@@ -110,6 +110,131 @@ documentation for feature flags.
When we have been using the new instance 100% of the time in production for a
while and there are no issues, we can proceed.
### Proposed solution: Migrate data by using MultiStore with the fallback strategy
We need a way to migrate users to a new Redis store without causing any inconveniences from UX perspective.
We also want the ability to fall back to the "old" Redis instance if something goes wrong with the new instance.
Migration Requirements:
- No downtime.
- No loss of stored data until the TTL for storing data expires.
- Partial rollout using Feature Flags or ENV vars or combinations of both.
- Monitoring of the switch.
- Prometheus metrics in place.
- Easy rollback without downtime in case the new instance or logic does not behave as expected.
It is somewhat similar to the zero-downtime DB table rename.
We need to write data into both Redis instances (old + new).
We read from the new instance, but we need to fall back to the old instance when pre-fetching from the new dedicated Redis instance that failed.
We need to log any issues or exceptions with a new instance, but still fall back to the old instance.
The proposed migration strategy is to implement and use the [MultiStore](https://gitlab.com/gitlab-org/gitlab/-/blob/fcc42e80ed261a862ee6ca46b182eee293ae60b6/lib/gitlab/redis/multi_store.rb).
We used this approach with [adding new dedicated Redis instance for session keys](https://gitlab.com/groups/gitlab-com/gl-infra/-/epics/579).
Also MultiStore comes with corresponding [specs](https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/lib/gitlab/redis/multi_store_spec.rb).
The MultiStore looks like a `redis-rb ::Redis` instance.
In the new Redis instance class you added in [Step 1](#step-1-support-configuring-the-new-instance),
override the [Redis](https://gitlab.com/gitlab-org/gitlab/-/blob/fcc42e80ed261a862ee6ca46b182eee293ae60b6/lib/gitlab/redis/sessions.rb#L20-28) method from the `::Gitlab::Redis::Wrapper`
```ruby
moduleGitlab
moduleRedis
classFoo<::Gitlab::Redis::Wrapper
...
defself.redis
# Don't use multistore if redis.foo configuration is not provided
MultiStore is initialized by providing the new Redis instance as a primary store, and [old (fallback-instance)](#fallback-instance) as a secondary store.
The third argument is `store_name` which is used for logs, metrics and feature flag names, in case we use MultiStore implementation for different Redis stores at the same time.
By default, the MultiStore reads and writes only from the default Redis store.
The default Redis store is `secondary_store` (the old fallback-instance).
This allows us to introduce MultiStore without changing the default behavior.
MultiStore uses two feature flags to control the actual migration:
For example, if our new Redis instance is called `Gitlab::Redis::Foo`, we can [create](../../../ee/development/feature_flags/#create-a-new-feature-flag) two feature flags by executing:
By enabling `use_primary_and_secondary_stores_for_foo` feature flag, our `Gitlab::Redis::Foo` will use `MultiStore` to write to both new Redis instance
and the [old (fallback-instance)](#fallback-instance).
If we fail to fetch data from the new instance, we will fallback and read from the old Redis instance.
We can monitor logs for `Gitlab::Redis::MultiStore::ReadFromPrimaryError`, and also the Prometheus counter `gitlab_redis_multi_store_read_fallback_total`.
Once we stop seeing them, this means that we are no longer relying on the data stored on the old Redis store.
At this point, we are probably safe to move the traffic to the new Redis store.
By enabling `use_primary_store_as_default_for_foo` feature flag, the `MultiStore` will use `primary_store` (new instance) as default Redis store.
Once this feature flag is enabled, we can disable `use_primary_and_secondary_stores_for_foo` feature flag.
This will allow the MultiStore to read and write only from the primary Redis store (new store), moving all the traffic to the new Redis store.
Once we have moved all our traffic to the primary store, our data migration is complete.
We can safely remove the MultiStore implementation and continue to use newly introduced Redis store instance.
#### Implementation details
MultiStore implements read and write Redis commands separately.
##### Read commands
-`get`
-`mget`
-`smembers`
-`scard`
##### Write commands
-`set`
-`setnx`
-`setex`
-`sadd`
-`srem`
-`del`
-`pipelined`
-`flushdb`
When a command outside of the supported list is used, `method_missing` will pass it to the old Redis instance and keep track of it.
This ensures that anything unexpected behaves like it would before.
NOTE:
By tracking `gitlab_redis_multi_store_method_missing_total` counter and `Gitlab::Redis::MultiStore::MethodMissingError`,
a developer will need to add an implementation for missing Redis commands before proceeding with the migration.
