59 lines
2.6 KiB
Markdown
59 lines
2.6 KiB
Markdown
---
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stage: Data Stores
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group: Database
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info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/product/ux/technical-writing/#assignments
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---
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# Database load balancing
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With database load balancing, read-only queries can be distributed across multiple
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PostgreSQL nodes to increase performance.
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This documentation provides a technical overview on how database load balancing
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is implemented in GitLab Rails and Sidekiq.
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## Nomenclature
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1. **Host**: Each database host. It could be a primary or a replica.
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1. **Primary**: Primary PostgreSQL host that is used for write-only and read-and-write operations.
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1. **Replica**: Secondary PostgreSQL hosts that are used for read-only operations.
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1. **Workload**: a Rails request or a Sidekiq job that requires database connections.
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## Components
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F few Ruby classes are involved in the load balancing process. All of them are
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in the namespace `Gitlab::Database::LoadBalancing`:
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1. `Host`
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1. `LoadBalancer`
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1. `ConnectionProxy`
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1. `Session`
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Each workload begins with a new instance of `Gitlab::Database::LoadBalancing::Session`.
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The `Session` keeps track of the database operations that have been performed. It then
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determines if the workload requires a connection to either the primary host or a replica host.
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When the workload requires a database connection through `ActiveRecord`,
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`ConnectionProxy` first redirects the connection request to `LoadBalancer`.
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`ConnectionProxy` requests either a `read` or `read_write` connection from the `LoadBalancer`
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depending on a few criteria:
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1. Whether the query is a read-only or it requires write.
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1. Whether the `Session` has recorded a write operation previously.
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1. Whether any special blocks have been used to prefer primary or replica, such as:
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- `use_primary`
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- `ignore_writes`
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- `use_replicas_for_read_queries`
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- `fallback_to_replicas_for_ambiguous_queries`
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`LoadBalancer` then yields the requested connection from the respective database connection pool.
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It yields either:
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- A `read_write` connection from the primary's connection pool.
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- A `read` connection from the replicas' connection pools.
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When responding to a request for a `read` connection, `LoadBalancer` would
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first attempt to load balance the connection across the replica hosts.
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It looks for the next `online` replica host and yields a connection from the host's connection pool.
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A replica host is considered `online` if it is up-to-date with the primary, based on
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either the replication lag size or time. The thresholds for these requirements are configurable.
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