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