golem-atomic-block-scala

name	golem-atomic-block-scala
description	Using atomic blocks, persistence control, idempotency, checkpoints, and oplog management in a Scala Golem project. Use when the user asks about atomically, persistence levels, idempotence mode, oplog commit, checkpoints, or idempotency keys.

Atomic Blocks and Durability Controls (Scala)

Overview

Golem provides automatic durable execution — all agents are durable by default. These APIs are advanced controls that most agents will never need. Only use them when you have specific requirements around persistence granularity, idempotency, or atomicity.

All guard and checkpoint APIs are Future-based — blocks must return Future[A].

Atomic Operations

Group external, observable side effects (HTTP calls, calls to other agents, file/network I/O) so that on a crash the whole group is replayed together. If the agent fails partway through the block, recovery will re-execute the entire block from the start instead of resuming from the middle — so any external effects performed before the crash will be performed again.

What this is NOT. atomically is not an STM/transaction primitive and not for grouping in-memory state mutations. Golem agents are single-threaded, and in-memory state is automatically rebuilt by oplog replay on recovery, so wrapping plain in-memory updates in atomically does nothing useful. The terminology overlaps with Haskell STM, Scala STM, database transactions, and synchronized blocks, but the semantics are different: this is purely about how durable, externally-observable effects are re-executed across a crash boundary.

It is also NOT how you reduce oplog size or speed up recovery. Despite the description's mention of "oplog management", "persistence control", and "checkpoints", atomically/persistence-level/idempotency-mode/checkpoint APIs do not shrink the oplog or skip replay. If your concern is that the oplog is growing too large or recovery/replay is becoming slow (long-running agents, heartbeats, polling, recurring tasks), use snapshot-based recovery instead — see golem-custom-snapshot-scala. You cannot opt out of oplog writes for a durable agent.

Use it only when you have two or more external side effects that must not be left in a "first one happened, second one didn't" state across a recovery.

Good use case — two external calls that must replay together:

import golem.Guards
import scala.concurrent.Future

// Reserve inventory and charge the customer — if we crash between them,
// we want recovery to re-run BOTH calls, not skip the reservation.
val order: Future[Order] = Guards.atomically {
  for {
    reservation <- inventoryApi.reserve(itemId, qty)
    charge      <- paymentApi.charge(customer, price)
  } yield Order(reservation, charge)
}

Bad use case — pure in-memory updates that already replay deterministically:

// DON'T do this. Wrapping in-memory mutations adds nothing — the oplog
// already rebuilds `state.balance` and `state.lastTx` deterministically.
Guards.atomically {
  state.balance -= amount
  state.lastTx = now
  Future.successful(())
}

Persistence Level Control

Adjust how the oplog is interpreted for a section of code. Setting the level to PersistNothing does not disable oplog recording — entries are still written, but they are treated only as an observable log and are not used for replay. On recovery, the side effects are not re-executed and not replayed; if the block naively runs the same side effects during replay, recovery will fail.

This is not a knob for application code. Its primary use case is authoring Golem-specific libraries that implement their own custom durability on top of raw side effects. Code inside such a block must:

1. Explicitly check whether the agent is in live or replay mode (via the durability API).
2. Skip the raw side effects during replay.
3. Use the durability APIs to record/recover state in a custom way.

import golem.{Guards, HostApi}
import scala.concurrent.Future

val result: Future[Unit] = Guards.withPersistenceLevel(HostApi.PersistenceLevel.PersistNothing) {
  // Oplog entries here are observable only, never used for replay.
  // The block MUST check live vs replay mode and use custom durability
  // primitives — naively running side effects will break recovery.
  Future.successful(())
}

Idempotence Mode

Default: true. Every outgoing HTTP request — including POST, PUT, PATCH, and DELETE — is treated as idempotent. This means status-code-keyed retry policies (see golem-retry-policies-scala) already work out of the box for POST requests. You do not need to wrap a POST in Guards.withIdempotenceMode(true) { ... } to make it retriable on a 5xx — that is the default.

Use Guards.withIdempotenceMode(false) { ... } only when you need to opt out for a specific call. The flag controls how WriteRemote host functions are replayed when their previous attempt's outcome is unknown after a crash:

• true (default): assume the previous attempt succeeded; do not re-invoke on replay. Combined with the host-side retry machinery, the request can be transparently re-sent when a matching retry policy fires.
• false: do not assume success; the worker traps so a higher-level retry decides what to do. Use this for non-idempotent side effects whose accidental duplication would be more harmful than missing the call entirely.

import golem.Guards
import scala.concurrent.Future

// Opt OUT of the default — the wrapped call is treated as non-idempotent.
val result: Future[Unit] = Guards.withIdempotenceMode(false) {
  // HTTP requests will not be automatically retried on uncertain outcomes
  Future.successful(())
}

Oplog Commit

Wait until the oplog is replicated to a specified number of replicas before continuing:

import golem.HostApi

// Ensure oplog is replicated to 3 replicas before proceeding
HostApi.oplogCommit(3)

Idempotency Key Generation

Generate a durable idempotency key that persists across agent restarts — safe for payment APIs and other exactly-once operations:

import golem.HostApi

val key = HostApi.generateIdempotencyKey()
// Use this key with external APIs to ensure exactly-once processing

Retry Policy

Override the default retry policy for a block of code:

import golem.Guards
import scala.concurrent.Future

Guards.withRetryPolicy(policy) {
  // Code with custom retry behavior
  Future.successful(())
}

Checkpoints

Capture an oplog position and revert execution to it on failure:

import golem.Checkpoint
import scala.concurrent.Future

// Create a checkpoint and use it manually
val cp = Checkpoint()
cp.assertOrRevert(condition)       // revert if false

// tryOrRevert — revert if the Future fails
val result: Future[Int] = cp.tryOrRevert {
  Future.successful(42)
}

// runOrRevert — revert if the Future resolves to a Left
val result: Future[Int] = cp.runOrRevert {
  Future.successful(Right(42))
}

Scoped checkpoints

import golem.Checkpoint
import scala.concurrent.Future

// withCheckpointTry — revert if the Future fails
val result: Future[Int] = Checkpoint.withCheckpointTry { cp =>
  cp.assertOrRevert(someCondition)
  Future.successful(42)
}

// withCheckpoint — revert if the Future resolves to a Left
val result: Future[Int] = Checkpoint.withCheckpoint { cp =>
  Future.successful(Right(42))
}

Resource-style Guards

For manual control, use the use* / markAtomicOperation methods which return a guard. Call drop() or close() when done:

import golem.Guards

val guard = Guards.usePersistenceLevel(HostApi.PersistenceLevel.PersistNothing)
// ... do work ...
guard.drop()