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add more explanation about FastDDS discovery server id maaping. #6107

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ahcorde merged 2 commits into from
Dec 19, 2025
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add more explanation about FastDDS discovery server id maaping. #6107

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7370a6b
add more explanation about FastDDS discovery server id maaping.
fujitatomoya Dec 19, 2025
a534ad1
fix wrong command line argument for "fastdds discovery".
fujitatomoya Dec 19, 2025
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30 changes: 24 additions & 6 deletions source/Tutorials/Advanced/Discovery-Server/Discovery-Server.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -203,13 +203,31 @@ In several terminals, run the following code to establish a communication with r

.. code-block:: console

$ fastdds discovery --server-id 0 --ip-address 127.0.0.1 --port 11811
$ fastdds discovery --server-id 0 --udp-address 127.0.0.1 --udp-port 11811

.. code-block:: console

$ fastdds discovery --server-id 1 --ip-address 127.0.0.1 --port 11888
$ fastdds discovery --server-id 1 --udp-address 127.0.0.1 --udp-port 11888

``--server-id N`` means server with id N. When referencing the servers with ``ROS_DISCOVERY_SERVER``, server ``0`` must be in first place and server ``1`` in second place.
.. important::

**Understanding Server ID Mapping**

The ``ROS_DISCOVERY_SERVER`` environment variable uses a **semicolon-separated list** where each position corresponds to a server ID.
The server ID is determined by the **index position** (0-based) in this semicolon-delimited list, NOT by the order servers appear.

* Server with ``--server-id 0``: First position (no leading semicolon needed)
* Server with ``--server-id 1``: Second position (one leading semicolon)
* Server with ``--server-id 2``: Third position (two leading semicolons)

**Examples:**

* For ``--server-id 0``: ``ROS_DISCOVERY_SERVER="127.0.0.1:11811"``
* For ``--server-id 1``: ``ROS_DISCOVERY_SERVER=";127.0.0.1:11888"``
* For ``--server-id 2``: ``ROS_DISCOVERY_SERVER=";;127.0.0.1:11999"``
* For multiple servers (0 and 1): ``ROS_DISCOVERY_SERVER="127.0.0.1:11811;127.0.0.1:11888"``

If the server ID doesn't match the position in the environment variable, clients will not be able to connect to the server.

.. tabs::

Expand Down Expand Up @@ -263,7 +281,7 @@ In different terminals, run the following code to establish a communication with

.. code-block:: console

$ fastdds discovery --server-id 0 --ip-address 127.0.0.1 --port 11811 --backup
$ fastdds discovery --server-id 0 --udp-address 127.0.0.1 --udp-port 11811 --backup

.. tabs::

Expand Down Expand Up @@ -324,13 +342,13 @@ Run the first server listening on localhost with the default port of 11811.

.. code-block:: console

$ fastdds discovery --server-id 0 --ip-address 127.0.0.1 --port 11811
$ fastdds discovery --server-id 0 --udp-address 127.0.0.1 --udp-port 11811

In another terminal run the second server listening on localhost using another port, in this case port 11888.

.. code-block:: console

$ fastdds discovery --server-id 1 --ip-address 127.0.0.1 --port 11888
$ fastdds discovery --server-id 1 --udp-address 127.0.0.1 --udp-port 11888

Now, run each node in a different terminal.
Use ``ROS_DISCOVERY_SERVER`` environment variable to decide which server they are connected to.
Expand Down