Scalingo Managed PostgreSQL and Application-Level Connection Pooling Interoperability

Answer

The recommended per‑container pool size is:

pool_per_container = floor( (effective_max_connections - reserved) / replica_count ) * safety_factor

Where:

effective_max_connections is the max_connections value reported by Scalingo’s PgBouncer (or PostgreSQL if bypassing PgBouncer).
reserved is a small number of connections kept for administrative tasks (typically 5).
replica_count is the expected number of running containers at peak scale.
safety_factor is a value < 1 (e.g., 0.8) to absorb bursts and avoid hitting the limit.

Confirmed facts from Scalingo’s managed offering

Scalingo provisions a PgBouncer instance between your application and the PostgreSQL database.
The connection limit enforced is the PgBouncer max_connections; exceeding it yields FATAL: too many connections.
At least a few connections should be reserved for platform maintenance and super‑user access.

Likely explanation – when external pooler overhead may not be worth it
Under the following load conditions the added latency and complexity of an external pooler (PgBouncer) can outweigh its concurrency benefits:

Low overall concurrent demand: total required connections (pool_per_container × replica_count) stays well below the effective limit, so the platform’s native connection handling is sufficient.
Latency‑sensitive workloads where every extra hop adds measurable overhead (e.g., sub‑millisecond OLTP).
Heavy use of prepared statements combined with PgBouncer in transaction mode, which discards session‑state on each release, forcing re‑prepare and increasing CPU usage.
Burst‑y traffic with short‑lived connections where the pool‑proxy’s recycle logic adds unnecessary churn.

Steps to apply the recommendation (only what’s needed for this case)

Obtain the current PgBouncer max_connections value via the Scalingo CLI or dashboard (scalingo --app my-app pgbouncer-info or equivalent).
Subtract the reserved connections (e.g., 5) to get the usable pool.
Divide by the expected maximum replica count (the highest number of containers you plan to run).
Apply a safety factor (0.8‑0.9) to accommodate traffic spikes.
Set the resulting integer as maximumPoolSize (HikariCP) or equivalent in your application’s pool configuration.

Missing diagnostic detail: If you cannot confirm the effective max_connections after PgBouncer (e.g., the platform hides it), please provide that value; the calculation changes only if the limit differs from the default PostgreSQL setting.