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MySQL 5.7 Reference Manual  /  ...  /  Configuring an Instance for Group Replication

17.2.1.2 Configuring an Instance for Group Replication

This section explains the configuration settings required for MySQL Server instances that you want to use for Group Replication.

Group Replication Server Settings

To install and use the Group Replication plugin you must configure the MySQL Server instance correctly. It is recommended to store the configuration in the instance's configuration file. See Section 4.2.6, “Using Option Files” for more information. Unless stated otherwise, what follows is the configuration for the first instance in the group, referred to as s1 in this procedure. The following section shows an example server configuration.

[mysqld]

# server configuration
datadir=<full_path_to_data>/data/s1
basedir=<full_path_to_bin>/mysql-5.7/

port=24801
socket=<full_path_to_sock_dir>/s1.sock

These settings configure MySQL server to use the data directory created earlier and which port the server should open and start listening for incoming connections.

Note

The non-default port of 24801 is used because in this tutorial the three server instances use the same hostname. In a setup with three different machines this would not be required.

Group Replication depends on a network connection between the members, which means that each member must be able to resolve the network address of all of the other members. For example in this tutorial all three instances run on one machine, so to ensure that the members can contact each other you could add a line to the option file such as report_host=127.0.0.1.

Replication Framework

The following settings configure replication according to the MySQL Group Replication requirements.

server_id=1
gtid_mode=ON
enforce_gtid_consistency=ON
master_info_repository=TABLE
relay_log_info_repository=TABLE
binlog_checksum=NONE
log_slave_updates=ON
log_bin=binlog
binlog_format=ROW

These settings configure the server to use the unique identifier number 1, to enable global transaction identifiers and to store replication metadata in system tables instead of files. Additionally, it instructs the server to turn on binary logging, use row-based format and disable binary log event checksums. For more details see Section 17.7.1, “Group Replication Requirements”.

Group Replication Settings

At this point the my.cnf file ensures that the server is configured and is instructed to instantiate the replication infrastructure under a given configuration. The following section configures the Group Replication settings for the server.

transaction_write_set_extraction=XXHASH64
loose-group_replication_group_name="aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"
loose-group_replication_start_on_boot=off
loose-group_replication_local_address= "127.0.0.1:24901"
loose-group_replication_group_seeds= "127.0.0.1:24901,127.0.0.1:24902,127.0.0.1:24903"
loose-group_replication_bootstrap_group=off
Note

The loose- prefix used for the group_replication variables above instructs the server to continue to start if the Group Replication plugin has not been loaded at the time the server is started.

  • Configuring transaction_write_set_extraction instructs the server that for each transaction it has to collect the write set and encode it as a hash using the XXHASH64 hashing algorithm. From MySQL 8.0.2, this setting is the default, so the system variable can be omitted.

  • Configuring group_replication_group_name tells the plugin that the group that it is joining, or creating, is named "aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa".

  • Configuring group_replication_start_on_boot instructs the plugin to not start operations automatically when the server starts. This is important when setting up Group Replication as it ensures you can configure the server before manually starting the plugin. Once the member is configured you can set group_replication_start_on_boot to on so that Group Replication starts automatically upon server boot.

  • Configuring group_replication_local_address tells the plugin to use the network address 127.0.0.1 and port 24901 for internal communication with other members in the group.

    Important

    Group Replication uses this address for internal member-to-member connections using XCom. This address must be different to the hostname and port used for SQL and it must not be used for client applications. It must be reserved for internal communication between the members of the group while running Group Replication.

    The network address configured by group_replication_local_address must be resolvable by all group members. For example, if each server instance is on a different machine with a fixed network address you could use the IP of the machine, such as 10.0.0.1. If you use hostnames then you must use a fully qualified name and ensure it is resolvable through DNS, correctly configured /etc/hosts files, or other name resolution processes. The recommended port for group_replication_local_address is 33061. In this tutorial we use three server instances running on one machine, thus ports 24901 to 24903 are used for the internal communication network address.

  • Configuring group_replication_group_seeds sets the hostname and port of the group members which are used by the new member to establish its connection to the group. These members are called the seed members. Once the connection is established, the group membership information is listed at performance_schema.replication_group_members. Usually the group_replication_group_seeds list contains the hostname:port of each of the group member's group_replication_local_address, but this is not obligatory and a subset of the group members can be chosen as seeds.

    Important

    The hostname:port listed in group_replication_group_seeds is the seed member's internal network address, configured by group_replication_local_address and not the SQL hostname:port used for client connections, and shown for example in performance_schema.replication_group_members table.

    The server that starts the group does not make use of this option, since it is the initial server and as such, it is in charge of bootstrapping the group. In other words, any existing data which is on the server bootstrapping the group is what is used as the data for the next joining member. The second server joining asks the one and only member in the group to join, any missing data on the second server is replicated from the donor data on the bootstrapping member, and then the group expands. The third server joining can ask any of these two to join, data is synchronized to the new member, and then the group expands again. Subsequent servers repeat this procedure when joining.

    Warning

    When joining multiple servers at the same time, make sure that they point to seed members that are already in the group. Do not use members that are also joining the group as as seeds, because they may not yet be in the group when contacted.

    It is good practice to start the bootstrap member first, and let it create the group. Then make it the seed member for the rest of the members that are joining. This ensures that there is a group formed when joining the rest of the members.

    Creating a group and joining multiple members at the same time is not supported. It may work, but chances are that the operations race and then the act of joining the group ends up in an error or a time out.

  • Configuring group_replication_bootstrap_group instructs the plugin whether to bootstrap the group or not.

    Important

    This option must only be used on one server instance at any time, usually the first time you bootstrap the group (or in case the entire group is brought down and back up again). If you bootstrap the group multiple times, for example when multiple server instances have this option set, then they could create an artificial split brain scenario, in which two distinct groups with the same name exist. Disable this option after the first server instance comes online.

Configuration for all servers in the group is quite similar. You need to change the specifics about each server (for example server_id, datadir, group_replication_local_address). This is illustrated later in this tutorial.


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