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.
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.
MergeRequestDiff
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.
#<MergeRequestDiff:0x00007fd1ed63b4d0
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,
patch_id_sha: "d504412d5b6e6739647e752aff8e468dde093f2f",
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.
MergeRequestDiff#commits_count
When MergeRequestDiff is saved, associated MergeRequestDiffCommit records are
counted and cached into the commits_count column. This number displays on the
merge request page as the counter for the Commits tab.
If MergeRequestDiffCommit records are deleted, the counter doesn’t update.
MergeRequestDiffCommit
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.
#<MergeRequestDiffCommit:0x00007fd1dfc6c4c0
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 <sample.user@example.com>\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
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.
#<MergeRequest::DiffCommitUser:0x00007fd1dff7c930
id: 19,
name: "Sample User",
email: "sample.user@example.com">
MergeRequestDiffFile
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.
External diff storage
By default, diff data of a MergeRequestDiffFile is stored in diff column in
the merge_request_diff_files table. On some installations, the table can grow
too large, so they’re configured to store diffs on external storage to save space.
To configure it, see Merge request diffs storage.
When configured to use external storage:
- The
diffcolumn in the database is leftNULL. - The associated
MergeRequestDiffrecord sets thestored_externallyattribute totrueon creation ofMergeRequestDiff.
A cron job named ScheduleMigrateExternalDiffsWorker is also scheduled at
minute 15 of every hour. This migrates diff that are still stored in the
database to external storage.
MergeRequestDiffDetail
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.
#<MergeRequestDiffFile:0x00007fd1ef7c9048
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",
diff:
"@@ -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>
Flow
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.
batch_diffs.json
The most common avenue for viewing diffs is the Changes
tab at the top 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: