VReplication features, design and options in a nutshell

VReplication is a core component of Vitess that can be used to compose many features. It can be used for the following use cases:

  • Data Migrations: Use MoveTables to migrate tables into Vitess or across Keyspaces with online revertable workflows.
  • Resharding: Use Reshard to scale Keyspaces up or down as needed with automated online revertable workflows.
  • Materialized Views: Use Materialize to create a view of the source table in a target keyspace. This materialization can use a different primary vindex than the source. It can also materialize a subset of the source columns, or add new expressions from the source. This view will be kept up-to-date in real time. One can also materialize reference tables onto all shards for improved data locality, allowing Vitess to perform efficient local joins with those materialized tables.
  • Realtime Rollups: Use Materialize with aggregation expressions in which case Vitess will create a rolled up version of the source table which can be used for realtime analytics.
  • Lookup Vindexes: Use CreateLookupVindex to create a new lookup vindex and backfill it from the existing data.
  • Online Schema Changes: Use ddl_stragegy=vitess for native online non-blocking schema migrations that are trackable, cancellable, revertible, and retryable. All being safe to run in production due to intelligent throttling and resource management.
  • Change Notifications (CDC): The VStream component of VReplication can be used for the application or a systems operator to subscribe to change notifications and use it to keep downstream systems up-to-date with the source.
  • Job Queues: VReplication can also be used to provide a job queue for asynchronous processing of data using the Messaging feature.

Feature Description #

VReplication works as a stream or set of streams. Each stream establishes replication from a source keyspace/shard to a target keyspace/shard.

A given stream can replicate multiple tables. For each table, you can specify a SELECT statement that represents both the transformation rule and the filtering rule. The SELECT expressions specify the transformation, and the WHERE clause specifies the filtering.

The SELECT expressions can be any non-aggregate MySQL expression, or they can also be COUNT or SUM as aggregate expressions. Aggregate expressions combined with the corresponding GROUP BY clauses will allow you to materialize real-time rollups of the source table, which can be used for analytics. The target table can have a different name from the source.

For a sharded system like Vitess, multiple VReplication streams may be needed to achieve the objective. This is because there can be multiple source shards and multiple destination shards, and the relationship between them may not be one to one.

VReplication performs the following essential functions:

  • Copy data from the source to the destination table in a consistent fashion. For a large table, this copy can be long-running. It can be interrupted and resumed. If interrupted, VReplication can keep the copied portion up-to-date with respect to the source, and it can resume the copy process at a point that is consistent with the current replication position.
  • After copying is finished, it can continuously replicate the data from the source to destination.
  • The copying rule can be expressed as a SELECT statement. The statement should be simple enough that the materialized table can be kept up-to-date from the data coming from the binlog. For example, joins in the SELECT statement are not supported today.
  • Correctness verification: VReplication supports the VDiff command which verifies that the target table is an exact representation of the SELECT statement from the source by capturing consistent snapshots of the source and target and comparing them against each other.
  • Journaling: If there is any kind of traffic cut-over where we start writing to a different table than we used to before, VReplication will save the current binlog positions into a journal table. This can be used by other streams to resume replication from the new source.
  • Routing rules: Although this feature is itself not a direct functionality of VReplication, it works hand in hand with it. It automatically manages sophisticated rules about where to route queries depending on the type of workflow being performed. For example, it is used to control the cut-over during MoveTables.

Other Properties of VReplication #

Fast Replay #

VReplication has the capability to batch transactions if the send rate of the source exceeds the replay rate of the destination. This allows it to catch up very quickly when there is a backlog. Load tests have shown a 3-20X improvement over traditional MySQL replication depending on the workload.

Accurate Lag Tracking #

The source VTTablet sends its current time along with every event. This allows the target to correct for clock skew while estimating replication lag. Additionally, the source starts sending heartbeats if there is nothing to send. If the target receives no events from the source at all, it knows that it's definitely lagged and starts reporting itself accordingly.

Self-Replication #

VReplication allows you to set the source keyspace/shard to be the same as the target. This is especially useful for performing schema rollouts: you can create the target table with the intended schema and vreplicate from the source table to the new target. Once caught up, you can cutover to write to the target table. In this situation, an apply on the target generates a binlog event that will be picked up by the source and sent to the target. Typically, it will be an empty transaction. In such cases, the target does not generally apply these transactions, because such an application will generate yet another event. However, there are situations where one needs to apply empty transactions, especially if it's a required stopping point. VReplication can differentiate between these situations and apply events only as needed.

Deadlocks and Lock Wait Timeouts #

It is possible that multiple streams can conflict with each other and cause deadlocks or lock waits. When such things happen, VReplication silently retries such transactions without reporting an error. It does increment a counter so that the frequency of such occurrences can be tracked.

Automatic Retries #

If any other error is encountered, the replication is retried after a short wait. Each time, the stream searches from the full list of available sources and picks one at random.

Handle DDL #

The MoveTables and Reshard commands allow you to specify a value for on-ddl. This allows you to specify what to do with DDL SQL statements when they are encountered in the replication stream from the source. The values can be as follows:

  • IGNORE: Ignore all DDLs (this is also the default, if a value for on-ddl is not provided).
  • STOP: Stop when DDL is encountered. This allows you to make any necessary changes to the target. Once changes are made, updating the state to Running will cause VReplication to continue from just after the point where it encountered the DDL.
  • EXEC: Apply the DDL, but stop if an error is encountered while applying it.
  • EXEC_IGNORE: Apply the DDL, but ignore any errors and continue replicating.

We caution against against using EXEC or EXEC_IGNORE for the following reasons:

  • You may want a different schema on the target
  • You may want to apply the DDL in a different way on the target
  • The DDL may take a long time to apply on the target and may disrupt replication, performance, and query execution while it is being applied (if serving traffic from the target)

Failover Continuation #

If a failover is performed on the target keyspace/shard, the new primary will automatically resume VReplication from where the previous primary left off.

Tablet Selection #

VReplication automatically chooses viable tablets for the source and target of a given stream. See tablet selection.

Throttling #

VReplication throttles operations when the source or target appear to be overloaded, indicated by replication lag. See throttling.

Monitoring and Troubleshooting #

VTAdmin #

VTAdmin provides views into the current workflows running within a Vitess cluster. See VTAdmin.

VTTablet /debug/status #

The first place to look at is the /debug/status page of the target primary VTtablet. The bottom of the page shows the status of all the VReplication streams.

Typically, if there is a problem, the Last Message column will display the error. Sometimes, it's possible that the stream cannot find a source. If so, the Source Tablet would be empty.

VTTablet Logfile #

If the errors are not clear or if they keep disappearing, the VTTablet's INFO logfile will contain information about what it's been doing with each stream.

Workflow Show #

The current status of the workflows and streams can also be fetched by using the vtctl client Workflow Show command.

Monitoring Variables #

VReplication also reports a set of metrics that can be scraped by monitoring tools like Prometheus.

Thresholds and alerts can be set to draw attention to potential problems.