Working with diffs

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

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

Deep Dive

In January 2019, Oswaldo Ferreira hosted a Deep Dive (GitLab team members only: on GitLab’s Diffs and Commenting on Diffs functionality to share his domain specific knowledge with anyone who may work in this part of the code base in the future. You can find the recording on YouTube, and the slides on Google Slides and in PDF. Everything covered in this deep dive was accurate as of GitLab 11.7, and while specific details may have changed since then, it should still serve as a good introduction.

Architecture overview

Merge request diffs

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. 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 larger than 10% of the value of ApplicationSettings#diff_max_patch_bytes are collapsed, we still keep them on PostgreSQL. However, diff files larger than defined safety limits (see the Diff limits section) are not persisted in the database.

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:
    • Highlight old and new file content
    • Know which viewer it should use for each file (text, image, deleted, etc)
    • Know if the file content changed
    • Know if it was stored externally
    • 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

Note diffs

When commenting on a diff (any comparison), we persist a truncated diff version on NoteDiffFile (which is associated with the actual DiffNote). So instead of hitting the repository every time we need the diff of the file, we:

  1. Check whether we have the NoteDiffFile#diff persisted and use it
  2. Otherwise, if it’s a current MR revision, use the persisted MergeRequestDiffFile#diff
  3. In the last scenario, go the repository and fetch the diff


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.

Note: All collection limit parameters are currently sent and applied on Gitaly. That is, once the limit is surpassed, Gitaly will only return the safe amount of data to be persisted on merge_request_diff_files.

Individual diff file limits

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

Expandable patches (collapsed)

Diff patches are collapsed when surpassing 10% of the value set in ApplicationSettings#diff_max_patch_bytes. That is, it’s equivalent to 10kb if the maximum allowed value is 100kb. The diff will still be persisted and expandable if the patch size doesn’t surpass ApplicationSettings#diff_max_patch_bytes.

Note: Although this nomenclature (Collapsing) is also used on Gitaly, this limit is only used on GitLab (hardcoded - not sent to Gitaly). Gitaly will only return Diff.Collapsed (RPC) when surpassing collection limits.

Not expandable patches (too large)

The patch not be rendered if it’s larger than ApplicationSettings#diff_max_patch_bytes. Users will see a This source diff could not be displayed because it is too large message.

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.

Note: This limit is currently hardcoded and only applied on GitLab.


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.