Non-Deterministic Values (sideEffect)

Non-deterministic values and durable execution

Durable execution replays a task from the start after an interruption, such as a restart or rolling deploy. Context calls that already completed return their cached results instead of re-running. A bare Date.now() or crypto.randomUUID() is not a context call, so it produces a different value on every replay and desynchronizes the run from its cached history.

ctx.sideEffect() runs your callback once and caches the returned value like any other context call. On replay, it returns the cached value and the callback does not run again.

sideEffect<T>(fn: () => T | Promise<T>): Promise<T>

Example

Generate a request id and timestamp once, then reuse them in a writeContract call:

import { TaskContext } from "compose";

export async function main(ctx: TaskContext) {
  // Computed once; the cached values are returned on any replay.
  const { requestId, timestamp } = await ctx.sideEffect(async () => ({
    requestId: crypto.randomUUID(),
    timestamp: Date.now(),
  }));

  const wallet = await ctx.evm.wallet({ name: "default" });
  // requestId and timestamp are passed in the contract call args
  await wallet.writeContract(
    ctx.evm.chains.base,
    ctx.env.CONTRACT_ADDRESS as `0x${string}`,
    "recordRequest(string,uint256)",
    [requestId, timestamp]
  );
}

Use ctx.sideEffect() for any value that must stay the same across replays: timestamps, random values, UUIDs, or any other source of non-determinism.

How context functions, caching, and retries work.

Compose

Guides

Reference

Non-deterministic values and durable execution

Example

Next Steps

callTask

Context overview

​Non-deterministic values and durable execution

​Example

​Next Steps

callTask

Context overview

Non-deterministic values and durable execution

Example

Next Steps