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Java CompletableFuture

What it is

java.util.concurrent.CompletableFuture is the standard Java async composition primitive, introduced in Java 8. It represents a future result that can be composed (thenApply, thenCompose, thenCombine), chained (supplyAsync(...).thenApply(...)), and terminated on deadline (orTimeout, completeOnTimeOut).

Why it shows up in timeout guidance

Zalando's timeouts post cites CompletableFuture.orTimeout as the standard-library implementation of the outer time- limiter wrap for chained calls:

"In Java, you could use CompletableFuture and several methods among which are orTimeout and completeOnTimeOut that provide built-in support for dealing with timeouts."

CompletableFuture
    .supplyAsync(orderService.placeOrder(...))
    .thenApply(paymentService.updateBalance(...))
    .orTimeout(1, TimeUnit.SECONDS);

(Source: sources/2023-07-25-zalando-all-you-need-to-know-about-timeouts)

This lets a Java service implement the chained-call time- limiter pattern without pulling in an external library — though Resilience4j's TimeLimiter module offers a more composable shape once other resilience primitives (circuit breaker, bulkhead, retry) are also in play.

The two timeout methods

• orTimeout(long timeout, TimeUnit unit) — if the future has not completed by the deadline, it completes exceptionally with a TimeoutException. This is the "request timeout" shape.
• completeOnTimeOut(T value, long timeout, TimeUnit unit) — if the future has not completed by the deadline, it completes normally with the supplied fallback value. This is the "request timeout + fallback" shape.

Seen in

• sources/2023-07-25-zalando-all-you-need-to-know-about-timeouts — canonical Zalando citation as the JDK-native implementation of the chained-call time-limiter pattern.

Related

• concepts/request-timeout — what orTimeout implements at the language-standard-library level.
• patterns/time-limiter-wrapping-chained-calls — the pattern orTimeout realises.
• patterns/explicit-timeout-on-remote-calls — the house- style rule this enables without external dependencies.
• systems/resilience4j — the library-level alternative with more composable primitives.