Inter-Executive Communication

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19 May 2015

This chapter describes how multiple interacting PLEXIL executives can be realized.


In many cases, it is useful to have multiple PLEXIL executives work together to execute a plan. For instance, in a distributed environment with many sub-systems, it is more efficient to have PLEXIL plans and executives on each sub-system than to have a single system containing all plans, which may rely on the network to send out low-level commands. By distributing plans and executives among sub-systems, the amount of network traffic can be limited to high-level communication and the system becomes more easily maintainable.

The PLEXIL executive can communicate with another executive via messages, commands, and lookups. An executive can be set up to receive messages, commands, and lookups via the interface configuration and the extended plexil constructs described below.

The communication framework utilizes Carnegie Mellon University's Inter-Process Communication (IPC) package (See plexil/third-party/ipc, for more information and the latest distribution of the IPC package).

Outgoing Communication

All communication save command return values are performed through a publish/subscribe model. That is, all communication that is sent to the central communication server is sent to every other executive connected to that server. Because of this, it is important that no two executives expect to handle the same command or lookup, as these are, by definition, intended for one and only one agent. Having two executives handling the same command or lookup results in undefined behavior.

Messages are published to all other executives via the SendMessage command, and commands and lookups are published via the normal methods of communicating with an external system. This allows executives to emulate external systems and act as simulators. These simulators can then be swapped out for a real application or more complex simulator with no change to the controlling plan. See the example section for an example plan that communicates with another executive.

Incoming Communication

When a message or command is received from an external executive, the action along with its parameters are stored in an internal queue. When an 'OnCommand', or 'OnMessage' node transitions to EXECUTING, it will immediately process the oldest matching action in the internal queue. If there are no matching actions in the internal queue, the command waits until one arrives. When a single action has been processed, the node completes.

Lookups are handled differently. The IpcAdapter holds a table of lookup names and values. When a request for a lookup is received, the adapter automatically returns the current value of that lookup.

When the parent node of an OnCommand or OnMessage node transitions from EXECUTING to any other state and the child node is in the EXECUTING state, the child node transitions to the FAILING state, canceling the request to handle new incoming actions.

OnCommand and OnMessage

For detailed semantics of the OnCommand and OnMessage Extended PLEXIL constructs used for receiving commands and messages, see the Plexil Reference.


Lookups are defined in the interface configuration for the IpcAdapter, and are updated via the command 'UpdateLookup'.

Lookup definitions are specified in the following format within the <Adapter> tag:

   <Lookup name="lookup name" type="variable type" value="initial value"/>


  • lookup name is the name of the lookup to implement
  • variable type is a PLEXIL data type (e.g. Integer, String, etc.)
  • initial value is the initial value of the lookup

The <Lookup> element requires all three attributes.

Multiple lookups can be specified one after another, inside the <ExternalLookup> element.

Example Lookup Configuration

This interface configuration fragment declares an Integer Lookup x whose initial value is 1, and a String Lookup str whose initial value is "Hello".

   <Lookup name="x" type="Integer" value="1" />
   <Lookup name="str" type="String" value="Hello" />


To enable inter-exec communication, the IpcAdapter must be set up correctly in the interface config file that is given to the PLEXIL Executive.

As a prerequisite for any communication, the IPC Central process must be running on any open port on any machine and the IpcAdapter must be specified in the interface configuration. A sample interface configuration fragment that ensures communication can be established, but does not declare specific communication features, is as follows:

   <Adapter AdapterType="IpcAdapter" Server="" AllowDuplicateMessages="true" />

This enables the IpcAdapter and connects it to the IPC Central process being hosted at IP address


To enable messaging, use a <CommandNames> block to register the commands "SendMessage" and "ReceiveMessage" with the IpcAdapter.


To enable the handling of incoming commands, use a <CommandNames> element to register the commands "ReceiveMessage", "GetParameters", and "SendReturnValues" with the IpcAdapter.

To enable the sending of commands, replace the <CommandNames> element with the element <DefaultCommandAdapter/>. This registers the IpcAdapter with every command not specified in a <CommandNames> block in any other adapter.


To enable the handling of incoming lookups, specify each lookup to be handled in an <ExternalLookups> block. Within this block, each lookup to be handled is specified in a <Lookup> element. For more information and an example, see Lookups.


Here is an example of two plans that communicate with each other. One acts as a simulator, and the other acts as a high-level controlling plan.

Controlling Plan

to be supplied

Simulator Plan

This plan implements a simulator that serves:

  • One integer lookup, x
  • The command MoveRight(Integer dX), which increases X by dX
  • The message Quit, which shuts down the simulator
 Command UpdateLookup(String, Integer);
 Command SendReturnValue(Integer);

    //The integer lookup, x
    Integer x = 1;
    Boolean continue = true;
      // Repeat the loop until the flag is false.
      RepeatCondition continue;
      // One iteration ends when either of the child nodes finishes.
      EndCondition RecMoveRight.state == FINISHED || RecQuit.state == FINISHED;

      //The handler for the command MoveRight(Integer dX)
      OnCommand "MoveRight" ( Integer modX ) {
        Increment: x = x + modX;
        SetExt: UpdateLookup("x", x);
        RespondMoveRight: SendReturnValue(x);
      //The handler for the message 'Quit'
      OnMessage "Quit" {
        Set: continue = false;

Here is the interface configuration for this executive:

    <Adapter AdapterType="OSNativeTime" />
    <Adapter AdapterType="IpcAdapter" Server="localhost" AllowDuplicateMessages="true">
        <Lookup name="x" type="Integer" value="1" />


Although not inherent in the system, the related problem of presence guarantees still remains. While a normal simulator interface directly connects to the simulator or is the simulator, a PLEXIL simulator operates on a subscription model. Because of this, there is no built-in way to check to ensure that one and only one simulator is going to process commands that are being broadcast.

Also, if an exec needs to send commands and lookups to multiple listening agents, there is no way to differentiate between the two. If the lookup “wind_speed” exists on two listening agents, any lookup “wind_speed” from the main plan will receive whichever response comes in first, leaving the second one to sit in the message queue.

Both of these limitations can be overcome with good planning and forethought, but the fact that they exist remains.

Configuration reference

The IpcAdapter takes the following parameters as attributes in the Adapter element:

  • Server is the name or IP address of the IPC central server. It defaults to localhost;
  • AllowDuplicateMessages is a Boolean value; true means multiple messages with the same name can be processed; false means they are ignored. The default is false;
  • TaskName is the identifier by which this executive will be known, for the purposes of sending and receiving IPC messages. The default is a randomly generated string.

The following elements may appear in the body of the Adapter element:

  • ExternalLookups is a list of Lookup elements describing the lookups that this executive will serve for itself and for other IPC clients. The Lookup element takes the following parameters as required attributes:
    • name is the name of the lookup;
    • type is the PLEXIL data type of the value returned, one of Boolean, Integer, Real, or String;
    • value is the initial value of this lookup.
  • CommandNames is a comma-separated list of command names that will be handled by the IpcAdapter. The adapter's built-in command names are automatically registered;
  • LookupNames is a comma-separated list of lookup names that will be routed to external IPC servers by the IpcAdapter;
  • DefaultCommandInterface causes the IpcAdapter to perform all commands not explicitly handled by other interfaces;
  • DefaultLookupInterface causes the IpcAdapter to perform all lookups not explicitly handled by other interfaces;
  • DefaultAdapter causes the IpcAdapter to perform all commands, lookups, and planner updates not explicitly handled by other interfaces.

An example Adapter element for a hypothetical robot mobility controller might look like this:

<Adapter AdapterType="IpcAdapter" Server="RobotCentral" TaskName="MobilityController" AllowDuplicateMessages = "true">
  <Lookup name="heading" type="Integer" value="0" />
  <Lookup name="speed" type="Real" value="0.0" />
  <Lookup name="odometer" type="Real" value="0.0" />

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