Whenever a member joins or rejoins a replication group, it must catch up with the transactions that were applied by the group members before it joined, or while it was away. This process is called distributed recovery.
The joining member begins by checking the relay log for its
group_replication_applier channel for any
transactions that it already received from the group but did not
yet apply. If the joining member was in the group previously, it
might find unapplied transactions from before it left, in which
case it applies these as a first step. A member that is new to the
group does not have anything to apply.
After this, the joining member connects to an online existing member to carry out state transfer. The joining member transfers all the transactions that took place in the group before it joined or while it was away, which are provided by the existing member (called the donor). Next, the joining member applies the transactions that took place in the group while this state transfer was in progress. When this process is complete, the joining member has caught up with the remaining servers in the group, and it begins to participate normally in the group.
Group Replication uses a combination of these methods for state transfer during distributed recovery:
A remote cloning operation using the clone plugin's function, which is available from MySQL 8.0.17. To enable this method of state transfer, you must install the clone plugin on the group members and the joining member. Group Replication automatically configures the required clone plugin settings and manages the remote cloning operation.
Replicating from a donor's binary log and applying the transactions on the joining member. This method uses a standard asynchronous replication channel named
group_replication_recoverythat is established between the donor and the joining member.
Group Replication automatically selects the best combination of
these methods for state transfer after you issue
START GROUP_REPLICATION on the
joining member. To do this, Group Replication checks which
existing members are suitable as donors, how many transactions the
joining member needs from a donor, and whether any required
transactions are no longer present in the binary log files on any
group member. If the transaction gap between the joining member
and a suitable donor is large, or if some required transactions
are not in any donor's binary log files, Group Replication begins
distributed recovery with a remote cloning operation. If there is
not a large transaction gap, or if the clone plugin is not
installed, Group Replication proceeds directly to state transfer
from a donor's binary log.
During a remote cloning operation, the existing data on the joining member is removed, and replaced with a copy of the donor's data. When the remote cloning operation is complete and the joining member has restarted, state transfer from a donor's binary log is carried out to get the transactions that the group applied while the remote cloning operation was in progress.
During state transfer from a donor's binary log, the joining member replicates and applies the required transactions from the donor's binary log, applying the transactions as they are received, up to the point where the binary log records that the joining member joined the group (a view change event). While this is in progress, the joining member buffers the new transactions that the group applies. When state transfer from the binary log is complete, the joining member applies the buffered transactions.
When the joining member is up to date with all the group's transactions, it is declared online and can participate in the group as a normal member, and distributed recovery is complete.
State transfer from the binary log is Group Replication's base mechanism for distributed recovery, and if the donors and joining members in your replication group are not set up to support cloning, this is the only available option. As state transfer from the binary log is based on classic asynchronous replication, it might take a very long time if the server joining the group does not have the group's data at all, or has data taken from a very old backup image. In this situation, it is therefore recommended that before adding a server to the group, you should set it up with the group's data by transferring a fairly recent snapshot of a server already in the group. This minimizes the time taken for distributed recovery, and reduces the impact on donor servers, since they have to retain and transfer fewer binary log files.