golem-mark-read-only-scala

name	golem-mark-read-only-scala
description	Marking Scala agent methods as read-only for a side-effect-free guarantee and result caching. Use when the user wants a cacheable query method, a method that must not write to the oplog, or HTTP GET endpoints that emit cache headers.

Marking Agent Methods as Read-Only (Scala)

Overview

A read-only agent method is one you promise is a pure read of the agent's already-loaded state: it must not mutate anything and its result must depend only on its inputs and the current state. Golem enforces the most important part of this contract — writes to persistent state, outgoing HTTP, and RPC calls trap at runtime with a ReadOnlyViolation agent error before they run — but it does not detect every source of impurity (in-memory mutation, clocks, randomness, env reads), so keeping the method pure is partly your responsibility (see What Works in a Read-Only Method). In exchange Golem:

• Caches the result per (method, normalized input, optional principal) on the worker.
• Bypasses the invocation queue on a cache hit — a read-only call returns immediately even while a slow write is being processed.
• Bypasses agent loading on a cache hit — a cached value is served even if the agent is currently evicted.
• Emits HTTP cache headers (Cache-Control / ETag / Vary) for read-only methods mapped to GET/HEAD.

Mark a method read-only with the @readOnly annotation.

Usage

import golem.runtime.annotations.{agentDefinition, readOnly}
import golem.BaseAgent

import scala.concurrent.Future

@agentDefinition()
trait CounterAgent extends BaseAgent {
  class Id(val name: String)

  // Non-read-only: writes shared state
  def increment(): Future[Long]

  // Read-only: pure read over already-loaded state
  @readOnly
  def getCount(): Future[Long]
}

Cache Policy

Pass cache = "..." (a string) to choose how long a cached result stays valid. The default is "until-write".

Policy	Annotation	Behavior
Until write (default)	@readOnly or @readOnly(cache = "until-write")	Cached until the next non-read-only invocation on the same agent
TTL	@readOnly(cache = "ttl(30 seconds)")	Expires after the given duration even without a write
No cache	@readOnly(cache = "no-cache")	Runs every time; still side-effect-free, never cached

The annotation takes a single cache: String argument. For TTL use the "ttl(<duration>)" form, where the duration parses as a Scala Duration (e.g. "ttl(30 seconds)", "ttl(500 millis)", "ttl(1 minute)").

import golem.runtime.annotations.readOnly

@readOnly(cache = "ttl(30 seconds)")
def recentSummary(): Future[Summary]

@readOnly(cache = "no-cache")
def pureCompute(x: Int, y: Int): Future[Int]

Per-Principal Caching (automatic)

There is no usesPrincipal argument on the annotation. Whether the cache is per-principal is derived automatically by the SDK from the method signature: if the method receives a Principal, the result is cached per principal and HTTP responses switch to Cache-Control: private with Vary: Authorization.

import golem.Principal

// Principal-aware: cached per principal (usesPrincipal auto-derived = true)
@readOnly
def myVisibleItems(principal: Principal): Future[List[Item]]

When the method does not receive a Principal, the result is shared across all callers and HTTP responses are Cache-Control: public (CDN-friendly). The Principal is auto-injected by the runtime — it is not part of the method's input schema and is not passed by callers (see golem-add-http-auth-scala).

What Works in a Read-Only Method

A read-only method must be a pure function of the agent's already-loaded state and the method inputs. The operations in the middle column go through Golem's durability layer and trap with a ReadOnlyViolation agent error before they run and before anything is persisted. The operations in the right column are not detected — they do not trap, but they still break the cache contract and must be avoided by you.

Allowed	Not allowed — traps with ReadOnlyViolation	Not allowed — not checked, your responsibility
Reading instance fields	Writing persistent state (storage, databases, …)	Mutating in-memory state
Computation over inputs	Outgoing HTTP requests	Reading the clock / current time
Returning derived values	RPC calls to other agents	Randomness
		Reading environment variables
		Remote / blob reads

Common Pitfalls

• Mutating state, reading a clock, randomness, or env in a read-only method is NOT detected. These do not trap — but they either mutate state that should be immutable here or make the result non-deterministic, which corrupts the cache. The runtime cannot catch them; keeping the method pure is your responsibility. If you need any of them, use a regular (non-read-only) method instead.
• Writes to persistent state, outgoing HTTP, and RPC do trap. Those go through the durability layer and raise ReadOnlyViolation before running.
• A method that mutates state must not be @readOnly — an in-memory mutation is not a host call, so it does not trap; nothing stops you at runtime, and keeping the method mutation-free is your responsibility.
• Read-only on an ephemeral agent fails to compile — ephemeral agents have no shared state to read, so the annotation has no effect. Remove @readOnly or make the agent durable (see golem-stateless-agent-scala).
• no-cache does not relax the contract — it only disables caching; writes to persistent state / HTTP / RPC still trap, and the same purity rules apply.

Key Points

• The annotation is per-method; an agent can mix read-only and regular methods freely.
• A cache policy can only be expressed via @readOnly — non-read-only methods cannot carry one.
• Read-only methods are the natural fit for HTTP GET/HEAD endpoints (load golem-add-http-endpoint-scala).
• A read-only method cannot call another agent via RPC; do read-only RPC fan-out from a regular method instead (see golem-call-another-agent-scala).