Horton

Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

Dilip Kumar <dilipbalaut@gmail.com>

View thread
From: Dilip Kumar <dilipbalaut@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Kuntal Ghosh <kuntalghosh.2007@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Michael Paquier <michael@paquier.xyz>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-05-15T10:50:20Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Tighten the concurrent abort check during decoding.

  2. Improve hash_create()'s API for some added robustness.

  3. Use HASH_BLOBS for xidhash.

  4. Fix initialization of RelationSyncEntry for streaming transactions.

  5. Remove unused function declaration in logicalproto.h.

  6. Add additional tests to test streaming of in-progress transactions.

  7. Fix inline marking introduced in commit 464824323e.

  8. Add support for streaming to built-in logical replication.

  9. Fix the SharedFileSetUnregister API.

  10. Fix comment in procarray.c

  11. Suppress compiler warning in non-cassert builds.

  12. Extend the BufFile interface.

  13. Mark a few logical decoding related variables with PGDLLIMPORT.

  14. Implement streaming mode in ReorderBuffer.

  15. Extend the logical decoding output plugin API with stream methods.

  16. WAL Log invalidations at command end with wal_level=logical.

  17. Immediately WAL-log subtransaction and top-level XID association.

  18. Allow logical replication to transfer data in binary format.

  19. Only superuser can set sslcert/sslkey in postgres_fdw user mappings

  20. Track statistics for spilling of changes from ReorderBuffer.

  21. Add logical_decoding_work_mem to limit ReorderBuffer memory usage.

  22. logical decoding: process ASSIGNMENT during snapshot build

  23. Emit invalidations to standby for transactions without xid.

On Fri, May 15, 2020 at 4:04 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Fri, May 15, 2020 at 2:47 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> >
> > > 6. I think it will be good if we can provide an example of streaming
> > > changes via test_decoding at
> > > https://www.postgresql.org/docs/devel/test-decoding.html. I think we
> > > can also explain there why the user is not expected to see the actual
> > > data in the stream.
> >
> > I have a few problems to solve here.
> > -  With streaming transaction also shall we show the actual values or
> > we shall do like it is currently in the patch
> > (appendStringInfo(ctx->out, "streaming change for TXN %u",
> > txn->xid);).  I think we should show the actual values instead of what
> > we are doing now.
> >
>
> I think why we don't want to display the tuple at this stage is
> because it is not clear by this time if the transaction will commit or
> abort.  I am not sure if displaying the contents of aborted
> transactions is a good idea but if there is a reason for doing so, we
> can do it later as well.

Ok.

>
> > - In the example we can not show a real example, because of the
> > in-progress transaction to show the changes, we might have to
> > implement a lot of tuple.  I think we can show the partial output?
> >
>
> I think we can display what API will actually display, what is the
> confusion here.

What, I meant is that even with the logical_decoding_work_mem=64kb, we
need to have quite a few changes in a transaction to stream it so the
example output will be quite big in size.  So I told we might not show
the real example instead we will just show a few lines and cut the
remaining.  But, I got your point we can just show how it will look
like.

>
> I have a few more comments on the previous version of patch
> v20-0005-Implement-streaming-mode-in-ReorderBuffer.  If you have fixed
> any, then leave those and fix others.
>
> Review comments:
> ------------------------------
> 1.
> @@ -1762,10 +1952,16 @@ ReorderBufferCommit(ReorderBuffer *rb,
> TransactionId xid,
>   }
>
>   case REORDER_BUFFER_CHANGE_MESSAGE:
> - rb->message(rb, txn, change->lsn, true,
> - change->data.msg.prefix,
> - change->data.msg.message_size,
> - change->data.msg.message);
> + if (streaming)
> + rb->stream_message(rb, txn, change->lsn, true,
> +    change->data.msg.prefix,
> +    change->data.msg.message_size,
> +    change->data.msg.message);
> + else
> + rb->message(rb, txn, change->lsn, true,
> +    change->data.msg.prefix,
> +    change->data.msg.message_size,
> +    change->data.msg.message);
>
> Don't we need to set any_data_sent flag while streaming messages as we
> do for other types of changes?

Actually, pgoutput plugin don't send any data on stream_message.  But,
I agree that how other plugin will handle.  I will analyze this part
again, maybe we have to such flag at the plugin level and whether stop
is sent to not can also be handled at the plugin level.

> 2.
> + if (streaming)
> + {
> + /*
> + * Set the last of the stream as the final lsn before calling
> + * stream stop.
> + */
> + if (!XLogRecPtrIsInvalid(prev_lsn))
> + txn->final_lsn = prev_lsn;
> + rb->stream_stop(rb, txn);
> + }
>
> I am not sure if it is good to use final_lsn for this purpose.  See
> comments for this variable in reorderbuffer.h.  Basically, it is used
> for a specific purpose on different occasions.  Now, if we want to
> start using it for a new purpose, we need to study its interaction
> with all other places and update the comments as well.  Can we pass an
> additional parameter to stream_stop() instead?

I think it was in sycn with the spill code right? I mean the last
change we spill is set as the final_lsn and same is done here.

Other comments looks fine so I will work on them and reply separatly.


-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com