entity/dag: proper base operation for simplified implementation

- reduce boilerplace necessary to implement an operation
- consolidate what an operation is in the core, which in turn pave the way for a generic cache layer mechanism
- avoid the previously complex unmarshalling process
- support operation metadata from the core
- simplified testing

12 files changed, 627 insertions, 180 deletions

diff --git a/entity/dag/common_test.go b/entity/dag/common_test.go
index c2177683..774acba8 100644
--- a/entity/dag/common_test.go
+++ b/entity/dag/common_test.go
@@ -18,78 +18,73 @@ import (
  Operations
 */
 
-type op1 struct {
-	author identity.Interface
+const (
+	_ OperationType = iota
+	Op1
+	Op2
+)
 
-	OperationType int               `json:"type"`
-	Field1        string            `json:"field_1"`
-	Files         []repository.Hash `json:"files"`
+type op1 struct {
+	OpBase
+	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}
+	return &op1{OpBase: NewOpBase(Op1, author, 0), Field1: field1, Files: files}
 }
 
-func (o *op1) Id() entity.Id {
-	data, _ := json.Marshal(o)
-	return entity.DeriveId(data)
+func (op *op1) Id() entity.Id {
+	return IdOperation(op, &op.OpBase)
 }
 
-func (o *op1) Validate() error { return nil }
+func (op *op1) Validate() error { return nil }
 
-func (o *op1) Author() identity.Interface {
-	return o.author
-}
-
-func (o *op1) GetFiles() []repository.Hash {
-	return o.Files
+func (op *op1) GetFiles() []repository.Hash {
+	return op.Files
 }
 
 type op2 struct {
-	author identity.Interface
-
-	OperationType int    `json:"type"`
-	Field2        string `json:"field_2"`
+	OpBase
+	Field2 string `json:"field_2"`
 }
 
 func newOp2(author identity.Interface, field2 string) *op2 {
-	return &op2{author: author, OperationType: 2, Field2: field2}
+	return &op2{OpBase: NewOpBase(Op2, author, 0), Field2: field2}
 }
 
-func (o *op2) Id() entity.Id {
-	data, _ := json.Marshal(o)
-	return entity.DeriveId(data)
+func (op *op2) Id() entity.Id {
+	return IdOperation(op, &op.OpBase)
 }
 
-func (o *op2) Validate() error { return nil }
-
-func (o *op2) Author() identity.Interface {
-	return o.author
-}
+func (op *op2) Validate() error { return nil }
 
-func unmarshaler(author identity.Interface, raw json.RawMessage, resolver identity.Resolver) (Operation, error) {
+func unmarshaler(raw json.RawMessage, resolver identity.Resolver) (Operation, error) {
 	var t struct {
-		OperationType int `json:"type"`
+		OperationType OperationType `json:"type"`
 	}
 
 	if err := json.Unmarshal(raw, &t); err != nil {
 		return nil, err
 	}
 
+	var op Operation
+
 	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
+	case Op1:
+		op = &op1{}
+	case Op2:
+		op = &op2{}
 	default:
 		return nil, fmt.Errorf("unknown operation type %v", t.OperationType)
 	}
+
+	err := json.Unmarshal(raw, &op)
+	if err != nil {
+		return nil, err
+	}
+
+	return op, nil
 }
 
 /*
diff --git a/entity/dag/entity.go b/entity/dag/entity.go
index f3229b7e..4ccf0e0e 100644
--- a/entity/dag/entity.go
+++ b/entity/dag/entity.go
@@ -26,7 +26,7 @@ type Definition struct {
 	// the Namespace in git references (bugs, prs, ...)
 	Namespace string
 	// a function decoding a JSON message into an Operation
-	OperationUnmarshaler func(author identity.Interface, raw json.RawMessage, resolver identity.Resolver) (Operation, error)
+	OperationUnmarshaler func(raw json.RawMessage, resolver identity.Resolver) (Operation, error)
 	// the expected format version number, that can be used for data migration/upgrade
 	FormatVersion uint
 }
diff --git a/entity/dag/entity_test.go b/entity/dag/entity_test.go
index 6d621bbe..e399b6c7 100644
--- a/entity/dag/entity_test.go
+++ b/entity/dag/entity_test.go
@@ -50,6 +50,8 @@ func TestWriteReadMultipleAuthor(t *testing.T) {
 }
 
 func assertEqualEntities(t *testing.T, a, b *Entity) {
+	t.Helper()
+
 	// testify doesn't support comparing functions and systematically fail if they are not nil
 	// so we have to set them to nil temporarily
 
diff --git a/entity/dag/example_test.go b/entity/dag/example_test.go
index 5d4ea34d..94850bd9 100644
--- a/entity/dag/example_test.go
+++ b/entity/dag/example_test.go
@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
+	"time"
 
 	"github.com/MichaelMure/git-bug/entity"
 	"github.com/MichaelMure/git-bug/entity/dag"
@@ -64,10 +65,8 @@ type Operation interface {
 	Apply(snapshot *Snapshot)
 }
 
-type OperationType int
-
 const (
-	_ OperationType = iota
+	_ dag.OperationType = iota
 	SetSignatureRequiredOp
 	AddAdministratorOp
 	RemoveAdministratorOp
@@ -75,37 +74,30 @@ const (
 
 // SetSignatureRequired is an operation to set/unset if git signature are required.
 type SetSignatureRequired struct {
-	author        identity.Interface
-	OperationType OperationType `json:"type"`
-	Value         bool          `json:"value"`
+	dag.OpBase
+	Value bool `json:"value"`
 }
 
 func NewSetSignatureRequired(author identity.Interface, value bool) *SetSignatureRequired {
-	return &SetSignatureRequired{author: author, OperationType: SetSignatureRequiredOp, Value: value}
+	return &SetSignatureRequired{
+		OpBase: dag.NewOpBase(SetSignatureRequiredOp, author, time.Now().Unix()),
+		Value:  value,
+	}
 }
 
 func (ssr *SetSignatureRequired) Id() entity.Id {
 	// the Id of the operation is the hash of the serialized data.
-	// we could memorize the Id when deserializing, but that will do
-	data, _ := json.Marshal(ssr)
-	return entity.DeriveId(data)
+	return dag.IdOperation(ssr, &ssr.OpBase)
 }
 
 func (ssr *SetSignatureRequired) Validate() error {
-	if ssr.author == nil {
-		return fmt.Errorf("author not set")
-	}
-	return ssr.author.Validate()
-}
-
-func (ssr *SetSignatureRequired) Author() identity.Interface {
-	return ssr.author
+	return ssr.OpBase.Validate(ssr, SetSignatureRequiredOp)
 }
 
 // Apply is the function that makes changes on the snapshot
 func (ssr *SetSignatureRequired) Apply(snapshot *Snapshot) {
 	// check that we are allowed to change the config
-	if _, ok := snapshot.Administrator[ssr.author]; !ok {
+	if _, ok := snapshot.Administrator[ssr.Author()]; !ok {
 		return
 	}
 	snapshot.SignatureRequired = ssr.Value
@@ -113,24 +105,20 @@ func (ssr *SetSignatureRequired) Apply(snapshot *Snapshot) {
 
 // AddAdministrator is an operation to add a new administrator in the set
 type AddAdministrator struct {
-	author        identity.Interface
-	OperationType OperationType        `json:"type"`
-	ToAdd         []identity.Interface `json:"to_add"`
-}
-
-// addAdministratorJson is a helper struct to deserialize identities with a concrete type.
-type addAdministratorJson struct {
-	ToAdd []identity.IdentityStub `json:"to_add"`
+	dag.OpBase
+	ToAdd []identity.Interface `json:"to_add"`
 }
 
 func NewAddAdministratorOp(author identity.Interface, toAdd ...identity.Interface) *AddAdministrator {
-	return &AddAdministrator{author: author, OperationType: AddAdministratorOp, ToAdd: toAdd}
+	return &AddAdministrator{
+		OpBase: dag.NewOpBase(AddAdministratorOp, author, time.Now().Unix()),
+		ToAdd:  toAdd,
+	}
 }
 
 func (aa *AddAdministrator) Id() entity.Id {
-	// we could memorize the Id when deserializing, but that will do
-	data, _ := json.Marshal(aa)
-	return entity.DeriveId(data)
+	// the Id of the operation is the hash of the serialized data.
+	return dag.IdOperation(aa, &aa.OpBase)
 }
 
 func (aa *AddAdministrator) Validate() error {
@@ -138,20 +126,13 @@ func (aa *AddAdministrator) Validate() error {
 	if len(aa.ToAdd) == 0 {
 		return fmt.Errorf("nothing to add")
 	}
-	if aa.author == nil {
-		return fmt.Errorf("author not set")
-	}
-	return aa.author.Validate()
-}
-
-func (aa *AddAdministrator) Author() identity.Interface {
-	return aa.author
+	return aa.OpBase.Validate(aa, AddAdministratorOp)
 }
 
 // Apply is the function that makes changes on the snapshot
 func (aa *AddAdministrator) Apply(snapshot *Snapshot) {
 	// check that we are allowed to change the config ... or if there is no admin yet
-	if !snapshot.HasAdministrator(aa.author) && len(snapshot.Administrator) != 0 {
+	if !snapshot.HasAdministrator(aa.Author()) && len(snapshot.Administrator) != 0 {
 		return
 	}
 	for _, toAdd := range aa.ToAdd {
@@ -161,25 +142,20 @@ func (aa *AddAdministrator) Apply(snapshot *Snapshot) {
 
 // RemoveAdministrator is an operation to remove an administrator from the set
 type RemoveAdministrator struct {
-	author        identity.Interface
-	OperationType OperationType        `json:"type"`
-	ToRemove      []identity.Interface `json:"to_remove"`
-}
-
-// removeAdministratorJson is a helper struct to deserialize identities with a concrete type.
-type removeAdministratorJson struct {
+	dag.OpBase
 	ToRemove []identity.Interface `json:"to_remove"`
 }
 
 func NewRemoveAdministratorOp(author identity.Interface, toRemove ...identity.Interface) *RemoveAdministrator {
-	return &RemoveAdministrator{author: author, OperationType: RemoveAdministratorOp, ToRemove: toRemove}
+	return &RemoveAdministrator{
+		OpBase:   dag.NewOpBase(RemoveAdministratorOp, author, time.Now().Unix()),
+		ToRemove: toRemove,
+	}
 }
 
 func (ra *RemoveAdministrator) Id() entity.Id {
 	// the Id of the operation is the hash of the serialized data.
-	// we could memorize the Id when deserializing, but that will do
-	data, _ := json.Marshal(ra)
-	return entity.DeriveId(data)
+	return dag.IdOperation(ra, &ra.OpBase)
 }
 
 func (ra *RemoveAdministrator) Validate() error {
@@ -188,26 +164,19 @@ func (ra *RemoveAdministrator) Validate() error {
 	if len(ra.ToRemove) == 0 {
 		return fmt.Errorf("nothing to remove")
 	}
-	if ra.author == nil {
-		return fmt.Errorf("author not set")
-	}
-	return ra.author.Validate()
-}
-
-func (ra *RemoveAdministrator) Author() identity.Interface {
-	return ra.author
+	return ra.OpBase.Validate(ra, RemoveAdministratorOp)
 }
 
 // Apply is the function that makes changes on the snapshot
 func (ra *RemoveAdministrator) Apply(snapshot *Snapshot) {
 	// check if we are allowed to make changes
-	if !snapshot.HasAdministrator(ra.author) {
+	if !snapshot.HasAdministrator(ra.Author()) {
 		return
 	}
 	// special rule: we can't end up with no administrator
 	stillSome := false
 	for admin, _ := range snapshot.Administrator {
-		if admin != ra.author {
+		if admin != ra.Author() {
 			stillSome = true
 			break
 		}
@@ -245,71 +214,52 @@ var def = dag.Definition{
 
 // operationUnmarshaller is a function doing the de-serialization of the JSON data into our own
 // concrete Operations. If needed, we can use the resolver to connect to other entities.
-func operationUnmarshaller(author identity.Interface, raw json.RawMessage, resolver identity.Resolver) (dag.Operation, error) {
+func operationUnmarshaller(raw json.RawMessage, resolver identity.Resolver) (dag.Operation, error) {
 	var t struct {
-		OperationType OperationType `json:"type"`
+		OperationType dag.OperationType `json:"type"`
 	}
 
 	if err := json.Unmarshal(raw, &t); err != nil {
 		return nil, err
 	}
 
-	var value interface{}
+	var op dag.Operation
 
 	switch t.OperationType {
 	case AddAdministratorOp:
-		value = &addAdministratorJson{}
+		op = &AddAdministrator{}
 	case RemoveAdministratorOp:
-		value = &removeAdministratorJson{}
+		op = &RemoveAdministrator{}
 	case SetSignatureRequiredOp:
-		value = &SetSignatureRequired{}
+		op = &SetSignatureRequired{}
 	default:
 		panic(fmt.Sprintf("unknown operation type %v", t.OperationType))
 	}
 
-	err := json.Unmarshal(raw, &value)
+	err := json.Unmarshal(raw, &op)
 	if err != nil {
 		return nil, err
 	}
 
-	var op Operation
-
-	switch value := value.(type) {
-	case *SetSignatureRequired:
-		value.author = author
-		op = value
-	case *addAdministratorJson:
-		// We need something less straightforward to deserialize and resolve identities
-		aa := &AddAdministrator{
-			author:        author,
-			OperationType: AddAdministratorOp,
-			ToAdd:         make([]identity.Interface, len(value.ToAdd)),
-		}
-		for i, stub := range value.ToAdd {
+	switch op := op.(type) {
+	case *AddAdministrator:
+		// We need to resolve identities
+		for i, stub := range op.ToAdd {
 			iden, err := resolver.ResolveIdentity(stub.Id())
 			if err != nil {
 				return nil, err
 			}
-			aa.ToAdd[i] = iden
+			op.ToAdd[i] = iden
 		}
-		op = aa
-	case *removeAdministratorJson:
-		// We need something less straightforward to deserialize and resolve identities
-		ra := &RemoveAdministrator{
-			author:        author,
-			OperationType: RemoveAdministratorOp,
-			ToRemove:      make([]identity.Interface, len(value.ToRemove)),
-		}
-		for i, stub := range value.ToRemove {
+	case *RemoveAdministrator:
+		// We need to resolve identities
+		for i, stub := range op.ToRemove {
 			iden, err := resolver.ResolveIdentity(stub.Id())
 			if err != nil {
 				return nil, err
 			}
-			ra.ToRemove[i] = iden
+			op.ToRemove[i] = iden
 		}
-		op = ra
-	default:
-		panic(fmt.Sprintf("unknown operation type %T", value))
 	}
 
 	return op, nil
diff --git a/entity/dag/op_noop.go b/entity/dag/op_noop.go
new file mode 100644
index 00000000..2bc53578
--- /dev/null
+++ b/entity/dag/op_noop.go
@@ -0,0 +1,39 @@
+package dag
+
+import (
+	"github.com/MichaelMure/git-bug/entity"
+	"github.com/MichaelMure/git-bug/identity"
+)
+
+var _ Operation = &NoOpOperation[Snapshot]{}
+var _ OperationDoesntChangeSnapshot = &NoOpOperation[Snapshot]{}
+
+// NoOpOperation is an operation that does not change the entity state. It can
+// however be used to store arbitrary metadata in the entity history, for example
+// to support a bridge feature.
+type NoOpOperation[SnapT Snapshot] struct {
+	OpBase
+}
+
+func NewNoOpOp[SnapT Snapshot](opType OperationType, author identity.Interface, unixTime int64) *NoOpOperation[SnapT] {
+	return &NoOpOperation[SnapT]{
+		OpBase: NewOpBase(opType, author, unixTime),
+	}
+}
+
+func (op *NoOpOperation[SnapT]) Id() entity.Id {
+	return IdOperation(op, &op.OpBase)
+}
+
+func (op *NoOpOperation[SnapT]) Apply(snapshot SnapT) {
+	// Nothing to do
+}
+
+func (op *NoOpOperation[SnapT]) Validate() error {
+	if err := op.OpBase.Validate(op, op.OperationType); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (op *NoOpOperation[SnapT]) DoesntChangeSnapshot() {}
diff --git a/entity/dag/op_noop_test.go b/entity/dag/op_noop_test.go
new file mode 100644
index 00000000..e2f5cde0
--- /dev/null
+++ b/entity/dag/op_noop_test.go
@@ -0,0 +1,13 @@
+package dag
+
+import (
+	"testing"
+
+	"github.com/MichaelMure/git-bug/identity"
+)
+
+func TestNoopSerialize(t *testing.T) {
+	SerializeRoundTripTest(t, func(author identity.Interface, unixTime int64) *NoOpOperation[*snapshotMock] {
+		return NewNoOpOp[*snapshotMock](1, author, unixTime)
+	})
+}
diff --git a/entity/dag/op_set_metadata.go b/entity/dag/op_set_metadata.go
new file mode 100644
index 00000000..bf32171f
--- /dev/null
+++ b/entity/dag/op_set_metadata.go
@@ -0,0 +1,68 @@
+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/util/text"
+)
+
+var _ Operation = &SetMetadataOperation[Snapshot]{}
+var _ OperationDoesntChangeSnapshot = &SetMetadataOperation[Snapshot]{}
+
+type SetMetadataOperation[SnapT Snapshot] struct {
+	OpBase
+	Target      entity.Id         `json:"target"`
+	NewMetadata map[string]string `json:"new_metadata"`
+}
+
+func NewSetMetadataOp[SnapT Snapshot](opType OperationType, author identity.Interface, unixTime int64, target entity.Id, newMetadata map[string]string) *SetMetadataOperation[SnapT] {
+	return &SetMetadataOperation[SnapT]{
+		OpBase:      NewOpBase(opType, author, unixTime),
+		Target:      target,
+		NewMetadata: newMetadata,
+	}
+}
+
+func (op *SetMetadataOperation[SnapT]) Id() entity.Id {
+	return IdOperation(op, &op.OpBase)
+}
+
+func (op *SetMetadataOperation[SnapT]) Apply(snapshot SnapT) {
+	for _, target := range snapshot.AllOperations() {
+		if target.Id() == op.Target {
+			// Apply the metadata in an immutable way: if a metadata already
+			// exist, it's not possible to override it.
+			for key, value := range op.NewMetadata {
+				target.setExtraMetadataImmutable(key, value)
+			}
+			return
+		}
+	}
+}
+
+func (op *SetMetadataOperation[SnapT]) Validate() error {
+	if err := op.OpBase.Validate(op, op.OperationType); err != nil {
+		return err
+	}
+
+	if err := op.Target.Validate(); err != nil {
+		return errors.Wrap(err, "target invalid")
+	}
+
+	for key, val := range op.NewMetadata {
+		if !text.SafeOneLine(key) {
+			return fmt.Errorf("metadata key is unsafe")
+		}
+		if !text.Safe(val) {
+			return fmt.Errorf("metadata value is not fully printable")
+		}
+	}
+
+	return nil
+}
+
+func (op *SetMetadataOperation[SnapT]) DoesntChangeSnapshot() {}
diff --git a/entity/dag/op_set_metadata_test.go b/entity/dag/op_set_metadata_test.go
new file mode 100644
index 00000000..4dab8a96
--- /dev/null
+++ b/entity/dag/op_set_metadata_test.go
@@ -0,0 +1,106 @@
+package dag
+
+import (
+	"testing"
+	"time"
+
+	"github.com/MichaelMure/git-bug/identity"
+	"github.com/MichaelMure/git-bug/repository"
+
+	"github.com/stretchr/testify/require"
+)
+
+type snapshotMock struct {
+	ops []Operation
+}
+
+func (s *snapshotMock) AllOperations() []Operation {
+	return s.ops
+}
+
+func TestSetMetadata(t *testing.T) {
+	snap := &snapshotMock{}
+
+	repo := repository.NewMockRepo()
+
+	rene, err := identity.NewIdentity(repo, "René Descartes", "rene@descartes.fr")
+	require.NoError(t, err)
+
+	unix := time.Now().Unix()
+
+	target1 := NewNoOpOp[*snapshotMock](1, rene, unix)
+	target1.SetMetadata("key", "value")
+	snap.ops = append(snap.ops, target1)
+
+	target2 := NewNoOpOp[*snapshotMock](1, rene, unix)
+	target2.SetMetadata("key2", "value2")
+	snap.ops = append(snap.ops, target2)
+
+	op1 := NewSetMetadataOp[*snapshotMock](2, rene, unix, target1.Id(), map[string]string{
+		"key":  "override",
+		"key2": "value",
+	})
+
+	op1.Apply(snap)
+	snap.ops = append(snap.ops, op1)
+
+	target1Metadata := snap.AllOperations()[0].AllMetadata()
+	require.Len(t, target1Metadata, 2)
+	// original key is not overrided
+	require.Equal(t, target1Metadata["key"], "value")
+	// new key is set
+	require.Equal(t, target1Metadata["key2"], "value")
+
+	target2Metadata := snap.AllOperations()[1].AllMetadata()
+	require.Len(t, target2Metadata, 1)
+	require.Equal(t, target2Metadata["key2"], "value2")
+
+	op2 := NewSetMetadataOp[*snapshotMock](2, rene, unix, target2.Id(), map[string]string{
+		"key2": "value",
+		"key3": "value3",
+	})
+
+	op2.Apply(snap)
+	snap.ops = append(snap.ops, op2)
+
+	target1Metadata = snap.AllOperations()[0].AllMetadata()
+	require.Len(t, target1Metadata, 2)
+	require.Equal(t, target1Metadata["key"], "value")
+	require.Equal(t, target1Metadata["key2"], "value")
+
+	target2Metadata = snap.AllOperations()[1].AllMetadata()
+	require.Len(t, target2Metadata, 2)
+	// original key is not overrided
+	require.Equal(t, target2Metadata["key2"], "value2")
+	// new key is set
+	require.Equal(t, target2Metadata["key3"], "value3")
+
+	op3 := NewSetMetadataOp[*snapshotMock](2, rene, unix, target1.Id(), map[string]string{
+		"key":  "override",
+		"key2": "override",
+	})
+
+	op3.Apply(snap)
+	snap.ops = append(snap.ops, op3)
+
+	target1Metadata = snap.AllOperations()[0].AllMetadata()
+	require.Len(t, target1Metadata, 2)
+	// original key is not overrided
+	require.Equal(t, target1Metadata["key"], "value")
+	// previously set key is not overrided
+	require.Equal(t, target1Metadata["key2"], "value")
+
+	target2Metadata = snap.AllOperations()[1].AllMetadata()
+	require.Len(t, target2Metadata, 2)
+	require.Equal(t, target2Metadata["key2"], "value2")
+	require.Equal(t, target2Metadata["key3"], "value3")
+}
+
+func TestSetMetadataSerialize(t *testing.T) {
+	SerializeRoundTripTest(t, func(author identity.Interface, unixTime int64) *SetMetadataOperation[*snapshotMock] {
+		return NewSetMetadataOp[*snapshotMock](1, author, unixTime, "message", map[string]string{
+			"key1": "value1",
+			"key2": "value2",
+		})
+	})
+}
diff --git a/entity/dag/operation.go b/entity/dag/operation.go
index a320859f..0227b3e0 100644
--- a/entity/dag/operation.go
+++ b/entity/dag/operation.go
@@ -1,11 +1,21 @@
 package dag
 
 import (
+	"crypto/rand"
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"github.com/pkg/errors"
+
 	"github.com/MichaelMure/git-bug/entity"
 	"github.com/MichaelMure/git-bug/identity"
 	"github.com/MichaelMure/git-bug/repository"
 )
 
+// OperationType is an operation type identifier
+type OperationType int
+
 // 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.
@@ -22,23 +32,39 @@ type Operation interface {
 	//   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
+	//   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
+	// Type return the type of the operation
+	Type() OperationType
 	// Validate check if the Operation data is valid
 	Validate() error
 	// Author returns the author of this operation
 	Author() identity.Interface
+	// Time return the time when the operation was added
+	Time() time.Time
+
+	// SetMetadata store arbitrary metadata about the operation
+	SetMetadata(key string, value string)
+	// GetMetadata retrieve arbitrary metadata about the operation
+	GetMetadata(key string) (string, bool)
+	// AllMetadata return all metadata for this operation
+	AllMetadata() map[string]string
+
+	// setId allow to set the Id, used when unmarshalling only
+	setId(id entity.Id)
+	// setAuthor allow to set the author, used when unmarshalling only
+	setAuthor(author identity.Interface)
+	// setExtraMetadataImmutable add a metadata not carried by the operation itself on the operation
+	setExtraMetadataImmutable(key string, value string)
 }
 
-// OperationWithFiles is an extended Operation that has files dependency, stored in git.
+// OperationWithFiles is an optional extension for an 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.
@@ -46,3 +72,201 @@ type OperationWithFiles interface {
 	// hash).
 	GetFiles() []repository.Hash
 }
+
+// OperationDoesntChangeSnapshot is an interface signaling that the Operation implementing it doesn't change the
+// snapshot, for example a metadata operation that act on other operations.
+type OperationDoesntChangeSnapshot interface {
+	DoesntChangeSnapshot()
+}
+
+// Snapshot is the minimal interface that a snapshot need to implement
+type Snapshot interface {
+	// AllOperations returns all the operations that have been applied to that snapshot, in order
+	AllOperations() []Operation
+}
+
+// OpBase implement the common feature that every Operation should support.
+type OpBase struct {
+	// Not serialized. Store the op's id in memory.
+	id entity.Id
+	// Not serialized
+	author identity.Interface
+
+	OperationType OperationType `json:"type"`
+	UnixTime      int64         `json:"timestamp"`
+
+	// mandatory random bytes to ensure a better randomness of the data used to later generate the ID
+	// len(Nonce) should be > 20 and < 64 bytes
+	// It has no functional purpose and should be ignored.
+	Nonce []byte `json:"nonce"`
+
+	Metadata map[string]string `json:"metadata,omitempty"`
+	// Not serialized. Store the extra metadata in memory,
+	// compiled from SetMetadataOperation.
+	extraMetadata map[string]string
+}
+
+func NewOpBase(opType OperationType, author identity.Interface, unixTime int64) OpBase {
+	return OpBase{
+		OperationType: opType,
+		author:        author,
+		UnixTime:      unixTime,
+		Nonce:         makeNonce(20),
+		id:            entity.UnsetId,
+	}
+}
+
+func makeNonce(len int) []byte {
+	result := make([]byte, len)
+	_, err := rand.Read(result)
+	if err != nil {
+		panic(err)
+	}
+	return result
+}
+
+func IdOperation(op Operation, base *OpBase) entity.Id {
+	if base.id == "" {
+		// something went really wrong
+		panic("op's id not set")
+	}
+	if base.id == entity.UnsetId {
+		// This means we are trying to get the op's Id *before* it has been stored, for instance when
+		// adding multiple ops in one go in an OperationPack.
+		// 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(op)
+		if err != nil {
+			panic(err)
+		}
+
+		base.id = entity.DeriveId(data)
+	}
+	return base.id
+}
+
+func (base *OpBase) Type() OperationType {
+	return base.OperationType
+}
+
+// Time return the time when the operation was added
+func (base *OpBase) Time() time.Time {
+	return time.Unix(base.UnixTime, 0)
+}
+
+// Validate check the OpBase for errors
+func (base *OpBase) Validate(op Operation, opType OperationType) error {
+	if base.OperationType == 0 {
+		return fmt.Errorf("operation type unset")
+	}
+	if base.OperationType != opType {
+		return fmt.Errorf("incorrect operation type (expected: %v, actual: %v)", opType, base.OperationType)
+	}
+
+	if op.Time().Unix() == 0 {
+		return fmt.Errorf("time not set")
+	}
+
+	if base.author == nil {
+		return fmt.Errorf("author not set")
+	}
+
+	if err := op.Author().Validate(); err != nil {
+		return errors.Wrap(err, "author")
+	}
+
+	if op, ok := op.(OperationWithFiles); ok {
+		for _, hash := range op.GetFiles() {
+			if !hash.IsValid() {
+				return fmt.Errorf("file with invalid hash %v", hash)
+			}
+		}
+	}
+
+	if len(base.Nonce) > 64 {
+		return fmt.Errorf("nonce is too big")
+	}
+	if len(base.Nonce) < 20 {
+		return fmt.Errorf("nonce is too small")
+	}
+
+	return nil
+}
+
+// IsAuthored is a sign post method for gqlgen
+func (base *OpBase) IsAuthored() {}
+
+// Author return author identity
+func (base *OpBase) Author() identity.Interface {
+	return base.author
+}
+
+// IdIsSet returns true if the id has been set already
+func (base *OpBase) IdIsSet() bool {
+	return base.id != "" && base.id != entity.UnsetId
+}
+
+// SetMetadata store arbitrary metadata about the operation
+func (base *OpBase) SetMetadata(key string, value string) {
+	if base.IdIsSet() {
+		panic("set metadata on an operation with already an Id")
+	}
+
+	if base.Metadata == nil {
+		base.Metadata = make(map[string]string)
+	}
+	base.Metadata[key] = value
+}
+
+// GetMetadata retrieve arbitrary metadata about the operation
+func (base *OpBase) GetMetadata(key string) (string, bool) {
+	val, ok := base.Metadata[key]
+
+	if ok {
+		return val, true
+	}
+
+	// extraMetadata can't replace the original operations value if any
+	val, ok = base.extraMetadata[key]
+
+	return val, ok
+}
+
+// AllMetadata return all metadata for this operation
+func (base *OpBase) AllMetadata() map[string]string {
+	result := make(map[string]string)
+
+	for key, val := range base.extraMetadata {
+		result[key] = val
+	}
+
+	// Original metadata take precedence
+	for key, val := range base.Metadata {
+		result[key] = val
+	}
+
+	return result
+}
+
+// setId allow to set the Id, used when unmarshalling only
+func (base *OpBase) setId(id entity.Id) {
+	if base.id != "" && base.id != entity.UnsetId {
+		panic("trying to set id again")
+	}
+	base.id = id
+}
+
+// setAuthor allow to set the author, used when unmarshalling only
+func (base *OpBase) setAuthor(author identity.Interface) {
+	base.author = author
+}
+
+func (base *OpBase) setExtraMetadataImmutable(key string, value string) {
+	if base.extraMetadata == nil {
+		base.extraMetadata = make(map[string]string)
+	}
+	if _, exist := base.extraMetadata[key]; !exist {
+		base.extraMetadata[key] = value
+	}
+}
diff --git a/entity/dag/operation_pack.go b/entity/dag/operation_pack.go
index 9b42f9bf..b2973343 100644
--- a/entity/dag/operation_pack.go
+++ b/entity/dag/operation_pack.go
@@ -314,10 +314,15 @@ func unmarshallPack(def Definition, resolver identity.Resolver, data []byte) ([]
 
 	for _, raw := range aux.Operations {
 		// delegate to specialized unmarshal function
-		op, err := def.OperationUnmarshaler(author, raw, resolver)
+		op, err := def.OperationUnmarshaler(raw, resolver)
 		if err != nil {
 			return nil, nil, err
 		}
+		// Set the id from the serialized data
+		op.setId(entity.DeriveId(raw))
+		// Set the author, taken from the OperationPack
+		op.setAuthor(author)
+
 		ops = append(ops, op)
 	}
 
diff --git a/entity/dag/operation_pack_test.go b/entity/dag/operation_pack_test.go
index 73960800..e438e4c5 100644
--- a/entity/dag/operation_pack_test.go
+++ b/entity/dag/operation_pack_test.go
@@ -11,13 +11,13 @@ import (
 )
 
 func TestOperationPackReadWrite(t *testing.T) {
-	repo, id1, _, resolver, def := makeTestContext()
+	repo, author, _, resolver, def := makeTestContext()
 
 	opp := &operationPack{
-		Author: id1,
+		Author: author,
 		Operations: []Operation{
-			newOp1(id1, "foo"),
-			newOp2(id1, "bar"),
+			newOp1(author, "foo"),
+			newOp2(author, "bar"),
 		},
 		CreateTime: 123,
 		EditTime:   456,
@@ -32,34 +32,26 @@ func TestOperationPackReadWrite(t *testing.T) {
 	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,
+	for _, op := range opp.Operations {
+		// force the creation of the id
+		op.Id()
 	}
-	require.Equal(t, opp.Id(), opp3.Id())
+	require.Equal(t, opp, opp2)
 }
 
 func TestOperationPackSignedReadWrite(t *testing.T) {
-	repo, id1, _, resolver, def := makeTestContext()
+	repo, author, _, resolver, def := makeTestContext()
 
-	err := id1.(*identity.Identity).Mutate(repo, func(orig *identity.Mutator) {
+	err := author.(*identity.Identity).Mutate(repo, func(orig *identity.Mutator) {
 		orig.Keys = append(orig.Keys, identity.GenerateKey())
 	})
 	require.NoError(t, err)
 
 	opp := &operationPack{
-		Author: id1,
+		Author: author,
 		Operations: []Operation{
-			newOp1(id1, "foo"),
-			newOp2(id1, "bar"),
+			newOp1(author, "foo"),
+			newOp2(author, "bar"),
 		},
 		CreateTime: 123,
 		EditTime:   456,
@@ -74,23 +66,15 @@ func TestOperationPackSignedReadWrite(t *testing.T) {
 	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,
+	for _, op := range opp.Operations {
+		// force the creation of the id
+		op.Id()
 	}
-	require.Equal(t, opp.Id(), opp3.Id())
+	require.Equal(t, opp, opp2)
 }
 
 func TestOperationPackFiles(t *testing.T) {
-	repo, id1, _, resolver, def := makeTestContext()
+	repo, author, _, resolver, def := makeTestContext()
 
 	blobHash1, err := repo.StoreData(randomData())
 	require.NoError(t, err)
@@ -99,10 +83,10 @@ func TestOperationPackFiles(t *testing.T) {
 	require.NoError(t, err)
 
 	opp := &operationPack{
-		Author: id1,
+		Author: author,
 		Operations: []Operation{
-			newOp1(id1, "foo", blobHash1, blobHash2),
-			newOp1(id1, "foo", blobHash2),
+			newOp1(author, "foo", blobHash1, blobHash2),
+			newOp1(author, "foo", blobHash2),
 		},
 		CreateTime: 123,
 		EditTime:   456,
@@ -117,6 +101,10 @@ func TestOperationPackFiles(t *testing.T) {
 	opp2, err := readOperationPack(def, repo, resolver, commit)
 	require.NoError(t, err)
 
+	for _, op := range opp.Operations {
+		// force the creation of the id
+		op.Id()
+	}
 	require.Equal(t, opp, opp2)
 
 	require.ElementsMatch(t, opp2.Operations[0].(OperationWithFiles).GetFiles(), []repository.Hash{
diff --git a/entity/dag/operation_testing.go b/entity/dag/operation_testing.go
new file mode 100644
index 00000000..29923eec
--- /dev/null
+++ b/entity/dag/operation_testing.go
@@ -0,0 +1,57 @@
+package dag
+
+import (
+	"encoding/json"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/MichaelMure/git-bug/entity"
+	"github.com/MichaelMure/git-bug/identity"
+	"github.com/MichaelMure/git-bug/repository"
+)
+
+// SerializeRoundTripTest realize a marshall/unmarshall round-trip in the same condition as with OperationPack,
+// and check if the recovered operation is identical.
+func SerializeRoundTripTest[OpT Operation](t *testing.T, maker func(author identity.Interface, unixTime int64) OpT) {
+	repo := repository.NewMockRepo()
+
+	rene, err := identity.NewIdentity(repo, "René Descartes", "rene@descartes.fr")
+	require.NoError(t, err)
+
+	op := maker(rene, time.Now().Unix())
+	// enforce having an id
+	op.Id()
+
+	rdt := &roundTripper[OpT]{Before: op, author: rene}
+
+	data, err := json.Marshal(rdt)
+	require.NoError(t, err)
+
+	err = json.Unmarshal(data, &rdt)
+	require.NoError(t, err)
+
+	require.Equal(t, op, rdt.after)
+}
+
+type roundTripper[OpT Operation] struct {
+	Before OpT
+	author identity.Interface
+	after  OpT
+}
+
+func (r *roundTripper[OpT]) MarshalJSON() ([]byte, error) {
+	return json.Marshal(r.Before)
+}
+
+func (r *roundTripper[OpT]) UnmarshalJSON(data []byte) error {
+	if err := json.Unmarshal(data, &r.after); err != nil {
+		return err
+	}
+	// Set the id from the serialized data
+	r.after.setId(entity.DeriveId(data))
+	// Set the author, as OperationPack would do
+	r.after.setAuthor(r.author)
+	return nil
+}