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Vitess, MySQL Replication, and Schema Changes
Statement vs Row Based Replication
MySQL supports two primary modes of replication in its binary logs: statement or row based. Vitess supports both these modes.
For schema changes, if the number of affected rows is greater > 100k (configurable), we don't allow direct application of DDLs. The recommended tools in such cases are gh-ost or pt-osc.
Not all statements are safe for Statement Based Replication (SBR): https://dev.mysql.com/doc/refman/8.0/en/replication-rbr-safe-unsafe.html. Vitess rewrites some of these statements to be safe for SBR, and others are explicitly failed. This is described in detail below.
With statement based replication, it becomes easier to perform offline advanced schema changes, or large data updates. Vitess’s solution is called schema swap (described below).
Rewriting Update Statements
Vitess rewrites ‘UPDATE’ SQL statements to always know what rows will be affected. For instance, this statement:
UPDATE <table> SET <set values> WHERE <clause>
Will be rewritten into:
SELECT <primary key columns> FROM <table> WHERE <clause> FOR UPDATE
UPDATE <table> SET <set values> WHERE <primary key columns> IN <result from previous SELECT> /* primary key values: … */
With this rewrite in effect, we know exactly which rows are affected, by primary key, and we also document them as a SQL comment.
The replication stream then doesn’t contain the expensive WHERE clauses, but only the UPDATE statements by primary key. In a sense, it is combining the best of row based and statement based replication: the slaves only do primary key based updates, but the replication stream is very friendly for schema changes.
Also, Vitess adds comments to the rewritten statements that identify the primary key affected by that statement. This allows us to produce an Update Stream (see section below).
Vitess Schema Swap
Within YouTube, we also use a combination of statement based replication and backups to apply long-running schema changes without disrupting ongoing operations. See the [schema swap tutorial]({% link user-guide/schema-swap.md %}) for a detailed example.
This operation, which is called schema swap, works as follows:
- Pick a slave, take it out of service. It is not used by clients any more.
- Apply whatever schema or large data change is needed, on the slave.
- Take a backup of that slave.
- On all the other slaves, one at a time, take them out of service, restore the backup, catch up on replication, put them back into service.
- When all slaves are done, reparent to a slave that has applied the change.
- The old master can then be restored from a backup too, and put back into service.
With this process, the only guarantee we need is for the change (schema or data) to be backward compatible: the clients won’t know if they talk to a server that has applied the change yet or not. This is usually fairly easy to deal with:
- When adding a column, clients cannot use it until the schema swap is done.
- When removing a column, all clients must stop referring to it before the schema swap begins.
- A column rename is still tricky: the best way to do it is to add a new column with the new name in one schema swap, then change the client to populate both (and backfill the values), then change the client again to use the new column only, then use another schema swap to remove the original column.
- A whole bunch of operations are really easy to perform though: index changes, optimize table, …
Note the real change is only applied to one instance. We then rely on the backup / restore process to propagate the change. This is a very good improvement from letting the changes through the replication stream, where they are applied to all hosts, not just one. This is also a very good improvement over the industry practice of online schema change, which also must run on all hosts. Since Vitess’s backup / restore and reparent processes are very reliable (they need to be reliable on their own, independently of this process!), this does not add much more complexity to a running system.
Update Stream
Since the SBR replication stream also contains comments of which primary key is affected by a change, it is possible to look at the replication stream and know exactly what objects have changed. This Vitess feature is called [Update Stream]({% link user-guide/update-stream.md %}).
By subscribing to the Update Stream for a given shard, one can know what values change. This stream can be used to create a stream of data changes (export to an Apache Kafka for instance), or even invalidate an application layer cache.
Note: the Update Stream only reliably contains the primary key values of the rows that have changed, not the actual values for all columns. To get these values, it is necessary to re-query the database.
We have plans to make this [Update Stream]({% link user-guide/update-stream.md %}) feature more consistent, very resilient, fast, and transparent to sharding.
Semi-Sync
If you tell Vitess to enforce semi-sync
(semisynchronous replication)
by passing the -enable_semi_sync flag to vttablets,
then the following will happen:
-
The master will only accept writes if it has at least one slave connected and sending semi-sync ACK. It will never fall back to asynchronous (not requiring ACKs) because of timeouts while waiting for ACK, nor because of having zero slaves connected (although it will fall back to asynchronous in case of shutdown, abrupt or graceful).
This is important to prevent split brain (or alternate futures) in case of a network partition. If we can verify all slaves have stopped replicating, we know the old master is not accepting writes, even if we are unable to contact the old master itself.
-
Slaves of replica type will send semi-sync ACK. Slaves of rdonly type will not send ACK. This is because rdonly slaves are not eligible to be promoted to master, so we want to avoid the case where a rdonly slave is the single best candidate for election at the time of master failure (though a split brain is possible when all rdonly slaves have transactions that none of replica slaves have).
These behaviors combine to give you the property that, in case of master failure, there is at least one other replica type slave that has every transaction that was ever reported to clients as having completed. You can then ([manually]({% link reference/vtctl.md %}#emergencyreparentshard), or with an automated tool like Orchestrator) pick the replica that is farthest ahead in GTID position and promote that to be the new master.
Thus, you can survive sudden master failure without losing any transactions that were reported to clients as completed. In MySQL 5.7+, this guarantee is strengthened slightly to preventing loss of any transactions that were ever committed on the original master, eliminating so-called phantom reads.
On the other hand these behaviors also give a requirement that each shard must have at least 2 tablets with type replica (with addition of the master that can be demoted to type replica this gives a minimum of 3 tablets with initial type replica). This will allow for the master to have a semi-sync acker when one of the replica tablets is down for any reason (for a version update, machine reboot, schema swap or anything else).
With regard to replication lag, note that this does not guarantee there is always at least one replica type slave from which queries will always return up-to-date results. Semi-sync guarantees that at least one slave has the transaction in its relay log, but it has not necessarily been applied yet. The only way to guarantee a fully up-to-date read is to send the request to the master.
Appendix: Adding support for RBR in Vitess
We are in the process of adding support for RBR in Vitess.
See [this document]({% link user-guide/row-based-replication.md %})) for more information.