Observed worker state inconsistencies under Apache mod_proxy_fcgi with connection reuse (PHP-FPM 8.5.6)

Description

Environment

• PHP-FPM: 8.5.6
• Process manager: pm = dynamic
• Typical pool configuration:
  • pm.max_children defined (value respected and stable)
  • pm.start_servers, pm.min_spare_servers, pm.max_spare_servers standard tuning
  • pm.max_requests = 500
• Backend: Apache HTTP Server
• FastCGI handler: mod_proxy_fcgi
• Transport: Unix socket
• Relevant proxy configuration:
  <Proxy "fcgi://handler/" enablereuse=on flushpackets=on max=10>

Additional working configuration (mitigating issue)

The following configuration appears to resolve or significantly reduce the observed behavior:

<Proxy "fcgi://handler/" enablereuse=on flushpackets=auto smax=2 max=12 acquire=1000 ttl=10 timeout=60 connectiontimeout=300ms>

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Observed behavior (problematic configuration)

When using the simplified proxy configuration with connection reuse enabled:

• Workers in PHP-FPM appear to remain in intermediate states such as reading headers longer than expected.
• /fpm-status shows discrepancies in perceived worker activity (notably higher "active" states than expected based on external observation).
• Idle/spare worker management appears delayed or not aligned with expected request completion cycles.
• max_spare_servers behavior appears inconsistent with expected worker availability under idle load conditions.
• The issue is reproducible and deterministic under sustained use of persistent FastCGI connection reuse.

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Observed behavior (tuned configuration)

With explicit tuning of Apache FastCGI proxy pooling parameters:

• Worker state transitions appear consistent and timely.
• Idle/spare worker management behaves as expected.
• No persistent reading headers state anomalies observed.
• /fpm-status metrics appear stable and consistent with external process observation.

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Notes on reproducibility

• Issue is reproducible under Apache mod_proxy_fcgi only with:
  • enablereuse=on
  • minimal or default proxy pool tuning
• Behavior changes significantly when introducing:
  • ttl
  • acquire
  • connectiontimeout
  • constrained pool size (smax, max)

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Hypothesis (not confirmed)

It is not confirmed whether this behavior is caused by:

• PHP-FPM internal worker state machine handling
• Apache mod_proxy_fcgi connection reuse behavior
• Interaction between FastCGI persistent connections and request lifecycle completion signaling

However, the observed behavior suggests a possible desynchronization between:

• actual request completion at PHP execution level
• worker state transition to idle within PHP-FPM
• FastCGI connection reuse lifecycle on the proxy side

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Relation to /fpm-status discrepancies

It is not confirmed whether this issue is directly related to known discrepancies observed in /fpm-status (e.g. inconsistencies in reported active worker counts under certain conditions).

However, both phenomena share a common dependency on:

• worker state transition timing
• FastCGI connection reuse behavior
• request lifecycle completion signaling (reading headers / transition states)

This suggests a possible shared underlying factor, although no direct causality is claimed.

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Important clarification

This report is based on observations from an Apache + PHP-FPM setup. Equivalent behavior under Nginx has not been directly evaluated in this context, although similar FastCGI reuse patterns may be relevant.

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Summary

• Reproducible worker state inconsistencies observed under Apache mod_proxy_fcgi with connection reuse enabled.
• Behavior strongly influenced by proxy connection pooling parameters.
• Mitigation achieved via explicit tuning of reuse, TTL, and connection limits.
• Root cause not confirmed; likely interaction between FastCGI reuse lifecycle and PHP-FPM worker state transitions.
• Possible, but unconfirmed, relation to /fpm-status inconsistencies.

PHP Version

PHP 8.5.7 (cli) (built: Jun 10 2026 20:19:22) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.5.7, Copyright (c) Zend Technologies
    with Zend OPcache v8.5.7, Copyright (c), by Zend Technologies

Operating System

No response

Inizia riproducendo il comportamento con PHP-FPM 8.5.7, Apache mod_proxy_fcgi, socket Unix e le configurazioni minima e ottimizzata descritte qui. Confronta gli stati dei worker in /fpm-status con le osservazioni esterne e stabilisci se la discrepanza risiede nelle transizioni delle richieste di PHP-FPM o nel riutilizzo delle connessioni Apache; il lavoro è completato quando sono disponibili una causa confermata e un test di regressione mirato o una correzione riproducibile.