RE: synchronized_standby_slots behavior inconsistent with quorum-based synchronous replication

On Wednesday, March 18, 2026 6:38 PM Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com> wrote:
> PFA patch that addresses above comments.

Thanks for the patch! Overall, I think this is a valuable feature as I've heard
requests from many customers for a way to avoid blocking logical replication
when only some instances are down when using synchronized_standby_slots.

I didn't find any bugs in the patch, but I have a few comments on the code and
tests:

1.

> /*
> * Return true if value starts with explicit FIRST syntax:
> *
> * FIRST N (...)
> *
> * This is used to distinguish explicit FIRST from simple list syntax whose
> * first slot name may start with "first".
> */
> static bool
> IsPrioritySyncStandbySlotsSyntax(const char *value)

I think adding a new function to manually parse the list isn't the most elegant
approach. Instead, it would be cleaner to have a new flag that distinguishes
when a plain name list is specified, and use that to mark the case
appropriately like:

/* syncrep_method of SyncRepConfigData */
#define SYNC_REP_PRIORITY 0
#define SYNC_REP_QUORUM 1
+#define SYNC_REP_IMPLICIT 2

standby_config:
- standby_list { $$ = create_syncrep_config("1", $1, SYNC_REP_PRIORITY); }
+ standby_list { $$ = create_syncrep_config("1", $1, SYNC_REP_IMPLICIT); }

2.

+ /*
+ * Allocate array to track slot states. Size it to the total number of
+ * configured slots since in the worst case all could have problem states.
+ */
+ slot_states = (SyncStandbySlotsStateInfo *)
+ palloc(sizeof(SyncStandbySlotsStateInfo) * synchronized_standby_slots_config->nslotnames);

I personally prefer building the list incrementally with List * and lappend
rather than pre-allocating, since the list may often be empty in success cases.

3.

+<synopsis>
+[FIRST] <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="parameter">slot_name</replaceable> [, ...] )
+ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="parameter">slot_name</replaceable> [, ...] )
+<replaceable class="parameter">slot_name</replaceable> [, ...]
+</synopsis>

The documentation mentions that the FIRST keyword is optional, but the code
and comments don't seem consistent. Some comments give the impression that
FIRST is required:

+ * FIRST N (slot1, slot2, ...) -- wait for first N in priority order

Additionally, IsPrioritySyncStandbySlotsSyntax() only checks for the FIRST
keyword and doesn't handle the "N (slot1, slot2)" case where FIRST is omitted.

4.

Regarding the new tests, I think we can avoid testing the plain slot list
since that's already covered extensively in 040_standby_failover_slots_sync.pl.

Instead, we could focus on testing edge cases for ANY and FIRST. I noticed
there are no tests for scenarios where logical decoding blocks when using ANY or
FIRST. For example, testing FIRST 1 (s1, s2) where s1 is alive but hasn't caught
up would be valuable (if possible to test in tap-test, maybe test via
recovery_min_apply_delay ?). This logic is more error-prone and required more
careful review than other cases.

BTW, Part E seems unnecessary since the patch reuses the sync_standby_names
parser. ISTM, testing the first_ prefix in this context doesn't add valuable
coverage.

Best Regards,
Hou zj