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docs: clarify cleaning PROMETHEUS_MULTIPROC_DIR #1167

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16 changes: 16 additions & 0 deletions docs/content/multiprocess/_index.md
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Expand Up @@ -32,6 +32,22 @@ The `PROMETHEUS_MULTIPROC_DIR` environment variable must be set to a directory
that the client library can use for metrics. This directory must be wiped
between process/Gunicorn runs (before startup is recommended).

## Cleaning `PROMETHEUS_MULTIPROC_DIR`

The multiprocess directory is not cleaned automatically. If it is not wiped before startup, stale files from a previous run can lead to incorrect metrics.

A common pattern is to wipe and recreate the directory in your process manager/entrypoint script **before** starting Gunicorn/workers:

```bash
rm -rf "$PROMETHEUS_MULTIPROC_DIR"
mkdir -p "$PROMETHEUS_MULTIPROC_DIR"
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@csmarchbanks csmarchbanks Apr 23, 2026

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Suggested change
rm -rf "$PROMETHEUS_MULTIPROC_DIR"
mkdir -p "$PROMETHEUS_MULTIPROC_DIR"
rm "$PROMETHEUS_MULTIPROC_DIR/*.db"

This might be a bit safer, and one less thing to do. I also wonder if the example should be in python rather than asking people to run bash before starting python?

```

Notes:
- Only wipe a directory that is dedicated to multiprocess metric files.
- In containerized deployments, `PROMETHEUS_MULTIPROC_DIR` is often set to a dedicated path such as `/tmp/prometheus_multiproc_dir`.
- If you run multiple independent apps on the same machine, each should use a separate multiprocess directory.

This environment variable should be set from a start-up shell script,
and not directly from Python (otherwise it may not propagate to child processes).

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