Intermittent Doctrine DBAL Connection Pool Exhaustion in Shopware 6

Diagnosing intermittent connection pool exhaustion

The goal is to determine why a Shopware 6 installation occasionally fails with 502/503 errors under high load, suspected to be caused by Doctrine DBAL’s limited connection pool.

Key constraints:

• Default pool size is 5 connections per connection type.
• Persistent connections are disabled by default but can be enabled via config/packages/doctrine.yaml.
• Pool size does not automatically scale with multiple PHP‑FPM workers or Docker containers.
• Shopware provides no built‑in per‑environment pool configuration.

Unresolved decisions or behaviors:

• Whether enabling persistent connections will reduce exhaustion or introduce stale data.
• How to determine the optimal pool size for a given traffic profile.
• Whether the current pool configuration scales correctly across containerized deployments.

Specific questions to guide investigation:

1. What is the maximum number of concurrent PHP processes that a 5‑connection pool can support without hitting exhaustion?
2. Does enabling persistent connections in a shared hosting environment lead to stale data or other side effects?
3. How does the pool size behave when running multiple PHP‑FPM workers or Docker containers—does each worker maintain its own pool or share a global pool?