WaitSet with Query Condition


Conditions and WaitSets provide another way for Connext to communicate status changes (including the arrival of data) to your application. While a Listener is used to provide a callback for asynchronous access, Conditions and WaitSets provide synchronous data access. In other words, Listeners are notification-based and Conditions are wait-based.

A WaitSet allows an application to wait until one or more attached Conditions becomes true (or until a timeout expires).

Briefly, your application can create a WaitSet, attach one or more Conditions to it, then call the WaitSet's wait() operation. The wait() blocks until one or more of the WaitSet's attached Conditions becomes TRUE.

A Condition has a trigger_value that can be TRUE or FALSE. You can retrieve the current value by calling the Condition's only operation, get_trigger_value().

There are three kinds of Conditions. A Condition is a root class for all the conditions that may be attached to a WaitSet. This basic class is specialized in three classes:

  • GuardConditions are created by your application. Each GuardCondition has a single, user-settable, boolean trigger_value. Your application can manually trigger the GuardCondition by calling set_trigger_value(). Connext does not trigger or clear this type of condition -- it is completely controlled by your application.
  • ReadConditions and QueryConditions are created by your application, but triggered by Connext. ReadConditions provide a way for you to specify the data samples that you want to wait for, by indicating the desired sample-states, view-states, and instance-states. A QueryCondition is a special ReadCondition that allows you to specify a query expression and parameters, so you can filter on the locally available (already received) data. QueryConditions use the same SQL-based filtering syntax as ContentFilteredTopics for query expressions, parameters, etc. Unlike ContentFilteredTopics, QueryConditions are applied to data already received, so they do not affect the reception of data.
  • StatusConditions are created automatically by Connext, one for each Entity. A StatusCondition is triggered by Connext when there is a change to any of that Entity's enabled statuses.

A WaitSet can be associated with more than one Entity (including multiple DomainParticipants). It can be used to wait on Conditions associated with different DomainParticipants. A WaitSet can only be in use by one application thread at a time.

Example Description

In this example we send a number called 'x' and a string called 'name'. The publisher will send the number and the name, which will be:

  • "EVEN" if the number is even.
  • "ODD" if the number is odd.

In the subscriber, like we are using WaitSet, we do not need to use the listeners, but we have to follow some steps to create the WaitSet.

  • Creating the Conditions we are going to attach to the WaitSet.
  • Creating the Waitset.
  • Attaching the conditions to the WaitSet.
  • Waiting for data.

When we are waiting for data, the WaitSet blocks the execution of the thread when we call its wait() function until one or more attached Conditions become true, or until a user-specified timeout expires.

This WaitSet uses a QueryCondition. This QueryCondition is: name MATCH %0 where %0 is a parameter.

Firstly, when we create the QueryCondition we set the parameter to the "EVEN" value. After 7 sec, we modify the QueryCondition and the parameter has the "ODD" value.

So, the subscriber receives the EVEN numbers in the first 7 sec of the execution, and then it receives the ODD numbers. Maybe it can receive a previous ODD number because we are using KEEP_LAST_HISTORY_QOS with depth=1 in the datareader_qos. That is why a DataReader with WaitSet receives all the samples and it is woken up when a sample satisfies the conditions. However, if a DataReader has stored some samples which satisfies the WaitSet's conditions, they will be processed too.

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