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Merge branch 'tc-geo-dev-docs' into 'master' 

Write some Geo dev docs

Closes #7685

See merge request gitlab-org/gitlab-ee!7452
parents b1df2b95 6cce6de0
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doc/development/README.md
View file @ 98a8e4d6
......@@ -53,6 +53,7 @@ description: 'Learn how to contribute to GitLab.'
- [Prometheus metrics](prometheus_metrics.md)
- [Guidelines for reusing abstractions](reusing_abstractions.md)
- [DeclarativePolicy framework](policies.md)
- [Geo development](geo.md)
## Performance guides
......
doc/development/geo.md
View file @ 98a8e4d6
# Geo (development)
Geo feature requires that we orchestrate a lot of components together.
For the Database we need to set up a streaming replication. Any operation on disk
is logged in an events table, that will leverage the database replication itself
from **Primary** to **Secondary** nodes. These events are processed by the
**Geo Log Cursor** daemon (on the Secondary) and asynchronous jobs takes care of
the changes.
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.
To keep track on the state of the replication, **Secondary** nodes includes an
additional PostgreSQL database, that includes metadata from all the tracked
repositories and assets. This additional database is required because we can't
do any writing operation on the replicated database.
## 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, etc. on the **secondary** node.
## Primary and Secondary
## Repository replication
There can be only one **Primary** node, which is the one that can do writing
operations. Both share the same codebase and are distinguished by a feature
toggle mechanism (see `Gitlab::Geo`).
Geo also replicates repositories. Each **secondary** node keeps track of
the state of every repository in the [tracking database](#tracking-database).
We use the values from `gitlab.yml`: `host`, `port`, `relative_url_root`
and search in the database to identify which node we are in
(see `Gitlab::Geo.current_node`).
There are a few ways a repository gets replicated by the:
Most of the Geo important methods are cached by the `RequestStore`, to reduce
the performance impact of using the methods throughout the codebase.
- [Repository Sync worker](#repository-sync-worker).
- [Geo Log Cursor](#geo-log-cursor).
To execute a piece of code in a **Primary** node use:
### Project Registry
```ruby
if Gitlab::Geo.primary?
# code goes here
end
```
You can do the same thing for a **Secondary** node:
```ruby
if Gitlab::Geo.secondary?
# code goes here
end
```
`.primary?` and `.secondary?` are not mutually exclusive, so you should never
take for granted that when one of them returns `false`, other will be true.
Both methods check if Geo is `.enabled?`, so there is a "third" state where
both will return false (when Geo is not enabled).
There is also an additional gotcha when dealing with `initializers` or with
things that happen during initialization time. We use in a few places the
`Gitlab::Geo.geo_database_configured?` to check if node has the additional
database which only happens in the secondary node, so we can overcome some
racing 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?`.
## Communication
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.
The communication channel has changed since first iteration, you can check here
historic decisions and why we moved to new implementations.
It records the following about repositories:
### Custom code (GitLab 8.6 and earlier)
- 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.
In GitLab versions before 8.6 custom code is used to handle
notification from **Primary** to **Secondary** by HTTP requests.
It also stores these attributes for project wikis in dedicated columns.
### System hooks (GitLab 8.7 till 9.5)
### Repository Sync worker
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.
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:
There is a specific **internal** endpoint in our api code (Grape),
that receives all requests from this System Hooks:
`/api/v4/geo/receive_events`.
- 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).
We switch and filter from each event by the `event_name` field.
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
### Geo Log Cursor (GitLab 10.0 and up)
The [Geo Log Cursor](#geo-log-cursor) is a separate process running on
each **secondary** node. It monitors the [Geo Event Log](#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.
Since GitLab 10.0, **System Webhooks** are no longer used, and Geo Log
Cursor is used instead. The Log Cursor traverses the `Geo::EventLog`
to see if there are changes since the last time the log was checked
and will handle repository updates, deletes, changes & renames.
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.
The table is within the replicated database. This has two advantages over the
old method:
## Uploads replication
1. Replication is synchronous and we preserve the order of events
2. Replication of the events happen at the same time as the changes in the
database
File uploads are also being replicated to the **secondary** node. To
track the state of syncing, the `Geo::FileRegistry` model is used.
### File Registry
## Read-only
Similar to the [Project Registry](#project-registry), there is a
`Geo::FileRegistry` model that tracks the synced uploads.
All **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 `Gitlab::Database.read_only?`
will always return `true` on a secondary node.
When some write actions are not allowed, because the node is a
secondary, consider the `Gitlab::Database.read_only?` or `Gitlab::Database.read_write?`
guard, instead of `Gitlab::Geo.secondary?`.
Database itself will already be read-only in a replicated setup, so we
don't need to take any extra step for that.
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
## File Transfers
Secondary Geo Nodes need to transfer files, such as LFS objects, attachments, avatars,
etc. from the primary. To do this, secondary nodes have a separate tracking database
that records which objects it needs to transfer.
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.
## Repository transfers
`/api/v4/geo/transfers/:type/:id` endpoint,
e.g. `/api/v4/geo/transfers/lfs/123`.
Secondary Geo Nodes fetch the repositories when they are updated. If you just
enabled a secondary node and connected it to the primary, the initial backfill
will be initialized.
## Authentication
When we fail to fetch a repository on the secondary several times, we try to do
a clone again 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.
To authenticate file transfers, each `GeoNode` record has two fields:
### Authentication
- A public access key (`access_key` field).
- A secret access key (`secret_access_key` field).
To authenticate file transfers, each GeoNode has two fields:
1. A public access key (`access_key`)
2. A secret access key (`secret_access_key`)
The secondary authenticates itself via a [JWT request](https://jwt.io/). When the
secondary wishes to download a file, it sends an HTTP request with the `Authorization`
header:
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:
```
Authorization: GL-Geo <access_key>:<JWT payload>
```
The primary uses the `access_key` to look up the corresponding Geo node and
decrypt the JWT payload, which contains additional information to identify the
file request. This ensures that the secondary downloads the right file for the
right database ID. For example, for an LFS object, the request must also
include the SHA256 of the file. An example JWT payload looks like:
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 data checks out, then the Geo primary 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.
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: **Note:**
JWT requires synchronized clocks between the machines
involved, otherwise it may fail with an encryption error.
## Using the Tracking Database
Please note that JWT requires synchronized clocks between involved machines,
otherwise it may fail with an encryption error.
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.
## Geo Tracking Database
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.
Secondary Geo nodes track data about what has been downloaded in a second
PostgreSQL database that is distinct from the production GitLab database.
The database configuration is set in `config/database_geo.yml`.
`ee/db/geo` contains the schema and migrations for this database.
### Configuration
The database configuration is set in [`config/database_geo.yml`](https://gitlab.com/gitlab-org/gitlab-ee/blob/master/config/database_geo.yml.postgresql).
The directory [`ee/db/geo`](https://gitlab.com/gitlab-org/gitlab-ee/tree/master/ee/db/geo)
contains the schema and migrations for this database.
To write a migration for the database, use the `GeoMigrationGenerator`:
......@@ -190,27 +161,30 @@ To migrate the tracking database, run:
bundle exec rake geo:db:migrate
```
In 10.1 we are introducing PostgreSQL FDW to bridge this database with the
replicated one, so we can perform queries joining tables from both instances.
### Foreign Data Wrapper
The use of [FDW](#fdw) was introduced in GitLab 10.1.
This is useful for the Geo Log Cursor and improves the performance of some
synchronization operations.
This is useful for the [Geo Log Cursor](#geo-log-cursor) and improves
the performance of some synchronization operations.
While FDW is available in older versions of Postgres, we needed to bump the
minimum required version to 9.6 as this includes many performance improvements
to the FDW implementation.
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
#### Refeshing the Foreign Tables
Whenever the database schema changes on the primary, the secondary will need to refresh
its foreign tables by running the following:
Whenever the database schema changes on the **primary** node, the
**secondary** node will need to refresh its foreign tables by running
the following:
```sh
bundle exec rake geo:db:refresh_foreign_tables
```
Failure to do this will prevent the secondary from functioning properly. The
secondary will generate error messages, as the following PostgreSQL error:
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
......@@ -222,3 +196,222 @@ STATEMENT: SELECT a.attname, format_type(a.atttypid, a.atttypmod)
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum
```
## Finders
Geo uses [Finders](https://gitlab.com/gitlab-org/gitlab-ee/tree/master/app/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](#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](../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).
### 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`](https://gitlab.com/gitlab-org/gitlab-ee/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.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](#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](#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.
ee/changelogs/unreleased/tc-geo-dev-docs.yml 0 → 100644
View file @ 98a8e4d6
---
title: Write some Geo development documentation
merge_request: 7452
author:
type: other
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