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WalShadow is a native replication engine for ClickHouse Managed Postgres that reads directly from the PostgreSQL Write-Ahead Log (WAL). It is optimized for lower latency and high-throughput workloads, delivering sub-second replication lag from Postgres to ClickHouse. It is developed in the open at ClickHouse/walshadow. WalShadow is one of two replication methods available under Sync to ClickHouse. The other, ClickPipes, is a fully managed data pipeline. Choose WalShadow when you need the lowest latency and the least replication overhead. When WalShadow is available for your service, a Private Preview banner appears on the Overview page. Click Configure WalShadow to get started.
Before you proceed, ensure that your Postgres service is accessible to ClickHouse. WalShadow connects to your destination ClickHouse service directly, so you will need a ClickHouse user and password with permission to create databases and tables.
To set up replication, click Sync to ClickHouse in the sidebar of your Postgres instance, then click Replicate data in ClickHouse.
1

Select the replication engine

Choose WalShadow as the replication engine, then click Next.
2

Configure the replication service

Fill in the replication settings:
  • ClickHouse service: Select an existing ClickHouse service or create a new one
  • Postgres database: Select the source database to replicate from, or create a new one
  • Replication method: Choose one of:
    • Initial load + CDC: Import existing data and keep tables updated with new changes (recommended)
    • Initial load only: One-time import of existing data with no ongoing updates
    • CDC only: Skip the initial import and only capture new changes going forward
Click Next to proceed.
3

Select tables to replicate

Choose a destination database and select which tables to replicate:
  • Destination database: Select an existing ClickHouse database or create a new one
  • Tables: Expand each schema and select individual tables, or toggle Select all tables. For each table you can customize the Destination table name and adjust Column settings.
  • ClickHouse user and ClickHouse password: WalShadow connects to the destination ClickHouse service directly, so it needs credentials with permission to create the target database and tables.
Click Replicate data to ClickHouse to start the replication.
4

Monitor WalShadow

After you start replication, WalShadow appears on the Sync to ClickHouse page. Its status is Creating while the pipeline is being provisioned and the initial load is prepared.Once provisioning completes, the status changes to Running. The page reports live metrics for the replication:
  • Status: The current state of the pipeline (for example, Running)
  • Rows synced: The total number of rows replicated to ClickHouse
  • Backfills pending: The number of tables still waiting for their initial load
  • Lag: The volume of WAL data still to be applied
  • Lag time: How far behind Postgres the destination currently is
  • Uptime: How long the pipeline has been running
  • Last updated: When the metrics were last reported

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Caveats

A few things to note while WalShadow is in Private Preview:
  • Only one WalShadow per Postgres service for now.
  • Operations and more detailed metrics are on the roadmap.
Last modified on September 10, 2026