This presentation provides an overview about Group Replication in MySQL 8.0. The primary election algorithm, Replication modes are described here. www.mydbops.com

1. Group Replication In MYSQL 8.0
A walk through
Presented by
VIGNESH PRABHU S
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2. About Mydbops
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3. Agenda
● Group Replication - Introduction
● Group Replication - Architecture
● Group Replication - Monitoring
● Group Replication - Modes
● Primary Member Change - Cases
● Primary Election Algorithm
● Conditions for using online UDF
● Communication Protocol
● Transaction Consistency

4. Group Replication - Introduction
● High Availability Solution
● To create a fault tolerant System.
● Supports Server side failover.
● Client side failover is not supported by GR.
● Distributed functionality.
● Based on the GCS protocol and paxos algorithm.
● Split brain issue is avoided by Quorum.

5. Group Replication - Introduction
Services of Group Replication:-
● Group Membership (VIEW)
● Failure Detection (Detect the failure & update the VIEW)
● Fault-tolerance ( QUORUM condition n = 2 x f + 1. )
● Monitoring ( performance schema tables )
All these Services are built into the GCS Protocol.

6. Group Replication - Architecture

7. Group Replication - Architecture
Functionality of Each Component:-
● Capture :Keep track of all the local transactions.
● Applier :Remote transactions.
● Recovery :Manage the distributed recovery.
● Replication :Define the replication protocol & handle the
conflict detection in the group.
● GCS & Xcom :GR Services & communication protocol.

8. Group Replication - ( Commit )

9. Group Replication - Monitoring
Tables in Perf schema:-
● replication_group_member_stats ( info about certification process)
● replication_group_members ( info about the group members )
● replication_connection_status ( info about the queue )
● replication_applier_status ( shows state of replication worker
thread )
Replication Channel:-
● group_replication_recovery - ( Channel for the recovery process)
● group_replication_applier - ( transactions from the other members
or from the group )

10. Group Replication - Monitoring
Group Replication Server States:-
● Online :will accept the connection from the client.
● Recovering :Applying the events from the donor.
● Offline :Member doesn’t belong to any group.
● ERROR :Member is in the group but not working properly.
“group_replication_exit_state_action”
● UNREACHABLE :It is mostly because of the network failure.

11. Group Replication - Modes
● Based on the variable - (group_replication_single_primary_mode)
● Single Primary Mode
○ One member accept the writes.
○ group_replication_enforce_update_everywhere_checks (off)
● Multi Primary Mode
○ All the members can accept the writes.
○ group_replication_enforce_update_everywhere_checks (ON)

12. Group Replication - Modes
● From MySQL version 8.0.13 we can change the modes in
online Using the UDF’s
○ group_replication_switch_to_single_primary_mode()
○ group_replication_switch_to_multi_primary_mode()
● From Version 8.0.13 to 8.0.16, no check for the safeguard.
● From Version 8.0.17, GR has safeguard check.
○ (i.e) Lower MySQL version member will become the
Primary.

13. Primary Member Change - Cases
● Primary leaves the group .
● While Changing the primary member manually using UDF.
● While Changing the multi primary to single primary.

14. Primary Election Algorithm
First Election Check:-
● Ordering based on MySQL major Version & patch Version.
Second Election Check:-
● Based on lowest member weight. ( group_replication_member_weight ).
Third Election Check:-
● Based on the lowest server UUID.

15. Conditions for using online UDF
1. All members must be in ONLINE state
2. No members can join in the group during a configuration change.
3. Only one configuration at once.
4. All members must be running MySQL 8.0.13 or higher

16. Communication Protocol
● From MySQL 8.0.16, Configure different versions of MySQL members.
● Group must use same communication protocol Version.
Reason:-
● From MySQL 5.7.14, allow compression of messages
● From MySQL 8.0.16, allow fragmentation of messages
Functions:-
● group_replication_get_communication_protocol();
● group_replication_set_communication_protocol(“version”);

17. Transaction Consistency.
● Controlled by group_replication_consistency variable.
○ EVENTUAL
○ BEFORE_ON_PRIMARY_FAILOVER
○ BEFORE ( RO trx wait for preceding trx to apply )
○ AFTER ( RW trx wait for preceding trx to apply in other members )
○ BEFORE_AND_AFTER

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