Agents (Java)

Agents (Java)

Agents in Akka are inspired by agents in Clojure.

Agents provide asynchronous change of individual locations. Agents are bound to a single storage location for their lifetime, and only allow mutation of that location (to a new state) to occur as a result of an action. Update actions are functions that are asynchronously applied to the Agent’s state and whose return value becomes the Agent’s new state. The state of an Agent should be immutable.

While updates to Agents are asynchronous, the state of an Agent is always immediately available for reading by any thread (using get) without any messages.

Agents are reactive. The update actions of all Agents get interleaved amongst threads in a thread pool. At any point in time, at most one send action for each Agent is being executed. Actions dispatched to an agent from another thread will occur in the order they were sent, potentially interleaved with actions dispatched to the same agent from other sources.

If an Agent is used within an enclosing transaction, then it will participate in that transaction. Agents are integrated with the STM - any dispatches made in a transaction are held until that transaction commits, and are discarded if it is retried or aborted.

Creating and stopping Agents

Agents are created by invoking new Agent(value, system) passing in the Agent’s initial value and a reference to the ActorSystem for your application. An ActorSystem is required to create the underlying Actors. See Actor Systems for more information about actor systems.

Here is an example of creating an Agent:

import akka.actor.ActorSystem;
import akka.agent.Agent;
ActorSystem system = ActorSystem.create("app");

Agent<Integer> agent = new Agent<Integer>(5, system);

An Agent will be running until you invoke close on it. Then it will be eligible for garbage collection (unless you hold on to it in some way).

agent.close();

Updating Agents

You update an Agent by sending a function that transforms the current value or by sending just a new value. The Agent will apply the new value or function atomically and asynchronously. The update is done in a fire-forget manner and you are only guaranteed that it will be applied. There is no guarantee of when the update will be applied but dispatches to an Agent from a single thread will occur in order. You apply a value or a function by invoking the send function.

import akka.japi.Function;
// send a value
agent.send(7);

// send a function
agent.send(new Function<Integer, Integer>() {
  public Integer apply(Integer i) {
    return i * 2;
  }
});

You can also dispatch a function to update the internal state but on its own thread. This does not use the reactive thread pool and can be used for long-running or blocking operations. You do this with the sendOff method. Dispatches using either sendOff or send will still be executed in order.

// sendOff a function
agent.sendOff(longRunningOrBlockingFunction);

Reading an Agent’s value

Agents can be dereferenced (you can get an Agent’s value) by calling the get method:

Integer result = agent.get();

Reading an Agent’s current value does not involve any message passing and happens immediately. So while updates to an Agent are asynchronous, reading the state of an Agent is synchronous.

Awaiting an Agent’s value

It is also possible to read the value after all currently queued sends have completed. You can do this with await:

import akka.util.Timeout;
import static java.util.concurrent.TimeUnit.SECONDS;
Integer result = agent.await(new Timeout(5, SECONDS));

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