Skip to content
Percona Software for MySQL Documentation
Rate this page
☆ ☆ ☆ ☆ ☆
Thanks for your feedback
Thank you! The feedback has been submitted.

For help, click the link below to get free database assistance or contact our experts for personalized support.

Get help from Percona

Manage group replication flow control

Flow control prevents one member from falling too far behind the cluster. This mechanism avoids excessive buffering in the relay log. Each member sends statistics to the group to coordinate throughput.

Flow control monitors two queues:

  • Transactions waiting in the certification queue

  • Transactions waiting in the applier queue

When transactions exceed the configured thresholds, flow control throttles writer members. This throttling matches writer throughput to the capacity of the slowest member.

Flow control operates asynchronously and depends on the following:

  • Monitoring the throughput and queue sizes of each member

  • Throttling members to prevent writes beyond available capacity

System variables

The following system variables configure flow control behavior for Group Replication:

group_replication_flow_control_mode

Specifies the mode used for flow control.

Option Description
Command-line --group-replication-flow-control-mode
Dynamic Yes
Scope Global
Data type Enumeration
Default QUOTA

The following values are valid:

  • DISABLED - Flow control is off

  • QUOTA - Flow control uses a quota-based throttling mechanism

  • MAJORITY - Flow control triggers when a majority of members exceed the threshold

-- Disable flow control
SET GLOBAL group_replication_flow_control_mode = 'DISABLED';

-- Enable quota-based flow control
SET GLOBAL group_replication_flow_control_mode = 'QUOTA';

-- Enable majority-based flow control
SET GLOBAL group_replication_flow_control_mode = 'MAJORITY';

group_replication_flow_control_certifier_threshold

Specifies the number of waiting transactions in the certifier queue that trigger flow control.

Option Description
Command-line --group-replication-flow-control-certifier-threshold
Dynamic Yes
Scope Global
Data type Integer
Default 25000
Range 0 to 2147483647

The default value of 25000 suits most workloads. Tune this value based on your hardware, workload, and latency requirements:

  • Lower values trigger flow control sooner, protecting slow secondaries at the cost of primary throughput

  • Higher values allow more lag before throttling, improving throughput but risking secondaries falling behind

Setting this variable to the maximum value effectively disables threshold-based triggering for this queue. This configuration risks memory exhaustion on slower members and extended failover times. Setting the value to 0 triggers flow control immediately when any transaction waits in the queue.

-- Set certifier threshold to 10000 transactions
SET GLOBAL group_replication_flow_control_certifier_threshold = 10000;

group_replication_flow_control_applier_threshold

Specifies the number of waiting transactions in the applier queue that trigger flow control.

Option Description
Command-line --group-replication-flow-control-applier-threshold
Dynamic Yes
Scope Global
Data type Integer
Default 25000
Range 0 to 2147483647

The default value of 25000 suits most workloads. Tune this value based on your hardware, workload, and latency requirements:

  • Lower values trigger flow control sooner, protecting slow secondaries at the cost of primary throughput

  • Higher values allow more lag before throttling, improving throughput but risking secondaries falling behind

Setting this variable to the maximum value effectively disables threshold-based triggering for this queue. This configuration risks memory exhaustion on slower members and extended failover times. Setting the value to 0 triggers flow control immediately when any transaction waits in the queue.

-- Set applier threshold to 10000 transactions
SET GLOBAL group_replication_flow_control_applier_threshold = 10000;

Flow control triggers when either threshold is exceeded on any member.

Status variables

Status variables are read-only and cannot be changed at runtime. These variables report the state of flow control on each member.

  • group_replication_flow_control_active - Reports whether flow control is throttling transactions. Returns ACTIVE when engaged or DISABLED when not throttling.

  • group_replication_flow_control_threshold_nodes - Lists members that trigger flow control. The format is (count/total) followed by a comma-separated list of member IDs.

  • group_replication_flow_control_throttle_quota - Reports the maximum transactions allowed per flow control period.

Query flow control status

The following query displays flow control status variables when throttling is active:

mysql> SHOW GLOBAL STATUS LIKE '%flow_control%';
+-----------------------------------------------+------------------------------------------+
| Variable_name                                 | Value                                    |
+-----------------------------------------------+------------------------------------------+
| group_replication_flow_control_active         | ACTIVE                                   |
| group_replication_flow_control_threshold_nodes| (2/3)node1:33061,node2:33061             |
| group_replication_flow_control_throttle_quota | 5000                                     |
+-----------------------------------------------+------------------------------------------+

The following output shows the status variables when flow control is not engaged:

mysql> SHOW GLOBAL STATUS LIKE '%flow_control%';
+-----------------------------------------------+------------------------------------------+
| Variable_name                                 | Value                                    |
+-----------------------------------------------+------------------------------------------+
| group_replication_flow_control_active         | DISABLED                                 |
| group_replication_flow_control_threshold_nodes| (0/3)                                    |
| group_replication_flow_control_throttle_quota | 0                                        |
+-----------------------------------------------+------------------------------------------+