Group Commit

The goal of Group Commit is to protect against data loss in case of single node failures or temporary outages. You achieve this by requiring more than one PGD node to successfully receive and confirm a transaction at COMMIT time.

Requirements

During normal operation, Group Commit is transparent to the application. Transactions that were in progress during failover need the reconciliation phase triggered or consolidated by either the application or a proxy in between. This activity currently happens only when either the origin node recovers or when it’s parted from the cluster. This behavior is the same as with Postgres legacy built-in synchronous replication.

Transactions committed with Group Commit use two-phase commit underneath. Therefore, configure max_prepared_transactions high enough to handle all such transactions originating per node.

Configuration

To use Group Commit, first define a Migration to commit scopes . The commit scope determines the PGD nodes involved in the commit of a transaction.

Behavior

The behavior of Group Commit depends on the configuration applied by the commit scope.

Commit decisions

You can configure Group Commit to decide commits in three different ways: group , partner , and raft .

Group Commit decision is done through the consensus specified using the same commit scope group settings used for the durability purposes. The difference is that the commit decision is made based on PREPARE replication while the durability checks COMMIT (PREPARED) replication.

The partner decision is what Commit At Most Once uses. This approach works only when there are two data nodes in the node group. These two nodes are partners of each other, and the replica rather than origin decides whether to commit something. This approach requires application changes to use the CAMO transaction protocol to work correctly, as the application is in some way part of the consensus. For more on this approach, see pgbench with CAMO/failover options .

The raft option uses the built-in Raft consensus to decide whether commit can happen. Currently, the global Raft is used. For this to work, the majority of nodes across the whole cluster must work.

Conflict resolution

Conflict resolution can be async or eager .

Async means that PGD does optimistic conflict resolution during replication using the row-level resolution as configured for given node. This happens regardless of whether the origin transaction committed or is still in progress. See Handling column conflicts using CRDT data types for details about how the asynchronous conflict resolution works.

Eager means that conflicts are resolved eagerly (as part of agreement on COMMIT), and conflicting transactions get aborted with a serialization error. This approach provides greater isolation than the asynchronous resolution at the price of performance. For the details about how Eager conflict resolution works, see Eager conflict resolution .

Aborts

To prevent a transaction that can’t get consensus on the COMMIT from hanging forever, the ABORT ON clause allows specifying timeout. After the timeout, the transaction abort is requested. If the transaction is already decided to be committed at the time the abort request is sent, the transaction does eventually COMMIT even though the client might receive an abort message.

Limitations

Group Commit transactions can’t yet execute DDL, and they don’t support explicit two-phase commit. Future releases might allow them. However, the TRUNCATE command is allowed.

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

Eager and CAMO transactions aren’t currently supported in combination with the decoding worker feature or with transaction streaming. Installations using Eager must keep enable_wal_decoder and streaming_mode disabled for the PGD node group.

Synchronous replication uses a mechanism for transaction confirmation different from Group Commit. The two aren’t compatible, and you must not use them together. Therefore, whenever you Group Commit transactions, make sure none of the PGD nodes are configured in synchronous_standby_names .

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.

Combining different commit decision options in the same transaction isn’t supported. Combining different conflict resolution options in the same transaction also isn’t supported.