Geo (development)

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 are described in more detail below.

Database replication

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.

Repository replication

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:

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 synced successfully.
  • If they need to be resynced.
  • When retry should be attempted.
  • The number of retries.
  • If and when the 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.

When we fail to fetch a repository on the secondary RETRIES_BEFORE_REDOWNLOAD times, Geo does a so-called redownload. 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.

Geo Log Cursor

The Geo Log Cursor is a separate process running on each secondary node. It monitors the Geo Event Log and handles all of the events. When it sees an unhandled event, it starts a background worker to handle that event, depending on the type of event.

When a repository receives an update, the Geo primary node creates a Geo event with an associated repository updated event. The cursor picks that up, and schedules a Geo::ProjectSyncWorker job which will use the Geo::RepositorySyncService class and Geo::WikiSyncService class to update the repository and the wiki.

Uploads replication

File uploads are also being replicated to the secondary node. To track the state of syncing, the Geo::FileRegistry model is used.

File Registry

Similar to the Project Registry, there is a Geo::FileRegistry model that tracks the synced uploads.

CI Job Artifacts are synced in a similar way as uploads or LFS objects, but they are tracked by Geo::JobArtifactRegistry model.

File Download Dispatch worker

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.

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. 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 Geo 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: JWT requires synchronized clocks between the machines involved, otherwise it may fail with an encryption error.

Using the Tracking Database

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.

Configuration

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:migrate

Foreign Data Wrapper

The use of FDW was introduced in GitLab 10.1.

This is useful for the Geo Log Cursor and improves the performance of some synchronization operations.

While FDW is available in older versions of PostgreSQL, we needed to raise the minimum required version to 9.6 as this includes many performance improvements to the FDW implementation.

Refeshing the Foreign Tables

Whenever the database schema changes on the primary node, the secondary node will need to refresh its foreign tables by running the following:

bundle exec rake geo:db:refresh_foreign_tables

Failure to do this will prevent the secondary node from functioning properly. The secondary node will generate error messages, as the following PostgreSQL error:

ERROR:  relation "gitlab_secondary.ci_job_artifacts" does not exist at character 323
STATEMENT:                SELECT a.attname, format_type(a.atttypid, a.atttypmod),
                          pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
                     FROM pg_attribute a LEFT JOIN pg_attrdef d
                       ON a.attrelid = d.adrelid AND a.attnum = d.adnum
                    WHERE a.attrelid = '"gitlab_secondary"."ci_job_artifacts"'::regclass
                      AND a.attnum > 0 AND NOT a.attisdropped
                    ORDER BY a.attnum

Finders

Geo uses Finders, which are classes take care of the heavy lifting of looking up projects/attachments/etc. in the tracking database and main database.

Finders Performance

The Finders need to compare data from the main database with data in the tracking database. For example, counting the number of synced projects normally involves retrieving the project IDs from one database and checking their state in the other database. This is slow and requires a lot of memory.

To overcome this, the Finders use FDW, or Foreign Data Wrappers. This allows a regular JOIN between the main database and the tracking 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 etc. 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.

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.

FDW

Foreign Data Wrapper, or FDW, is a feature built-in in PostgreSQL. It allows data to be queried from different data sources. In Geo, it’s used to query data from different PostgreSQL instances.

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 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.

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 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.

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 was decided to move away from custom code and integrate by using System Webhooks. More people are 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)

Since GitLab 10.0, 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.