# Resilience and Performance — Spring Boot

Source: https://www.skillbyai.com/en/spring-boot/s-resilience

> Timeouts, pools, virtual threads.

## Fail fast and use resources well

Set **timeouts** on every outbound HTTP and database call; use retries with backoff only for safe, idempotent operations, and circuit breakers (for example Resilience4j) for flaky dependencies. Size connection pools (HikariCP) deliberately. On Java 21+, enabling **virtual threads** (`spring.threads.virtual.enabled=true`) lets blocking request handling scale to many concurrent requests with simple code. Load-test before launch and watch pool and thread metrics.

## Resilience settings (sketch)

Not part of the built demo; check the Spring documentation for your version.

```properties
spring.threads.virtual.enabled=true
spring.datasource.hikari.maximum-pool-size=20
spring.datasource.hikari.connection-timeout=3000
spring.mvc.async.request-timeout=10s
# outbound HTTP clients: always configure connect and read timeouts
```

## Timeouts everywhere

A single call without a timeout can hold threads and connections until the whole service stalls.

**Quiz:** What do virtual threads (Java 21+) help with?

- [ ] Removing the need for a database
- [ ] Reducing JSON size
- [ ] Encrypting data
- [x] Handling many concurrent blocking requests with simple code

*Answer:* Handling many concurrent blocking requests with simple code. Cheap threads for blocking I/O.