Creating Ephemeral (Stateless) Agents (Scala)

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 fresh constructor call — 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

Pass mode = DurabilityMode.Ephemeral to the @agentDefinition annotation:

import golem.runtime.annotations.{agentDefinition, agentImplementation, DurabilityMode}
import golem.BaseAgent

import scala.concurrent.Future

@agentDefinition(mode = DurabilityMode.Ephemeral)
trait RequestHandler extends BaseAgent {
  def handle(input: String): Future[String]
}

@agentImplementation()
final class RequestHandlerImpl extends RequestHandler {
  override def handle(input: String): Future[String] =
    Future.successful(s"processed: $input")
}

What "Fresh Instance Per Invocation" Means

Consider a durable agent vs an ephemeral one:

// DURABLE (default) — state accumulates across calls
@agentDefinition()
trait DurableCounter extends BaseAgent {
  class Id(val name: String)
  def increment(): Future[Int]
}

@agentImplementation()
final class DurableCounterImpl(name: String) extends DurableCounter {
  private var count: Int = 0
  override def increment(): Future[Int] = Future.successful {
    count += 1
    count
  }
}
// Call increment() three times → returns 1, 2, 3

// EPHEMERAL — state resets every call
@agentDefinition(mode = DurabilityMode.Ephemeral)
trait EphemeralCounter extends BaseAgent {
  class Id(val name: String)
  def increment(): Future[Int]
}

@agentImplementation()
final class EphemeralCounterImpl(name: String) extends EphemeralCounter {
  private var count: Int = 0
  override def increment(): Future[Int] = Future.successful {
    count += 1
    count
  }
}
// Call increment() three times → returns 1, 1, 1

Each invocation of an ephemeral agent:

1. Creates a fresh instance via the constructor
2. Executes the method
3. Discards the instance entirely

Combining with HTTP Endpoints

Ephemeral agents are a natural fit for HTTP request handlers:

import golem.runtime.annotations._
import golem.BaseAgent

import scala.concurrent.Future

@agentDefinition(mode = DurabilityMode.Ephemeral, mount = "/api/convert/{name}")
trait ConverterAgent extends BaseAgent {
  class Id(val name: String)

  @post("/to-upper")
  def toUpper(@body input: String): Future[String]

  @post("/to-lower")
  def toLower(@body input: String): Future[String]
}

@agentImplementation()
final class ConverterAgentImpl(private val name: String) extends ConverterAgent {
  override def toUpper(input: String): Future[String] =
    Future.successful(input.toUpperCase)

  override def toLower(input: String): Future[String] =
    Future.successful(input.toLowerCase)
}

When to Use Ephemeral Agents

Use Case	Why Ephemeral?
Stateless HTTP API (REST adapter, proxy)	No state to persist between requests
Data transformation / format conversion	Pure function — input in, output out
Validation service	Each validation is independent
Webhook receiver that forwards events	No need to remember previous webhooks
Stateless computation (math, encoding)	No side effects worth persisting

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