Docker Runtime Error Analysis

Analyze running PHP containers for runtime failures and provide targeted diagnosis and fixes.

Runtime Error Categories

Error	Symptom	Root Cause
502 Bad Gateway	Nginx returns 502	PHP-FPM not running or crashed
OOM Killed	Container restarts	memory_limit or container limit exceeded
Connection refused	Service unreachable	Container not ready or wrong network
Permission denied	File operation fails	UID/GID mismatch or read-only fs
Segmentation fault	Process crashes	Extension bug or memory corruption
Slow requests	Timeouts, 504 errors	PHP-FPM pool exhaustion

Detection Patterns

1. 502 Bad Gateway (PHP-FPM Not Running)

# FIX: Add healthcheck and proper depends_on
services:
  php-fpm:
    healthcheck:
      test: ["CMD-SHELL", "php-fpm-healthcheck || exit 1"]
      interval: 10s
      timeout: 5s
      retries: 3
  nginx:
    depends_on:
      php-fpm:
        condition: service_healthy

2. OOM Killed

; php.ini -- match memory_limit to container limit
; Container limit 256MB -> PHP memory_limit should be lower
memory_limit = 128M

# docker-compose.yml memory constraints
services:
  php-fpm:
    deploy:
      resources:
        limits:
          memory: 512M
        reservations:
          memory: 256M

3. Connection Refused

# FIX: Proper service dependencies and networking
services:
  php-fpm:
    depends_on:
      redis: { condition: service_healthy }
      database: { condition: service_healthy }
    networks: [app-network]
  redis:
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
    networks: [app-network]
  database:
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U $$POSTGRES_USER"]
    networks: [app-network]
networks:
  app-network:
    driver: bridge

4. Permission Denied (File Ownership)

# FIX: Set correct ownership in Dockerfile
RUN mkdir -p /var/www/var/cache /var/www/var/log \
    && chown -R www-data:www-data /var/www/var
COPY --chown=www-data:www-data . /var/www
USER www-data

5. PHP-FPM Pool Exhaustion

; Sizing formula: max_children = (available_memory - overhead) / avg_process_memory
; Example: (512MB - 64MB) / 32MB = ~14
[www]
pm = dynamic
pm.max_children = 14
pm.start_servers = 4
pm.min_spare_servers = 2
pm.max_spare_servers = 6
pm.max_requests = 500

6. Segmentation Fault

; Mitigation: disable problematic extensions one by one
; Common cause: OPcache + Xdebug conflict
opcache.enable=0  ; Test if OPcache causes segfault
opcache.fast_shutdown=0

Grep Patterns

# Find PHP-FPM configuration
Grep: "pm\.(max_children|start_servers|max_requests)" --glob "**/www.conf" --glob "**/php-fpm*.conf"

# Find memory_limit settings
Grep: "memory_limit" --glob "**/php.ini" --glob "**/*.ini"

# Find healthcheck definitions
Grep: "healthcheck" --glob "**/docker-compose*.yml"

# Find depends_on without condition
Grep: "depends_on:" --glob "**/docker-compose*.yml"

# Find error logging configuration
Grep: "error_log|log_errors" --glob "**/php.ini" --glob "**/*.ini"

PHP-FPM Specific Diagnostics

Log Pattern	Diagnosis	Fix
server reached pm.max_children	Pool exhaustion	Increase max_children or optimize code
child exited on signal 11	Segmentation fault	Check extensions, disable OPcache
child exited with code 255	Fatal PHP error	Check error log for details
execution timed out	Slow script	Profile code, check DB queries
unable to open primary script	Wrong document root	Fix Nginx fastcgi_param SCRIPT_FILENAME

Severity Classification

Pattern	Severity	Impact
OOM Killed (recurring)	Critical	Service unavailable, data loss risk
PHP-FPM pool exhaustion	Critical	All requests blocked
Segmentation fault	Critical	Process crash, intermittent failures
502 Bad Gateway	Major	Service unavailable
Connection refused	Major	Dependent service failures
Permission denied (runtime)	Major	Feature degradation
Slow requests (< threshold)	Minor	User experience impact

Output Format

### Runtime Error: [Category]

**Severity:** Critical/Major/Minor
**Container:** `<service_name>`
**Log Pattern:** `<error message from logs>`

**Diagnosis:**
[Root cause analysis with evidence from logs]

**Steps to Verify:**
1. [Command to confirm the issue]
2. [Command to check related state]

**Fix:**
```yaml
# docker-compose.yml or config change

Monitoring: [How to detect this issue early in production]