A Guide to MySQL Galera Cluster Streaming Replication: Part One

Paul Namuag

Streaming Replication is a new feature which was introduced with the 4.0 release of Galera Cluster. Galera uses replication synchronously across the entire cluster, but before this release write-sets greater than 2GB were not supported. Streaming Replication allows you to now replicate large write-sets, which is perfect for bulk inserts or loading data to your database.

In a previous blog we wrote about Handling Large Transactions with Streaming Replication and MariaDB 10.4, but as of writing this blog Codership had not yet released their version of the new Galera Cluster. Percona has, however, released their experimental binary version of Percona XtraDB Cluster 8.0 which highlights the following features...

  • Streaming Replication supporting large transactions

  • The synchronization functions allow action coordination (wsrep_last_seen_gtid, wsrep_last_written_gtid, wsrep_sync_wait_upto_gtid)

  • More granular and improved error logging. wsrep_debug is now a multi-valued variable to assist in controlling the logging, and logging messages have been significantly improved.

  • Some DML and DDL errors on a replicating node can either be ignored or suppressed. Use the wsrep_ignore_apply_errors variable to configure.

  • Multiple system tables help find out more about the state of the cluster state.

  • The wsrep infrastructure of Galera 4 is more robust than that of Galera 3. It features a faster execution of code with better state handling, improved predictability, and error handling.

What's New With Galera Cluster 4.0?

The New Streaming Replication Feature

With Streaming Replication, transactions are replicated gradually in small fragments during transaction processing (i.e. before actual commit, we replicate a number of small size fragments). Replicated fragments are then applied in slave threads, preserving the transaction’s state in all cluster nodes. Fragments hold locks in all nodes and cannot be conflicted later.

Galera SystemTables 

Database Administrators and clients with access to the MySQL database may read these tables, but they cannot modify them as the database itself will make any modifications needed. If your server doesn’t have these tables, it may be that your server is using an older version of Galera Cluster.

#> show tables from mysql like 'wsrep%';

+--------------------------+

| Tables_in_mysql (wsrep%) |

+--------------------------+

| wsrep_cluster            |

| wsrep_cluster_members    |

| wsrep_streaming_log      |

+--------------------------+

3 rows in set (0.12 sec)

New Synchronization Functions 

This version introduces a series of SQL functions for use in wsrep synchronization operations. You can use them to obtain the Global Transaction ID which is based on either the last write or last seen transaction. You can also set the node to wait for a specific GTID to replicate and apply, before initiating the next transaction.

Intelligent Donor Selection

Some understated features that have been present since Galera 3.x include intelligent donor selection and cluster crash recovery. These were originally planned for Galera 4, but made it into earlier releases largely due to customer requirements. When it comes to donor node selection in Galera 3, the State Snapshot Transfer (SST) donor was selected at random. However with Galera 4, you get a much more intelligent choice when it comes to choosing a donor, as it will favour a donor that can provide an Incremental State Transfer (IST), or pick a donor in the same segment. As a Database Administrator, you can force this via setting wsrep_sst_donor.

Why Use MySQL Galera Cluster Streaming Replication?

Long-Running Transactions

Galera's problems and limitations always revolved around how it handled long-running transactions and oftentimes caused the entire cluster to slow down due to large write-sets being replicated. It's flow control often goes high, causing the writes to slow down or even terminating the process in order to revert the cluster back to its normal state. This is a pretty common issue with previous versions of Galera Cluster.

Codership advises to use Streaming Replication for your long-running transactions to mitigate these situations. Once the node replicates and certifies a fragment, it is no longer possible for other transactions to abort it.

Large Transactions

This is very helpful when loading data to your report or analytics. Creating bulk inserts, deletes, updates, or using LOAD DATA statement to load large quantity of data can fall down in this category. Although it depends on how your manage your data for retrieval or storage. You must take into account that Streaming Replication has its limitations such that certification keys are generated from record locks. 

Without Streaming Replication, updating a large number of records would result in a conflict and the whole transaction would have to be rolled back. Slaves that are also replicating large transactions are subject to the flow control as it hits the threshold and starts slowing down the entire cluster to process any writes as they tend to relax receiving incoming transactions from the synchronous replication. Galera will relax the replication until the write-set is manageable as it allows to continue replication again. Check this external blog by Percona to help you understand more about flow control within Galera.

With Streaming Replication, the node begins to replicate the data with each transaction fragment, rather than waiting for the commit. This means that there's no way for any conflicting transactions running within the other nodes to abort since this simply affirms that the cluster has certified the write-set for this particular fragment. It’s free to apply and commit other concurrent transactions without blocking and process large transaction with a minimal impact on the cluster.

Hot Records/Hot Spots

Hot records or rows are those rows in your table that gets constantly get updated. These data could be the most visited and highly gets the traffic of your entire database (e.g. news feeds, a counter such as number of visits or logs). With Streaming Replication, you can force critical updates to the entire cluster. 

As noted by the Galera Team at Codership

“Running a transaction in this way effectively locks the hot record on all nodes, preventing other transactions from modifying the row. It also increases the chances that the transaction will commit successfully and that the client in turn will receive the desired outcome.”

This comes with limitations as it might not be persistent and consistent that you'll have successful commits. Without using Streaming Replication, you'll end up high chances or rollbacks and that could add overhead to the end user when experiencing this issue in the application's perspective.

Things to Consider When Using Streaming Replication

  • Certification keys are generated from record locks, therefore they don’t cover gap locks or next key locks. If the transaction takes a gap lock, it is possible that a transaction, which is executed on another node, will apply a write set which encounters the gap log and will abort the streaming transaction.
  • When enabling Streaming Replication, write-set logs are written to wsrep_streaming_log table found in the mysql system database to preserve persistence in case crash occurs, so this table serves upon recovery. In case of excessive logging and elevated replication overhead, streaming replication will cause degraded transaction throughput rate. This could be a performance bottleneck when high peak load is reached. As such, it’s recommended that you only enable Streaming Replication at a session-level and then only for transactions that would not run correctly without it.
  • Best use case is to use streaming replication for cutting large transactions
  • Set fragment size to ~10K rows
  • Fragment variables are session variables and can be dynamically set
  • Intelligent application can set streaming replication on/off on need basis

Conclusion

Thanks for reading, in part two we will discuss how to enable Galera Cluster Streaming Replication and what the results could look like for your setup.

 

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