Merge request diffs development guide

This document explains the backend design and flow of merge request diffs. It should help contributors:

  • Understand the code design.
  • Identify areas for improvement through contribution.

It’s intentional that it doesn’t contain too many implementation details, as they can change often. The code better explains these details. The components mentioned here are the major parts of the application for how merge request diffs are generated, stored, and returned to users.

This page is a living document. Update it accordingly when the parts of the codebase touched in this document are changed or removed, or when new components are added.

Data model

Four main ActiveRecord models represent what we collectively refer to as diffs. These database-backed records replicate data contained in the project’s Git repository, and are in part a cache against excessive access requests to Gitaly. Additionally, they provide a logical place for:

  • Calculated and retrieved metadata about the pieces of the diff.
  • General class- and instance- based logic.
erDiagram MergeRequest ||--|{ MergeRequestDiff: "" MergeRequestDiff |{--|{ MergeRequestDiffCommit: "" MergeRequestDiff |{--|| MergeRequestDiffDetail: "" MergeRequestDiff |{--|{ MergeRequestDiffFile: "" MergeRequestDiffCommit |{--|| MergeRequestDiffCommitUser: ""


MergeRequestDiff is defined in app/models/merge_request_diff.rb. This class holds metadata and context related to the diff resulting from a set of commits. It defines methods that are the primary means for interacting with diff contents, individual commits, and the files containing changes.

 id: 28,
 state: "collected",
 merge_request_id: 28,
 created_at: Tue, 06 Sep 2022 18:56:02.509469000 UTC +00:00,
 updated_at: Tue, 06 Sep 2022 18:56:02.754201000 UTC +00:00,
 base_commit_sha: "ae73cb07c9eeaf35924a10f713b364d32b2dd34f",
 real_size: "9",
 head_commit_sha: "bb5206fee213d983da88c47f9cf4cc6caf9c66dc",
 start_commit_sha: "0b4bc9a49b562e85de7cc9e834518ea6828729b9",
 commits_count: 6,
 external_diff: "diff-28",
 external_diff_store: 1,
 stored_externally: nil,
 files_count: 9,
 sorted: true,
 diff_type: "regular",
 verification_checksum: nil>

Diff content is usually accessed through this class. Logic is often applied to diff, file, and commit content before it is returned to a user.


MergeRequestDiffCommit is defined in app/models/merge_request_diff_commit.rb. This class corresponds to a single commit contained in its corresponding MergeRequestDiff, and holds header information about the commit.

  authored_date: Wed, 06 Aug 2022 06:35:52.000000000 UTC +00:00,
  committed_date: Wed, 06 Aug 2022 06:35:52.000000000 UTC +00:00,
  merge_request_diff_id: 28,
  relative_order: 0,
  sha: "bb5206fee213d983da88c47f9cf4cc6caf9c66dc",
  message: "Feature conflcit added\n\nSigned-off-by: Sample User <>\n",
  trailers: {},
  commit_author_id: 19,
  committer_id: 19>

Every MergeRequestDiffCommit has a corresponding MergeRequest::DiffCommitUser record it :belongs_to, in ActiveRecord parlance. These records are :commit_author and :committer, and could be distinct individuals.


MergeRequest::DiffCommitUser is defined in app/models/merge_request/diff_commit_user.rb. It captures the name and email of a given commit, but contains no connection itself to any User records.

  id: 19,
  name: "Sample User",
  email: "">


MergeRequestDiffFile is defined in app/models/merge_request_diff_file.rb. This record of this class represents the diff of a single file contained in the MergeRequestDiff. It holds both meta and specific information about the file’s relationship to the change, such as:

  • Whether it is added or renamed.
  • Its ordering in the diff.
  • The raw diff output itself.


MergeRequestDiffDetail is defined in app/models/merge_request_diff_detail.rb. This class provides verification information for Geo replication, but otherwise is not used for user-facing diffs.

  merge_request_diff_id: 28,
  relative_order: 0,
  new_file: true,
  renamed_file: false,
  deleted_file: false,
  too_large: false,
  a_mode: "0",
  b_mode: "100644",
  new_path: "files/ruby/feature.rb",
  old_path: "files/ruby/feature.rb",
   "@@ -0,0 +1,4 @@\n+# This file was changed in feature branch\n+# We put different code here to make merge conflict\n+class Conflict\n+end\n",
  binary: false,
  external_diff_offset: nil,
  external_diff_size: nil>


These flowcharts should help explain the flow from the controllers down to the models for different features. This page is not intended to document the entirety of options for access and working with diffs, focusing solely on the most common.


The most common avenue for viewing diffs is the Changes tab in the top navigation bar of merge request pages in the GitLab UI. When selected, the diffs themselves are loaded via a paginated request to /-/merge_requests/:id/batch_diffs.json, which is served by Projects::MergeRequests::DiffsController#diffs_batch:

sequenceDiagram Note over .#diffs_batch: Preload diffs and ivars .#diffs_batch->>+.#define_diff_vars: &nbsp; .#define_diff_vars ->>+ @merge_request: @merge_request_diffs = Note right of @merge_request: An ordered collection of all diffs in MR @merge_request-->>-.#define_diff_vars: &nbsp; .#define_diff_vars ->>+ @merge_request: @merge_request_diff = Note right of @merge_request: Most recent merge_request_diff (or commit) @merge_request-->>-.#define_diff_vars: &nbsp; .#define_diff_vars ->>+ .#define_diff_vars: @compare = Note right of .#define_diff_vars:: param-filtered merge_request_diff(s) .#define_diff_vars -->>- .#diffs_batch: &nbsp; Note over .#diffs_batch: Preloading complete .#diffs_batch->>+@merge_request: Calculate unfoldable diff lines Note right of @merge_request: note_positions_for_paths.unfoldable @merge_request-->>-.#diffs_batch: &nbsp; Note over .#diffs_batch: Build options hash Note over .#diffs_batch: Build cache_context Note over .#diffs_batch: Unfold files in diff .#diffs_batch->>+Gitlab_Diff_FileCollection_MergeRequestDiffBase: diffs.write_diff Gitlab_Diff_FileCollection_MergeRequestDiffBase->>+Gitlab_Diff_HighlightCache: Highlight diff Gitlab_Diff_HighlightCache -->>-Gitlab_Diff_FileCollection_MergeRequestDiffBase: Return highlighted diff Note over Gitlab_Diff_FileCollection_MergeRequestDiffBase: Cache diff Gitlab_Diff_FileCollection_MergeRequestDiffBase-->>-.#diffs_batch: &nbsp; Note over .#diffs_batch: render JSON