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
• Correlation between requests and replies

Replier: receiving requests and sending replies

• Basic Replier example
• SimpleReplier example

Note

To use Request-Reply you need to build and link your application with the additional rticonnextmsgcpp2 library.

Requester Creation

• Setting up a DomainParticipant
• Creating a Requester

      using namespace rti::request;
  
      dds::domain::DomainParticipant participant(domain_id);
  
      Requester<Foo, Bar> requester(participant, "TestService");
  

Creating a Requester with optional parameters

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

      using namespace rti::request;
  
      // Create a DomainParticipant
      dds::domain::DomainParticipant participant(domain_id);
  
      // Create a Requester with a QoS profile (located for example in
      // USER_QOS_PROFILES.xml, in the current working directory, which the
      // default QosProvider will load)
      RequesterParams requester_params(participant);
      requester_params.service_name("TestService");
  
      dds::core::QosProvider qos_provider = dds::core::QosProvider::Default();
      requester_params.datareader_qos(
              qos_provider.datareader_qos("RequestReplyExampleProfiles::RequesterExampleProfile"));
      requester_params.datawriter_qos(
              qos_provider.datawriter_qos("RequestReplyExampleProfiles::RequesterExampleProfile"));
  
  
      Requester<Foo, Bar> requester(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 rti::request;
  
      // Send request
      Foo request("A Request");
      requester.send_request(request);
  
      // Receive reply (wait for it and get the sample)
      auto replies = requester.receive_replies(MAX_WAIT);
  
      if (replies.length() == 0) {
          std::cout << "Reply not received\n";
      }
  
      for (const auto& reply : replies) {
          if (reply.info().valid()) {
              std::cout << "Received reply: " << reply.data() << std::endl;
          } else {
              std::cout << "Received invalid reply\n";
          }
      }
  

  See also

  Basic Replier example

  SimpleReplier example

Correlating requests and replies

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

  
      using namespace rti::request;
  
      // Create requests
      Foo request1("Request 1"), request2("Request 2");
  
      // Send requests and save request ID
      rti::core::SampleIdentity request_id1 = requester.send_request(request1);
      rti::core::SampleIdentity request_id2 = requester.send_request(request2);
  
      // Wait for a reply to the second request
      bool received = requester.wait_for_replies(1, MAX_WAIT, request_id2);
  
      if (!received) {
          std::cout << "Did not receive reply for request 2" << std::endl;
          return;
      }
  
      // Take that reply
      dds::sub::LoanedSamples<Bar> replies = requester.take_replies(request_id2);
      if (replies.length() == 0) {
          throw std::runtime_error("did not receive any replies");
      }
  
      // This postcondition should always be true
      if (replies[0].info()->related_original_publication_virtual_sample_identity()
              != request_id2) {
          throw std::runtime_error("postcondition failed");
      }
  
      if (replies[0].info().valid()) {
          std::cout << "Received reply for request 2: "
                    << replies[0].data() << std::endl;
      }
  
      // Wait for a reply to the first request
      received = requester.wait_for_replies(1, MAX_WAIT, request_id1);
  
      if (!received) {
          std::cout << "Did not receive reply for request 1" << std::endl;
          return;
      }
  
      // Take that reply
      replies = requester.take_replies(request_id1);
      if (replies.length() == 0) {
          throw std::runtime_error("did not receive any replies");
      }
  
      // This postcondition should always be true
      if (replies[0].info()->related_original_publication_virtual_sample_identity()
              != request_id1) {
          throw std::runtime_error("postcondition failed");
      }
  
      if (replies[0].info().valid()) {
          std::cout << "Received reply for request 1: "
                    << replies[0].data() << std::endl;
      }
  

• Example 2) Correlating a reply after receiving it

  
      using namespace rti::request;
  
      // Create requests
      Foo request1("Request 1"), request2("Request 2");
  
      // Send requests and save request ID
      rti::core::SampleIdentity request_id1 = requester.send_request(request1);
      rti::core::SampleIdentity request_id2 = 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;
      }
  
      // Get all the replies
      auto 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 API that evaluates to
      // true when a Sample's related_identity equals the identity that
      // is passed to the constructor (request_id1, in this case)
      auto it = std::find_if(
              replies.begin(),
              replies.end(),
              IsReplyRelatedPredicate<Bar>(request_id1));
  
      if (it != replies.end()) {
          std::cout << "Received reply for request 1: "
                    << it->data() << std::endl;
      }
  
      // Find the reply for request 2
      it = std::find_if(
              replies.begin(),
              replies.end(),
              IsReplyRelatedPredicate<Bar>(request_id2));
  
      if (it != replies.end()) {
          std::cout << "Received reply for request 2: "
                    << it->data() << std::endl;
      }
  

  See also

  Basic Requester example

  Basic Replier example

Creating a Replier

• Setting up a DomainParticipant
• Create a Replier

      using namespace rti::request;
  
      dds::domain::DomainParticipant participant(domain_id);
  
      // Use the same service name as the Requester's
      Replier<Foo, Bar> replier(participant, "TestService");
  

Basic Replier example

• Creating a Replier
• Basic Replier example

  
      using namespace rti::request;
  
      // Receive requests
      dds::sub::LoanedSamples<Foo> requests = replier.receive_requests(MAX_WAIT);
      for (const auto& request : requests) {
          if (!request.info().valid()) {
              continue;
          }
  
          Bar reply("Reply for " + request.data().message());
  
          // Send a reply for this request, identified by the ID in
          // request.info()
          replier.send_reply(reply, request.info());
  
          // Note: a replier can send more than one reply for the same request
          // replier.send_reply(Bar("Another reply"), request.info());
      }
  

  See also

  Basic Requester example

SimpleReplier example

    using namespace rti::request;

    SimpleReplier<Foo, Bar> replier(
        participant,
        "TestService",
        [](const Foo& request)
        {
            return Bar(std::string("Simple reply for ") + request.message());
        }
    );

    // After creation the SimpleReplier is already active and the functor will
    // be called upon receiving a request.

See also

Basic Requester example

Configuring Request-Reply QoS profiles

If you do not specify your own QoS parameters (in RequesterParams and ReplierParams), a rti::request::Requester and rti::request::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"?>

<dds xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation="../../../resource/schema/rti_dds_qos_profiles.xsd">

    <qos_library name="RequestReplyExampleProfiles">

        <!-- Default QoS:

             This profile contains the QoS that Requesters and Repliers
             would use by default. We can use it as a base profile to inherit
             from and override some parameters
         -->
        <qos_profile name="default">
            <datawriter_qos>

                <!-- Strict reliable -->
                <reliability>
                    <kind>RELIABLE_RELIABILITY_QOS</kind>
                    <max_blocking_time>
                        <sec>10</sec>
                        <nanosec>0</nanosec>
                    </max_blocking_time>
                </reliability>

                <history>
                    <kind>KEEP_ALL_HISTORY_QOS</kind>
                </history>

                <!-- These are typical protocol parameters for a reliable
                     DataWriter -->
                <protocol>
                    <rtps_reliable_writer>
                        <max_heartbeat_retries>
                            LENGTH_UNLIMITED
                        </max_heartbeat_retries>
                        <heartbeats_per_max_samples>
                            2
                        </heartbeats_per_max_samples>
                        <heartbeat_period>
                            <sec>0</sec>
                            <nanosec>100000000</nanosec> <!--100ms -->
                        </heartbeat_period>
                        <fast_heartbeat_period>
                            <sec>0</sec>
                            <nanosec>10000000</nanosec> <!--10ms -->
                        </fast_heartbeat_period>
                        <late_joiner_heartbeat_period>
                            <sec>0</sec>
                            <nanosec>10000000</nanosec> <!--10ms -->
                        </late_joiner_heartbeat_period>
                        <max_nack_response_delay>
                            <sec>0</sec>
                            <nanosec>0</nanosec>
                        </max_nack_response_delay>
                        <min_nack_response_delay>
                            <sec>0</sec>
                            <nanosec>0</nanosec>
                        </min_nack_response_delay>
                        <max_send_window_size>32</max_send_window_size>
                        <min_send_window_size>32</min_send_window_size>
                    </rtps_reliable_writer>
                </protocol>

                <writer_resource_limits>
                    <!-- This setting enables efficient communication
                         between a replier and an arbitrary number of requesters
                     -->
                    <max_remote_reader_filters>
                        LENGTH_UNLIMITED
                    </max_remote_reader_filters>
                </writer_resource_limits>
            </datawriter_qos>

            <datareader_qos>
                <!-- Strict reliable -->
                <reliability>
                    <kind>RELIABLE_RELIABILITY_QOS</kind>
                    <max_blocking_time>
                        <sec>10</sec>
                        <nanosec>0</nanosec>
                    </max_blocking_time>
                </reliability>

                <history>
                    <kind>KEEP_ALL_HISTORY_QOS</kind>
                </history>

                <!-- These are typical protocol parameters for a reliable
                     DataReader -->
                <protocol>
                    <rtps_reliable_reader>
                        <max_heartbeat_response_delay>
                            <sec>0</sec>
                            <nanosec>0</nanosec>
                        </max_heartbeat_response_delay>
                        <min_heartbeat_response_delay>
                            <sec>0</sec>
                            <nanosec>0</nanosec>
                        </min_heartbeat_response_delay>
                    </rtps_reliable_reader>
                </protocol>

            </datareader_qos>

        </qos_profile>

        <!-- This is the profile used by the Requester.
             It inherits from "default", defined above,
             and overrides some QoS -->
        <qos_profile name="RequesterExampleProfile"
                     base_name="default">

            <!-- QoS for the data writer that sends requests -->
            <datawriter_qos>
                <durability>
                    <kind>TRANSIENT_LOCAL_DURABILITY_QOS</kind>
                </durability>
            </datawriter_qos>

            <!-- QoS for the data reader that receives replies -->
            <datareader_qos>
                <durability>
                    <kind>VOLATILE_DURABILITY_QOS</kind>
                </durability>
            </datareader_qos>
        </qos_profile>

        <!-- This is the profile used by the Replier.
             It inherits from "default", defined above,
             and overrides some QoS -->
        <qos_profile name="ReplierExampleProfile"
                     base_name="default">

            <!-- QoS for the data writer that sends replies -->
            <datawriter_qos>
                <durability>
                    <kind>VOLATILE_DURABILITY_QOS</kind>
                </durability>
            </datawriter_qos>

            <!-- QoS for the data reader that receives requests -->
            <datareader_qos>
                <durability>
                    <kind>TRANSIENT_LOCAL_DURABILITY_QOS</kind>
                </durability>
            </datareader_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 rti::request::Requester using this profile.

See also

Creating a Requester with optional parameters

Configuring QoS Profiles with XML