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Re: MDEV-7502 Automatic provisioning of slave


Martin Kaluznik <martin.kaluznik@xxxxxxxxx> writes:

> different places is not acceptable. I have decided to use regular
> replication IO thread as Kristian Nielsen suggested before and add
> special handling where necessary.

Right. Being able to re-use the code for talking on the network with the
master will be a huge benefit, I think.

> Now when the IO thread requests binlog dump from master, it sends
> packet with following structure
> master position | binlog flags | server id | log name

This is the COM_BINLOG_DUMP client packet, right?

> Master needs to receive information, that slaves IO thread is
> expecting provisioning data along with current binlog. I have created
> new BINLOG_PROVISIONING_MODE flag, but I believe that the name is
> incorrect and it doesn’t belong to binlog flags field. My question is,
> what is better for compatibility, change binlog flags to any flags and
> use it for sending information about provisioning request to master
> (as in current implementation) or add another byte to packet after log
> name containing provisioning mode flag and possibly more flags later.

It's not really a flag for the binlog, but for the COM_BINLOG_DUMP
request. It seems fine to me to use that flag field. Maybe a better name

This stuff is part of the client-server protocol, which is tricky to change
due to the requirement of both forward and backwards compatibility with
clients. So a minimal change of adding a flag bit seems safest. You might
want to consider adding the flag at the end (0x8000) to reduce risk of
clashing with any future flag added by MySQL@Oracle (those people don't care
about compatibility with the rest of the world).

The alternative is to add a new packet type (COM_SLAVE_PROVISION or
whatever). MySQL 5.6 similarly added a new command for their GTID
implementation (COM_BINLOG_DUMP_GTID). But I think using the existing
command with a flag is fine. The provisioning is replicating events same way
as a normal slave, while also interleaving new provision data. So it seems a
natural extension of the existing protocol.

One thing that occurs to me is how to handle if the user tries to LOAD DATA
FROM MASTER from a master that does not support the new feature. This should
give an error, but the old master will just ignore the new flag and attempt
to process the COM_BINLOG_DUMP in the normal way. But we can detect this
error in another way, I think, for example by adding an extra query to

> My current implementation progress can be seen at
> https://github.com/f4rnham/server/commits/MDEV-7502
> diff of all commits at once
> https://github.com/MariaDB/server/compare/10.1...f4rnham:MDEV-7502

I'm happy to see that you have already started to write test cases!

> There is one more decision I am not sure about. What should result of
> provisioning on non-empty slave be? For example, slave contains table
> with same name in same database as master, but with different columns.
> Should slave detect already existing tables and fail with error, or
> silently drop everything what could cause conflicts (add optional
> FORCE part of provisioning start command?). Which one from these
> solutions should be default? Or maybe there is third one and better.

Good point.

It is actually a valid use case to provision into a non-empty slave, eg. for
multi-source replication, provisioning from two separate masters and then
replicating from both. But for most users, provisioning into a non-empty
slave will be a mistake, ending up with a useless mix of old and new data.

I think the simplest solution is to just give an error whenever a conflict
is found (provisioning an object like a table which already exists on
slave). The user will be expected to DROP DATABASE/TABLE/... before running
the command. Automatically dropping conflicting tables may not be that
useful, as it can still leave stray tables that happened to not exist on the

And this point can be refined in a later version. For example, it could be
nice if the slave would get the list of objects to be created at the start,
so that error about conflicts could be given before making any changes to
the slave. But that is not a requirement of a first version I think, just
giving the error whenever the conflict is detected is fine.

The tricky part seems to be what to do about the system tables in the
`mysql` schema. Like user accounts. I am not sure about what to do there,
but I suggest looking at what happens when a user uses mysqldump to dump a
server and provision a new slave from it. Probably doing the same for the
new provisioning feature would be reasonable. Or maybe just ignore the
`mysql` schema completely - the user will in any case need to have set up
accounts on the new slave-to-be, to be able to connect the slave to the
master and run the LOAD DATA FROM MASTER command.

 - Kristian.