Class: Gitlab::Database::LoadBalancing::Session

Inherits:
Object
  • Object
show all
Defined in:
lib/gitlab/database/load_balancing/session.rb

Overview

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.

Constant Summary collapse

CACHE_KEY =
:gitlab_load_balancer_session

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initializeSession

Returns a new instance of Session.


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# File 'lib/gitlab/database/load_balancing/session.rb', line 26

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

Class Method Details

.clear_sessionObject


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# File 'lib/gitlab/database/load_balancing/session.rb', line 18

def self.clear_session
  RequestStore.delete(CACHE_KEY)
end

.currentObject


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# File 'lib/gitlab/database/load_balancing/session.rb', line 14

def self.current
  RequestStore[CACHE_KEY] ||= new
end

.without_sticky_writes(&block) ⇒ Object


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# File 'lib/gitlab/database/load_balancing/session.rb', line 22

def self.without_sticky_writes(&block)
  current.ignore_writes(&block)
end

Instance Method Details

#fallback_to_replicas_for_ambiguous_queries(&blk) ⇒ Object

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


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# File 'lib/gitlab/database/load_balancing/session.rb', line 92

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

#fallback_to_replicas_for_ambiguous_queries?Boolean

Returns:

  • (Boolean)

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# File 'lib/gitlab/database/load_balancing/session.rb', line 100

def fallback_to_replicas_for_ambiguous_queries?
  @fallback_to_replicas_for_ambiguous_queries == true && !use_primary? && !performed_write?
end

#ignore_writes(&block) ⇒ Object


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# File 'lib/gitlab/database/load_balancing/session.rb', line 52

def ignore_writes(&block)
  @ignore_writes = true

  yield
ensure
  @ignore_writes = false
end

#performed_write?Boolean

Returns:

  • (Boolean)

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# File 'lib/gitlab/database/load_balancing/session.rb', line 112

def performed_write?
  @performed_write
end

#use_primary(&blk) ⇒ Object


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# File 'lib/gitlab/database/load_balancing/session.rb', line 44

def use_primary(&blk)
  used_primary = @use_primary
  @use_primary = true
  yield
ensure
  @use_primary = used_primary || @performed_write
end

#use_primary!Object


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# File 'lib/gitlab/database/load_balancing/session.rb', line 40

def use_primary!
  @use_primary = true
end

#use_primary?Boolean Also known as: using_primary?

Returns:

  • (Boolean)

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# File 'lib/gitlab/database/load_balancing/session.rb', line 34

def use_primary?
  @use_primary
end

#use_replicas_for_read_queries(&blk) ⇒ Object

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.


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# File 'lib/gitlab/database/load_balancing/session.rb', line 69

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

#use_replicas_for_read_queries?Boolean

Returns:

  • (Boolean)

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# File 'lib/gitlab/database/load_balancing/session.rb', line 77

def use_replicas_for_read_queries?
  @use_replicas_for_read_queries == true
end

#write!Object


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# File 'lib/gitlab/database/load_balancing/session.rb', line 104

def write!
  @performed_write = true

  return if @ignore_writes

  use_primary!
end