sourcegraph/cmd/gitserver

gitserver

Mirrors repositories from their code host. All other Sourcegraph services talk to gitserver when they need data from git. Requests for fetch operations, however, go through repo-updater.

gitserver exposes an "exec" API over HTTP for running git commands against clones of repositories. gitserver also exposes APIs for the management of clones.

The management of clones comprises most of the complexity in gitserver since:

• We want to avoid concurrent clones and fetches of the same repository.
• We want to limit the number of concurrent clones and fetches.
• When adding/removing/modifying a clone, concurrent attempts to run commands needs to be gracefully dealt with.
• We need to be robust against the many ways git clones can degrade. (gc, interrupted clones)

Additionally we have invested heavily in the observability of gitserver. Nearly every operation Sourcegraph does runs one or more git commands. So we have detailed oberservability in prometheus, net/event, jaeger, honeycomb and stderr logs.

We normalize repository names when storing them on disk. Always use protocol.NormalizeRepo. The $GIT_DIR of a repository is at reposRoot/normalized_name/.git.

When doing an operation on a file or directory which may be concurrently read/written please use atomic filesystem patterns. This usually involves heavy use of os.Rename. Search for existing uses of os.Rename to see examples.

Scaling

gitserver's memory usage consists of short lived git subprocesses.

This is an IO and compute heavy service since most Sourcegraph requests will trigger 1 or more git commands. As such we shard requests for a repo to a specific replica. This allows us to horizontally scale out the service.

The service is stateful (maintaining git clones). However, it only contains data mirrored from upstream code hosts.