What’s New in MySQL Galera Cluster 4.0

Krzysztof Ksiazek

MySQL Galera Cluster 4.0 is the new kid on the database block with very interesting new features. Currently it is available only as a part of MariaDB 10.4 but in the future it will work as well with MySQL 5.6, 5.7 and 8.0. In this blog post we would like to go over some of the new features that came along with Galera Cluster 4.0.

Galera Cluster Streaming Replication

The most important new feature in this release is streaming replication. So far the certification process for the Galera Cluster worked in a way that whole transactions had to be certified after they completed. 

This process was not ideal in several scenarios...

  1. Hotspots in tables, rows which are very frequently updated on multiple nodes - hundreds of fast transactions running on multiple nodes, modifying the same set of rows result in frequent deadlocks and rollback of transactions
  2. Long running transactions - if a transaction takes significant time to complete, this seriously increases chances that some other transaction, in the meantime, on another node, may modify some of the rows that were also updated by the long transaction. This resulted in a deadlock during certification and one of the transactions having to be rolled back.
  3. Large transactions - if a transaction modifies a significant number of rows, it is likely that another transaction, at the same time, on a different node, will modify one of the rows already modified by the large transaction. This results in a deadlock during certification and one of the transactions has to be rolled back. In addition to this, large transactions will take additional time to be processed, sent to all nodes in the cluster and certified. This is not an ideal situation as it adds delay to commits and slows down the whole cluster.

Luckily, streaming replication can solve these problems. The main difference is that the certification happens in chunks where there is no need to wait for the whole transaction to complete. As a result, even if a transaction is large or long, majority (or all, depending on the settings we will discuss in a moment) of rows are locked on all of the nodes, preventing other queries from modifying them.

MySQL Galera Cluster Streaming Replication Options

There are two configuration options for streaming replication: 

wsrep_trx_fragment_size 

This tells how big a fragment should be (by default it is set to 0, which means that the streaming replication is disabled)

wsrep_trx_fragment_unit 

This tells what the fragment really is. By default it is bytes, but it can also be a ‘statements’ or ‘rows’. 

Those variables can (and should) be set on a session level, making it possible for user to decide which particular query should be replicated using streaming replication. Setting unit to ‘statements’ and size to 1 allow, for example, to use streaming replication just for a single query which, for example, updates a hotspot.

You can configure Galera 4.0 to certify every row that you have modified and grab the locks on all of the nodes while doing so. This makes streaming replication great at solving problems with frequent deadlocks which, prior to Galera 4.0, were possible to solve only by redirecting all writes to a single node.

WSREP Tables

Galera 4.0 introduces several tables, which will help to monitor the state of the cluster:

  • wsrep_cluster
  • wsrep_cluster_members
  • wsrep_streaming_log

All of them are located in the ‘mysql’ schema. wsrep_cluster will provide insight into the state of the cluster. wsrep_cluster_members will give you information about the nodes that are part of the cluster. wsrep_streaming_log helps to track the state of the streaming replication.

Galera Cluster Upcoming Features

Codership, the company behind the Galera, isn’t done yet. We were able to get a preview of the roadmap  from CEO, Seppo Jaakola which was given at Percona Live earlier this year. Apparently, we are going to see features like XA transaction support and gcache encryption. This is really good news. 

Support for XA transactions will be possible thanks to the streaming replication. In short, XA transactions are the distributed transactions which can run across multiple nodes. They utilize two-phase commit, which requires to first acquire all required locks to run the transaction on all of the nodes and then, once it is done, commit the changes. In previous versions Galera did not have means to lock resources on remote nodes, with streaming replication this has changed.

Gcache is a file which stores writesets. Its contents are sent to joiner nodes which asks for a data transfer. If all data is stored in the gcache, joiner will receive just the missing transactions in the process called Incremental State Transfer (IST). If gcache does not contain all required data, State Snapshot Transfer (SST) will be required and the whole dataset will have to be transferred to the joining node. 

Gcache contains information about recent changes, therefore it’s great to see its contents encrypted for better security. With better security standards being introduced through more and more regulations, it is crucial that the software will become better at achieving compliance.

Conclusion

We are definitely looking forward to see how Galera Cluster 4.0 will work out on databases than MariaDB. Being able to deploy MySQL 5.7 or 8.0 with Galera Cluster will be really great. After all, Galera is one of the most widely tested synchronous replication solutions that are available on the market.

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