Hi,

On 11/08/16 13:34, Stas Kelvich wrote:


* max_logical_replication_workers mentioned everywhere in docs, but guc.c 
defines
variable called max_logical_replication_processes for postgresql.conf


Ah changed it in code but not in docs, will fix.

* Since pg_subscription already shared across the cluster, it can be also handy 
to
share pg_publications too and allow publication of tables from different 
databases. That
is rare scenarios but quite important for virtual hosting use case — tons of 
small databases
in a single postgres cluster.

You can't decode changes from multiple databases in one slot so I don't see the usefulness there. The pg_subscription is currently shared because it's technical necessity (as in I don't see how to solve the need to access the catalog from launcher in any other way) not because I think it's great design :)


* There is no way to see attachecd tables/schemas to publication through \drp


That's mostly intentional as publications for table are visible in \d, but I am not against adding it to \drp.

* As far as I understand there is no way to add table/tablespace right in CREATE
PUBLICATION and one need explicitly do ALTER PUBLICATION right after creation.
May be add something like WITH TABLE/TABLESPACE to CREATE?


Yes, as I said to Masahiko Sawada, it's just not there yet but I plan to have that.

* So binary protocol goes into core. Is it still possible to use it as decoding 
plugin for
manually created walsender? May be also include json as it was in pglogical? 
While
i’m not arguing that it should be done, i’m interested about your opinion on 
that.


Well the plugin is bit more integrated into the publication infra so if somebody would want to use it directly they'd have to use that part as well. OTOH the protocol itself is provided as API so it's reusable by other plugins if needed.

JSON plugin is something that would be nice to have in core as well, but I don't think it's part of this patch.

* Also I’ve noted that you got rid of reserved byte (flags) in protocol 
comparing to
pglogical_native. It was very handy to use it for two phase tx decoding (0 — 
usual
commit, 1 — prepare, 2 — commit prepared), because both prepare and commit
prepared generates commit record in xlog.

Hmm maybe commit message could get it back. PGLogical has them sprinkled all around the protocol which I don't really like so I want to limit them to the places where they are actually useful.


On 05 Aug 2016, at 18:00, Petr Jelinek <p...@2ndquadrant.com> wrote:

- DDL, I see several approaches we could do here for 10.0. a) don't
  deal with DDL at all yet, b) provide function which pushes the DDL
  into replication queue and then executes on downstream (like
  londiste, slony, pglogical do), c) capture the DDL query as text
  and allow user defined function to be called with that DDL text on
  the subscriber

* Since here DDL is mostly ALTER / CREATE / DROP TABLE (or am I wrong?) may be
we can add something like WITH SUBSCRIBERS to statements?


Not sure I follow. How does that help?

* Talking about exact mechanism of DDL replication I like you variant b), but 
since we
have transactional DDL, we can do two phase commit here. That will require two 
phase
decoding and some logic about catching prepare responses through logical 
messages. If that
approach sounds interesting i can describe proposal in more details and create 
a patch.


I'd think that such approach is somewhat more interesting with c) honestly. The difference between b) and c) is mostly about explicit vs implicit. I definitely would like to see the 2PC patch updated to work with this. But maybe it's wise to wait a while until the core of the patch stabilizes during the discussion.

* Also I wasn’t able actually to run replication itself =) While regression 
tests passes, TAP
tests and manual run stuck. pg_subscription_rel.substate never becomes ‘r’. 
I’ll investigate
that more and write again.

Interesting, please keep me posted. It's possible for tables to stay in 's' state for some time if there is nothing happening on the server, but that should not mean anything is stuck.


* As far as I understand sync starts automatically on enabling publication. May 
be split that
logic into a different command with some options? Like don’t sync at all for 
example.


I think SYNC should be option of subscription creation just like INITIALLY ENABLED/DISABLED is. And then there should be interface to resync a table manually (like pglogical has). Not yet sure how that interface should look like in terms of DDL though.

* When I’m trying to create subscription to non-existent publication, CREATE 
SUBSRITION
creates replication slot and do not destroys it:

# create subscription sub connection 'host=127.0.0.1 dbname=postgres' 
publication mypub;
NOTICE:  created replication slot "sub" on provider
ERROR:  could not receive list of replicated tables from the provider: ERROR:  
cache lookup failed for publication 0
CONTEXT:  slot "sub", output plugin "pgoutput", in the list_tables callback

after that:

postgres=# drop subscription sub;
ERROR:  subscription "sub" does not exist
postgres=# create subscription sub connection 'host=127.0.0.1 dbname=postgres' 
publication pub;
ERROR:  could not crate replication slot "sub": ERROR:  replication slot "sub" 
already exists


See the TODO in CreateSubscription function :)

* Also can’t drop subscription:

postgres=# \drs
                          List of subscriptions
 Name | Database | Enabled | Publication |            Conninfo
------+----------+---------+-------------+--------------------------------
 sub  | postgres | t       | {mypub}     | host=127.0.0.1 dbname=postgres
(1 row)

postgres=# drop subscription sub;
ERROR:  unrecognized object class: 6102

Yes that has been already reported.


* Several time i’ve run in a situation where provider's postmaster ignores 
Ctrl-C until subscribed
node is switched off.


Hmm I guess there is bug in signal processing code somewhere.


* Patch with small typos fixed attached.

I’ll do more testing, just want to share what i have so far.


Thanks for both.

--
  Petr Jelinek                  http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services


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