| stage | group | info |
|---|---|---|
Enablement |
Geo |
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 |
Geo connects GitLab instances together. One GitLab instance is designated as a primary node and can be run with multiple secondary nodes. Geo orchestrates quite a few components that can be seen on the diagram below and are described in more detail within this document.
Geo handles replication for different components:
- Database: includes the entire application, except cache and jobs.
- Git repositories: includes both projects and wikis.
- Uploaded blobs: includes anything from images attached on issues to raw logs and assets from CI.
With the exception of the Database replication, on a secondary node, everything is coordinated by the Geo Log Cursor.
The Geo Log Cursor daemon is a separate process running on each secondary node. It monitors the Geo Event Log for new events and creates background jobs for each specific event type.
For example when a repository is updated, the Geo primary node creates
a Geo event with an associated repository updated event. The Geo Log Cursor daemon
picks the event up and schedules a Geo::ProjectSyncWorker job which will
use the Geo::RepositorySyncService and Geo::WikiSyncService classes
to update the repository and the wiki respectively.
The Geo Log Cursor daemon can operate in High Availability mode automatically. The daemon will try to acquire a lock from time to time and once acquired, it will behave as the active daemon.
Any additional running daemons on the same node, will be in standby mode, ready to resume work if the active daemon releases its lock.
We use the ExclusiveLease lock type with a small TTL, that is renewed at every
pooling cycle. That allows us to implement this global lock with a timeout.
At the end of the pooling cycle, if the daemon can't renew and/or reacquire the lock, it switches to standby mode.
Geo uses streaming replication to replicate the database from the primary to the secondary nodes. This replication gives the secondary nodes access to all the data saved in the database. So users can log in on the secondary and read all the issues, merge requests, etc. on the secondary node.
Geo also replicates repositories. Each secondary node keeps track of the state of every repository in the tracking database.
There are a few ways a repository gets replicated by the:
The Geo::ProjectRegistry class defines the model used to track the
state of repository replication. For each project in the main
database, one record in the tracking database is kept.
It records the following about repositories:
- The last time they were synced.
- The last time they were successfully synced.
- If they need to be resynced.
- When a retry should be attempted.
- The number of retries.
- If and when they were verified.
It also stores these attributes for project wikis in dedicated columns.
The Geo::RepositorySyncWorker class runs periodically in the
background and it searches the Geo::ProjectRegistry model for
projects that need updating. Those projects can be:
- Unsynced: Projects that have never been synced on the secondary node and so do not exist yet.
- Updated recently: Projects that have a
last_repository_updated_attimestamp that is more recent than thelast_repository_successful_sync_attimestamp in theGeo::ProjectRegistrymodel. - Manual: The admin can manually flag a repository to resync in the Geo admin panel.
When we fail to fetch a repository on the secondary RETRIES_BEFORE_REDOWNLOAD
times, Geo does a so-called re-download. It will do a clean clone
into the @geo-temporary directory in the root of the storage. When
it's successful, we replace the main repo with the newly cloned one.
File uploads are also being replicated to the secondary node. To
track the state of syncing, the Geo::UploadRegistry model is used.
Similar to the Project Registry, there is a
Geo::UploadRegistry model that tracks the synced uploads.
CI Job Artifacts and LFS objects are synced in a similar way as uploads,
but they are tracked by Geo::JobArtifactRegistry, and Geo::LfsObjectRegistry
models respectively.
Also similar to the Repository Sync worker,
there is a Geo::FileDownloadDispatchWorker class that is run
periodically to sync all uploads that aren't synced to the Geo
secondary node yet.
Files are copied via HTTP(s) and initiated via the
/api/v4/geo/transfers/:type/:id endpoint,
e.g. /api/v4/geo/transfers/lfs/123.
To authenticate file transfers, each GeoNode record has two fields:
- A public access key (
access_keyfield). - A secret access key (
secret_access_keyfield).
The secondary node authenticates itself via a JWT request.
When the secondary node wishes to download a file, it sends an
HTTP request with the Authorization header:
Authorization: GL-Geo <access_key>:<JWT payload>
The primary node uses the access_key field to look up the
corresponding secondary node and decrypts the JWT payload,
which contains additional information to identify the file
request. This ensures that the secondary node downloads the right
file for the right database ID. For example, for an LFS object, the
request must also include the SHA256 sum of the file. An example JWT
payload looks like:
{"data": {sha256: "31806bb23580caab78040f8c45d329f5016b0115"}, iat: "1234567890"}If the requested file matches the requested SHA256 sum, then the Geo primary node sends data via the X-Sendfile feature, which allows NGINX to handle the file transfer without tying up Rails or Workhorse.
NOTE: Note: JWT requires synchronized clocks between the machines involved, otherwise it may fail with an encryption error.
The Git Push Proxy exists as a functionality built inside the gitlab-shell component.
It is active on a secondary node only. It allows the user that has cloned a repository
from the secondary node to push to the same URL.
Git push requests directed to a secondary node will be sent over to the primary node,
while pull requests will continue to be served by the secondary node for maximum efficiency.
HTTPS and SSH requests are handled differently:
- With HTTPS, we will give the user a
HTTP 302 Redirectpointing to the project on the primary node. The Git client is wise enough to understand that status code and process the redirection. - With SSH, because there is no equivalent way to perform a redirect, we have to proxy the request.
This is done inside
gitlab-shell, by first translating the request to the HTTP protocol, and then proxying it to the primary node.
The gitlab-shell daemon knows when to proxy based on the response
from /api/v4/allowed. A special HTTP 300 status code is returned and we execute a "custom action",
specified in the response body. The response contains additional data that allows the proxied push operation
to happen on the primary node.
Along with the main database that is replicated, a Geo secondary node has its own separate Tracking database.
The tracking database contains the state of the secondary node.
Any database migration that needs to be run as part of an upgrade needs to be applied to the tracking database on each secondary node.
The database configuration is set in config/database_geo.yml.
The directory ee/db/geo
contains the schema and migrations for this database.
To write a migration for the database, use the GeoMigrationGenerator:
rails g geo_migration [args] [options]To migrate the tracking database, run:
bundle exec rake geo:db:migrateGeo uses Finders, which are classes take care of the heavy lifting of looking up projects/attachments/etc. in the tracking database and main database.
Redis on the secondary node works the same as on the primary node. It is used for caching, storing sessions, and other persistent data.
Redis data replication between primary and secondary node is not used, so sessions etc. aren't shared between nodes.
GitLab can optionally use Object Storage to store data it would otherwise store on disk. These things can be:
- LFS Objects
- CI Job Artifacts
- Uploads
Objects that are stored in object storage, are not handled by Geo. Geo ignores items in object storage. Either:
- The object storage layer should take care of its own geographical replication.
- All secondary nodes should use the same storage node.
Repositories are verified with a checksum.
The primary node calculates a checksum on the repository. It
basically hashes all Git refs together and stores that hash in the
project_repository_states table of the database.
The secondary node does the same to calculate the hash of its clone, and compares the hash with the value the primary node calculated. If there is a mismatch, Geo will mark this as a mismatch and the administrator can see this in the Geo admin panel.
A primary node is the single node in a Geo setup that read-write capabilities. It's the single source of truth and the Geo secondary nodes replicate their data from there.
In a Geo setup, there can only be one primary node. All secondary nodes connect to that primary.
A secondary node is a read-only replica of the primary node running in a different geographical location.
Geo depends on the streaming replication feature of PostgreSQL. It completely replicates the database data and the database schema. The database replica is a read-only copy.
Streaming replication depends on the Write Ahead Logs, or WAL. Those logs are copied over to the replica and replayed there.
Since streaming replication also replicates the schema, the database migration do not need to run on the secondary nodes.
A database on each Geo secondary node that keeps state for the node on which it resides. Read more in Using the Tracking database.
The Geo primary stores events in the geo_event_log table. Each
entry in the log contains a specific type of event. These type of
events include:
- Repository Deleted event
- Repository Renamed event
- Repositories Changed event
- Repository Created event
- Hashed Storage Migrated event
- Lfs Object Deleted event
- Hashed Storage Attachments event
- Job Artifact Deleted event
- Upload Deleted event
The process running on the secondary node that looks for new
Geo::EventLog rows.
Small utility methods related to Geo go into the
ee/lib/gitlab/geo.rb
file.
Many of these methods are cached using the RequestStore class, to
reduce the performance impact of using the methods throughout the
codebase.
The class method .current_node returns the GeoNode record for the
current node.
We use the host, port, and relative_url_root values from
gitlab.yml and search in the database to identify which node we are
in (see GeoNode.current_node).
To determine whether the current node is a primary node or a
secondary node use the .primary? and .secondary? class
methods.
It is possible for these methods to both return false on a node when
the node is not enabled. See Enablement.
There is also an additional gotcha when dealing with things that
happen during initialization time. In a few places, we use the
Gitlab::Geo.geo_database_configured? method to check if the node has
the tracking database, which only exists on the secondary
node. This overcomes race conditions that could happen during
bootstrapping of a new node.
We consider Geo feature enabled when the user has a valid license with the feature included, and they have at least one node defined at the Geo Nodes screen.
See Gitlab::Geo.enabled? and Gitlab::Geo.license_allows? methods.
All Geo secondary nodes are read-only.
The general principle of a read-only database
applies to all Geo secondary nodes. So the
Gitlab::Database.read_only? method will always return true on a
secondary node.
When some write actions are not allowed because the node is a
secondary, consider adding the Gitlab::Database.read_only? or
Gitlab::Database.read_write? guard, instead of Gitlab::Geo.secondary?.
The database itself will already be read-only in a replicated setup, so we don't need to take any extra step for that.
As GitLab evolves, we constantly need to add new resources to the Geo replication system. The implementation depends on resource specifics, but there are several things that need to be taken care of:
- Event generation on the primary site. Whenever a new resource is changed/updated, we need to create a task for the Log Cursor.
- Event handling. The Log Cursor needs to have a handler for every event type generated by the primary site.
- Dispatch worker (cron job). Make sure the backfill condition works well.
- Sync worker.
- Registry with all possible states.
- Verification.
- Cleaner. When sync settings are changed for the secondary site, some resources need to be cleaned up.
- Geo Node Status. We need to provide API endpoints as well as some presentation in the GitLab Admin Area.
- Health Check. If we can perform some pre-cheсks and make node unhealthy if something is wrong, we should do that.
The
rake gitlab:geo:checkcommand has to be updated too.
We started developing a new Geo self-service framework (alpha) which makes it a lot easier to add a new data type.
The communication channel has changed since first iteration, you can check here historic decisions and why we moved to new implementations.
In GitLab versions before 8.6, custom code is used to handle notification from primary node to secondary nodes by HTTP requests.
Later, it was decided to move away from custom code and begin using system hooks. More people were using them, so many would benefit from improvements made to this communication layer.
There is a specific internal endpoint in our API code (Grape),
that receives all requests from this System Hooks:
/api/v4/geo/receive_events.
We switch and filter from each event by the event_name field.
In GitLab 10.0 and later, System Webhooks are no longer
used and Geo Log Cursor is used instead. The Log Cursor traverses the
Geo::EventLog rows to see if there are changes since the last time
the log was checked and will handle repository updates, deletes,
changes, and renames.
The table is within the replicated database. This has two advantages over the old method:
- Replication is synchronous and we preserve the order of events.
- Replication of the events happen at the same time as the changes in the database.
If you want to add easy Geo replication of a resource you're working on, check out our self-service framework.