Root cause of a real cutover run's failure: the standby_leader's basebackup
from legacy completed, but its Postgres process then got permanently stuck
in "starting" because legacy had no replication slot reserving WAL — normal
WAL recycling (checkpoint_timeout=300s) deleted the segment needed to
resume streaming during the 8-11 min basebackup window. This also explained
why the cascade replica's own basebackup (which targets the standby_leader)
failed with "database system is starting up" — one root cause, not two.
Fix: created a physical replication slot (standby_leader_slot) on live
production legacy, and added primary_slot_name: standby_leader_slot under
bootstrap.dcs.standby_cluster in both patroni-0 and patroni-1's
SPILO_CONFIGURATION so Patroni pins the slot automatically. Also bumped
wal_keep_size to 4GB on legacy as defense-in-depth.
Full incident detail documented in this file's header for future reference.
Phase 2's cascade-replica bootstrap does its own full basebackup FROM the
new standby_leader (an extra hop beyond Phase 1's legacy-direct copy), so
per operator request its window is extended further than Phase 1's —
from 1200s (20 min, matched to observed 8-11 min legacy-direct timing) to
2100s (35 min), giving more margin for the additional hop. Phase 1's
window is intentionally left unchanged at 1200s since it already has
comfortable headroom against the timing we've actually observed for that
specific bootstrap path.
Two real-run bugs found and fixed:
1. Phase 1 hardcoded patroni-1 as the expected standby_leader. The
bootstrap-race winner is actually nondeterministic (Patroni/etcd lock
race) — the original prod attempt had patroni-0 win it instead, which
the dry run never exercised. Fixed by polling BOTH patroni-0 and
patroni-1 each iteration and capturing whichever wins into
$LEADER_HOST, with the other becoming $REPLICA_HOST. Every later phase
(2,3,4,5,6,8,10) now references $LEADER_HOST/$REPLICA_HOST instead of
hardcoded hostnames.
2. Phase 1's wait window (240s) and Phase 2's (300s) were both far shorter
than the observed real basebackup duration for a ~42GB cluster
(8-11 minutes per prior dry-run/live polling). The role only flips to
standby_leader/replica AFTER the full copy completes, so both timeouts
could fire — and did — while a legitimate basebackup was still
in-progress, triggering a false-negative rollback. Both windows
extended to 1200s (20 min), with per-poll data-dir size logging
(timeout-guarded du -sh) so progress is observable instead of a silent
binary wait.
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