Journaling Results

Restate uses an execution log for replay after failures and suspensions. This means that non-deterministic results (e.g. database responses, UUID generation) need to be stored in the execution log. The SDK offers some functionalities to help you with this:

1. Journaled actions: Run any block of code and store the result in Restate. Restate replays the result instead of re-executing the block on retries.
2. Awaitable combinators: Log the order in which Awaitables were resolved/rejected, to ensure deterministic replay.
3. Random generators: Built-in helpers for generating stable UUIDs and random numbers.

Journaled actions

You can store the result of a (non-deterministic) operation in the Restate execution log (e.g. database requests, HTTP calls, etc). Restate replays the result instead of re-executing the operation on retries.

Here is an example of a database request for which the string response is stored in Restate:

Java

String output = ctx.run(JsonSerdes.STRING, () -> doDbRequest());

You can use Restate's built-in CoreSerdes to serialize primitive types. Have a look at the serialization docs for other options.

Kotlin

val output: String = ctx.runBlock { doDbRequest() }

By default, Kotlin serialization is used to serialize the result. Have a look at the serialization docs, for other options.

You cannot use the Restate context within a ctx.run function. This includes actions such as getting state, calling another service, and nesting other journaled actions.

Failure semantics

Failures in ctx.run have the same semantics as failures in the rest of your handler code. By default, the ctx.run function is retried infinitely on failure, unless you throw a terminal error.

Awaitable combinators

Operations such as calls, awakeables, and sleep return a Awaitable. The SDK provides combinators for working with Awaitable. Restate then logs the order in which they are resolved or rejected, to make them deterministic on replay.

Java

Await all creates an Awaitable that awaits for all of the provided Awaitables to resolve. The semantics are similar to CompleteableFuture.allOf(), but the outcome is stored in the Restate journal to be deterministically replayable.

Awaitable.all(a1, a2, a3).await();

Await any creates an Awaitable that awaits any of the provided Awaitables to resolve. The semantics are similar to CompleteableFuture.anyOf(), but the outcome is stored in the Restate journal to be deterministically replayable.

boolean res = (boolean) Awaitable.any(a1, a2, a3).await();

Kotlin

Await all

Creates an Awaitable that awaits for all of the provided Awaitables to resolve. The semantics are similar to CompleteableFuture.allOf(), but the outcome is stored in the Restate journal to be deterministically replayable.

listOf(a1, a2, a3).awaitAll()

Await any

Creates an Awaitable that awaits any of the provided Awaitables to resolve.

The semantics are similar to CompleteableFuture.anyOf(), but the outcome is stored in the Restate journal to be deterministically replayable.

val res = Awaitable.any(a1, a2, a3).await() as Boolean

// Or using the select statement
val resSelect = select {
  a1.onAwait { it }
  a2.onAwait { it }
  a3.onAwait { it }
}

Generating randoms

The SDK provides helper functions for the deterministic generation of UUIDs and random numbers. Restate seeds the random number generator with the invocation ID, so it always returns the same value on retries.

Generating UUIDs

You can use these UUIDs to generate stable idempotency keys, to deduplicate operations. For example, you can use this to let a payment service avoid duplicate payments during retries.

Do not use this in cryptographic contexts.

Java

UUID uuid = ctx.random().nextUUID();

Kotlin

val uuid: UUID = ctx.random().nextUUID()

Generating random numbers

This returns a new pseudorandom float within the range [0,1]. This is the equivalent of Java's Math.random() but deterministically replayable.

You can use any of the methods of java.util.Random to generate random numbers: for example, nextBoolean(), nextLong(), nextFloat(), etc.

Java

int value = ctx.random().nextInt();

Kotlin

val value: Int = ctx.random().nextInt()