Content

Getting Started

akka-serial uses sbt as build system. To get started, include a dependency to akka-serial in your project:

libraryDependencies += "ch.jodersky" %% "akka-serial-core" % "4.1.2"

Next, you need to include akka-serial’s native library that supports communication for serial devices.

Including Native Library

There are two options to include the native library:

  1. Using a pre-packaged dependency, available only for certain OSes but easily included.

  2. Including the library manually for maximum portability.

It is recommended that you use the first option for testing purposes or end-user applications. The second option is recomended for libraries, since it leaves more choice to the end-user.

The Easy Way

In case your kernel/architecture combination is present in the “supported platforms” table in the downloads section, add a second dependency to your project:

libraryDependencies += "ch.jodersky" % "akka-serial-native" % "4.1.2" % "runtime"

This will add a jar to your classpath containing native libraries for various platforms. At run-time, the correct library for the current platform is selected, extracted and loaded. This solution enables running applications seamlessly, as if they were pure JVM applications.

Maximum Portability

Start by obtaining a copy of the native library, either by building akka-serial or by downloading a native archive. In order to work with this version of akka-serial, native libraries need to be of major version 1 and minor version greater or equal to 0.

Then, for every end-user application that relies on akka-serial, manually add the native library for the current platform to the JVM’s library path. This can be achieved through various ways, notably:


Communication Protocol

The following is a general guide on the usage of akak-serial. If you prefer a complete example, check out the code contained in the samples directory.

akka-serial’s API follows that of an actor based system, where each actor is assigned specific functions involved in serial communication. The two main actor types are:

  1. Serial “manager”. The manager is a singleton actor that is instantiated once per actor system, a reference to it may be obtained with IO(Serial). It is typically used to open serial ports (see following section).

  2. Serial “operators”. Operators are created once per open serial port and serve as an intermediate between client code and native code dealing with serial data transmission and reception. They isolate the user from threading issues and enable the reactive dispatch of incoming data. A serial operator is said to be “associated” to its underlying open serial port.

The messages understood by akka-serial’s actors are all contained in the akka.serial.Serial object. They are well documented and should serve as the entry point when searching the API documentation.

Opening a Port

A serial port is opened by sending an Open message to the serial manager. The response varies on the outcome of opening the underlying serial port.

  1. In case of failure, the serial manager will respond with a CommandFailed message to the original sender. The message contains details on the reason to why the opening failed.

  2. In case of success, the sender is notified with an Opened message. This message is sent from an operator actor, spawned by the serial manager. It is useful to capture the sender (i.e. the operator) of this message as all further communication with the newly opened port must pass through the operator.

import akka.serial.{ Serial, SerialSettings, AccessDeniedException }

val port = "/dev/ttyXXX"
val settings = SerialSettings(
  baud = 115200,
  characterSize = 8,
  twoStopBits = false,
  parity = Parity.None
)

IO(Serial) ! Serial.Open(port, settings)

def receive = {
  case Serial.CommandFailed(cmd: Serial.Open, reason: AccessDeniedException) =>
    println("You're not allowed to open that port!")
  case Serial.CommandFailed(cmd: Serial.Open, reason) =>
	println("Could not open port for some other reason: " + reason.getMessage)
  case Serial.Opened(settings) => {
    val operator = sender
    //do stuff with the operator, e.g. context become opened(op)
  }
}

Writing Data

Writing data is as simple as sending a Write message to an operator. The data to send is an instance of akka.util.ByteString:

operator ! Serial.Write(data)

Optionally, an acknowledgement for sent data can be requested by adding an ack parameter to a Write message. The ack parameter is of type Int => Serial.Event, i.e. a function that takes the number of actual bytes written and returns an event. Note that “bytes written” refers to bytes enqueued in a kernel buffer; no guarantees can be made on the actual transmission of the data.


case class MyPacketAck(wrote: Int) extends Serial.Event

operator ! Serial.Write(data, MyPacketAck(_))
operator ! Serial.Write(data, n => MyPacketAck(n))

def receive = {
  case MyPacketAck(n) => println("Wrote " + n + " bytes of data")
}

Receiving Data

The actor that opened a serial port (referred to as the client), exclusively receives incomming messages from the operator. These messages are in the form of akka.util.ByteStrings and wrapped in a Received object.

def receive = {
  case Serial.Received(data) => println("Received data: " + data.toString)
}

Closing a Port

A port is closed by sending a Close message to its operator:

operator ! Serial.Close

The operator will close the underlying serial port and respond with a final Closed message before terminating.

Resources and Error Handling

The operator has a deathwatch on the client actor that opened the port, this means that if the latter crashes, the operator closes the port and equally terminates, freeing any allocated resources.

The opposite is not true by default, i.e. if the operator crashes (this can happen for example on IO errors) it dies silently and the client is not informed. Therefore, it is recommended that the client keep a deathwatch on the operator.


Watching Ports

akka-serial can watch directories for new files. On most unix systems this can be used for watching for new serial ports in /dev/. Watching happens through a message-based, publish-subscribe protocol as explained in the sections below.

Subscribing

A client actor may watch – i.e subscribe to notifications on – a directory by sending a Watch command to the serial manager.

Should an error be encountered whilst trying to obtain the watch, the manager will respond with a CommandFailed message. Otherwise, the client may be considered “subscribed” to the directory and the serial manager will thenceforth notify the client on new files.

IO(Serial) ! Serial.Watch("/dev/")

def receive = {
  case Serial.CommandFailed(w: Watch, reason) =>
    println(s"Cannot obtain a watch on ${w.directory}: ${reason.getMessage}")
}

Notifications

Whilst subscribed to a directory, a client actor is informed of any new files in said directory by receiving Connected messages from the manager.

def receive = {
  case Serial.Connected(port) if port matches "/dev/ttyUSB\\d+" =>
    // do something with the available port, e.g.
    // IO(Serial) ! Open(port, settings)
}

Unsubscribing

Unsubscribing from events on a directory is done by sending an Unsubscribe message to the serial manager.

IO(Serial) ! Unwatch("/dev/")

Resource Handling

Note that the manager has a deathwatch on every subscribed client. Hence, should a client die, any underlying resources will be freed.


Stream Support

akka-serial provides support for Akka streams and thus can be interfaced with reactive-streams. Support is implemented in a separate module, which needs to be added as a library dependency:

libraryDependencies += "ch.jodersky" %% "akka-serial-stream" % "4.1.2"

The main entry point for serial streaming is akka.serial.stream.Serial. Its API is also well documented and should serve as the starting point when searching documentation on serial streaming.

Opening a Port

Connection is established by materializing a Flow[ByteString, ByteString, Future[Connection]] obtained by calling Serial().open()

val serial: Flow[ByteString, ByteString, Future[Connection]] = Serial().open("/dev/ttyUSB0", settings)

val source: Source[ByteString, _] = // some source
val sink: Sink[ByteString, _] = // some sink

source.viaMat(serial)(Keep.right).toMat(sink)(Keep.left).run() onComplete {
  case Success(connection) => // a serial connection has been established
  case Failure(error) => // connection could not be established due to error
}

The materialized future will be completed with a Success in case the port is opened or a Failure in case an error is encountered whilst opening.

Communication

Any data pushed to the Flow’s inlet will be sent to the serial port and any data received by the port will be emitted by the Flow’s outlet.

Note that backpressure is only available for writing, to add backpressure on the receiving side a higher-level protocol needs to be implemented on top of serial communication.

Closing a Port

The underlying serial port is closed when its materialized serial flow is closed.

Errors and Resource Handling

Any errors described in akka-serial-core can also be encountered in akka-serial-stream. When thrown, they will be wrapped as the cause of a StreamSerialException and cause the the serial Flow stage to fail.

As with akka-serial-core, native resources are handled by underlying Akka mechanisms and any crashes in user code will automatically case the resources to be freed.