An enterprise maintains a primary Cloud SQL for PostgreSQL database in us-central1 and a cross-region read replica in europe-west1 for regional reporting and disaster recovery readiness. During heavy transaction processing periods, operations teams observe growing replication delay. A review of Cloud Monitoring telemetry reveals: Primary and replica CPU utilization levels remain healthy at 55% and 40%, respectively. The database/replication/network_lag metric is elevated near the 25-second reporting threshold. The database/postgres/replication/replica_byte_lag metric with replica_lag_type: sent_location displays a steadily accumulating byte backlog. Which configuration change should you implement on the primary database to reduce network lag and improve cross-region replication throughput?

Enable and configure the wal_compression database flag on the primary instance: ### What WAL compression is **WAL compression** (`wal_compression`) is a PostgreSQL database configuration flag that instructs the primary database engine to compress full page images written to the **Write Ahead Log (WAL)** using algorithms such as `lz4`, `zstd`, or `pglz` before writing them to disk and transmitting them across the network. ### How it meets the requirements - **Reduces cross-region network payload**: Compressing full-page writes within WAL segments substantially shrinks the raw number of bytes transmitted across high-latency cross-region network links from `us-central1` to `europe-west1`. - **Resolves `sent_location` byte backlog**: Because the `replica_byte_lag` metric specifically tracks unsent WAL data at `sent_location`, reducing the physical byte volume allows the WAL sender process to transmit logs much faster over constrained bandwidth. - **Maintains primary stability**: With primary CPU utilization safely at 55%, the primary instance has sufficient compute overhead to absorb the minimal CPU cost of compression algorithms like LZ4 or Zstandard without degrading production query throughput. ### Key advantages and capabilities - **Bandwidth optimization**: Drastically minimizes egress traffic volume over inter-region VPC paths. - **Reduced disk I/O**: Decreases the volume of WAL data flushed to local persistent storage on both primary and replica nodes. - **Transparent recovery**: The read replica automatically decompresses WAL records during log replay without requiring custom application changes. - **Targeted bottleneck remediation**: Directly addresses network transfer bottlenecks without requiring intrusive schema modifications. ### Why this is optimal Alternative solutions such as scaling up hardware or altering client concurrency do not fix the underlying bottleneck caused by large, uncompressed WAL streams traveling over inter-region boundaries. Enabling `wal_compression` strikes the best balance between available CPU resources and network transmission efficiency.

Cloud Sql Postgres Wal Compression Network Lag | professional-cloud-data-engineer

Cloud Sql Postgres Wal Compression Network Lag

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An enterprise maintains a primary Cloud SQL for PostgreSQL database in us-central1 and a cross-region read replica in europe-west1 for regional reporting and disaster recovery readiness. During heavy transaction processing periods, operations teams observe growing replication delay.

A review of Cloud Monitoring telemetry reveals:

Primary and replica CPU utilization levels remain healthy at 55% and 40%, respectively.
The database/replication/network_lag metric is elevated near the 25-second reporting threshold.
The database/postgres/replication/replica_byte_lag metric with replica_lag_type: sent_location displays a steadily accumulating byte backlog.

Which configuration change should you implement on the primary database to reduce network lag and improve cross-region replication throughput?

Increase the threshold of the cloudsql.googleapis.com/database/replication/network_lag alerting metric beyond 25 seconds

Enable and configure the wal_compression database flag on the primary instance

Increase the max_connections database flag on the primary instance and the replica instance

Deploy an intermediate read replica in us-central1 and configure cascading replication to europe-west1

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Cloud Sql Postgres Wal Compression Network Lag

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