golem-configure-durability-moonbit

name	golem-configure-durability-moonbit
description	Choosing between durable and ephemeral agents in a MoonBit Golem project. Use when the user asks to change durability mode, switch between durable and ephemeral, or configure persistence behavior.

Configuring Agent Durability (MoonBit)

Durable Agents (Default)

By default, all Golem agents are durable:

• State persists across invocations, failures, and restarts
• Every side effect is recorded in an oplog (operation log)
• On failure, the agent is transparently recovered by replaying the oplog
• No special code needed — durability is automatic

You cannot opt out of oplog writes for a durable agent. The oplog is how durability works — every side effect must be recorded. If you are worried about oplog volume or replay cost (long-running agents, heartbeats, polling, recurring tasks), do not try to skip persistence. Use durable with periodic snapshots instead (see below).

A standard durable agent:

#derive.agent
struct CounterAgent {
  name: String
  mut count: UInt
}

fn CounterAgent::new(name: String) -> CounterAgent {
  { name, count: 0 }
}

pub fn CounterAgent::increment(self: Self) -> UInt {
  self.count = self.count + 1
  self.count
}

pub fn CounterAgent::get_count(self: Self) -> UInt {
  self.count
}

Durable with Periodic Snapshots

Same durability guarantees as the default durable mode, but recovery starts from the latest snapshot instead of replaying the full oplog from the beginning. Use this whenever the oplog grows unboundedly — long-running agents, high-frequency state changes, heartbeats, polling loops, recurring tasks.

#derive.agent(snapshotting="every_n(10)")    // snapshot every 10 successful calls
struct CounterAgent { ... }

See golem-custom-snapshot-moonbit for snapshotting modes and the Snapshottable trait.

Ephemeral Agents

Use ephemeral mode for stateless, per-invocation agents where persistence is not needed:

• State is discarded after each invocation completes
• No oplog replay — lower overhead (an oplog is still recorded lazily for debugging via golem agent oplog, but never replayed)
• Each invocation calls new() to create a fresh instance, executes the method, then discards the instance
• Useful for pure functions, request handlers, or adapters

#derive.agent("ephemeral")
struct StatelessHandler {
}

fn StatelessHandler::new() -> StatelessHandler {
  {  }
}

pub fn StatelessHandler::handle(self: Self, input: String) -> String {
  "processed: " + input
}

When to Choose Which

Use Case	Mode
Counter, shopping cart, workflow orchestrator	Durable (default)
Stateless request processor, transformer	Ephemeral
Long-running saga or multi-step pipeline	Durable (default)
Pure computation, no side effects worth persisting	Ephemeral
Agent that calls external APIs with at-least-once semantics	Durable (default)
Long-running agent with heartbeats, polling, or recurring tasks	Durable + periodic snapshots
Any durable agent whose oplog grows so large that replay is slow	Durable + periodic snapshots

When in doubt, use the default (durable). Ephemeral mode is an optimization for agents that genuinely don't need persistence. Add periodic snapshots whenever recovery time matters — see golem-custom-snapshot-moonbit.

Switching Between Modes

To make a durable agent ephemeral, change:

#derive.agent
struct MyAgent {

to:

#derive.agent("ephemeral")
struct MyAgent {

To make an ephemeral agent durable, change #derive.agent("ephemeral") back to #derive.agent.

After changing the annotation, rebuild with golem build to regenerate derived files. Never edit generated files — golem_reexports.mbt, golem_agents.mbt, and golem_derive.mbt are auto-generated.