debian-mirror-gitlab/doc/development/geo.md

Geo (development) [PREMIUM ONLY]

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.

Replication layer

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.

Geo Log Cursor daemon

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.

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:

• Repository Sync worker.
• 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.

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.

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 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.

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, 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.

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

Introduced in GitLab 10.1.

Foreign Data Wrapper (FDW) is used by the Geo Log Cursor and improves the performance of many synchronization operations.

FDW is a PostgreSQL extension (postgres_fdw) that is enabled within the Geo Tracking Database (on a secondary node), which allows it to connect to the readonly database replica and perform queries and filter data from both instances.

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.

This persistent connection is configured as an FDW server named gitlab_secondary. This configuration exists within the database's user context only. To access the gitlab_secondary, GitLab needs to use the same database user that had previously been configured.

The Geo Tracking Database accesses the readonly database replica via FDW as a regular user, limited by its own restrictions. The credentials are configured as a USER MAPPING associated with the SERVER mapped previously (gitlab_secondary).

FDW configuration and credentials definition are managed automatically by the Omnibus GitLab gitlab-ctl reconfigure command.

Refeshing the Foreign Tables

Whenever a new Geo node is configured or the database schema changes on the primary node, you must refresh the foreign tables on the secondary node 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

Accessing data from a Foreign Table

At the SQL level, all you have to do is SELECT data from gitlab_secondary.*.

Here's an example of how to access all projects from the Geo Tracking Database's FDW:

SELECT * FROM gitlab_secondary.projects;

As a more real-world example, this is how you filter for unarchived projects on the Tracking Database:

SELECT project_registry.*
  FROM project_registry
  JOIN gitlab_secondary.projects
    ON (project_registry.project_id = gitlab_secondary.projects.id 
   AND gitlab_secondary.projects.archived IS FALSE)

At the ActiveRecord level, we have additional Models that represent the foreign tables. They must be mapped in a slightly different way, and they are read-only.

Check the existing FDW models in ee/app/models/geo/fdw for reference.

From a developer's perspective, it's no different than creating a model that represents a Database View.

With the examples above, you can access the projects with:

Geo::Fdw::Project.all

and to access the ProjectRegistry filtering by unarchived projects:

# We have to use Arel here:
project_registry_table = Geo::ProjectRegistry.arel_table
fdw_project_table = Geo::Fdw::Project.arel_table

project_registry_table.join(fdw_project_table)
                      .on(project_registry_table[:project_id].eq(fdw_project_table[:id]))
                      .where((fdw_project_table[:archived]).eq(true)) # if you append `.to_sql` you can check generated query

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, 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)

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.