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Sharding workflow state reference

This document can be used as reference when going through the Sharding quickstart. It covers the various states you may enter during the workflow.

copying

During this state, Vitess is copying rows from your source keyspace to your target keyspace using a combination of SELECT * FROM table and binlog-based replication.

running

Once copying is complete, we switch to running state. Replication lag must be low enough for VReplication to advance into the running state. During this state, we are running pure binlog-based replication.

At this point, your original source keyspace tables are serving all primary and replica traffic. VReplication is replicating all writes to the tables in the target keyspace. Routing rules are routing all queries/writes from the target keyspace to the source keyspace.

There are some additional substates you may enter during the running state:

  • running with not verified data: You can verify replicated data is accurate before switching any traffic
  • running with verified data: You have run the verification process in the last 24 hours
  • running with verification stale: You have run the verification process over 24 hours ago

switched_replicas

The switched_replicas state is an optional intermediate step between running and switched_primaries. During the switched_replicas state, the new target keyspace is serving replica traffic, and the source keyspace continues serving all primary traffic.

Because writes are still doing to the source keyspace, data is still being replicated to the target keyspace.

switched_primaries

During the switched_primaries state, the target keyspace is serving replica and primary traffic.

Vitess has now created a reverse workflow that replicates from the target keyspace back to the source keyspace.

At this point:

  • The original, "forward" workflow is paused (stopped)
  • Vitess has initialized the sequence tables
  • The routing rules are in place, as they will be critical for reversed_cutover state.

If you click "reverse traffic", here's what happens:

  • The workflow goes back to a running state
  • Depending on which way you're reversing, "forward" workflow is started again or the "reverse" workflow is stopped

cutover

  • This is tantamount to MoveTables Complete --keep-data=false --keep-routing-rules=false
    • User cannot cancel the workflow at this point. This is because the underlying workflow is actually complete, and the source tables have been deleted
  • The underlying Vitess workflow is finished.
  • We reset the routing rules to what they were before the workflow started
  • User must have updated their application code to no longer target a keyspace in order for their app to still work at this step.
    • This is because the routing rules have been cleaned up, and can no longer route queries from source_keyspace.table to target_keyspace.table and we must rely on Vitess global routing.
  • If the user's app is failing at this point, have no fear! There is a reverse cutover button

reversed_cutover

  • User has pressed "reverse cutover"
  • We put back the routing rules we saved during switched_primaries
    • These rules route queries to the source keyspace table, to the target keyspace table so that queries work again
  • User now has more time to update/fix their client application code

complete

  • Does nothing except say, "I'm all done and I don't need to reverse cutover anymore".
  • Allows you to do another workflow. since we limit to 1 active workflow per branch at a time

cancelled

  • User clicked "cancel"
  • We issued a MoveTables Cancel --keep-data=false --keep-routing-rules=false
    • target tables will be deleted
    • routing rules will be removed
  • Vitess workflow deleted

error

  • Some sort of error happened. User can click "retry" to try again.

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