Re: POC: enable logical decoding when wal_level = 'replica' without a server restart

Hi Sawada-San,

Here are some more review comments for v18.

(WIP)

======
src/backend/access/transam/xact.c

MarkCurrentTransactionIdLoggedIfAny:

1.
- * This returns true if wal_level >= logical and we are inside a valid
- * subtransaction, for which the assignment was not yet written to any WAL
- * record.
+ * This returns true if effective WAL level is logical and we are inside
+ * a valid subtransaction, for which the assignment was not yet written to
+ * any WAL record.

Maybe it is better to always substitute 'wal_level' with
'effective_wal_level' for consistency?

~~~

2.
- /* wal_level has to be logical */
+ /* effective WAL level has to be logical */

ditto: maybe just say: "effective_wal_level has to be logical".

~~~

3.
+ * When effective WAL level is logical, guarantee that a subtransaction's
+ * xid can only be seen in the WAL stream if its toplevel xid has been
+ * logged before. If necessary we log an xact_assignment record with fewer

ditto: maybe say "When effective_wal_level is logical..."

======
src/backend/access/transam/xlog.c

4.
+const char *
+show_effective_wal_level(void)
+{
+ char *str;
+
+ if (wal_level == WAL_LEVEL_MINIMAL)
+ return "minimal";
+
+ /*
+ * During the recovery, we need to check the shared status instead of the
+ * local XLogLogicalInfo cache as we don't synchronously update it during
+ * the recovery.
+ */
+ if (RecoveryInProgress())
+ return IsXLogLogicalInfoEnabled() ? "logical" : "replica";
+
+ if (wal_level == WAL_LEVEL_REPLICA)
+ {
+ /*
+ * With wal_level='replica', XLogLogicalInfo indicates the actual WAL
+ * level.
+ */
+ if (IsXLogLogicalInfoEnabled())
+ str = "logical";
+ else
+ str = "replica";
+ }
+ else
+ str = "logical";
+
+ return str;
+}

It would be simpler and have more consistent ternary usage to have
direct returns in all places instead of just half or them. Then remove
the 'str' var:

SUGGESTION
...
if (wal_level == WAL_LEVEL_MINIMAL)
return "minimal";
...
if (RecoveryInProgress())
return IsXLogLogicalInfoEnabled() ? "logical" : "replica";
...
if (wal_level == WAL_LEVEL_REPLICA)
return IsXLogLogicalInfoEnabled() ? "logical" : "replica";
...
return "logical";

======
src/backend/commands/publicationcmds.c

5.
- if (wal_level != WAL_LEVEL_LOGICAL)
+ if (!IsLogicalDecodingEnabled())
ereport(WARNING,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
- errmsg("\"wal_level\" is insufficient to publish logical changes"),
- errhint("Set \"wal_level\" to \"logical\" before creating subscriptions.")));
+ errmsg("logical decoding should be allowed to publish logical changes"),
+ errhint("Before creating subscriptions, set \"wal_level\" >=
\"logical\" or create a logical replication slot when \"wal_level\" =
\"replica\".")));

Why not just say errmsg "\"effective_wal_level\" is insufficient to
publish logical changes"

======
src/backend/commands/tablecmds.c

6.
- /* should only get here if wal_level >= logical */
+ /* should only get here if effective WAL level is 'logical' */
Assert(XLogLogicalInfoActive());
ditto from earlier: say "should only get here if effective_wal_level is logical"

======
src/backend/postmaster/checkpointer.c

7.
+ /*
+ * Disable logical decoding if someone requested to disable logical
+ * decoding. See comments atop logicalctl.c.
+ */
+ DisableLogicalDecodingIfNecessary();
+

The comment seems a bit repetitive.

SUGGESTION
Disable logical decoding if someone requested it. See comments atop
logicalctl.c.

======
src/backend/replication/logical/decode.c

8.
- errmsg("logical decoding on standby requires \"wal_level\" >=
\"logical\" on the primary")));
+ errmsg("logical decoding on standby requires \"effective_wal_level\"
>= \"logical\" on the primary")));

Is that >= really necessary?

Why not say: "... requires \"effective_wal_level\" to be \"logical\" on the .."

======
src/backend/replication/logical/logical.c

9.
+ errmsg("logical decoding on standby requires \"effective_wal_level\"
>= \"logical\" on the primary"),
+ errhint("Set \"wal_level\" >= \"logical\" or create at least one
logical slot when \"wal_level\" = \"replica\".")));

ditto above. Those >= seem to make it more complex than necessary.

errmsg can be: "...requires \"effective_wal_level\" to be \"logical\"
on the ..."
errhint can be: "Set \"wal_level\" = \"logical\" or..."

======
src/backend/replication/logical/slotsync.c

10.
ereport(elevel,
errcode(ERRCODE_INVALID_PARAMETER_VALUE),
- errmsg("replication slot synchronization requires \"wal_level\" >=
\"logical\""));
+ errmsg("replication slot synchronization requires
\"effective_wal_level\" >= \"logical\" on the primary"),
+ errhint("To enable logical decoding on primary, set \"wal_level\" >=
\"logical\" or create at least one logical slot when \"wal_level\" =
\"replica\"."));

ditto above. Those >= seem to make it more complex than necessary.

errmsg can be: "...requires \"effective_wal_level\" to be \"logical\"
on the ..."
errhint can be: "...set \"wal_level\" = \"logical\" or..."

======
src/backend/replication/slot.c

ReplicationSlotCleanup:

11.
+ if (SlotIsLogical(s))
+ nlogicalslots++;
+

How about assign SlotIsLogical(s) to a var like other places, instead
of calling it 2x?

~~

ReportSlotInvalidation:

12.
case RS_INVAL_WAL_LEVEL:
- appendStringInfoString(&err_detail, _("Logical decoding on standby
requires \"wal_level\" >= \"logical\" on the primary server."));
+ appendStringInfoString(&err_detail, _("Logical decoding on standby
requires the primary server to have either \"wal_level\" >=
\"logical\" or at least one logical slot created."));

12a.
ditto previous comments about the >=

~

12b.
This error message does not seem to be as good as previous ones. e.g.
I think it should be mentioning about" replica", because creating
slots with "minimal" is no good, right?

~~~

InvalidateObsoleteReplicationSlots:

13.
- * - RS_INVAL_WAL_LEVEL: is logical and wal_level is insufficient
+ * - RS_INVAL_WAL_LEVEL: is logical and logical decoding is disabled due to
+ * insufficient WAL level or no logical slot presence.

Maybe it is just easier to say: "...is logical and effective_wal_level
is not logical"

======
src/backend/storage/ipc/standby.c

14.
- if (wal_level < WAL_LEVEL_LOGICAL)
+ if (!IsLogicalDecodingEnabled())
LWLockRelease(ProcArrayLock);

recptr = LogCurrentRunningXacts(running);

/* Release lock if we kept it longer ... */
- if (wal_level >= WAL_LEVEL_LOGICAL)
+ if (IsLogicalDecodingEnabled())
LWLockRelease(ProcArrayLock);

~

Would it be better to assign IsLogicalDecodingEnabled() to a var here
instead of 2x calls?

======
src/backend/utils/cache/inval.c

15.
+ * When effective WAL level is 'logical', write invalidations into WAL at
+ * each command end to support the decoding of the in-progress transactions.
+ * See CommandEndInvalidationMessages.

ditto some previous review comment: Say "when effective_wal_level is
logical, write..."

======
Kind Regards,
Peter Smith.
Fujitsu Australia