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Request-Reply communication

This section explains how to configure a Request-Reply communication in Fast DDS between two applications. A Client application will send a Request to the Server application and this one, after processing the request, will send a Reply to the Client application.

.. uml::
   :align: center

    node "Client application" as client {
    }

    node "Server application" as server {
    }

    client --> server : Request
    server --> client : Reply

Overview

This kind of communication involves in the DDS paradigm the usage of two Topics: one for sending requests (Request Topic) and the other one for sending replies (Reply Topic). For managing these Topics four DDS entities are involved: a DataReader and a DataWriter for each Topic. The DDS communication schema will be:

.. uml::
   :align: center

    node "Server application" {
        cloud "Processing" as Processing
        [Request DataReader] -right-> Processing
        Processing -right-> [Reply DataWriter]
    }

    node "Client application" {
        [Reply DataReader]
        [Request DataWriter]
    }

    [Request DataWriter] -down-> [Request DataReader] : Request Topic
    [Reply DataWriter] --> [Reply DataReader] : Reply Topic

The key for making Request-Reply work is relating the Request with the Reply in the client's side. Fast DDS API provides |SampleIdentity-api| to achieve this.

A full example can be found in Fast DDS repository.

Getting started

For Request-Reply communication perform the following steps:

  1. Define two structures in an IDL file. One structure will be used as Request Topic's data type and the other one as Reply Topic's data type.

    enum OperationType
{
    ADDITION,
    SUBSTRACTION,
    MULTIPLICATION,
    DIVISION
};

struct RequestType
{
    OperationType operation;
    long x;
    long y;
};

struct ReplyType
{
    long long z;
};

  2. In the client application, create a DataWriter for the request and a DataReader for the reply.

    .. literalinclude:: ../../../../code/DDSCodeTester.cpp
   :language: c++
   :dedent: 4
   :start-after: //REQUEST_REPLY_EXAMPLE_CLIENT_CREATE_ENTITIES
   :end-before: //!

  3. In the server application, create a DataWriter for the reply and a DataReader for the request.

    .. literalinclude:: ../../../../code/DDSCodeTester.cpp
   :language: c++
   :dedent: 4
   :start-after: //REQUEST_REPLY_EXAMPLE_SERVER_CREATE_ENTITIES
   :end-before: //!

Sending the request and storing the assigned identifier

For sending the request, the client application should retrieve and store the internal identifier assigned to the published sample. Therefore the sample should be published using the overloaded |DataWriter::write-api| function which second argument is a reference to a |WriteParams-api| object. The assigned identifier will be stored in the |WriteParams-api|'s attribute |WriteParams-sample_identity-api|.

.. literalinclude:: ../../../../code/DDSCodeTester.cpp
   :language: c++
   :dedent: 4
   :start-after: //REQUEST_REPLY_EXAMPLE_CLIENT_RETRIEVE_ID
   :end-before: //!

Receiving the request and sending a reply associated with it

When the server application receives the request (for example through |DataReaderListener::on_data_available-api|), it has to retrieve the request's identifier using |SampleInfo::sample_identity-api|.

.. literalinclude:: ../../../../code/DDSCodeTester.cpp
   :language: c++
   :dedent: 4
   :start-after: //REQUEST_REPLY_EXAMPLE_SERVER_GET_ID
   :end-before: //!

After processing the request, the server should send the reply to the client with the related request attached. This is done assigning the stored identifier in |WriteParams-related_sample_identity-api|.

.. literalinclude:: ../../../../code/DDSCodeTester.cpp
   :language: c++
   :dedent: 4
   :start-after: //REQUEST_REPLY_EXAMPLE_SERVER_SEND_REPLY
   :end-before: //!

Identifying the reply for the client

When the client application receives a reply (for example through |DataReaderListener::on_data_available-api|), the client application should identify if the received reply is the one expected for its request. For this the client application has to compare the stored |SampleIdentity-api| with the incoming |SampleInfo::related_sample_identity-api|.

.. literalinclude:: ../../../../code/DDSCodeTester.cpp
   :language: c++
   :dedent: 4
   :start-after: //REQUEST_REPLY_EXAMPLE_CLIENT_RECEIVE_REPLY
   :end-before: //!