Limitations v5

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The following limitations apply to the use of commit scopes and the various durability options they enable.

General limitations

  • Replacing a node with its physical standby doesn't work for nodes that use CAMO/Eager/Group Commit. We don't recommend combining physical standbys and EDB Postgres Distributed, even if it's possible.

  • Legacy synchronous replication uses a mechanism for transaction confirmation different from the one used by CAMO, Eager, and Group Commit. The two aren't compatible, so don't use them together. Whenever you use Group Commit, CAMO or Eager, make sure none of the PGD nodes are configured in synchronous_standby_names.

  • Postgres two-phase commit (2PC) transactions (that is, PREPARE TRANSACTION) can't be used with CAMO, Group Commit, or Eager because those features use two-phase commit underneath.

Group Commit

Group Commit enables configurable synchronous commits over nodes in a group. If you use this feature, take the following limitations into account:

  • Not all DDL can run when you use Group Commit. If you use unsupported DDL, a warning is logged, and the transactions commit scope is set to local. The only supported DDL operations are:

    • Nonconcurrent CREATE INDEX
    • Nonconcurrent DROP INDEX
    • Nonconcurrent REINDEX of an individual table or index
    • CLUSTER (of a single relation or index only)
    • ANALYZE
    • TRUNCATE

  • Explicit two-phase commit is not supported by Group Commit as it already uses two-phase commit.

  • Combining different commit decision options in the same transaction or combining different conflict resolution options in the same transaction isn't supported.

  • Currently, Raft commit decisions are extremely slow, producing very low TPS. We recommended using them only with the eager conflict resolution setting to get the Eager All-Node Replication behavior of PGD 4 and older.

Eager

Eager is available through Group Commit. It avoids conflicts by eagerly aborting transactions that might clash. It's subject to the same limitations as Group Commit.

  • Eager doesn't allow the NOTIFY SQL command or the pg_notify() function. It also don't allow LISTEN or UNLISTEN.

CAMO

Commit At Most Once (CAMO) is a feature that aims to prevent applications committing more than once. If you use this feature, take these limitations into account when planning:

  • CAMO is designed to query the results of a recently failed COMMIT on the origin node. In case of disconnection the application must request the transaction status from the CAMO partner. Ensure that you have as little delay as possible after the failure before requesting the status. Applications must not rely on CAMO decisions being stored for longer than 15 minutes.

  • If the application forgets the global identifier assigned, for example, as a result of a restart, there's no easy way to recover it. Therefore, we recommend that applications wait for outstanding transactions to end before shutting down.

  • For the client to apply proper checks, a transaction protected by CAMO can't be a single statement with implicit transaction control. You also can't use CAMO with a transaction-controlling procedure or in a DO block that tries to start or end transactions.

  • CAMO resolves commit status but doesn't resolve pending notifications on commit. CAMO doesn't allow the NOTIFY SQL command or the pg_notify() function. They also don't allow LISTEN or UNLISTEN.

  • When replaying changes, CAMO transactions might detect conflicts just the same as other transactions. If timestamp-conflict detection is used, the CAMO transaction uses the timestamp of the prepare-on-the-origin node, which is before the transaction becomes visible on the origin node itself.

  • CAMO isn't currently compatible with transaction streaming. Be sure to disable transaction streaming when planning to use CAMO. You can configure this option globally or in the PGD node group. See Transaction streaming configuration.

  • CAMO isn't currently compatible with decoding worker. Be sure to not enable decoding worker when planning to use CAMO. You can configure this option in the PGD node group. See Decoding worker disabling.

  • Not all DDL can run when you use CAMO. If you use unsupported DDL, a warning is logged and the transactions commit scope is set to local only. The only supported DDL operations are:

    • Nonconcurrent CREATE INDEX
    • Nonconcurrent DROP INDEX
    • Nonconcurrent REINDEX of an individual table or index
    • CLUSTER (of a single relation or index only)
    • ANALYZE
    • TRUNCATE

  • Explicit two-phase commit is not supported by CAMO as it already uses two-phase commit.

  • You can combine only CAMO transactions with the DEGRADE TO clause for switching to asynchronous operation in case of lowered availability.

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