Historian writer

Optional separate process. The runtime rotates the spool into ready/; the writer drains those files into durable storage.

Env

Variable	Meaning
VION_SPOOL_DIR	Required. Same spool root as the runtime
VION_HISTORIAN	columnar or postgres
VION_HISTORIAN_DIR	Columnar VIONCOL1 root
VION_HISTORIAN_DATABASE_URL	Historian PostgreSQL (not the engineering DB)
VION_HISTORIAN_RETENTION_DAYS	Drop older samples (source_ts)
VION_HISTORIAN_RETENTION_SECS	Same window in seconds (tests; secs wins if both set)

Embedded path

If runtime has VION_HISTORIAN_DIR, it flushes its own spool to columnar files. Do not run both embedded flush and historian-writer on the same spool.

History reads: runtime GET /api/v1/tags/{id}/history. With VION_HISTORIAN_DATABASE_URL the runtime reads PG (it does not append).