118 lines
3.6 KiB
Ruby
118 lines
3.6 KiB
Ruby
# frozen_string_literal: true
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module Gitlab
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module Database
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module LoadBalancing
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# Tracking of load balancing state per user session.
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#
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# A session starts at the beginning of a request and ends once the request
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# has been completed. Sessions can be used to keep track of what hosts
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# should be used for queries.
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class Session
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CACHE_KEY = :gitlab_load_balancer_session
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def self.current
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RequestStore[CACHE_KEY] ||= new
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end
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def self.clear_session
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RequestStore.delete(CACHE_KEY)
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end
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def self.without_sticky_writes(&block)
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current.ignore_writes(&block)
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end
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def initialize
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@use_primary = false
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@performed_write = false
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@ignore_writes = false
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@fallback_to_replicas_for_ambiguous_queries = false
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@use_replicas_for_read_queries = false
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end
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def use_primary?
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@use_primary
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end
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alias_method :using_primary?, :use_primary?
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def use_primary!
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@use_primary = true
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end
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def use_primary(&blk)
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used_primary = @use_primary
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@use_primary = true
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yield
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ensure
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@use_primary = used_primary || @performed_write
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end
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def ignore_writes(&block)
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@ignore_writes = true
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yield
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ensure
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@ignore_writes = false
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end
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# Indicates that the read SQL statements from anywhere inside this
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# blocks should use a replica, regardless of the current primary
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# stickiness or whether a write query is already performed in the
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# current session. This interface is reserved mostly for performance
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# purpose. This is a good tool to push expensive queries, which can
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# tolerate the replica lags, to the replicas.
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#
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# Write and ambiguous queries inside this block are still handled by
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# the primary.
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def use_replicas_for_read_queries(&blk)
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previous_flag = @use_replicas_for_read_queries
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@use_replicas_for_read_queries = true
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yield
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ensure
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@use_replicas_for_read_queries = previous_flag
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end
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def use_replicas_for_read_queries?
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@use_replicas_for_read_queries == true
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end
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# Indicate that the ambiguous SQL statements from anywhere inside this
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# block should use a replica. The ambiguous statements include:
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# - Transactions.
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# - Custom queries (via exec_query, execute, etc.)
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# - In-flight connection configuration change (SET LOCAL statement_timeout = 5000)
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#
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# This is a weak enforcement. This helper incorporates well with
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# primary stickiness:
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# - If the queries are about to write
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# - The current session already performed writes
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# - It prefers to use primary, aka, use_primary or use_primary! were called
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def fallback_to_replicas_for_ambiguous_queries(&blk)
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previous_flag = @fallback_to_replicas_for_ambiguous_queries
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@fallback_to_replicas_for_ambiguous_queries = true
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yield
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ensure
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@fallback_to_replicas_for_ambiguous_queries = previous_flag
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end
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def fallback_to_replicas_for_ambiguous_queries?
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@fallback_to_replicas_for_ambiguous_queries == true && !use_primary? && !performed_write?
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end
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def write!
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@performed_write = true
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return if @ignore_writes
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use_primary!
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end
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def performed_write?
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@performed_write
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end
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end
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end
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end
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end
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