2.1. DDS Domain ID change¶

A very typical scenario within the DDS ecosystem is that of two applications running under different DDS domain IDs, so that they are isolated from each other; however, it might be required to bridge some of the topics published by one of the applications, so that a subscriber on a different domain ID can consume this information. This is where the Integration Service plays a fundamental role, by allowing to bridge two DDS dataspaces easily.

The steps described below allows a Fast DDS HelloWorld publisher application, running under a certain domain ID, to communicate with a Fast DDS HelloWorld subscriber application, which is running under a different domain ID.

2.1.1. Requirements¶

To prepare the deployment and setup the environment, you need to have Integration Service correctly installed in your system. To do so, please follow the steps delineated in the Installation section.

Also, to get this example working, the following requirements must be met:

Having Fast DDS (v.2.0.0 or superior) installed and the Integration Service DDSHelloWorld example working. This example can be found in the main Integration Service repository, under the examples/utils/dds/DDSHelloWorld folder; to compile it, you can either compile the whole Integration Service project using colcon with the CMake flag BUILD_EXAMPLES enabled; or execute the following steps:
```
cd ~/is-workspace/src/Integration-Service/examples/utils/dds/DDSHelloWorld
mkdir build && cd build
cmake .. -DBUILD_EXAMPLES=ON && make
```
Having the Fast DDS System Handle installed. You can download it from the dedicated repository into the is-workspace where you have Integration Service installed:
```
cd ~/is-workspace
git clone https://github.com/eProsima/FastDDS-SH.git src/FastDDS-SH
```

After you have everything correctly installed in your is-workspace, build the packages by running:

colcon build --cmake-args -DBUILD_FASTDDS_EXAMPLES=ON

2.1.2. Deployment¶

Below we explain how to deploy an example of this use case. To do so, open three terminals:

In the first terminal, execute the DDSHelloWorld example, as a subscriber running under DDS domain ID 3 and subscribed to the topic hello_domain_3:
```
cd ~/is-workspace/build/is-examples/dds/DDSHelloWorld
./DDSHelloWorld -m subscriber -n hello_domain_3 -d 3
```
In the second terminal, execute the DDSHelloWorld example, as a publisher running under DDS domain ID 5 and publishing data to the topic hello_domain_3:
```
cd ~/is-workspace/build/is-examples/dds/DDSHelloWorld
./DDSHelloWorld -m publisher -n hello_domain_3 -d 5
```

Up to this point, no communication should be seen between the publisher and the subscriber, due to the domain ID change. This is where Integration Service comes into play to make the communication possible.

In the third terminal, go to the is-workspace folder, source the local installations, and execute Integration Service with the integration-service command followed by the fastdds__domain_id_change.yaml configuration file located in the src/Integration-Service/examples/basic folder:
```
cd ~/is-workspace
source install/setup.bash
integration-service src/Integration-Service/examples/basic/fastdds__domain_id_change.yaml
```

Once the last command is executed, the two applications will start communicating.