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Document metrics that help detect when WAL apply lag is increasing #191

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@mtopolnik mtopolnik commented May 26, 2025

Adds three new metrics to the table of all metrics:

  1. counter: questdb_wal_apply_seq_txn_total
  2. counter: questdb_wal_apply_writer_txn_total
  3. gauge: questdb_suspended_tables

TODO: document how to use these metrics:

To track increasing WAL apply lag not caused by suspended tables, monitor the difference questdb_wal_apply_seq_txn_total - questdb_wal_apply_writer_txn_total. If it's persistently growing, it is likely that at least one table is falling seriously behind.

The gauge questdb_suspended_tables is simpler, any value above zero is a sign of trouble.

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@mtopolnik mtopolnik changed the title Add metrics that help detect when WAL apply lag is increasing Document metrics that help detect when WAL apply lag is increasing May 26, 2025
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