Signal

Signals are tools provided by the QodlyScript language to manage interactions and avoid conflicts between worker processes in a multiprocess application. Signals allow you to make sure one or more process(es) will wait for a specific task to be completed before continuing execution. Any process can wait and/or release a signal.

info

Semaphores can also be used to manage interactions. Semaphores allow you to make sure that two or more processes do not modify the same resource (file, record...) at the same time. Only the process that sets the semaphore can remove it.

Signal object

A signal is a shared object that must be passed as a parameter to commands that call or create workers.

A 4D.Signal object contains the following built-in methods and properties:

• .wait()
• .trigger()
• .signaled
• .description.

Any worker/process calling the .wait() method will suspend its execution until the .signaled property is true. While waiting for a signal, the calling process does not use any CPU. This can be very interesting for performance in multiprocess applications. The .signaled property becomes true when any worker/process calls the .trigger() method.

Note that to avoid blocking situations, the .wait() can also return after a defined timeout has been reached.

Signal objects are created with the newSignal command.

Working with signals

In QodlyScript, you create a new signal object by calling the newSignal command. Once created, this signal must be passed as a parameter to the callWorker command so that it can modify it when it has finished the task you want to wait for.

• signal.wait() must be called from the worker that needs another worker to finish a task in order to continue.
• signal.trigger() must be called from the worker that finished its execution in order to release all others.

Once a signal has been released using a signal.trigger() call, it cannot be reused again. If you want to set another signal, you need to call the newSignal command again.

Since a signal object is a shared object, you can use it to return results from called workers, provided that you do not forget to write values within a use...end structure (see example).

Example

 var signal : 4D.Signal

  // Creation of a signal
 signal = newSignal

  // call main process and execute OpenForm method
 callWorker(1,"OpenForm",signal)
  // do another calculation
 ...
  // Waiting for the end of the process
 signaled = signal.wait()

  // Processing of the results
 calc = signal.result+...

OpenForm method :

 declare (signal : 4D.Signal)  
 var form : object
 form = newObject("value",0)

  // Process the form object
 form = ...

  // Add a new attribute to your signal shared object to pass your result to the other process:
 use(signal)
    signal.result = form.value
 end

  // Trigger the signal to the waiting process
 signal.trigger()

Functions and properties

.description

.description* : string

Description

The .description property contains a custom description for the signal object..

.description can be set at the creation of the signal object or at any moment. Note that since the signal object is a shared object, any write-mode access to the .description property must be surrounded by a use...end structure.

This property is read-write.

.signaled

.signaled* : Boolean

Description

The .signaled property contains the current state of the signal object. When the signal is created, .signaled is false. It becomes true when the .trigger( ) is called on the object.

This property is read-only.

.trigger()

*.trigger( )**

Parameter	Type		Description
			Does not require any parameters

Description

The .trigger( ) function sets the signaled property of the signal object to true and awakens all workers waiting for this signal.

If the signal is already in the signaled state (i.e., the signaled property is already true), the function does nothing.

.wait()

.wait*( { timeout : number } ) : boolean

Parameter	Type		Description
timeout	number	→	Maximum waiting time for the signal in seconds
Result	boolean	←	State of the .signaled property

Description

The .wait() function makes the current process wait until the .signaled property of the signal object to become true or the optional timeout to expire.

To prevent blocking code, you can pass a maximum waiting time in seconds in the timeout parameter (decimals are accepted).

caution

Calling .wait() without a timeout in the Qodly main process is not recommended because it could freeze the whole Qodly application.

If the signal is already in the signaled state (i.e. the .signaled property is already true), the function returns immediately, without waiting.

The function returns the value of the .signaled property. Evaluating this value allows knowing if the function returned because the .trigger( ) has been called (.signaled is true) or if the timeout expired (.signaled is false).

The state of a process that waits for a signal is Waiting for internal flag.