Module: DistributeReads

Defined in:
lib/distribute_reads.rb,
lib/distribute_reads/version.rb,
lib/distribute_reads/cache_store.rb,
lib/distribute_reads/job_methods.rb,
lib/distribute_reads/global_methods.rb,
lib/distribute_reads/appropriate_pool.rb

Defined Under Namespace

Modules: AppropriatePool, GlobalMethods, JobMethods Classes: CacheStore, Error, NoReplicasAvailable, TooMuchLag

Constant Summary collapse

VERSION = 
"0.3.0"

Class Attribute Summary collapse

Class Method Summary collapse

Class Attribute Details

.by_default ⇒ Object

Returns the value of attribute by_default.



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# File 'lib/distribute_reads.rb', line 17

def by_default
  @by_default
end

.default_options ⇒ Object

Returns the value of attribute default_options.



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# File 'lib/distribute_reads.rb', line 18

def default_options
  @default_options
end

.logger ⇒ Object 



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# File 'lib/distribute_reads.rb', line 27

def self.logger
  unless defined?(@logger)
    @logger = ActiveRecord::Base.logger
  end
  @logger
end

Class Method Details

.default_to_primary ⇒ Object

legacy



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# File 'lib/distribute_reads.rb', line 123

def self.default_to_primary
  !by_default
end

.default_to_primary=(value) ⇒ Object

legacy



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# File 'lib/distribute_reads.rb', line 128

def self.default_to_primary=(value)
  self.by_default = !value
end

.log(message) ⇒ Object 



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# File 'lib/distribute_reads.rb', line 99

def self.log(message)
  logger.info("[distribute_reads] #{message}") if logger
end

.makara3? ⇒ Boolean

private

Returns:

  • (Boolean) 


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# File 'lib/distribute_reads.rb', line 115

def self.makara3?
  unless defined?(@makara3)
    @makara3 = Gem::Version.new(Makara::VERSION.to_s) < Gem::Version.new("0.4.0")
  end
  @makara3
end

.replication_lag(connection: nil) ⇒ Object 



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# File 'lib/distribute_reads.rb', line 34

def self.replication_lag(connection: nil)
  connection ||= ActiveRecord::Base.connection

  replica_pool = connection.instance_variable_get(:@slave_pool)
  if replica_pool && replica_pool.connections.size > 1
    log "Multiple replicas available, lag only reported for one"
  end

  with_replica do
    case connection.adapter_name
    when "PostgreSQL", "PostGIS"
      # cache the version number
      @server_version_num ||= {}
      cache_key = connection.pool.object_id
      @server_version_num[cache_key] ||= connection.execute("SHOW server_version_num").first["server_version_num"].to_i

      lag_condition =
        if @server_version_num[cache_key] >= 100000
          "pg_last_wal_receive_lsn() = pg_last_wal_replay_lsn()"
        else
          "pg_last_xlog_receive_location() = pg_last_xlog_replay_location()"
        end

      connection.execute(
        "SELECT CASE
          WHEN NOT pg_is_in_recovery() OR #{lag_condition} THEN 0
          ELSE EXTRACT (EPOCH FROM NOW() - pg_last_xact_replay_timestamp())
        END AS lag".squish
      ).first["lag"].to_f
    when "MySQL", "Mysql2", "Mysql2Spatial", "Mysql2Rgeo"
      @aurora_mysql ||= {}
      cache_key = connection.pool.object_id

      unless @aurora_mysql.key?(cache_key)
        # makara doesn't send SHOW queries to replica by default
        @aurora_mysql[cache_key] = connection.exec_query("SHOW VARIABLES LIKE 'aurora_version'").to_hash.any?
      end

      if @aurora_mysql[cache_key]
        status = connection.exec_query("SELECT Replica_lag_in_msec FROM mysql.ro_replica_status WHERE Server_id = @@aurora_server_id").to_hash.first
        status ? status["Replica_lag_in_msec"].to_f / 1000.0 : 0.0
      else
        status = connection.exec_query("SHOW SLAVE STATUS").to_hash.first
        if status
          if status["Seconds_Behind_Master"].nil?
            # replication stopped
            # https://dev.mysql.com/doc/refman/8.0/en/show-slave-status.html
            nil
          else
            status["Seconds_Behind_Master"].to_f
          end
        else
          # not a replica
          0.0
        end
      end
    when "SQLite"
      # never a replica
      0.0
    else
      raise DistributeReads::Error, "Option not supported with this adapter"
    end
  end
end

.with_replica ⇒ Object

private



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# File 'lib/distribute_reads.rb', line 104

def self.with_replica
  previous_value = Thread.current[:distribute_reads]
  begin
    Thread.current[:distribute_reads] = {replica: true, failover: false}
    yield
  ensure
    Thread.current[:distribute_reads] = previous_value
  end
end