+func NewMergeInvalidStatus(id Id, reason string) MergeResult {
 	return MergeResult{
 		Id:     id,
-		Status: status,
+		Status: MergeStatusInvalid,
+		Reason: reason,
+	}
+}
 
-		// Entity is not set for an invalid merge result
+func NewMergeUpdatedStatus(id Id, entity Interface) MergeResult {
+	return MergeResult{
+		Id:     id,
+		Status: MergeStatusUpdated,
 		Entity: entity,
 	}
 }
 
-func NewMergeInvalidStatus(id Id, reason string) MergeResult {
+func NewMergeNothingStatus(id Id) MergeResult {
 	return MergeResult{
 		Id:     id,
-		Status: MergeStatusInvalid,
-		Reason: reason,
+		Status: MergeStatusNothing,
+	}
+}
+
+func NewMergeError(err error, id Id) MergeResult {
+	return MergeResult{
+		Id:     id,
+		Status: MergeStatusError,
+		Err:    err,
 	}
 }
diff --git a/entity/refs.go b/entity/refs.go
index 82b1741b..070d4dba 100644
--- a/entity/refs.go
+++ b/entity/refs.go
@@ -2,17 +2,19 @@ package entity
 
 import "strings"
 
+// RefsToIds parse a slice of git references and return the corresponding Entity's Id.
 func RefsToIds(refs []string) []Id {
 	ids := make([]Id, len(refs))
 
 	for i, ref := range refs {
-		ids[i] = refToId(ref)
+		ids[i] = RefToId(ref)
 	}
 
 	return ids
 }
 
-func refToId(ref string) Id {
+// RefsToIds parse a git reference and return the corresponding Entity's Id.
+func RefToId(ref string) Id {
 	split := strings.Split(ref, "/")
 	return Id(split[len(split)-1])
 }