Merge request diffs frontend overview

This document provides an overview on how the frontend diffs Vue application works, and the various different parts that exist. It should help contributors:

  • Understand how the diffs Vue app is set up.
  • Identify any areas that need improvement.

This document is a living document. Update it whenever anything significant changes in the diffs application.

Diffs Vue app


The Vue app for rendering diffs uses many different Vue components, some of which get shared with other areas of the GitLab app. The below chart shows the direction for which components get rendered.

Issue #388843 is open to generate a Mermaid graph of the components diagram. An image version of the diagram is available in the issue.

Some of the components are rendered more than others, but the main component is diff_row.vue. This component renders every diff line in a diff file. For performance reasons, this component is a functional component. However, when we upgrade to Vue 3, this is no longer required.

The main diff app component is the main entry point to the diffs app. One of the most important parts of this component is to dispatch the action that assigns discussions to diff lines. This action gets dispatched after the metadata request is completed, and after the batch diffs requests are finished. There is also a watcher set up to watches for changes in both the diff files array and the notes array. Whenever a change happens here, the set discussion action gets dispatched.

The DiffRow component is set up in a way that allows for us to store the diff line data in one format. Previously, we had to request two different formats for inline and side-by-side. The DiffRow component then uses this standard format to render the diff line data. With this standard format, the user can then switch between inline and side-by-side without the need to re-fetch any data.

For this component, a lot of the data used and rendered gets memoized and cached, based on various conditions. It is possible that data sometimes gets cached between each different component render.

Vuex store

The Vuex store for the diffs app consists of 3 different modules:

  • Notes
  • Diffs
  • Batch comments

The notes module is responsible for the discussions, including diff discussions. In this module, the discussions get fetched, and the polling for new discussions is setup. This module gets shared with the issue app as well, so changes here need to be tested in both issues and merge requests.

The diffs module is responsible for the everything related to diffs. This includes, but is not limited to, fetching diffs, assigning diff discussions to lines, and creating diff discussions.

Finally, the batch comments module is not complex, and is responsible only for the draft comments feature. However, this module does dispatch actions in the notes and diff modules whenever draft comments are published.

API Requests


The diffs metadata endpoint exists to fetch the base data the diffs app requires quickly, without the need to fetch all the diff files. This includes, but is not limited to:

  • Diff file names, including some extra meta data for diff files
  • Added and removed line numbers
  • Branch names
  • Diff versions

The most important part of the metadata response is the diff file names. This data allows the diffs app to render the file browser inside of the diffs app, without waiting for all batch diffs requests to complete.

When the metadata response is received, the diff file data gets sent to a web worker. The web worker exists to allow for this data, which for larger merge requests could be huge, to be processed off the main thread. Processing this data involves getting the data into the correct structure that the frontend requires to render the file browser in either tree view or list view.

graph TD A[fetchDiffFilesMeta] B[Create web worker] C[Fetch data] D[SET_DIFF_METADATA] E[Post worker data] F[Worker message event listener] K[SET_TREE_DATA] G[TreeWorker] H[onMessage] I[generateTreeList] J[postMessage sortTree] A -->B E -->F B -->C C -->D D -->E G -->H H -->I I -->J J -->F F -->K

The structure for this file object is:

  "key": "",
  "path": "",
  "name": "",
  "type": "",
  "tree": [],
  "changed": true,
  "tempFile": false,
  "deleted": false,
  "fileHash": "",
  "addedLines": 1,
  "removedLines": 1,
  "parentPath": "/",
  "submodule": false

Batch diffs

To reduce the response size for the diffs endpoint, we are splitting this response up into different requests, to:

  • Reduces the response size of each request.
  • Allows the diffs app to start rendering diffs as quickly as the first request finishes.

To make the first request quicker, the request gets sent asking for a small amount of diffs. The number of diffs requested then increases, until the maximum number of diffs per request is 30.

When the request finishes, the diffs app formats the data received into a format that makes it easier for the diffs app to render the diffs lines.

graph TD A[fetchDiffFilesBatch] --> B[commit SET_DIFF_DATA_BATCH] --> C[prepareDiffData] --> D[prepareRawDiffFile] --> E[ensureBasicDiffFileLines] --> F[prepareDiffFileLines] --> G[finalizeDiffFile] --> H[deduplicateFilesList]

After this has been completed, the diffs app can now begin to render the diff lines. However, before anything can be rendered the diffs app does one more format. It takes the diff line data, and maps the data into a format for easier switching between inline and side-by-side modes. This formatting happens in a computed property inside the diff_content.vue component.

Render queue

This might not be required any more. Some investigation work is required to decide the future of the render queue. The virtual scroll bar we created has probably removed any performance benefit we got from this approach.

To render diffs quickly, we have a render queue that allows the diffs to render only if the browser is idle. This saves the browser getting frozen when rendering a lot of large diffs at once, and allows us to reduce the total blocking time.

This pipeline of rendering files happens only if all the below conditions are true for every diff file. If any of these are false, then this render queue does not happen and the diffs get rendered as expected.

  • Are the diffs in this file already rendered?
  • Does this diff have a viewer? (Meaning, is it not a download?)
  • Is the diff expanded?

This chart gives a brief overview of the pipeline that happens:

graph TD A[startRenderDiffsQueue] -->B B[commit RENDER_FILE current file index] -->C C[canRenderNextFile?] C -->|Yes| D[Render file] -->B C -->|No| E[Re-run requestIdleCallback] -->C

The checks that happen:

  • Is the idle time remaining less than 5 ms?
  • Have we already tried to render this file 4 times?

After these checks happen, the file is marked in Vuex as renderable, which allows the diffs app to start rendering the diff lines and discussions.