Marking Agent Methods as Read-Only (MoonBit)

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 #derive.read_only attribute, placed before the method (the same place as #derive.prompt_hint).

Usage

#derive.agent
struct CounterAgent {
  name: String
  mut count: UInt64
}

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

/// Increments the counter and returns the new value.
pub fn CounterAgent::increment(self: Self) -> UInt64 {
  self.count += 1
  self.count
}

/// Returns the current counter value.
#derive.read_only
pub fn CounterAgent::get_count(self: Self) -> UInt64 {
  self.count
}

Cache Policy

Pass cache=... to choose how long a cached result stays valid. The default is "until_write".

Policy	Attribute	Behavior
Until write (default)	#derive.read_only or #derive.read_only(cache="until_write")	Cached until the next non-read-only invocation on the same agent
TTL	#derive.read_only(cache="ttl", ttl="30000000000")	Expires after the given duration even without a write
No cache	#derive.read_only(cache="no_cache")	Runs every time; still side-effect-free, never cached

For TTL, pass cache="ttl" together with a separate ttl argument giving the duration in nanoseconds (e.g. "30000000000" = 30 seconds).

/// A summary that may be recomputed at most every 30 seconds (30s = 30_000_000_000 ns).
#derive.read_only(cache="ttl", ttl="30000000000")
pub fn CounterAgent::recent_summary(self: Self) -> Summary {
  // ...
}

/// A pure computation that is never cached.
#derive.read_only(cache="no_cache")
pub fn CounterAgent::pure_compute(self: Self, x: UInt, y: UInt) -> UInt {
  x + y
}

Per-Principal Caching (automatic)

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

// Principal-aware: cached per principal (uses_principal auto-derived = true)
#derive.read_only
pub fn CounterAgent::my_visible_items(self: Self, principal: Principal) -> Array[Item] {
  // ...
}

When the method has no Principal parameter, the result is shared across all callers and HTTP responses are Cache-Control: public (CDN-friendly). The Principal parameter 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-moonbit).

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 struct fields	Writing persistent state (storage, databases, …)	Mutating in-memory state (mut fields)
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 #derive.read_only — assigning to a mut field is a plain in-memory write, 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 attribute has no effect. Remove #derive.read_only or make the agent durable (see golem-stateless-agent-moonbit).
• 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 attribute is per-method; an agent can mix read-only and regular methods freely.
• A cache policy can only be expressed via #derive.read_only — non-read-only methods cannot carry one.
• Read-only methods are the natural fit for HTTP GET/HEAD endpoints (load golem-add-http-endpoint-moonbit).
• 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-moonbit).