Re: logical replication restrictions

Hi Euler, a long time ago you ask me a few questions about my previous
review [1].

Here are my replies, plus a few other review comments for patch v7-0001.

======

1. doc/src/sgml/catalogs.sgml

+ <para>
+ Application delay of changes by a specified amount of time. The
+ unit is in milliseconds.
+ </para></entry>

The wording sounds a bit strange still. How about below

SUGGESTION
The length of time (ms) to delay the application of changes.

=======

2. Other documentation?

Maybe should say something on the Logical Replication Subscription
page about this? (31.2 Subscription)

=======

3. doc/src/sgml/ref/create_subscription.sgml

+ synchronized, this may lead to apply changes earlier than expected.
+ This is not a major issue because a typical setting of this parameter
+ are much larger than typical time deviations between servers.

Wording?

SUGGESTION
... than expected, but this is not a major issue because this
parameter is typically much larger than the time deviations between
servers.

~~~

4. Q/A

From [2] you asked:

> Should there also be a big warning box about the impact if using
> synchronous_commit (like the other streaming replication page has this
> warning)?
Impact? Could you elaborate?

~

I noticed the streaming replication docs for recovery_min_apply_delay
has a big red warning box saying that setting this GUC may block the
synchronous commits. So I was saying won’t a similar big red warning
be needed also for this min_apply_delay parameter if the delay is used
in conjunction with a publisher wanting synchronous commit because it
might block everything?

~~~

4. Example

+<programlisting>
+CREATE SUBSCRIPTION foo
+ CONNECTION 'host=192.168.1.50 port=5432 user=foo dbname=foodb'
+ PUBLICATION baz
+ WITH (copy_data = false, min_apply_delay = '4h');
+</programlisting></para>

If the example named the subscription/publication as ‘mysub’ and
‘mypub’ I think it would be more consistent with the existing
examples.

======

5. src/backend/commands/subscriptioncmds.c - SubOpts

@@ -89,6 +91,7 @@ typedef struct SubOpts
bool disableonerr;
char *origin;
XLogRecPtr lsn;
+ int64 min_apply_delay;
} SubOpts;

I feel it would be better to be explicit about the storage units. So
call this member ‘min_apply_delay_ms’. E.g. then other code in
parse_subscription_options will be more natural when you are
converting using and assigning them to this member.

~~~

6. - parse_subscription_options

+ /*
+ * If there is no unit, interval_in takes second as unit. This
+ * parameter expects millisecond as unit so add a unit (ms) if
+ * there isn't one.
+ */

The comment feels awkward. How about below

SUGGESTION
If no unit was specified, then explicitly add 'ms' otherwise the
interval_in function would assume 'seconds'

~~~

7. - parse_subscription_options

(This is a repeat of [1] review comment #12)

+ if (opts->min_apply_delay < 0 && IsSet(supported_opts,
SUBOPT_MIN_APPLY_DELAY))
+ ereport(ERROR,
+ errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
+ errmsg("option \"%s\" must not be negative", "min_apply_delay"));

Why is this code here instead of inside the previous code block where
the min_apply_delay was assigned in the first place?

======

8. src/backend/replication/logical/worker.c - apply_delay

+ * When min_apply_delay parameter is set on subscriber, we wait long enough to
+ * make sure a transaction is applied at least that interval behind the
+ * publisher.

"on subscriber" -> "on the subscription"

~~~

9.

+ * Apply delay only after all tablesync workers have reached READY state. A
+ * tablesync worker are kept until it reaches READY state. If we allow the

Wording ??

"A tablesync worker are kept until it reaches READY state." ??

~~~

10.

10a.
+ /* nothing to do if no delay set */

Uppercase comment
/* Nothing to do if no delay set */

~

10b.
+ /* set apply delay */

Uppercase comment
/* Set apply delay */

~~~

11. - apply_handle_stream_prepare / apply_handle_stream_commit

The previous concern about incompatibility with the "Parallel Apply"
work (see [1] review comments #17, #18) is still a pending issue,
isn't it?

======

12. src/backend/utils/adt/timestamp.c interval_to_ms

+/*
+ * Given an Interval returns the number of milliseconds.
+ */
+int64
+interval_to_ms(const Interval *interval)

SUGGESTION
Returns the number of milliseconds in the specified Interval.

~~~

13.

+ /* adds portion time (in ms) to the previous result. */

Uppercase comment
/* Adds portion time (in ms) to the previous result. *

======

14. src/bin/pg_dump/pg_dump.c - getSubscriptions

+ {
+ appendPQExpBufferStr(query, " s.suborigin,\n");
+ appendPQExpBufferStr(query, " s.subapplydelay\n");
+ }

This could be done using just a single appendPQExpBufferStr if you
want to have 1 call instead of 2.

======

15. src/bin/psql/describe.c - describeSubscriptions

+ /* origin and min_apply_delay are only supported in v16 and higher */

Uppercase comment
/* Origin and min_apply_delay are only supported in v16 and higher */

======

16. src/include/catalog/pg_subscription.h

+ int64 subapplydelay; /* Replication apply delay */
+

Consider renaming this as 'subapplydelayms' to make the units perfectly clear.

======

17. src/test/regress/sql/subscription.sql

Is [1] review comment 21 (There are some test cases for CREATE
SUBSCRIPTION but there are no
test cases for ALTER SUBSCRIPTION changing this new parameter.) still
a pending item?

------
[1] My v4 review -
https://www.postgresql.org/message-id/CAHut%2BPvugkna7avUQLydg602hymc8qMp%3DCRT2ZCTGbi8Bkfv%2BA%40mail.gmail.com
[2] Euler's reply to my v4 review -
https://www.postgresql.org/message-id/acfc1946-a73e-4e9d-86b3-b19cba225a41%40www.fastmail.com

Kind Regards,
Peter Smith.
Fujitsu Australia