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diff --git a/doc/merge1.png b/doc/merge1.png Binary files differnew file mode 100644 index 00000000..7ba24173 --- /dev/null +++ b/doc/merge1.png diff --git a/doc/merge2.png b/doc/merge2.png Binary files differnew file mode 100644 index 00000000..614be5e8 --- /dev/null +++ b/doc/merge2.png diff --git a/doc/model.md b/doc/model.md index c4252e6c..da76761c 100644 --- a/doc/model.md +++ b/doc/model.md @@ -1,111 +1,127 @@ -# Data model +Entities data model +=================== If you are not familiar with [git internals](https://git-scm.com/book/en/v1/Git-Internals), you might first want to read about them, as the `git-bug` data model is built on top of them. -The biggest problem when creating a distributed bug tracker is that there is no central authoritative server (doh!). This implies some constraints. +## Entities (bugs, ...) are a series of edit operations -## Anybody can create and edit bugs at the same time as you +As entities are stored and edited in multiple process at the same time, it's not possible to store the current state like it would be done in a normal application. If two process change the same entity and later try to merge the states, we wouldn't know which change takes precedence or how to merge those states. -To deal with this problem, you need a way to merge these changes in a meaningful way. +To deal with this problem, you need a way to merge these changes in a meaningful way. Instead of storing the final bug data directly, we store a series of edit `Operation`s. This is a common idea, notably with [Operation-based CRDTs](https://en.wikipedia.org/wiki/Conflict-free_replicated_data_type#Operation-based_CRDTs). -Instead of storing the final bug data directly, we store a series of edit `Operation`s. + -Note: In git-bug internally it is a golang struct, but in the git repo it is stored as JSON, as seen later. +To get the final state of an entity, we apply these `Operation`s in the correct order on an empty state to compute ("compile") our view. -These `Operation`s are aggregated in an `OperationPack`, a simple array. An `OperationPack` represents an edit session of a bug. We store this pack in git as a git `Blob`; that consists of a string containing a JSON array of operations. One such pack -- here with two operations -- might look like this: +## Entities are stored in git objects + +An `Operation` is a piece of data including: +- a type identifier +- an author (a reference to another entity) +- a timestamp (there is also 1 or 2 Lamport time that we will describe later) +- all the data required by that operation type (a message, a status ...) +- a random nonce to ensure we have enough entropy, as the operation identifier is a hash of that data (more on that later) + +These `Operation`s are aggregated in an `OperationPack`, a simple array. An `OperationPack` represents an edit session of a bug. As the operation's author is the same for all the `OperationPack` we only store it once. + +We store this pack in git as a git `Blob`; that consists of a string containing a JSON array of operations. One such pack -- here with two operations -- might look like this: ```json -[ - { - "type": "SET_TITLE", - "author": { - "id": "5034cd36acf1a2dadb52b2db17f620cc050eb65c" - }, - "timestamp": 1533640589, - "title": "This title is better" +{ + "author": { + "id": "04bf6c1a69bb8e9679644874c85f82e337b40d92df9d8d4176f1c5e5c6627058" }, - { - "type": "ADD_COMMENT", - "author": { - "id": "5034cd36acf1a2dadb52b2db17f620cc050eb65c" + "ops": [ + { + "type": 3, + "timestamp": 1647377254, + "nonce": "SRQwUWTJCXAmQBIS+1ctKgOcbF0=", + "message": "Adding a comment", + "files": null }, - "timestamp": 1533640612, - "message": "A new comment" - } -] + { + "type": 4, + "timestamp": 1647377257, + "nonce": "la/HaRPMvD77/cJSJOUzKWuJdY8=", + "status": 1 + } + ] +} ``` -To reference our `OperationPack`, we create a git `Tree`; it references our `OperationPack` `Blob` under `"\ops"`. If any edit operation includes a media (for instance in a message), we can store that media as a `Blob` and reference it here under `"/media"`. +To reference our `OperationPack`, we create a git `Tree`; it references our `OperationPack` `Blob` under `"/ops"`. If any edit operation includes a media (for instance in a message), we can store that media as a `Blob` and reference it here under `"/media"`. To complete the picture, we create a git `Commit` that references our `Tree`. Each time we add more `Operation`s to our bug, we add a new `Commit` with the same data-structure to form a chain of `Commit`s. This chain of `Commit`s is made available as a git `Reference` under `refs/bugs/<bug-id>`. We can later use this reference to push our data to a git remote. As git will push any data needed as well, everything will be pushed to the remote, including the media. -For convenience and performance, each `Tree` references the very first `OperationPack` of the bug under `"/root"`. That way we can easily access the very first `Operation`, the `CREATE` operation. This operation contains important data for the bug, like the author. - Here is the complete picture: -``` - refs/bugs/<bug-id> - | - | - | - +-----------+ +-----------+ "ops" +-----------+ - | Commit |----------> Tree |---------+------------| Blob | (OperationPack) - +-----------+ +-----------+ | +-----------+ - | | - | | - | | "root" +-----------+ - +-----------+ +-----------+ +------------| Blob | (OperationPack) - | Commit |----------> Tree |-- ... | +-----------+ - +-----------+ +-----------+ | - | | - | | "media" +-----------+ +-----------+ - | +------------| Tree |---+--->| Blob | bug.jpg - +-----------+ +-----------+ +-----------+ | +-----------+ - | Commit |----------> Tree |-- ... | - +-----------+ +-----------+ | +-----------+ - +--->| Blob | demo.mp4 - +-----------+ -``` - -Now that we have this, we can easily merge our bugs without conflict. When pulling bug updates from a remote, we will simply add our new operations (that is, new `Commit`s), if any, at the end of the chain. In git terms, it's just a `rebase`. + -## You can't have a simple consecutive index for your bugs +## Time is unreliable -The same way git can't have a simple counter as identifier for its commits as SVN does, we can't have consecutive identifiers for bugs. +It would be very tempting to use the `Operation`'s timestamp to give us the order to compile the final state. However, you can't rely on the time provided by other people (their clock might be off) for anything other than just display. This is a fundamental limitation of distributed system, and even more so when actors might want to game the system. -`git-bug` uses as identifier the hash of the first commit in the chain of commits of the bug. As this hash is ultimately computed with the content of the `CREATE` operation that includes title, message and a timestamp, it will be unique and prevent collision. +Instead, we are going to use [Lamport logical clock](https://en.wikipedia.org/wiki/Lamport_timestamps). A Lamport clock is a simple counter of events. This logical clock gives us a partial ordering: +- if L1 < L2, L1 happened before L2 +- if L1 > L2, L1 happened after L2 +- if L1 == L2, we can't tell which happened first: it's a concurrent edition -The same way as git does, this hash is displayed truncated to a 7 characters string to a human user. Note that when specifying a bug id in a command, you can enter as few character as you want, as long as there is no ambiguity. If multiple bugs match your prefix, `git-bug` will complain and display the potential matches. -## You can't rely on the time provided by other people (their clock might by off) for anything other than just display +Each time we are appending something to the data (create an Entity, add an `Operation`) a logical time will be attached, with the highest time value we are aware of plus one. This declares a causality in the event and allows ordering entities and operations. -When in the context of a single bug, events are already ordered without the need of a timestamp. An `OperationPack` is an ordered array of operations. A chain of commits orders `OperationPack`s amongst each other. +The first commit of an Entity will have both a creation time and edit time clock, while a later commit will only have an edit time clock. These clocks value are serialized directly in the `Tree` entry name (for example: `"create-clock-4"`). As a Tree entry needs to reference something, we reference the git `Blob` with an empty content. As all of these entries will reference the same `Blob`, no network transfer is needed as long as you already have any entity in your repository. -Now, to be able to order bugs by creation or last edition time, `git-bug` uses a [Lamport logical clock](https://en.wikipedia.org/wiki/Lamport_timestamps). A Lamport clock is a simple counter of events. When a new bug is created, its creation time will be the highest time value we are aware of plus one. This declares a causality in the event and allows to order bugs. - -When bugs are pushed/pulled to a git remote, it might happen that bugs get the same logical time. This means that they were created or edited concurrently. In this case, `git-bug` will use the timestamp as a second layer of sorting. While the timestamp might be incorrect due to a badly set clock, the drift in sorting is bounded by the first sorting using the logical clock. That means that if users synchronize their bugs regularly, the timestamp will rarely be used, and should still provide a kinda accurate sorting when needed. - -These clocks are stored in the chain of commits of each bug, as entries in each main git `Tree`. The first commit will have both a creation time and edit time clock, while a later commit will only have an edit time clock. A naive way could be to serialize the clock in a git `Blob` and reference it in the `Tree` as `"create-clock"` for example. The problem is that it would generate a lot of blobs that would need to be exchanged later for what is basically just a number. - -Instead, the clock value is serialized directly in the `Tree` entry name (for example: `"create-clock-4"`). As a Tree entry needs to reference something, we reference the git `Blob` with an empty content. As all of these entries will reference the same `Blob`, no network transfer is needed as long as you already have any bug in your repository. - - -Example of Tree of the first commit of a bug: +Example of Tree of the first commit of an entity: ``` 100644 blob e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 create-clock-14 100644 blob e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 edit-clock-137 100644 blob a020a85baa788e12699a4d83dd735578f0d78c75 ops -100644 blob a020a85baa788e12699a4d83dd735578f0d78c75 root ``` Note that both `"ops"` and `"root"` entry reference the same OperationPack as it's the first commit in the chain. - -Example of Tree of a later commit of a bug: +Example of Tree of a later commit of an entity: ``` 100644 blob e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 edit-clock-154 100644 blob 68383346c1a9503f28eec888efd300e9fc179ca0 ops -100644 blob a020a85baa788e12699a4d83dd735578f0d78c75 root ``` -Note that the `"root"` entry still references the same root `OperationPack`. Also, all the clocks reference the same empty `Blob`. + +## Entities and Operation's ID + +`Operation`s can be referenced in the data model or by users with an identifier. This identifier is computed from the `Operation`'s data itself, with a hash of that data: `id = hash(json(op))` + +For entities, `git-bug` uses as identifier the hash of the first `Operation` of the entity, as serialized on disk. + +The same way as git does, this hash is displayed truncated to a 7 characters string to a human user. Note that when specifying a bug id in a command, you can enter as few characters as you want, as long as there is no ambiguity. If multiple entities match your prefix, `git-bug` will complain and display the potential matches. + +## Entities support conflict resolution + +Now that we have all that, we can finally merge our entities without conflict and collaborate with other users. Let's start by getting rid of two simple scenario: +- if we simply pull updates, we move forward our local reference. We get an update of our graph that we read as usual. +- if we push fast-forward updates, we move forward the remote reference and other users can update their reference as well. + +The tricky part happens when we have concurrent edition. If we pull updates while we have local changes (non-straightforward in git term), git-bug create the equivalent of a merge commit to merge both branches into a DAG. This DAG has a single root containing the first operation, but can have branches that get merged back into a single head pointed by the reference. + +As we don't have a purely linear series of commits/`Operations`s, we need a deterministic ordering to always apply operations in the same order. + +git-bug apply the following algorithm: +1. load and read all the commits and the associated `OperationPack`s +2. make sure that the Lamport clocks respect the DAG structure: a parent commit/`OperationPack` (that is, towards the head) cannot have a clock that is higher or equal than its direct child. If such a problem happen, the commit is refused/discarded. +3. individual `Operation`s are assembled together and ordered given the following priorities: + 1. the edition's lamport clock if not concurrent + 2. the lexicographic order of the `OperationPack`'s identifier + +Step 2 is providing and enforcing a constraint over the `Operation`'s logical clocks. What that means is that we inherit the implicit ordering given by the DAG. Later, logical clocks refine that ordering. This, coupled with signed commit has the nice property of limiting how this data model can be abused. + +Here is an example of such an ordering. We can see that: +- Lamport clocks respect the DAG structure +- the final `Operation` order is [A,B,C,D,E,F], according to those clocks + + + +When we have a concurrent edition, we apply a secondary ordering based on the `OperationPack`'s identifier: + + + +This secondary ordering doesn't carry much meaning, but it's unbiased and hard to abuse.
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