Add Node to Participant mapping article (#250)

Signed-off-by: ivanpauno <[email protected]>

Author: ivanpauno

articles/151_node_to_participant_mapping.md

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+---
+layout: default
+title: Node to Participant mapping
+permalink: articles/Node_to_Participant_mapping.html
+abstract: This article analyzes the performance implications of enforcing a one-to-one mapping between ROS Nodes and DDS Participants, and proposes alternative implementation approaches.
+author: '[Ivan Paunovic](https://github.com/ivanpauno)'
+published: true
+categories: Middleware
+---
+
+{:toc}
+
+# {{ page.title }}
+
+<div class="abstract" markdown="1">
+{{ page.abstract }}
+</div>
+
+Original Author: {{ page.author }}
+
+## Background
+
+### Node
+
+In ROS, a Node is an entity used to group other entities.
+For example: Publishers, Subscriptions, Servers, Clients.
+Nodes ease organization and code reuse, as they can be composed in different ways.
+
+### Domain Participant
+
+A Participant is a type of DDS entity.
+Participant also group other entities, like Publishers, Subscribers, Data Writters, Data Readers, etc.
+Creating more Participants adds overhead to an application:
+
+- Each Participant participates in discovery.
+  Creating more than one Participant usually increases CPU usage and network IO load.
+- Each Participant keeps track of other DDS entities.
+  Using more than one within a single process may result in data duplication.
+- Each Participant may create multiple threads for event handling, discovery, etc.
+  The number of threads created per Participant depends on the DDS vendor (e.g.: [RTI Connext](https://community.rti.com/best-practices/create-few-domainParticipants-possible)).
+
+For those reasons, a Participant is a heavyweight entity.
+
+Note: This might actually depend on the DDS vendor, some of them share these resources between Participants (e.g. `OpenSplice`).
+Many DDS vendors, however, do not perform this kind of optimization (e.g.: `RTI Connext` and `Fast-RTPS`), and actually recommend creating just one Participant per process.
+
+### Context
+
+In ROS, a Context is the non-global state of an init-shutdown cycle.
+It also encapsulates shared state between Nodes and other entities.
+In most applications, there is only one ROS Context in a process.
+
+## Behavior pre-Foxy
+
+There is a one-to-one mapping between Nodes and DDS Participants.
+This simplified the original implementation, as DDS Participants provide many features equivalent to the ones of ROS Nodes.
+The drawback of this approach is the overhead that comes with creating many Participants.
+Furthermore, the maximum number of Domain Participants is rather small.
+For example, in [RTI Connext](https://community.rti.com/kb/what-maximum-number-Participants-domain) it is limited to 120 Participants per Domain.
+
+## Proposed approach
+
+The goal of this proposal is to improve overall performance by avoiding the creation of one Participant per Node.
+API changes will be avoided, if possible.
+
+### Mapping of DDS Participant to a ROS entity
+
+There are two alternatives, besides the one-to-one Node to Participant mapping used pre-Foxy:
+- Using one Participant per process.
+- Using one Participant per Context.
+
+The second approach is much more flexible, allowing more than one Participant in a single application for those that need it e.g. domain bridge applications.
+Thus, a one-to-one Participant to Context mapping was chosen.
+
+When multiple Nodes are running in a single process, there are different options for grouping them by - ranging from a separate context for each Node, over grouping a few Nodes in the same context, to using a single context for all Nodes.
+For most applications, only one Context is created.
+
+### Discovery information
+
+If a one-to-one Node to Participant mapping is not used, extra discovery information is needed to be able to match other entities to a Node e.g. Publishers, Subscriptions, etc.
+Several approaches can be used to share this information.
+The proposed approach uses a topic for it.
+Each Participant publishes a message with all the information needed to match an entity to a Node.
+The message structure is the following:
+
+* ParticipantInfo
+  * gid
+  * NodeInfo
+    * Node namespace
+    * Node name
+    * Reader gid
+    * writed gid
+
+When one entity is updated (e.g.: a Publisher is created or destroyed), a new message is sent.
+
+Identification of Clients and Servers happens according to the ROS conventions for their topic names (see [	
+Topic and Service name mapping to DDS](140_topic_and_service_name_mapping.md)).
+
+This topic is considered an implementation detail, and not all `rmw` implementations have to use it.
+Thus, all the necessary logic has to be in the rmw implementation itself or in an upstream package.
+Implementing this logic in `rcl` would make it part of the API, and not an implementation detail.
+
+To avoid code repetition, a common implementation of this logic is provided by the [rmw_dds_common](https://github.com/ros2/rmw_dds_common/) package.
+
+#### Details of the ROS discovery topic
+
+- topic name: `ros_discovery_info`
+- Writer qos:
+  - durability: transient local
+  - history: keep last
+  - history depth: 1
+  - reliability: reliable
+- Reader qos:
+  - durability: transient local
+  - history: keep all
+  - history depth: 1
+  - reliability: reliable
+
+## Other implications
+
+#### Security
+
+Previously, each Node could have different security artifacts.
+That was possible because each Node was mapped to one Participant.
+The new approach allows to specify different security artifacts for each process.
+For more details, see [ROS 2 Security Enclaves](ros2_security_enclaves.md).
+
+#### Ignore local publications option
+
+There is an `ignore_local_publications` option that can be set when [creating a Subscription](https://github.com/ros2/rmw/blob/2250b3eee645d90f9e9d6c96d71ce3aada9944f3/rmw/include/rmw/rmw.h#L517).
+That option avoids receiving messages from Publishers within the same Node.
+This wasn't implemented in all the rmw implementations (e.g.: [FastRTPS](https://github.com/ros2/rmw_fastrtps/blob/099f9eed9a0f581447405fbd877c6d3b15f1f26e/rmw_fastrtps_cpp/src/rmw_Subscription.cpp#L118)).
+
+After this change, implementing this feature will be less direct.
+Some extra logic needs to be added in order to identify from which Node a Publisher was created.
+
+## Alternative implementations
+
+### Using keyed topics
+
+Keyed topics could be used, with the Participant gid as the key.
+That would allow the Reader side of the topic to use a keep last, depth 1 history.
+
+### Using Participant/Writer/Reader userData QoS
+
+Instead of using a custom topic to share the extra ROS specific discovery information, a combination of Participant/Reader/Writer `userData` could be used:
+
+- Participant `userData`: list of Nodes created by the Participant (updated when Node created destroyed).
+- Reader user `userData`: Node name/namespace.
+- Writer user `userData`: Node name/namespace.
+
+That information would be enough to satisfy ROS required graph API.
+This mechanism would also preclude having a topic that has to be read and written by all Nodes, which is better from a security perspective.
+
+This alternative wasn't implemented because of lack of support in some of the currently supported DDS vendors.
+
+## Further work
+
+This optimization has been applied to `rmw_cyclonedds`, `rmw_fastrtps_cpp` and `rmw_fastrtps_dynamic_cpp`.
+Other DDS based `rmw` implementations, like `rmw_connext_cpp` could use the same approach.