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Replication is asynchronous by default; slaves do not need to be connected permanently to receive updates from the master. Depending on the configuration, you can replicate all databases, selected databases, or even selected tables within a database. Scale-out solutions - spreading the load among multiple slaves to improve performance. In this environment, all writes and updates must take place on the master server. Reads, however, may take place on one or more slaves.
This model can improve the performance of writes since the master is dedicated to updates , while dramatically increasing read speed across an increasing number of slaves. Data security - because data is replicated to the slave, and the slave can pause the replication process, it is possible to run backup services on the slave without corrupting the corresponding master data.
Analytics - live data can be created on the master, while the analysis of the information can take place on the slave without affecting the performance of the master. Long-distance data distribution - you can use replication to create a local copy of data for a remote site to use, without permanent access to the master.
The traditional method is based on replicating events from the master's binary log, and requires the log files and positions in them to be synchronized between master and slave. The newer method based on global transaction identifiers GTIDs is transactional and therefore does not require working with log files or positions within these files, which greatly simplifies many common replication tasks. Replication using GTIDs guarantees consistency between master and slave as long as all transactions committed on the master have also been applied on the slave.
Replication in MySQL supports different types of synchronization. The original type of synchronization is one-way, asynchronous replication, in which one server acts as the master, while one or more other servers act as slaves. There are a number of solutions available for setting up replication between servers, and the best method to use depends on the presence of data and the engine types you are using.
Replication is controlled through a number of different options and variables. You can use replication to solve a number of different problems, including performance, supporting the backup of different databases, and as part of a larger solution to alleviate system failures.
This section describes how to configure the different types of replication available in MySQL and includes the setup and configuration required for a replication environment, including step-by-step instructions for creating a new replication environment.
The major components of this section are:. Events in the binary log are recorded using a number of formats. Once started, the replication process should require little administration or monitoring. The information in the binary log is stored in different logging formats according to the database changes being recorded. Slaves are configured to read the binary log from the master and to execute the events in the binary log on the slave's local database. Each slave receives a copy of the entire contents of the binary log.
It is the responsibility of the slave to decide which statements in the binary log should be executed. Unless you specify otherwise, all events in the master binary log are executed on the slave.
If required, you can configure the slave to process only events that apply to particular databases or tables. Each slave keeps a record of the binary log coordinates: This means that multiple slaves can be connected to the master and executing different parts of the same binary log.
Because the slaves control this process, individual slaves can be connected and disconnected from the server without affecting the master's operation. Also, because each slave records the current position within the binary log, it is possible for slaves to be disconnected, reconnect and then resume processing. The master and each slave must be configured with a unique ID using the server-id option. In addition, each slave must be configured with information about the master host name, log file name, and position within that file.
This section describes how to set up a MySQL server to use binary log file position based replication. There are a number of different methods for setting up replication, and the exact method to use depends on how you are setting up replication, and whether you already have data within your master database.
On the master, you must enable binary logging and configure a unique server ID. This might require a server restart. On each slave that you want to connect to the master, you must configure a unique server ID. Optionally, create a separate user for your slaves to use during authentication with the master when reading the binary log for replication.
Before creating a data snapshot or starting the replication process, on the master you should record the current position in the binary log. You need this information when configuring the slave so that the slave knows where within the binary log to start executing events.
If you already have data on the master and want to use it to synchronize the slave, you need to create a data snapshot to copy the data to the slave. The storage engine you are using has an impact on how you create the snapshot. When you are using MyISAM , you must stop processing statements on the master to obtain a read-lock, then obtain its current binary log coordinates and dump its data, before permitting the master to continue executing statements.
If you do not stop the execution of statements, the data dump and the master status information will not match, resulting in inconsistent or corrupted databases on the slaves. If you are using InnoDB , you do not need a read-lock and a transaction that is long enough to transfer the data snapshot is sufficient. Configure the slave with settings for connecting to the master, such as the host name, login credentials, and binary log file name and position. If you do not have this privilege, it might not be possible to enable replication.
To configure a master to use binary log file position based replication, you must enable binary logging and establish a unique server ID.
If this has not already been done, a server restart is required. Binary logging must be enabled on the master because the binary log is the basis for replicating changes from the master to its slaves. If binary logging is not enabled on the master using the log-bin option, replication is not possible. Each server within a replication group must be configured with a unique server ID. How you organize and select the numbers is your choice.
Within the [mysqld] section of the configuration file, add the log-bin and server-id options. If these options already exist, but are commented out, uncomment the options and alter them according to your needs. For example, to enable binary logging using a log file name prefix of mysql-bin , and configure a server ID of 1, use these lines:. Ensure that the skip-networking option is not enabled on your replication master.
If networking has been disabled, the slave cannot communicate with the master and replication fails. Each slave connects to the master using a MySQL user name and password, so there must be a user account on the master that the slave can use to connect. You can choose to create a different account for each slave, or connect to the master using the same account for each slave.
Therefore, you may want to create a separate account that has privileges only for the replication process, to minimize the possibility of compromise to other accounts. For example, to set up a new user, repl , that can connect for replication from any host within the mydomain. To configure the slave to start the replication process at the correct point, you need the master's current coordinates within its binary log.
In that case, the values that you need to use later when specifying the slave's log file and position are the empty string '' and 4. If the master has been binary logging previously, use this procedure to obtain the master binary log coordinates:. If you exit the client, the lock is released. The File column shows the name of the log file and the Position column shows the position within the file.
In this example, the binary log file is mysql-bin. You need them later when you are setting up the slave. They represent the replication coordinates at which the slave should begin processing new updates from the master. You now have the information you need to enable the slave to start reading from the binary log in the correct place to start replication.
The next step depends on whether you have existing data on the master. Choose one of the following options:. If you have existing data that needs be to synchronized with the slave before you start replication, leave the client running so that the lock remains in place. This prevents any further changes being made, so that the data copied to the slave is in synchrony with the master. If you are setting up a new master and slave replication group, you can exit the first session to release the read lock.
If the master database contains existing data it is necessary to copy this data to each slave. There are different ways to dump the data from the master database. The following sections describe possible options. To select the appropriate method of dumping the database, choose between these options:. Use the mysqldump tool to create a dump of all the databases you want to replicate. This is the recommended method, especially when using InnoDB. If your database is stored in binary portable files, you can copy the raw data files to a slave.
This can be more efficient than using mysqldump and importing the file on each slave, because it skips the overhead of updating indexes as the INSERT statements are replayed. With storage engines such as InnoDB this is not recommended. To create a snapshot of the data in an existing master database, use the mysqldump tool. Once the data dump has been completed, import this data into the slave before starting the replication process.
The following example dumps all databases to a file named dbdump. If you do not use --master-data , then it is necessary to lock all tables in a separate session manually. It is possible to exclude certain databases from the dump using the mysqldump tool.
If you want to choose which databases to include in the dump, do not use --all-databases. Choose one of these options:. Exclude all the tables in the database using --ignore-table option. Name only those databases which you want dumped using the --databases option. To import the data, either copy the dump file to the slave, or access the file from the master when connecting remotely to the slave. This section describes how to create a data snapshot using the raw files which make up the database.
How the storage engine responds to this depends on its crash recovery abilities. This command records the log name and offset corresponding to the snapshot to be used on the slave.