Agents - Version 2.4.17

Agents

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 an ExecutionContext. 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 threads.

Creating Agents

Agents are created by invoking new Agent<ValueType>(value, executionContext) – passing in the Agent's initial value and providing an ExecutionContext to be used for it:

import scala.concurrent.ExecutionContext;
import akka.agent.Agent;
import akka.dispatch.ExecutionContexts;
ExecutionContext ec = ExecutionContexts.global();
Agent<Integer> agent = Agent.create(5, ec);

Reading an Agent's value

Agents can be dereferenced (you can get an Agent's value) by invoking the Agent with get() like this:

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.

You can also get a Future to the Agents value, that will be completed after the currently queued updates have completed:

import scala.concurrent.Future;
Future<Integer> future = agent.future();

See Futures for more information on Futures.

Updating Agents (send & alter)

You update an Agent by sending a function (akka.dispatch.Mapper) 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.dispatch.Mapper;
// send a value, enqueues this change
// of the value of the Agent
agent.send(7);

// send a Mapper, enqueues this change
// to the value of the Agent
agent.send(new Mapper<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.

import akka.dispatch.Mapper;
// sendOff a function
agent.sendOff(longRunningOrBlockingFunction,
              theExecutionContextToExecuteItIn);

All send methods also have a corresponding alter method that returns a Future. See Futures for more information on Futures.

import akka.dispatch.Mapper;
import scala.concurrent.Future;
// alter a value
Future<Integer> f1 = agent.alter(7);

// alter a function (Mapper)
Future<Integer> f2 = agent.alter(new Mapper<Integer, Integer>() {
    public Integer apply(Integer i) {
        return i * 2;
    }
});
import akka.dispatch.Mapper;
import scala.concurrent.Future;
// alterOff a function (Mapper)
Future<Integer> f3 = agent.alterOff(longRunningOrBlockingFunction,
                                    theExecutionContextToExecuteItIn);

Configuration

There are several configuration properties for the agents module, please refer to the reference configuration.

Deprecated Transactional Agents

Agents participating in enclosing STM transaction is a deprecated feature in 2.3.

If an Agent is used within an enclosing Scala STM transaction, then it will participate in that transaction. If you send to an Agent within a transaction then the dispatch to the Agent will be held until that transaction commits, and discarded if the transaction is aborted.

Contents