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Runbook — QuestDB WAL apply stall

Symptoms:

  • questdb_wal_apply_lag_seconds Prometheus metric is above 60s.
  • New dbt model materialization runs hang on INSERT.
  • The QuestDB UI shows WAL applied = N is no longer advancing.

Root cause​

A long-running query or external table lock has blocked the WAL apply worker. The new QuestDB documentation explicitly warns: "Non-partitioned tables cannot use WAL" — the AlphaSwarm custom questdb_table materialization forces PARTITION BY DAY to avoid the most common form, but mis-configured external tables can still trip the apply loop.

Recovery​

  1. Identify the offending table from the Prometheus alert label:

    {table="equities_minute_bars"}

  2. Suspend writers to that table:

    alphaswarm ratelimit admin halt-pool questdb_writer:equities_minute_bars

  3. Resume the WAL apply loop:

    ALTER TABLE equities_minute_bars RESUME WAL;

  4. Once the lag drops back below 5s, lift the writer halt:

    alphaswarm ratelimit admin resume-pool questdb_writer:equities_minute_bars

Prevention​

The Phase 2 alphaswarm/dagster/dagster.yaml reserves a per-table questdb_writer:<table> pool with limit=1 so concurrent writers to the same table are impossible. Verify that pool is present + has limit=1.