Request-Reply Examples

Examples on how to use the request-reply API. More...

Examples on how to use the request-reply API.

Request-Reply code examples.

Request-Reply Examples

Requesters and Repliers provide a way to use the Request-Reply communication pattern on top of the DDS entities. An application uses a Requester to send requests to a Replier; another application using a Replier receives a request and can send one or more replies for that request. The Requester that sent the request (and only that one) will receive the reply (or replies).

DDS Types

RTI Connext uses DDS data types for sending and receiving requests and replies. Valid types are those generated by the rtiddsgen code generator, the DDS built-in types, and DynamicData. Refer to the Core Libraries User's Manual and the following links for more information:

• Code Generator User's Manual,
• Using the DDS built-in types,
• Using DynamicData

Set up

• Create a DomainParticipant
• Create a requester
• Create a requester with parameters
• Create a replier

Requester: sending requests and receiving replies

• Basic Requester example
• Taking loaned samples
• Taking samples by copy
• Correlation between requests and replies

Replier: receiving requests and sending replies

• Basic Replier example
• SimpleReplier example
• Taking loaned samples
• Taking samples by copy

Error handling

• Error handling example

Note

To use Request-Reply you need to build and link your application with the additional rticonnextmsgcpp library and include ndds/ndds_requestreply_cpp.h.

Requester Creation

• Setting up a participant
• Creating a Requester

      using namespace connext;
  
      // Note: error checking omitted for simplicity
  
      // Create a DomainParticipant
      DDS::DomainParticipant * participant =
          DDS::DomainParticipantFactory::get_instance()->create_participant(
              domain_id, DDS::PARTICIPANT_QOS_DEFAULT,
              NULL, DDS::STATUS_MASK_NONE);
  
      // Create a Requester (if creation fails, the constructor throws)
      Requester<Foo, Bar> * requester =
          new Requester<Foo, Bar>(participant, "TestService");
  

Creating a Requester with optional parameters

• Setting up a participant
• Creating a Requester with additional parameters

      using namespace connext;
  
      // Note: error checking omitted for simplicity
  
      // Create a DomainParticipant
      DDS::DomainParticipant * participant =
          DDS::DomainParticipantFactory::get_instance()->create_participant(
              domain_id, DDS::PARTICIPANT_QOS_DEFAULT,
              NULL, DDS::STATUS_MASK_NONE);
  
      // Create a Requester with a QoS profile (located for example in
      // USER_QOS_PROFILES.xml, in the current working directory)
      RequesterParams requester_params(participant);
      requester_params.service_name("TestService");
      requester_params.qos_profile(
          "RequestReplyExampleProfiles", "RequesterExampleProfile");
  
      Requester<Foo, Bar> * requester = new Requester<Foo, Bar>(requester_params);
  

  See also

  Requester Creation

  Configuring Request-Reply QoS profiles

Basic Requester example

• Requester Creation
• Creating a Requester with optional parameters
• Basic Requester example

      using namespace connext;
  
      // Send request
      WriteSample<Foo> request;
      strcpy(request.data().message, "A Request");
  
      requester->send_request(request);
  
      // Create a reply Sample
      Sample<Bar> reply;
  
      // Receive reply (wait for it and get the sample)
      bool received = requester->receive_reply(reply, MAX_WAIT);
  
      if (received) {
          if(reply.info().valid_data) {
              std::cout << "Received reply: " << reply.data().message << std::endl;
          } else {
              std::cout << "Received invalid reply" << std::endl;
          }
      } else {
          std::cout << "Reply not received" << std::endl;
      }
  

• Basic Requester example using the String built-in type

  
      using namespace connext;
  
      // Create a Requester (this time we create it on the stack)
      // The template parameter std::string maps to the String DDS built-in type
      Requester<std::string, std::string> requester(participant, "TestServiceString");
  
      // Send a request
      requester.send_request("A string request");
  
      // Receive reply
      LoanedSamples<std::string> replies = requester.receive_replies(MAX_WAIT);
  
      for (LoanedSamples<std::string>::iterator it = replies.begin();
           it != replies.end();
           ++it) {
          if (it->info().valid_data) {
              std::cout << "Received string reply: " << it->data() << std::endl;
          }
      }
  
      // When replies go out of scope, the loan is automatically returned
  

  See also

  Basic Replier example

  SimpleReplier example

Taking loaned samples

• Requester Creation
• Get iterator to loaned replies (no copies)

  
      using namespace connext;
  
      // Get loaned samples
      // Unlike receive_replies(), take_replies() doesn't block and returns an
      // empty collection if there are no replies at that moment.
      // (before that we can call requester->wait_for_replies)
      LoanedSamples<Bar> replies = requester->take_replies();
  
      // Iterate through the container of loaned samples.
      // If there were no replies, the container is empty
      for (LoanedSamples<Bar>::const_iterator it = replies.begin();
          it != replies.end();
          ++it) {
  
          if (it->info().valid_data) {
              std::cout << "Received reply: " << it->data().message << std::endl;
          }
      }
  
      // When replies go out of scope, the loan is automatically returned
  

  See also

  Basic Requester example

  Basic Replier example

  Taking samples by copy

Taking samples by copy

• Requester Creation
• Get copies of the replies:

  
      using namespace connext;
  
      // Get loaned samples
      LoanedSamples<Bar> replies = requester->take_replies();
  
      // Copy the contents using the STL-compliant iterator in LoanedSamples
      std::list<Sample<Bar> > my_list;
      std::copy(replies.begin(), replies.end(), std::back_inserter(my_list));
  
      // We can copy to a container of Bar rather than Sample<Bar>,
      // thanks to the implicit conversion from Sample<Bar> to Bar.
      // If our application works with instances, we may have received
      // invalid data (e.g. disposed instance). We can iterate through valid
      // samples only using an iterator adapter.
      std::vector<Bar> my_vector;
      std::copy(
          make_valid_sample_iterator(replies.begin()),
          make_valid_sample_iterator(replies.end()),
          std::back_inserter(my_vector));
  
      // When replies go out of scope, the loan is automatically returned
  

  See also

  Basic Replier example

  Taking loaned samples

Correlating requests and replies

• Requester Creation
• Example 1) Waiting for a reply to a specific request

  
      using namespace connext;
  
      // Create requests
      WriteSample<Foo> request1, request2;
      strcpy(request1.data().message, "Request 1");
      strcpy(request2.data().message, "Request 2");
  
      // Send requests using WriteSample
      requester->send_request(request1);
      requester->send_request(request2);
  
      // Wait for a reply to the second request
      Sample<Bar> reply;
      bool received = requester->wait_for_replies(
          1, MAX_WAIT, request2.identity());
  
      if (!received) {
          std::cout << "Did not receive reply for request 2" << std::endl;
          return;
      }
  
      // Take that reply
      requester->take_reply(reply, request2.identity());
  
      // This postcondition should always be true
      if (reply.related_identity() != request2.identity()) {
          throw std::runtime_error("postcondition failed");
      }
  
      if (reply.info().valid_data) {
          std::cout << "Received reply for request 2: "
                    << reply.data().message << std::endl;
      }
  
      // Wait for a reply to the first request
      received = requester->wait_for_replies(1, MAX_WAIT, request1.identity());
  
      if (!received) {
          std::cout << "Did not receive reply for request 1" << std::endl;
          return;
      }
  
      // Take that reply
      requester->take_reply(reply, request1.identity());
  
      // This postcondition should always be true
      if (reply.related_identity() != request1.identity()) {
          throw std::runtime_error("postcondition failed");
      }
  
      if (reply.info().valid_data) {
          std::cout << "Received reply for request 1: "
                    << reply.data().message << std::endl;
      }
  

• Example 2) Correlating a reply after receiving it

  
      using namespace connext;
  
      // Create requests
      WriteSample<Foo> request1, request2;
      strcpy(request1.data().message, "Request 1");
      strcpy(request2.data().message, "Request 2");
  
      // Send requests using WriteSample
      requester->send_request(request1);
      requester->send_request(request2);
  
      // Wait for two replies. In this case we don't mind the
      // reception order
      bool received = requester->wait_for_replies(2, MAX_WAIT);
      if (!received) {
          std::cout << "Replies not received" << std::endl;
          return;
      }
  
      LoanedSamples<Bar>::iterator it;
  
      // Get all the replies
      LoanedSamples<Bar> replies = requester->take_replies();
  
      // Find the reply for request 1.
      //
      // We search using the STL find_if algorithm. The search range is all
      // the elements in the replies container (from beginning to end).
      // We use a predicate included in the Connext namespace that evaluates to
      // true when a Sample's related_identity equals the identity that
      // is passed to the constructor (request1's, in this case)
      it = std::find_if(replies.begin(), replies.end(),
                        IsReplyRelatedPredicate<Bar>(request1.identity()));
  
      if (it != replies.end()) {
          std::cout << "Received reply for request 1: "
                    << it->data().message << std::endl;
      }
  
      // Find the reply for request 2
      it = std::find_if(replies.begin(), replies.end(),
                        IsReplyRelatedPredicate<Bar>(request2.identity()));
  
      if (it != replies.end()) {
          std::cout << "Received reply for request 2: "
                    << it->data().message << std::endl;
      }
  
      // When replies go out of scope, the loan is automatically returned
  

  See also

  Basic Requester example

  Basic Replier example

Basic Requester example using SampleRef

This example is similar to Basic Requester example except it uses a SampleRef instance instead of a Sample instance.

• Assuming the variables you want to use already exist:

      Foo request_data;
      DDS::WriteParams_t request_params;
  
      Bar reply_data;
      DDS::SampleInfo reply_info;
  
      // ...
  

• You can use WriteSampleRef and SampleRef:

      using namespace connext;
  
      // Create a request WriteSampleRef
      WriteSampleRef<Foo> request;
      request.set_data(request_data);
      request.set_info(request_params);
  
      // Send request
      requester->send_request(request);
  
      // Create a reply SampleRef
      SampleRef<Bar> reply;
      reply.set_data(reply_data);
      reply.set_info(reply_info);
  
      // Receive reply
      bool received = requester->receive_reply(reply, MAX_WAIT);
  
      if (received) {
          if(reply_info.valid_data) {
              std::cout << "Received reply: " << reply_data.message << std::endl;
          } else {
              std::cout << "Received invalid reply" << std::endl;
          }
      } else {
          std::cout << "Reply not received" << std::endl;
      }
  

  See also

  connext::SampleRef

  Basic Requester example

Creating a Replier

• Setting up a participant
• Create a Replier

      using namespace connext;
  
      // Note: error checking omitted for simplicity
  
      // Create a DomainParticipant
      DDS::DomainParticipant * participant =
          DDS::DomainParticipantFactory::get_instance()->create_participant(
              domain_id, DDS::PARTICIPANT_QOS_DEFAULT,
              NULL, DDS::STATUS_MASK_NONE);
  
      // Create a Replier (if creation fails, the constructor throws exception)
      Replier<Foo, Bar> * replier =
          new Replier<Foo, Bar>(participant, "TestService");
  

Basic Replier example

• Creating a Replier
• Basic Replier example

  
      using namespace connext;
  
      Sample<Foo> request;
  
      // Receive one request
      bool received = replier->receive_request(request, MAX_WAIT);
  
      if (!received) {
          std::cout << "Requests not received" << std::endl;
          return false;
      }
  
      // A WriteSample automatically initializes and finalizes
      // the data using the TypeSupport (in this case BarTypeSupport)
      WriteSample<Bar> reply;
  
      if (request.info().valid_data) {
          sprintf(reply.data().message, "Reply for %s", request.data().message);
  
          // Send a reply for that request
          replier->send_reply(reply, request.identity());
  
          // Note: a replier can send more than one reply for the same request
      }
  

  See also

  Basic Requester example

SimpleReplier example

• Implement a listener

  class MySimpleReplierListener : public SimpleReplierListener<Foo, Bar>
  {
  public:
      Bar * on_request_available(SampleRef<Foo> request)
      {
          if (!request.info().valid_data) {
              // In this example we don't reply to invalid data samples.
              // In other cases, it could make sense to reply
              // to a disposed instance, for example.
              return NULL;
          }
  
          sprintf(_reply.data().message, "Simple reply for %s",
                  request.data().message);
  
          return &_reply.data();
      }
  
      void return_loan(Bar * reply)
      {
          // nothing to do
      }
  
  private:
      // By using a WriteSample (instead of just Bar), the sample is
      // automatically initialized and released.
      WriteSample<Bar> _reply;
  };

• And create a SimpleReplier with the listener

  
      using namespace connext;
  
      // Note: error checking omitted for simplicity
  
      MySimpleReplierListener * listener = new MySimpleReplierListener();
  
      SimpleReplier<Foo, Bar> * replier = new SimpleReplier<Foo, Bar>(
          participant, "TestService", *listener);
  
      // After creation the SimpleReplier is already active and may call
      // the listener
  

• This next example shows how to implement a listener for a SimpleReplier using the std::string type

  class MyStringSimpleReplierListener :
      public SimpleReplierListener<std::string, std::string>
  {
  public:
      // Note that the actual received type is const char*,
      // because internally, the middleware uses char*
      std::string * on_request_available(SampleRef<const char*> request)
      {
          if (!request.info().valid_data) {
              return NULL;
          }
  
          _reply = std::string("Simple string reply for ") + request.data();
          return &_reply;
      }
  
      void return_loan(std::string * reply)
      {
          // nothing to do
      }
  
  private:
      std::string _reply;
  };

  See also

  Basic Requester example

Error handling example

• Catching an exception from the request-reply API

      using namespace connext;
  
      // All the operations in the request-reply API
      // throw exceptions in case of error.
  
      // You can also set the verbosity level to warning for
      // additional information
      NDDSConfigLogger::get_instance()->set_verbosity(
          NDDS_CONFIG_LOG_VERBOSITY_WARNING);
  
      //
      // For example, this replier is sending a reply for a nonexistent request
      //
      WriteSample<Bar> reply;
      DDS::SampleIdentity_t invalid_request_id;
  
      try {
          replier->send_reply(reply, invalid_request_id);
      } catch (const BadParameterException& ex) {
          std::cout << "Exception while sending reply: "
                    << ex.what() << std::endl;
      }
  
      // Exceptions inherit from std::runtime_error or std::logic_error,
      // so you can have a more general catch statement:
      //
      // This piece of code produces the same result as the previous one:
      //
      try {
          replier->send_reply(reply, invalid_request_id);
      } catch (const std::exception& ex) {
          std::cout << "Exception while sending request: "
                    << ex.what() << std::endl;
      }
  

Configuring Request-Reply QoS profiles

If you do not specify your own QoS parameters (in connext::RequesterParams and connext::ReplierParams), a connext::Requester and connext::Replier are created using a default configuration. That configuration is equivalent to the one in the following QoS profile called "default":

<?xml version="1.0"?>
<!-- 
Description
XML QoS Profile for HelloWorld

(c) Copyright, Real-Time Innovations, 2012.  All rights reserved.
RTI grants Licensee a license to use, modify, compile, and create derivative
works of the software solely for use with RTI Connext DDS. Licensee may
redistribute copies of the software provided that all such copies are
subject to this license. The software is provided "as is", with no warranty
of any type, including any warranty for fitness for any purpose. RTI is
under no obligation to maintain or support the software. RTI shall not be
liable for any incidental or consequential damages arising out of the use
or inability to use the software.

The QoS configuration of the DDS entities in the generated example is loaded 
from this file.

This file is used only when it is in the current working directory or when the
environment variable NDDS_QOS_PROFILES is defined and points to this file.

The profile in this file inherits from the builtin QoS profile 
BuiltinQosLib::Generic.StrictReliable. That profile, along with all of the 
other built-in QoS profiles can be found in the 
BuiltinProfiles.documentationONLY.xml file located in the 
$NDDSHOME/resource/xml/ directory.

You may use any of these QoS configurations in your application simply by 
referring to them by the name shown in the 
BuiltinProfiles.documentationONLY.xml file.

Also, following the QoS Profile composition pattern you can use QoS Snippets
to easily create your final QoS Profile. For further information visit:
https://community.rti.com/best-practices/qos-profile-inheritance-and-composition-guidance

There is a QoS Snippet library that contains a collection of
QoS Snippets that set some specific features or configurations. You can find
them in the BuiltinProfiles.documentationONLY.xml file as well.

You should not edit the file BuiltinProfiles.documentationONLY.xml directly.
However, if you wish to modify any of the values in a built-in profile, the
recommendation is to create a profile of your own and inherit from the built-in
profile you wish to modify. The NDDS_QOS_PROFILES.example.xml file (contained in 
the same directory as the BuiltinProfiles.documentationONLY.xml file) shows how
to inherit from the built-in profiles. 

For more information about XML QoS Profiles see the "Configuring QoS with
XML" chapter in the RTI Connext DDS Core Libraries User's Manual.
-->

<dds xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="file:////rti/jenkins/workspace/connextdds_htmldocs_release_connextdds_7.0.0/build/nddsgen.2.0/resource/schema/rti_dds_qos_profiles.xsd">
    <!-- QoS Library containing the QoS profile used in the generated example.

        A QoS library is a named set of QoS profiles.
    -->
    <qos_library name="HelloWorld_Library">

        <!-- QoS profile used to configure reliable communication between the DataWriter 
             and DataReader created in the example code.

             A QoS profile groups a set of related QoS.
        -->
        <qos_profile name="HelloWorld_Profile" base_name="BuiltinQosLib::Generic.StrictReliable" is_default_qos="true">
            <!-- QoS used to configure the data writer created in the example code -->                
            <datawriter_qos>
                <publication_name>
                    <name>HelloWorldDataWriter</name>
                </publication_name>
            </datawriter_qos>

            <!-- QoS used to configure the data reader created in the example code -->                
            <datareader_qos>
                <subscription_name>
                    <name>HelloWorldDataReader</name>
                </subscription_name>
            </datareader_qos>
            <domain_participant_qos>
                <!--
                The participant name, if it is set, will be displayed in the
                RTI tools, making it easier for you to tell one
                application from another when you're debugging.
                -->
                <participant_name>
                    <name>HelloWorldParticipant</name>
                    <role_name>HelloWorldParticipantRole</role_name>
                </participant_name>

            </domain_participant_qos>
        </qos_profile>

    </qos_library>
</dds>

You can use the profile called "RequesterExampleProfile", which modifies some parameters from the default. The example Creating a Requester with optional parameters shows how to create a connext::Requester using this profile.

See also

Creating a Requester with optional parameters

Configuring QoS Profiles with XML