How would you size a HikariCP connection pool for a Spring Boot service?

Sizes per-instance database concurrency from workload and database capacity instead of equating more connections with more throughput.

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I size the pool from the database's safe connection and work capacity, divided across application replicas and other clients, then validate it under realistic load. The pool should support expected concurrent database operations, not total HTTP requests. A small bounded pool often performs better than a large one because excessive active queries contend for database CPU, locks, memory, and I/O.

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HikariCP pool limits, replica multiplication, database capacity, transaction duration, queueing, timeouts, and measurement.

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A connection pool limits concurrent database work; increasing it can move queueing into the database and make the whole system slower.
I size the pool from the database's safe connection and work capacity, divided across application replicas and other clients, then validate it under realistic load. The pool should support expected concurrent database operations, not total HTTP requests. A small bounded pool often performs better than a large one because excessive active queries contend for database CPU, locks, memory, and I/O.
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I look at transaction duration, number of database calls per request, peak concurrency, and the acceptable wait before acquisition times out. Scaling from five to twenty replicas multiplies the possible connections even if each instance keeps the same pool size. I keep transactions short and never hold a connection while waiting on a remote API. Metrics for active, idle, pending, acquisition time, timeouts, query latency, and database utilization show whether the pool or the database is the constraint.

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Setting the pool equal to expected request concurrency.
Ignoring the number of application replicas.
Increasing the pool before measuring database saturation.

How would you size a HikariCP connection pool for a Spring Boot service?

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