Toggle logical decoding dynamically based on logical slot presence. Previously logical decoding required wal_level to be set to 'logical' at server start. This meant that users had to incur the overhead of logical-level WAL logging even when no logical replication slots were in use.
This commit adds functionality to automatically control logical decoding availability based on logical replication slot presence. The newly introduced module logicalctl.c allows logical decoding to be dynamically activated when needed when wal_level is set to 'replica'. When the first logical replication slot is created, the system automatically increases the effective WAL level to maintain logical-level WAL records. Conversely, after the last logical slot is dropped or invalidated, it decreases back to 'replica' WAL level. While activation occurs synchronously right after creating the first logical slot, deactivation happens asynchronously through the checkpointer process. This design avoids a race condition at the end of recovery; a concurrent deactivation could happen while the startup process enables logical decoding at the end of recovery, but WAL writes are still not permitted until recovery fully completes. The checkpointer will handle it after recovery is done. Asynchronous deactivation also avoids excessive toggling of the logical decoding status in workloads that repeatedly create and drop a single logical slot. On the other hand, this lazy approach can delay changes to effective_wal_level and the disabling logical decoding, especially when the checkpointer is busy with other tasks. We chose this lazy approach in all deactivation paths to keep the implementation simple, even though laziness is strictly required only for end-of-recovery cases. Future work might address this limitation either by using a dedicated worker instead of the checkpointer, or by implementing synchronous waiting during slot drops if workloads are significantly affected by the lazy deactivation of logical decoding. The effective WAL level, determined internally by XLogLogicalInfo, is allowed to change within a transaction until an XID is assigned. Once an XID is assigned, the value becomes fixed for the remainder of the transaction. This behavior ensures that the logging mode remains consistent within a writing transaction, similar to the behavior of GUC parameters. A new read-only GUC parameter effective_wal_level is introduced to monitor the actual WAL level in effect. This parameter reflects the current operational WAL level, which may differ from the configured wal_level setting. Bump PG_CONTROL_VERSION as it adds a new field to CheckPoint struct. Reviewed-by: Shveta Malik <[email protected]> Reviewed-by: Amit Kapila <[email protected]> Reviewed-by: Hayato Kuroda <[email protected]> Reviewed-by: Bertrand Drouvot <[email protected]> Reviewed-by: Peter Smith <[email protected]> Reviewed-by: Shlok Kyal <[email protected]> Reviewed-by: Ashutosh Bapat <[email protected]> Discussion: https://postgr.es/m/cad21aocvlelyq09pqpaws+jwdni5fuj8v2jgq-u9_ufbcp6...@mail.gmail.com Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/67c20979ce72b8c236622e5603f9775968ff501c Modified Files -------------- doc/src/sgml/config.sgml | 43 ++ doc/src/sgml/logical-replication.sgml | 4 +- doc/src/sgml/logicaldecoding.sgml | 45 +- doc/src/sgml/ref/pg_createsubscriber.sgml | 12 +- doc/src/sgml/system-views.sgml | 5 +- src/backend/access/heap/heapam.c | 8 +- src/backend/access/rmgrdesc/xlogdesc.c | 13 +- src/backend/access/transam/xact.c | 27 +- src/backend/access/transam/xlog.c | 105 +++- src/backend/commands/publicationcmds.c | 11 +- src/backend/commands/tablecmds.c | 2 +- src/backend/postmaster/checkpointer.c | 19 + src/backend/postmaster/postmaster.c | 4 +- src/backend/replication/logical/Makefile | 1 + src/backend/replication/logical/decode.c | 41 +- src/backend/replication/logical/logical.c | 29 +- src/backend/replication/logical/logicalctl.c | 639 +++++++++++++++++++++ src/backend/replication/logical/meson.build | 1 + src/backend/replication/logical/slotsync.c | 8 +- src/backend/replication/slot.c | 186 +++++- src/backend/replication/slotfuncs.c | 7 + src/backend/replication/walsender.c | 7 + src/backend/storage/ipc/ipci.c | 2 + src/backend/storage/ipc/procsignal.c | 3 + src/backend/storage/ipc/standby.c | 7 +- src/backend/utils/activity/wait_event_names.txt | 1 + src/backend/utils/cache/inval.c | 8 +- src/backend/utils/init/postinit.c | 3 + src/backend/utils/misc/guc_parameters.dat | 9 + src/backend/utils/misc/guc_tables.c | 1 + src/bin/pg_basebackup/pg_createsubscriber.c | 6 +- src/bin/pg_basebackup/t/040_pg_createsubscriber.pl | 2 - src/bin/pg_upgrade/check.c | 6 +- src/bin/pg_upgrade/t/002_pg_upgrade.pl | 4 + src/include/access/xlog.h | 17 +- src/include/catalog/pg_control.h | 4 +- src/include/replication/logicalctl.h | 33 ++ src/include/replication/slot.h | 1 + src/include/storage/lwlocklist.h | 1 + src/include/storage/procsignal.h | 2 + src/include/utils/guc_hooks.h | 1 + src/test/recovery/meson.build | 1 + .../recovery/t/035_standby_logical_decoding.pl | 5 +- src/test/recovery/t/051_effective_wal_level.pl | 404 +++++++++++++ src/test/regress/expected/publication.out | 4 - src/test/subscription/t/001_rep_changes.pl | 2 +- src/tools/pgindent/typedefs.list | 1 + 47 files changed, 1599 insertions(+), 146 deletions(-)
