[Slony1-hackers] [Slony1-patches] Removal of session replication role for Logical Replication compatibility

[Steve Singer]

On Mon, 24 Feb 2020, Steve Singer wrote:

As a follow up this this.

I hadn't committed this because I was struggling with a how to update 
(https://www.slony.info/documentation/2.2/triggers.html) to reflect the 
proposed behaviour. I was thinking maybe this page needs a rewrite.

However I've since noticed that this patch causes the ExecuteScript disorder 
test to fail.  I get 'duplicate key value violates unique 
constraint "do_customer_pkey' with this patch applied.

This test performs INSERT's into a replicated table from inside the execute 
script (among other things)

Slonik set session_replication_role to local before doing the slonik work.

The patch causes the data to both show up in sl_log_1 and sl_log_script.

Steve





> On Thu, 20 Feb 2020, Richard Yen wrote:
>
> I've CC's slony1-hackers since I don't think many people are subscribed to 
> -patches.
>
> I'm inclined to apply this patch. I don't think the value of the extra check 
> is worth making like difficult for users that want to run the slony triggers 
> on a logical replica.
>
> If someone things that the extra guard in the log trigger should be 
> configurable instead of just removed they should speak up.
>
>
>
>> 
>> 
>> On Thu, Feb 20, 2020 at 4:16 AM Steve Singer <steve at ssinger.info> wrote:
>>       On Thu, 20 Feb 2020, Richard Yen wrote:
>>
>>       So with your patch, what happens on node C?
>>       Do the replication triggers fire there as well?
>> 
>> 
>> No, on node C, the replication triggers do not fire (i.e., sl_log_{1,2} are 
>> empty).  This is because the
>> triggers themselves are disabled on the tables (Slony's doing, not mine):
>> 
>> pgbench=# select tgname, tgenabled  from pg_trigger where tgname like 
>> '_slony_example%' order by 2;
>>              tgname             | tgenabled
>> --------------------------------+-----------
>>  _slony_example_truncatetrigger | D
>>  _slony_example_logtrigger      | D
>>  _slony_example_truncatetrigger | D
>>  _slony_example_logtrigger      | D
>>  _slony_example_truncatetrigger | D
>>  _slony_example_logtrigger      | D
>>  _slony_example_truncatedeny    | O
>>  _slony_example_truncatedeny    | O
>>  _slony_example_denyaccess      | O
>>  _slony_example_denyaccess      | O
>>  _slony_example_denyaccess      | O
>>  _slony_example_truncatedeny    | O
>> (12 rows)
>> 
>> Granted, my patch is like taking the guardrails off, but I'd argue that the 
>> ENABLE/DISABLE syntax on PG's
>> ALTER TABLE command is quite comprehensive, in terms of letting Slony do 
>> what it needs to do to make sure
>> the triggers are properly turned on/off, based on the publisher/subscriber 
>> status.
>> 
>> There are other locations where there's this redundancy of 1) setting 
>> triggers to fire on replica-only
>> and 2) enforcing replica role identity in the C-side of the trigger code 
>> (i.e., logApply):
>> 
>> pgbench=# \d _slony_example.sl_log_1;
>>                Table "_slony_example.sl_log_1"
>>       Column      |  Type   | Collation | Nullable | Default
>> ------------------+---------+-----------+----------+---------
>>  log_origin       | integer |           |          |
>> ...<snip>...
>> Triggers firing on replica only:
>>     apply_trigger BEFORE INSERT ON _slony_example.sl_log_1 FOR EACH ROW 
>> EXECUTE PROCEDURE
>> _slony_example.logapply('_slony_example')
>> 
>> To avoid inserting more risk, I didn't take those checks out--just enough 
>> to get Slony working nicely
>> with Logical Replication.
>> 
>> With that said, getting Slony to work with Logical Replication (with my 
>> patch installed) isn't a trivial
>> task.  Between the "set add table" slonik command and the "subscribe set" 
>> slonik command, there needs to
>> be a manual execution of "ALTER TABLE ENABLE TRIGGER [ALWAYS|REPLICA]" on 
>> each *_logtrigger and
>> *_truncatetrigger for each table, along with an "ALTER TABLE DISABLE 
>> TRIGGER apply_trigger" for
>> sl_log_{1,2} on the Slony-provider/LR-subscriber side (node B in my 
>> example).  But at least with my
>> patch, it becomes *possible* to set up such an architecture.
>> 
>> Thanks for considering,
>> --Richard
>> 
>> 
>> 
>> 
>>
>>       > I recently came across a customer who sought to set up Slony on the 
>> tail end of a
>>       cascaded-replication
>>       > setup:
>>       > Node A -> Logical Replication -> Node B -> Slony -> Node C
>>       >
>>       > We discovered that this does not work because the client (WAL 
>> replay process) logs in as a
>>       Replica user,
>>       > but logTrigger won't fire unless replication role is Origin.
>>       >
>>       > Seeing that the "replication role == Origin" check is a bit dated, 
>> and Slony will
>>       enable/disable triggers
>>       > based on subscriber/provider status, I'm thinking that the attached 
>> patch might be worth a
>>       discussion. 
>>       > After all, the "replication role == Origin" check basically negates 
>> all the replication
>>       role features
>>       > that are now baked into Postgres core.
>>       >
>>       > Please let me know if there are other factors to consider.
>>       >
>>       > Cheers,
>>       > --Richard
>>       >
>>       >
>>       >
>>       >
>> 
>> 
>> 
>> --
>> Richard Yen
>> Principal Support Engineer
>> 
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