MySQL Replication. Rick Golba and Stephane Combaudon April 15, 2015

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1 MySQL Replication Rick Golba and Stephane Combaudon April 15, 2015

2 Agenda What is, and what is not, MySQL Replication Replication Use Cases Types of replication Replication lag Replication errors Replication demonstration

3 MySQL Replication Replication enables data from one MySQL database to be copied to one or more other MySQL database servers Database that is the source is the master Database that is the target is/are the slave(s) Replication is asynchronous or semi-synchronous, meaning slaves do not need to be permanently connected to receive updates Synchronous replication is a feature of Galera cluster You can replicate all databases, selected databases, or selected tables within a database

4 MySQL Replication It is not a full High Availability solution There is no automatic failover with replication It is not an always available solution There could be lag as a result of the replication

5 Use Cases for Replication Scale-out All writes and updates take place on the master Reads take place on one or more slaves Improves write performance and read speed across an increasing number of slaves Data security Replicate to slave, stop replication, backup slave only

6 Use Cases for Replication High availability If master fails, can access data on slave Analytics Data loaded on the master, copied to the slave Data analysis performed on the slave Data distribution Create a local copy of the data to have data close to end user or for disaster recovery

7 GTID Replication As of version 5.6.5, replication can be managed through the use of Global Transaction IDs Replication is transactional Guarantees consistency provided all transactions committed on the master have also been applied on the slave

8 Binary Log Classic replication is based on the binary log file created by MySQL Events recorded on the master Slaves read the binlog on the master and repeat the events locally A slave may process all events or only events that apply to a particular databases or tables

9 Replication Synchronization By default, replication is asynchronous As of version 5.7, replication can be semi synchronous A commit performed on the master blocks returning to the session until at least one slave acknowledges that it has received and logged the events for the transaction Mixed option is also available

10 Replication Formats Statement Based Replicates entire SQL statements Default before MySQL Row Based Replicates only the changed rows Default for MySQL and forward

11 Replication Lag While it is possible to force lag, it also can happen unintentionally Unintended lag is often inconsistent and unnoticed

12 Reasons for Lag Transactions are serialized on slave Hardware issues on master or slave MySQL configuration changes MySQL restart needs to initialize buffer pool Increased read activity on slave Increased replication load

13 Diagnosing Lag MySQL replication works with two threads IO_THREAD connects to a master, reads binary log events from the master and copies them to a local log file called relay log SQL_THREAD reads events from a relay log stored locally on the replication slave (the file that was written by IO thread) and then applies them as fast as possible

14 Resolving Lag If issue is found in IO_THREAD, it is likely network based If issue is found in SQL_THREAD, it is likely query based

15 Replication Errors If a statement produces the same error on both the master and the slave, the error is logged, but replication continues If a statement produces different errors on the master and the slave, the slave replication thread terminates Slave writes a message to its error log and waits for the DBA to decide what to do

16 Demonstration