Creating Ephemeral (Stateless) Agents (MoonBit)

Overview

An ephemeral agent is a Golem agent that gets a fresh instance for every invocation. Unlike the default durable agents, ephemeral agents:

• No shared state: Each invocation starts from a clean new() — field values set in one call are gone by the next
• No replay: An oplog is still recorded lazily (useful for debugging via golem agent oplog), but it is never used for replay — no automatic recovery on failure
• No persistence: The agent’s memory is discarded after each invocation completes
• Same identity model: The agent is still addressed by its constructor parameters, but every call behaves as if the agent was just created

This makes ephemeral agents ideal for pure request handlers, stateless transformers, adapters, and serverless-style functions where each call is independent.

How to Create an Ephemeral Agent

Use #derive.agent("ephemeral") instead of #derive.agent:

#derive.agent("ephemeral")
struct RequestHandler {
}
 
fn RequestHandler::new() -> RequestHandler {
  {  }
}
 
pub fn RequestHandler::handle(self: Self, input: String) -> String {
  "processed: " + input
}

What “Fresh Instance Per Invocation” Means

Consider a durable agent vs an ephemeral one:

// DURABLE (default) — state accumulates across calls
#derive.agent
struct DurableCounter {
  mut count: UInt
}
 
fn DurableCounter::new() -> DurableCounter {
  { count: 0 }
}
 
pub fn DurableCounter::increment(self: Self) -> UInt {
  self.count = self.count + 1
  self.count
}
 
// Call increment() three times → returns 1, 2, 3
 
// EPHEMERAL — state resets every call
#derive.agent("ephemeral")
struct EphemeralCounter {
  mut count: UInt
}
 
fn EphemeralCounter::new() -> EphemeralCounter {
  { count: 0 }
}
 
pub fn EphemeralCounter::increment(self: Self) -> UInt {
  self.count = self.count + 1
  self.count
}
 
// Call increment() three times → returns 1, 1, 1

Each invocation of an ephemeral agent:

1. Calls new() to create a fresh instance
2. Executes the method
3. Discards the instance entirely

When to Use Ephemeral Agents

When NOT to Use Ephemeral Agents

• Counters, accumulators, shopping carts — need state across calls → use durable (default)
• Workflow orchestrators, sagas — need oplog for recovery → use durable (default)
• Agents calling external APIs where at-least-once semantics matter → use durable (default)
• Any agent where one call’s result depends on a previous call → use durable (default)

Key Points

• Ephemeral mode is set at the agent type level — all instances of the type are ephemeral
• Constructor parameters still define identity — you can have multiple ephemeral agents with different parameters
• Ephemeral agents can still call other agents via RPC, make HTTP requests, and use all Golem APIs
• The oplog is still recorded lazily, so you can inspect what an ephemeral agent did via golem agent oplog — but it is never replayed
• Never edit generated files — golem_reexports.mbt, golem_agents.mbt, and golem_derive.mbt are auto-generated by golem build