89 lines
4.5 KiB
Markdown
89 lines
4.5 KiB
Markdown
---
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stage: Create
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group: Source Code
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info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
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---
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# Git LFS **(FREE)**
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## Deep Dive
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In April 2019, Francisco Javier López hosted a Deep Dive (GitLab team members only: `https://gitlab.com/gitlab-org/create-stage/issues/1`)
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on the GitLab [Git LFS](../topics/git/lfs/index.md) implementation to share domain-specific
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knowledge with anyone who may work in this part of the codebase in the future.
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You can find the <i class="fa fa-youtube-play youtube" aria-hidden="true"></i> [recording on YouTube](https://www.youtube.com/watch?v=Yyxwcksr0Qc),
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and the slides on [Google Slides](https://docs.google.com/presentation/d/1E-aw6-z0rYd0346YhIWE7E9A65zISL9iIMAOq2zaw9E/edit)
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and in [PDF](https://gitlab.com/gitlab-org/create-stage/uploads/07a89257a140db067bdfb484aecd35e1/Git_LFS_Deep_Dive__Create_.pdf).
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This deep dive was accurate as of GitLab 11.10, and while specific
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details may have changed, it should still serve as a good introduction.
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## Including LFS blobs in project archives
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> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/15079) in GitLab 13.5.
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The following diagram illustrates how GitLab resolves LFS files for project archives:
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```mermaid
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sequenceDiagram
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autonumber
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Client->>+Workhorse: GET /group/project/-/archive/master.zip
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Workhorse->>+Rails: GET /group/project/-/archive/master.zip
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Rails->>+Workhorse: Gitlab-Workhorse-Send-Data git-archive
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Workhorse->>Gitaly: SendArchiveRequest
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Gitaly->>Git: git archive master
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Git->>Smudge: OID 12345
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Smudge->>+Workhorse: GET /internal/api/v4/lfs?oid=12345&gl_repository=project-1234
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Workhorse->>+Rails: GET /internal/api/v4/lfs?oid=12345&gl_repository=project-1234
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Rails->>+Workhorse: Gitlab-Workhorse-Send-Data send-url
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Workhorse->>Smudge: <LFS data>
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Smudge->>Git: <LFS data>
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Git->>Gitaly: <streamed data>
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Gitaly->>Workhorse: <streamed data>
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Workhorse->>Client: master.zip
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```
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1. The user requests the project archive from the UI.
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1. Workhorse forwards this request to Rails.
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1. If the user is authorized to download the archive, Rails replies with
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an HTTP header of `Gitlab-Workhorse-Send-Data` with a base64-encoded
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JSON payload prefaced with `git-archive`. This payload includes the
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`SendArchiveRequest` binary message, which is encoded again in base64.
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1. Workhorse decodes the `Gitlab-Workhorse-Send-Data` payload. If the
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archive already exists in the archive cache, Workhorse sends that
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file. Otherwise, Workhorse sends the `SendArchiveRequest` to the
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appropriate Gitaly server.
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1. The Gitaly server calls `git archive <ref>` to begin generating
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the Git archive on-the-fly. If the `include_lfs_blobs` flag is enabled,
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Gitaly enables a custom LFS smudge filter via the `-c
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filter.lfs.smudge=/path/to/gitaly-lfs-smudge` Git option.
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1. When `git` identifies a possible LFS pointer using the
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`.gitattributes` file, `git` calls `gitaly-lfs-smudge` and provides the
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LFS pointer via the standard input. Gitaly provides `GL_PROJECT_PATH`
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and `GL_INTERNAL_CONFIG` as environment variables to enable lookup of
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the LFS object.
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1. If a valid LFS pointer is decoded, `gitaly-lfs-smudge` makes an
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internal API call to Workhorse to download the LFS object from GitLab.
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1. Workhorse forwards this request to Rails. If the LFS object exists
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and is associated with the project, Rails sends `ArchivePath` either
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with a path where the LFS object resides (for local disk) or a
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pre-signed URL (when object storage is enabled) via the
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`Gitlab-Workhorse-Send-Data` HTTP header with a payload prefaced with
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`send-url`.
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1. Workhorse retrieves the file and send it to the `gitaly-lfs-smudge`
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process, which writes the contents to the standard output.
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1. `git` reads this output and sends it back to the Gitaly process.
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1. Gitaly sends the data back to Rails.
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1. The archive data is sent back to the client.
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In step 7, the `gitaly-lfs-smudge` filter must talk to Workhorse, not to
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Rails, or an invalid LFS blob is saved. To support this, GitLab
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13.5 [changed the default Omnibus configuration to have Gitaly talk to
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the Workhorse](https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/4592)
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instead of Rails.
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One side effect of this change: the correlation ID of the original
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request is not preserved for the internal API requests made by Gitaly
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(or `gitaly-lfs-smudge`), such as the one made in step 8. The
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correlation IDs for those API requests are random values until [this
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Workhorse issue](https://gitlab.com/gitlab-org/gitlab-workhorse/-/issues/309) is
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resolved.
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