2020-03-13 15:44:24 +05:30
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# Redis guidelines
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GitLab uses [Redis](https://redis.io) for three distinct purposes:
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- Caching via `Rails.cache`.
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- As a job processing queue with [Sidekiq](sidekiq_style_guide.md).
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- To manage the shared application state.
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Every application process is configured to use the same Redis servers, so they
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can be used for inter-process communication in cases where [PostgreSQL](sql.md)
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2020-04-08 14:13:33 +05:30
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is less appropriate. For example, transient state or data that is written much
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2020-03-13 15:44:24 +05:30
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more often than it is read.
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If [Geo](geo.md) is enabled, each Geo node gets its own, independent Redis
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database.
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## Key naming
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Redis is a flat namespace with no hierarchy, which means we must pay attention
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to key names to avoid collisions. Typically we use colon-separated elements to
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provide a semblence of structure at application level. An example might be
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`projects:1:somekey`.
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Although we split our Redis usage into three separate purposes, and those may
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map to separate Redis servers in a [Highly Available](../administration/high_availability/redis.md)
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configuration, the default Omnibus and GDK setups share a single Redis server.
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This means that keys should **always** be globally unique across the three
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purposes.
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It is usually better to use immutable identifiers - project ID rather than
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full path, for instance - in Redis key names. If full path is used, the key will
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stop being consulted if the project is renamed. If the contents of the key are
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invalidated by a name change, it is better to include a hook that will expire
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the entry, instead of relying on the key changing.
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We don't use [Redis Cluster](https://redis.io/topics/cluster-tutorial) at the
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moment, but may wish to in the future: [#118820](https://gitlab.com/gitlab-org/gitlab/issues/118820).
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This imposes an additional constraint on naming: where GitLab is performing
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operations that require several keys to be held on the same Redis server - for
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instance, diffing two sets held in Redis - the keys should ensure that by
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enclosing the changeable parts in curly braces, such as, `project:{1}:set_a` and
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`project:{1}:set_b`.
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