Replication and sequence tuning v6.3.1

Geo-distributed clusters replicate changes across high-latency network links, which makes replication throughput and sequence uniqueness more important than in single-location setups. Configure these settings at the node group level to control how changes are applied on remote nodes and how sequences generate unique values across locations.

Parallel Apply

Parallel Apply allows each node to apply incoming changes using multiple writer processes concurrently rather than sequentially, reducing apply lag under high write volumes. It's enabled by default with bdr.writers_per_subscription set to two writers per node. Use num_writers at the node group level to set the number of writers per subgroup, within the limit set by bdr.max_writers_per_subscription.

1. Increase throughput for your geo-distributed cluster by modifying the number of num_writers per location subgroup. Higher values improve throughput but increase memory usage. Repeat for each location subgroup.

   → WrapCopy

   SELECT bdr.alter_node_group_option(
       node_group_name := '<location_a>',
       config_key := 'num_writers',
       config_value := '4'
   );

2. Set streaming_mode to writer so that large transactions are sent directly to a writer process rather than being written to disk first, reducing I/O overhead on the receiving node:

   → WrapCopy

   SELECT bdr.alter_node_group_option(
       node_group_name := '<location_a>',
       config_key := 'streaming_mode',
       config_value := 'writer'
   );

   The default is auto, which lets PGD choose between file and writer based on available resources. Setting writer explicitly is recommended for geo-distributed clusters with consistently high write volumes. The trade-off is additional resource usage. In writer mode, each large transaction requires a dedicated writer process. In file mode, PGD writes to temporary disk files before applying.

   streaming_mode = 'writer' is incompatible with CAMO. If your cluster uses CAMO, leave streaming_mode at its default auto.

Apply delay

Use apply_delay on subscriber-only subgroups to create a lagging read replica, giving you a recovery window against accidental data loss or logical corruption. The delay is measured from the transaction's original commit time at the origin. The default is 0s (no delay).

SELECT bdr.alter_node_group_option(
    node_group_name := '<delayed_read_replica_group>',
    config_key := 'apply_delay',
    config_value := '30min'
);

Sequences

Standard Postgres sequences are local to each node and cause primary key conflicts when multiple locations write simultaneously. Configure your geo-distributed cluster to use PGD global sequences, which guarantee uniqueness across all nodes. The bdr.default_sequence_kind configuration parameter controls which type applies to newly created sequences and serial columns:

• snowflakeid — generates unique values without inter-node communication, using a timestamp-based algorithm. Works only for 64-bit bigint or bigserial columns. Resilient to network partitions and cross-region latency.
• galloc — allocates ranges of values from a cluster-wide counter, requiring quorum when a new range is needed. Supports smallint, integer, and bigint, but is sensitive to quorum availability.
• distributed (default) — automatically uses snowflakeid for bigint sequences and galloc for smallint and integer sequences.

For geo-distributed cluster, we recommend using the default distributed setting. It avoids the quorum dependency of galloc for large sequences where possible, while still supporting smaller integer types.

See Sequences for full details on global sequence types and behavior.