Polling With Query Condition

Concept

In RTI Connext DDS, you have the option to query for data that is already in the DataReader's queue using a QueryCondition object. This lets you retrieve data from the queue based on the values of one or more fields inside of that data. You do this by:

1. Specifying the query that you want to use. 

This either looks like the WHERE clause in SQL, such as x < 34 OR y > 2 or it can be a string parameter that can be compared using a MATCH comparison such as symbol MATCH 'IBM'. This query can have parameters to it, represented with %0, %1, etc. These parameters can be changed at runtime.

2. Creating the QueryCondition object 

You use your DataReader to create a QueryCondition object, specifying the query that you want to run, and parameters to that query.

3. Using the QueryCondition to retrieve the data subset

You can call read_w_condition() or take_w_condition() to retrieve only the data that passes the query from the DataReader's cache.

4. Optionally, changing the parameters at runtime

You can update the parameters to your query by calling set_query_parameters. Note that a QueryCondition requires that the single quotes around a string are inside the QueryCondition parameter. For example:

DDS_StringSeq queryParameters;
queryParameters.ensure_length(1,1); 

// DON'T FORGET THE SINGLE QUOTES INSIDE THE PARAMETER 
queryParameters[0] = DDS_String_dup("'Initial String'");
query_condition = _reader->create_querycondition(DDS_ANY_SAMPLE_STATE, 
    DDS_ANY_VIEW_STATE, DDS_ALIVE_INSTANCE_STATE, 
    DDS_String_dup("stringField MATCH %0"), queryParameters); 

//... 

// DON'T FORGET THE SINGLE QUOTES INSIDE THE PARAMETER 
queryParameters[0] = DDS_String_dup("'Changed String'"); 
_queryForFlights->set_query_parameters(queryParameters);

Download Example

Browse Example