We build highly available and fault tolerant MySQL database infrastructure operations for some of the largest internet properties in this planet, Our consulting team spend several hours daily researching on MySQL documentation and MySQL blogs to understand what are the best possible ways we can build optimal, scalable, highly available and reliable database infrastructure operations for planet-scale web properties. The most common approach towards building a fault-tolerant system is to make all the components in the ecosystem redundant, To make it even simple, component can be removed and system should continue to operate as expected. MySQL replication is an proven method to build redundant database infrastructure operations, operationally these systems are highly complex, requiring maintenance and administration of several servers instead of just one, You need Sr. DBAs to manage such systems.
MySQL Group Replication can operate in both single-primary mode with automatic primary election, where only one server accepts updates at a time and multi-primary mode, where all servers can accept updates, even if they are issued concurrently. The built-in group membership service retains view of the group consistent and available for all servers at any given point in time, Servers can leave and join the group and the view is updated accordingly. If servers leave the group unexpectedly, the failure detection mechanism detects this and notifies the group that the view has changed, All this happens automatically !
For any transaction to commit, the majority of group members have to agree on the order of a given transaction in the global sequence of transactions. Decision to commit or abort a transaction is taken by respective servers, but all servers make same decision. In the case of systems built on network partition, When the members of the group are unable to reach agreement on a transaction, the system halt till the issue is resolved, This guarantees split-brain protection mechanism.
All these fault tolerance mechanisms are powered by Group Communication System (GCS) protocols, a group membership service, and completely safe ordered message delivery system. Group Replication is powered by Paxos algorithm (https://en.wikipedia.org/wiki/Paxos_(computer_science)), which acts as the group communication engine.
Till now, we have just posted about the capabilities of Group Replication, Now we have mentioned below (in bullets) about all the limitations of Group Replication:
- set –binlog-checksum=NONE – Group replication cannot benefit from –binlog-checksum due to the design limitation of replication event checksums.
- Gap Locks not supported – The information about gap locks are not available outside InnoDB so certification process cannot acknowledge gap locks .
- Table Locks and Named Locks not supported – Certification process will not acknowledge table / named locks.
- Concurrent DDL versus DML Operations – Concurrent data definition statements and data manipulation statements executing against the same object but on different servers is not supported when using multi-primary mode. During execution of Data Definition Language (DDL) statements on an object, executing concurrent Data Manipulation Language (DML) on the same object but on a different server instance has the risk of conflicting DDL executing on different instances not being detected.
- Very Large Transactions. Individual transactions that result in GTID contents which are large enough that it cannot be copied between group members over the network within a 5 second window can cause failures in the group communication. To avoid this issue try and limit the size of your transactions as much as possible. For example, split up files used with LOAD DATA INFILE into smaller chunks.
- Multi-primary Mode Deadlock. When a group is operating in multi-primary mode, SELECT … FOR UPDATE statements can result in a deadlock. This is because the lock is not shared across the members of the group, therefore the expectation for such a statement might not be reached.
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