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Mention rmw_specific_{pub,sub}_payload in rmw impl tutorial #5967

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fujitatomoya merged 1 commit into from
Oct 20, 2025
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Mention rmw_specific_{pub,sub}_payload in rmw impl tutorial #5967

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7 changes: 5 additions & 2 deletions source/Tutorials/Advanced/Creating-An-RMW-Implementation.rst
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Expand Up @@ -455,8 +455,11 @@ These tests can use ``rmw``'s ``rmw_get_implementation_identifier()`` `function
Middleware- and ``rmw`` implementation-specific configuration
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The ``rmw`` interface as defined by that package does not support passing arbitrary configuration data to the implementation or the underlying middleware.
To get around that and introduce some flexibility, some implementations use environment variables: ``RMW_FASTRTPS_*``, ``RMW_CONNEXT_*``, etc.
The ``rmw`` interface allows providing arbitrary implementation-specific configuration payloads for publishers and subscriptions through the type-erased ``rmw_specific_publisher_payload`` / ``rmw_specific_subscription_payload`` fields in ``rmw_publisher_options_t`` / ``rmw_subscription_options_t``.
This is set by users through ``RMWImplementationSpecificPublisherPayload`` / ``RMWImplementationSpecificSubscriptionPayload`` in ``rclcpp``, for example.
This is an advanced, non-portable feature that is not currently used by any (tier 1) implementations.
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@christophebedard christophebedard Oct 20, 2025

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I thought it was interesting to point out that this is currently not used by any common/tier 1 implementation, just to discourage people from using it, but I'm open to removing that part of the sentence.

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@fujitatomoya fujitatomoya Oct 20, 2025

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IMO this can stay here as it is implemented. to be honest, i did not even know this interface to pass any payload to the rmw... thanks for pointing that out 👍


For a bit more flexibility, some implementations use environment variables: ``RMW_FASTRTPS_*``, ``RMW_CONNEXT_*``, etc.
The underlying middleware may also be configurable through environment variables: ``FASTDDS_*``, ``ZENOH_*``, ``CYCLONEDDS_*``, ``EMAIL_*``, etc.
For example, the ``CYCLONEDDS_URI``, ``FASTDDS_DEFAULT_PROFILES_FILE``, and ``ZENOH_SESSION_CONFIG_URI`` environment variables can be used to provide a path to a full configuration file if the relevant middleware is used.

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