Re: Stream consistent snapshot via a logical decoding plugin as a series of INSERTs - Mailing list pgsql-hackers

On 15 January 2016 at 16:30, Shulgin, Oleksandr <oleksandr.shulgin@zalando.de> wrote:

 

I'd like to propose generic functions (probably in an extension, or in core if not possible otherwise) to facilitate streaming existing data from the database *in the same format* that one would get if these would be the changes decoded by a logical decoding plugin.

So effectively produce synthetic logical decoding callbacks to run a bunch of fake INSERTs, presumably with a fake xid etc?

The idea is to use a snapshot returned from CREATE_REPLICATION_SLOT command of the replication protocol to get a consistent snapshot of the database, then start listening to new changes on the slot.

My impression is that you want to avoid the current step of "synchronize database initial contents" when using logical decoding for replication.

But I guess you're looking to then populate that empty schema in-band via logical decoding, rather than having to do a --data-only dump or use COPY.  Right?

That won't help you for schema; presumably you'd still do a pg_dump --schema-only | pg_restore for that.

Just like when restoring a --data-only dump or using COPY you'd have to disable FKs during sync, but that's pretty much unavoidable.

The way this initial export phase is implemented there is by providing a SQL-callable set returning function which is using SPI to run "SELECT * FROM mytable" behind the scenes and runs the resulting tuples through the INSERT callback of the logical decoding plugin, which lives in the same loadable module as this SQL function.

o_O

What about the reorder buffer, the logical decoding memory context, etc?

Bottled Water logical decoding plugin uses binary protocol based on Avro data serialization library.  As an experiment I was adding support for JSON output format to it, and for that I had to re-implement the aforementioned SRF to export initial data to convert tuples to JSON instead.

Have you taken a look at what's been done with pglogical and pglogical_output? 

We've got extensible protocol support there, and if Avro offers compelling benefits over the current binary serialization I'm certainly interested in hearing about it.

What do you say?

Interesting idea. As outlined I think it sounds pretty fragile though; I really, really don't like the idea of lying to the insert callback by passing it a fake insert with (presumably) fake reorder buffer txn, etc.

What we've done in pglogical is take a --schema-only dump then, on the downstream, attach to the exported snapshot and use COPY ... TO STDOUT over a libpq connection to the upstream feed that to COPY ... FROM STDIN on another libpq connection to "ourselves", i.e. the downstream. Unless Petr changed it to use COPY innards directly on the downstream; I know he talked about it but haven't checked if he did. Anyway, either way it's not pretty and requires a sideband non-replication connection to sync initial state. The upside is that it can be relatively easily parallelized for faster sync using multiple connections.

To what extent are you setting up a true logical decoding context here?

Where does the xact info come from? The commit record? etc.

You're presumably not forming a reorder buffer then decoding it since it could create a massive tempfile on disk, so are you just dummying this info up?

Or hoping the plugin won't look at it?

The functionality is good and I think that for the SQL level you'd have to use SET TRANSACTION SNAPSHOT as you show. But I think it should really be usable from the replication protocol too - and should try to keep the state as close to that of a normal decoding session as possible. We'd at least need a new walsender protocol command equivalent that took the snapshot identifier, relation info and the other decoding params instead of a slot name. Or, ideally, a variant on START_REPLICATION ... LOGICAL ... that omits SLOT and instead takes TABLES as an argument, with a list of relation(s) to sync. Unlike normal START_REPLICATION ... LOGICAL ... it'd return to walsender protocol mode on completion, like the phys rep protocol does when it's time for a timeline switch.

Rather than lie to the insert callback I'd really rather define a new logical decoding callback for copying initial records. It doesn't get any xact info (since it's not useful/relevant) or a full reorder buffer. No ReorderBufferChange is passed; instead we pass something like a ReorderBufferSync that contains the new tuple ReorderBufferTupleBuf, origin id, origin lsn and commit timestamp (if known) and the RelFileNode affected. The LogicalDecodingContext that's set up for the callback gets ctx->reorder = NULL .  There's no ReorderBufferTxn argument and none is defined.

Since it's a new callback the plugin knows the rules, knows it's getting initial state data to sync over, etc. It doesn't have to try to guess if it's seeing a real insert and act differently with respect to xact identity etc.

Obviously that's 9.6 material at the soonest, and only 9.6 if it could be done ... well, right about now. So that won't meet your immediate needs, but I think the same is true of the interface you propose above.

What I suggest doing in the mean time is specifying a new callback function interface for tuple copies as described above, to be implemented by logical decoding modules that support this extension. In each decoding plugin we then define a SQL-callable function with 'internal' return type that returns a pointer to the callback so you can obtain the hook function address via a fmgr call via pg_proc. The callback would expect a state much like I describe above and we'd use a SQL-callable function like what you outlined to set up a fake logical decoding state for it, complete with decoding context etc. Probably copying & pasting a moderately painful amount of the logical decoding guts into an ext in the process :( since I don't think you can easily set up much of the decoding state using the decoding backend code without having a slot to use. Still, that'd let us prototype this and prove the idea for inclusion in 9.7 (?) in-core while retaining the capability via an extension for earlier versions.

You'd have to do much of the same hoop jumping to call an arbitrary output plugin's insert callback directly, if not more.

Alternately, you could just use COPY ;)