Maybe a better way to track the release history of a Git repo

While I was still at ZipRecruiter, our Git monorepo was developing an annoying problem: it had too many refs.

Whenever there was a deployment of any of the applications in the repo, to staging or to production, the deployment tool would create a tag object with a name like this:

refs/release/stg.www.our-app-2019-09-06T114410.954-0700-treesha-179dab6fd11bee5195f0f8e31cea3edfb86ae23b

The tag contained the name of the target tier of the deployment (stg here, for “staging”), the app name and class of hosts on which it ran, www.our-app in this case, the date and time, and the hash of the tree that was deployed. This made a complete record of what was deployed, where, and when.

After we had been doing this for a while, there were thousands of these tag objects. Git is well-designed for handling thousands of commits, but not for handling thousands of refs. Perhaps this has changed, but at the time, Git would store refs in a file called packed-refs, which, for each ref, contained an entry with the ref's name and the ID of the object to which it referred.

(As with all things Git, the actual situation is somewhat more complicated. These additional ramifications don't affect the following discussion.)

Any operation that required resolving a ref name to its object had to search through the packed-refs file, which was taking an appreciable amount of time, far more than was desirable for an operation that everyone did many times a day. This affected not just local operations but operations on the Git server as well, and it was clear that it was only going to get slower as time went on. Also, the outputs of commands like git fetch and git branch were becoming increasingly cluttered with the names of tag objects whose only purpose was to record release history.

It seemed reasonable to me record the deployment history in the repository, but using thousands of tag objects clearly had serious drawbacks. In addition to the performance problems, the metadata itself was purely a convention in the tag name format, not legible to Git itself. There was a tree ID there, but nothing enforcing the rule that the tag should refer to a commit with that tree.

Also, the tag objects themselves seemed poorly-designed for the purpose. One good reason for storing history data in the respoitory is to render it immutable. But tags aren't immutable! They can be deleted and new ones can be inserted at any time, with any names desired.

And more important: consider the most natural sort of question that someone might want answered by the release history:

When did we last deploy the www.our-app app to production?

The only way to answer this question was to get the full list of all tags, filter it for prod.www.our-app, then sort by date and take the last item.

Or similarly: What if the most recent staging deployment had turned out to be bad, and we wanted to immediately revert to the version before that? Again, we (or the reversion process) would have had to filter and sort a list of thousands of tags, then pick out the second-last one.

I proposed this alternative in 2019. It stores the deployment history of each app as a series of unnamed but related Git objects, using Git's own history mechanism, so it's simple, efficient, and meshes well with Git's commands.

I don't think it was ever adopted, and it might turn out to have serious drawbacks I haven't foreseen. But it might be useful to someone in a similar situation.

Currently, each time a new version of an app is deployed, we create a ref:

refs/release/stg.www.our-app-2019-09-06T114410.954-0700-treesha-179dab6fd11bee5195f0f8e31cea3edfb86ae23b

As these refs proliferate, many operations in the Git repo become slower and slower.

We can record this information in a way that is at least as useful and which doesn't clutter up the ref list.

Current practice

At present, the release procedure creates a ref something like this:

• A new release is created at commit H

• A new tag object is created, pointing to H, something like

  git tag -a refs/release/stg.www.our-app-2019-09-.... H

Every new tag object has a name in the ref database. This adds a name for every release. The structure is something like this:

One ref object per release, each pointing to its own head commit

Alternative proposal

• Each (tier, app) pair is represented by a linked list of release commits
• The head of the list is the most recent release
• One ref points to the head of the list
• Older releases can be found by following the links back in time

Main point of this proposal: There is only one ref for each (tier, app) pair, no matter how many releases have been done.

It will look like this:

A chain of metainfo commits, each with the previous metainfo commit as first parent and the actual release commit as second parent

The one ref (which might or might not be an actual ref object) is named stg.www.our-app. That points not to the actual release commit, but to a commit that contains metainformation, such as the treesha. The metainformation could be stored in any of several ways. (See below.) The exact time of the release doesn't need to be stored explicitly because it is exactly the time at which the metainfo commit was created.

The metainfo commit has two parent commits. One is the actual release commit, the one that, in the current system, is the target of the named ref object. The other parent is the metainfo commit from the previous release.

The metainfo commits are chained together exactly the same way Git normally chains commit history, so the usual Git commands will work on the release history in reasonable and useful ways. In particular, the command git log stg.www.our-app will produce a listing of the release history.

Technical details

Creating this sort of two-parent commit is easy. Suppose the ref stg.www.our-app points to the current metainfo commit. And suppose we want to release new commit H. We do:

(Write some useful commit message into /tmp/message.$$.)

git commit-tree -p refs/release/stg.www.our-app -p H \
    H^{tree} < /tmp/message.$$

This creates a new metainfo commit with two parents, given by the arguments of the -p options. The first parent is the metainfo commit for the current release and the second is new commit H.

The git commit-tree command creates the new metainfo commit and writes its SHA on standard output. Say this SHA is N.

git push origin N:refs/release/stg.www.our-app

The release ref now points to the metainfo commit for the new release.

Usage

This will be easier to use than what we have now, because Git's command set was designed to manage a list of commits linked in this way; this is exactly how Git normally expects to represent history. This natural fit with Git's command set shows up in many places.

You can fetch a complete release history for any project with a single fetch:

git fetch origin refs/release/stg.www.our-app:HISTORY

After this, git log HISTORY will list the entire release history in reverse chronological order. All the usual git-log options make sense to filter or search this list. For example, if we want to find the last release before a certain date, we can use:

git log -1 --until=date HISTORY

If we want a forward-chronological list of release commit SHAs with their release dates, we could use something like:

    git log --reverse --format="%P %cd" HISTORY

At all times:

• The current release commit is accessible at HISTORY^2.
• The metainfo commit from n releases ago is at HISTORY~n.
• The release commit from n releases ago is at HISTORY~n^2.

Notice: if M is a metainfo commit that describes a release, M^ is the metainfo commit for the release before that, and M^{2} is the exact commit that was actually released. We don't have to record the tree IDs in the ref names; the tree that was released in release M is M^{2}^{tree}. These notations will work as expected in all git commands; see git-rev-parse.

Because Git naturally understands this way of organizing history, it will be easy to locate a particular previous release for rollback. When we roll back, we can roll back to the exact same commit as before: the new metainfo commit and the old one can share the exact same release commit object.

We can see the differences between the last two releases, without knowing their exact dates, with something like:

    git diff HISTORY~1^2 HISTORY^2

The metainfo commit is a full commit with a full set of Git commit metadata, such as a creation date. It has a commit message where we can record any unstructured text that we want. We could also require the commit message to be in TOML format, or we could use the tooling provided by Git to store semi-structured information in the commit message; see git-interpret-trailers.

If the built-in commit structure is not big enough for everything we want to record, the metainfo commit also has an associated tree where we could record any amount of structured information. In the example above I had its tree be identical to the release commit's tree, which might be convenient. (That way, if you wanted to see the files that were released, you could look in either the release commit or the metainfo commit; this would make some of the commands simpler.) But the tree doesn't have to be the same. We could also add metainformation files to this tree, or we could commit an entirely unrelated tree with nothing in it other than structured metainformation.

Performance?

Ploni Almoni asks about the performance issues of tracing out a linked list of commits. But this is exactly the same mechanism that Git normally uses to record history. When we do git log, Git is tracing a linked list of commits in exactly the way suggested here. If any operation in Git is going to be fast, it will be this one.

Git currently takes a few seconds to trace the 275,000-commit history of our monorepo. Tracing the similarly-structured history of a few hundred release commits is not going to be an issue.

I wrote this originally, but my only copy was in the form of a PDF. I had Claude convert the PDF to Markdown, which is the native format accepted by my blog, and the two diagrams to SVG. I then substantially edited the text and the diagrams. Along the way Claude pointed out three minor errors, which I corrected. I also took Claude's advice on how to adjust the colors from the original to make them more legible to color-blind readers.

Every word was written by my brain using my fingers.

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