441 lines
17 KiB
Markdown
441 lines
17 KiB
Markdown
---
|
||
stage: Enablement
|
||
group: Geo
|
||
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
|
||
---
|
||
|
||
# Geo (development) **(PREMIUM SELF)**
|
||
|
||
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 Architecture Diagram](../administration/geo/replication/img/geo_architecture.png)
|
||
|
||
## Replication layer
|
||
|
||
Geo handles replication for different components:
|
||
|
||
- [Database](#database-replication): includes the entire application, except cache and jobs.
|
||
- [Git repositories](#repository-replication): includes both projects and wikis.
|
||
- [Uploaded blobs](#uploads-replication): 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](#geo-log-cursor).
|
||
|
||
### Geo Log Cursor daemon
|
||
|
||
The [Geo Log Cursor daemon](#geo-log-cursor-daemon) is a separate process running on
|
||
each **secondary** node. It monitors the [Geo Event Log](#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`](https://www.rubydoc.info/github/gitlabhq/gitlabhq/Gitlab/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.
|
||
|
||
### Database replication
|
||
|
||
Geo uses [streaming replication](#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, and so on, on the **secondary** node.
|
||
|
||
### Repository replication
|
||
|
||
Geo also replicates repositories. Each **secondary** node keeps track of
|
||
the state of every repository in the [tracking database](#tracking-database).
|
||
|
||
There are a few ways a repository gets replicated by the:
|
||
|
||
- [Repository Sync worker](#repository-sync-worker).
|
||
- [Geo Log Cursor](#geo-log-cursor).
|
||
|
||
#### Project Registry
|
||
|
||
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.
|
||
|
||
#### Repository Sync worker
|
||
|
||
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_at`
|
||
timestamp that is more recent than the `last_repository_successful_sync_at`
|
||
timestamp in the `Geo::ProjectRegistry` model.
|
||
- Manual: The admin can manually flag a repository to resync in the
|
||
[Geo admin panel](../user/admin_area/geo_nodes.md).
|
||
|
||
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 repository with the newly cloned one.
|
||
|
||
### Uploads replication
|
||
|
||
File uploads are also being replicated to the **secondary** node. To
|
||
track the state of syncing, the `Geo::UploadRegistry` model is used.
|
||
|
||
#### Upload Registry
|
||
|
||
Similar to the [Project Registry](#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.
|
||
|
||
#### File Download Dispatch worker
|
||
|
||
Also similar to the [Repository Sync worker](#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,
|
||
for example, `/api/v4/geo/transfers/lfs/123`.
|
||
|
||
## Authentication
|
||
|
||
To authenticate file transfers, each `GeoNode` record has two fields:
|
||
|
||
- A public access key (`access_key` field).
|
||
- A secret access key (`secret_access_key` field).
|
||
|
||
The **secondary** node authenticates itself via a [JWT request](https://jwt.io/).
|
||
When the **secondary** node wishes to download a file, it sends an
|
||
HTTP request with the `Authorization` header:
|
||
|
||
```plaintext
|
||
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:
|
||
|
||
```yaml
|
||
{"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](https://www.nginx.com/resources/wiki/start/topics/examples/xsendfile/)
|
||
feature, which allows NGINX to handle the file transfer without tying
|
||
up Rails or Workhorse.
|
||
|
||
NOTE:
|
||
JWT requires synchronized clocks between the machines
|
||
involved, otherwise it may fail with an encryption error.
|
||
|
||
## Git Push to Geo secondary
|
||
|
||
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 Redirect` pointing 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`](https://gitlab.com/gitlab-org/gitlab-shell), by first translating the request
|
||
to the HTTP protocol, and then proxying it to the **primary** node.
|
||
|
||
The [`gitlab-shell`](https://gitlab.com/gitlab-org/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.
|
||
|
||
## Using the Tracking Database
|
||
|
||
Along with the main database that is replicated, a Geo **secondary**
|
||
node has its own separate [Tracking database](#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.
|
||
|
||
### Configuration
|
||
|
||
The database configuration is set in [`config/database_geo.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/database_geo.yml.postgresql).
|
||
The directory [`ee/db/geo`](https://gitlab.com/gitlab-org/gitlab/-/tree/master/ee/db/geo)
|
||
contains the schema and migrations for this database.
|
||
|
||
To write a migration for the database, use the `GeoMigrationGenerator`:
|
||
|
||
```shell
|
||
rails g geo_migration [args] [options]
|
||
```
|
||
|
||
To migrate the tracking database, run:
|
||
|
||
```shell
|
||
bundle exec rake geo:db:migrate
|
||
```
|
||
|
||
## Finders
|
||
|
||
Geo uses [Finders](https://gitlab.com/gitlab-org/gitlab/-/tree/master/app/finders),
|
||
which are classes take care of the heavy lifting of looking up
|
||
projects/attachments/ and so on, in the tracking database and main database.
|
||
|
||
## Redis
|
||
|
||
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 and so on, aren't shared between nodes.
|
||
|
||
## Object Storage
|
||
|
||
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.
|
||
|
||
## Verification
|
||
|
||
### Repository verification
|
||
|
||
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](../user/admin_area/geo_nodes.md).
|
||
|
||
## Glossary
|
||
|
||
### Primary node
|
||
|
||
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**.
|
||
|
||
### Secondary node
|
||
|
||
A **secondary** node is a read-only replica of the **primary** node
|
||
running in a different geographical location.
|
||
|
||
### Streaming replication
|
||
|
||
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.
|
||
|
||
### Tracking database
|
||
|
||
A database on each Geo **secondary** node that keeps state for the node
|
||
on which it resides. Read more in [Using the Tracking database](#using-the-tracking-database).
|
||
|
||
## Geo Event Log
|
||
|
||
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
|
||
|
||
### Geo Log Cursor
|
||
|
||
The process running on the **secondary** node that looks for new
|
||
`Geo::EventLog` rows.
|
||
|
||
## Code features
|
||
|
||
### `Gitlab::Geo` utilities
|
||
|
||
Small utility methods related to Geo go into the
|
||
[`ee/lib/gitlab/geo.rb`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/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.
|
||
|
||
#### Current node
|
||
|
||
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`).
|
||
|
||
#### Primary or secondary
|
||
|
||
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](#enablement).
|
||
|
||
#### Geo Database configured?
|
||
|
||
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.
|
||
|
||
#### Enablement
|
||
|
||
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.
|
||
|
||
#### Read-only
|
||
|
||
All Geo **secondary** nodes are read-only.
|
||
|
||
The general principle of a [read-only database](verifying_database_capabilities.md#read-only-database)
|
||
applies to all Geo **secondary** nodes. So the
|
||
`Gitlab::Database.main.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.main.read_only?` or
|
||
`Gitlab::Database.main.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.
|
||
|
||
## Steps needed to replicate a new data type
|
||
|
||
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:check` command has to be updated too.
|
||
|
||
## History of communication channel
|
||
|
||
The communication channel has changed since first iteration, you can
|
||
check here historic decisions and why we moved to new implementations.
|
||
|
||
### Custom code (GitLab 8.6 and earlier)
|
||
|
||
In GitLab versions before 8.6, custom code is used to handle
|
||
notification from **primary** node to **secondary** nodes by HTTP
|
||
requests.
|
||
|
||
### System hooks (GitLab 8.7 to 9.5)
|
||
|
||
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.
|
||
|
||
### Geo Log Cursor (GitLab 10.0 and up)
|
||
|
||
In GitLab 10.0 and later, [System Webhooks](#system-hooks-gitlab-87-to-95) 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.
|
||
|
||
## Self-service framework
|
||
|
||
If you want to add easy Geo replication of a resource you're working
|
||
on, check out our [self-service framework](geo/framework.md).
|