Working with Merge Request diffs

Currently we rely on different sources to present merge request diffs, these include:

  • Rugged gem
  • Gitaly service
  • Database (through merge_request_diff_files)
  • Redis (cached highlighted diffs)

We're constantly moving Rugged calls to Gitaly and the progress can be followed through Gitaly repo.

Architecture overview

When refreshing a Merge Request (pushing to a source branch, force-pushing to target branch, or if the target branch now contains any commits from the MR) we fetch the comparison information using Gitlab::Git::Compare, which fetches base and head data using Gitaly and diff between them through Gitlab::Git::Diff.between (which uses Gitaly if it's enabled, otherwise Rugged). The diffs fetching process limits single file diff sizes and the overall size of the whole diff through a series of constant values. Raw diff files are then persisted on merge_request_diff_files table.

Even though diffs higher than 10kb are collapsed (Gitlab::Git::Diff::COLLAPSE_LIMIT), we still keep them on Postgres. However, diff files over safety limits (see the Diff limits section) are not persisted.

In order to present diffs information on the Merge Request diffs page, we:

  1. Fetch all diff files from database merge_request_diff_files
  2. Fetch the old and new file blobs in batch to:
    1. Highlight old and new file content
    2. Know which viewer it should use for each file (text, image, deleted, etc)
    3. Know if the file content changed
    4. Know if it was stored externally
    5. Know if it had storage errors
  3. If the diff file is cacheable (text-based), it's cached on Redis using Gitlab::Diff::FileCollection::MergeRequestDiff

Diff limits

As explained above, we limit single diff files and the size of the whole diff. There are scenarios where we collapse the diff file, and cases where the diff file is not presented at all, and the user is guided to the Blob view. Here we'll go into details about these limits.

Diff collection limits

Limits that act onto all diff files collection. Files number, lines number and files size are considered.

Gitlab::Git::DiffCollection.collection_limits[:safe_max_files] = Gitlab::Git::DiffCollection::DEFAULT_LIMITS[:max_files] = 100

File diffs will be collapsed (but be expandable) if 100 files have already been rendered.

Gitlab::Git::DiffCollection.collection_limits[:safe_max_lines] = Gitlab::Git::DiffCollection::DEFAULT_LIMITS[:max_lines] = 5000 

File diffs will be collapsed (but be expandable) if 5000 lines have already been rendered.

Gitlab::Git::DiffCollection.collection_limits[:safe_max_bytes] = Gitlab::Git::DiffCollection.collection_limits[:safe_max_files] * 5.kilobytes = 500.kilobytes 

File diffs will be collapsed (but be expandable) if 500 kilobytes have already been rendered.

Gitlab::Git::DiffCollection.collection_limits[:max_files] = Commit::DIFF_HARD_LIMIT_FILES = 1000 

No more files will be rendered at all if 1000 files have already been rendered.

Gitlab::Git::DiffCollection.collection_limits[:max_lines] = Commit::DIFF_HARD_LIMIT_LINES = 50000 

No more files will be rendered at all if 50,000 lines have already been rendered.

Gitlab::Git::DiffCollection.collection_limits[:max_bytes] = Gitlab::Git::DiffCollection.collection_limits[:max_files] * 5.kilobytes = 5000.kilobytes 

No more files will be rendered at all if 5 megabytes have already been rendered.

Individual diff file limits

Limits that act onto each diff file of a collection. Files number, lines number and files size are considered.

Gitlab::Git::Diff::COLLAPSE_LIMIT = 10.kilobytes

File diff will be collapsed (but be expandable) if it is larger than 10 kilobytes.

Gitlab::Git::Diff::SIZE_LIMIT = 100.kilobytes

File diff will not be rendered if it's larger than 100 kilobytes.

Commit::DIFF_SAFE_LINES = Gitlab::Git::DiffCollection::DEFAULT_LIMITS[:max_lines] = 5000

File diff will be suppressed (technically different from collapsed, but behaves the same, and is expandable) if it has more than 5000 lines.


Diff Viewers, which can be found on models/diff_viewer/* are classes used to map metadata about each type of Diff File. It has information whether it's a binary, which partial should be used to render it or which File extensions this class accounts for.

DiffViewer::Base validates blobs (old and new versions) content, extension and file type in order to check if it can be rendered.