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18.6.3 Message Compression

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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.

Table 18.4 LZ4 Compression Ratios for Different Binary Log Formats

Workload

Ratio for ROW

Ratio for STMT

mysqlslapd

4,5

4,1

sysbench

3,4

2,9


The TCP peer-to-peer nature of the interconnections between N participants on the group makes the sender send the same amount of data N times. Furthermore, binary logs are likely to exhibit a high compression ratio (see table above). This makes compression a compelling feature for workloads that contain large transaction.

Figure 18.15 Compression Support

The MySQL Group Replication plugin architecture is shown as described in an earlier topic, with the five layers of the plugin positioned between the MySQL server and the replication group. Compression and decompression are handled by the Group Communication System API, which is the fourth layer of the Group Replication plugin. The group communication engine (the fifth layer of the plugin) and the group members use the compressed transactions with the smaller data size. The MySQL Server core and the three higher layers of the Group Replication plugin (the APIs, the capture, applier, and recovery components, and the replication protocol module) use the original transactions with the larger data size.

Compression happens at the group communication engine level, before the data is handed over to the group communication thread, so it happens within the context of the mysql user session thread. Transaction payloads may be compressed before being sent out to the group and decompressed when received. Compression is conditional and depends on a configured threshold. By default compression is enabled.

In addition, there is no requirement that all servers in the group have compression enabled to be able to work together. 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.

The compression algorithm used is LZ4. Compression is enabled by default with threshold of 1000000 bytes. The compression threshold, in bytes, may be set to something larger than default. In that case, only transactions that have a payload larger than the threshold are compressed. Below is an example of how to set a compression threshold.

  1. STOP GROUP_REPLICATION;
  2. SET GLOBAL group_replication_compression_threshold= 2097152;
  3. START GROUP_REPLICATION;

This sets the compression threshold to 2MB. If a transaction generates a replication message with a payload larger than 2MB, for example a binary log transaction entry larger than 2MB, then it is compressed. To disable compression set threshold to 0.


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Document created the 26/06/2006, last modified the 26/10/2018
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