For messages sent between online group members, Group Replication enables message compression by default. Whether a specific message is compressed depends on the threshold that you configure using the group_replication_compression_threshold system variable. Messages that have a payload larger than the specified number of bytes are compressed.

The default compression threshold is 1000000 bytes. You could use the following statements to increase the compression threshold to 2MB, for example:

STOP GROUP_REPLICATION;
SET GLOBAL group_replication_compression_threshold = 2097152;
START GROUP_REPLICATION;

If you set group_replication_compression_threshold to zero, message compression is disabled.

Group Replication uses the LZ4 compression algorithm to compress messages sent in the group. Note that the maximum supported input size for the LZ4 compression algorithm is 2113929216 bytes. This limit is lower than the maximum possible value for the group_replication_compression_threshold system variable, which is matched to the maximum message size accepted by XCom. The LZ4 maximum input size is therefore a practical limit for message compression, and transactions above this size cannot be committed when message compression is enabled. With the LZ4 compression algorithm, do not set a value greater than 2113929216 bytes for group_replication_compression_threshold.

The value of group_replication_compression_threshold is not required by Group Replication to be the same on all group members. However, it is advisable to set the same value on all group members in order to avoid unnecessary rollback of transactions, failure of message delivery, or failure of message recovery.

You can also configure compression for messages sent for distributed recovery by the method of state transfer from a donor's binary log. Compression for these messages, which are sent from a donor already in the group to a joining member, is controlled separately using the group_replication_recovery_compression_algorithms and group_replication_recovery_zstd_compression_level system variables. For more information, see Section 6.2.8, “Connection Compression Control”.

Binary log transaction compression, enabled by the binlog_transaction_compression system variable, can also be used to save bandwidth. The transaction payloads remain compressed when they are transferred between group members. If you use binary log transaction compression in combination with Group Replication's message compression, message compression has less opportunity to act on the data, but can still compress headers and those events and transaction payloads that are uncompressed. For more information on binary log transaction compression, see Section 7.4.4.5, “Binary Log Transaction Compression”.

Compression for messages sent in the group happens at the group communication engine level, before the data is handed over to the group communication thread, so it takes place within the context of the mysql user session thread. If the message payload size exceeds the threshold set by group_replication_compression_threshold, the transaction payload is compressed before being sent out to the group, and decompressed when it is received. Upon receiving a message, the member checks the message envelope to verify whether it is compressed or not. If needed, then the member decompresses the transaction, before delivering it to the upper layer. This process is shown in the following figure.

When network bandwidth is a bottleneck, message compression can provide up to 30-40% throughput improvement at the group communication level. This is especially important within the context of large groups of servers under load. The TCP peer-to-peer nature of the interconnections between N participants in the group makes the sender send the same amount of data N times. Furthermore, binary logs are likely to exhibit a high compression ratio. This makes compression a compelling feature for Group Replication workloads that contain large transactions.