1. Entities

The monitoring functions of this application relay on tracking the activity of certain Entities retrieved by the Fast DDS Monitor, as well as on storing connections and interchanged data between them for their presentation to the user. These entities represent DDS communication entities, or different physical elements related to DDS entities.

In the following diagram one can see the different kinds of entities being tracked by the monitor. The arrows in this schema represent a direct connection (not a 1:n in every case) between both kinds of entities. Those entities (or types of entities) that do not have a direct relationship are also related, as intermediate entities make it possible to relate any entity in the graph to one another indirectly, i.e. one DomainParticipant is related with its User by the relation with its Process.


1.1. DDS Entities

These entities represent the DDS entities that manage the communication. That is, the DomainParticipants, DataWriters and DataReaders. Each DataReader/DataWriter has one or more associated Locator entities. Locators are the network addresses through which DataReaders/DataWriters communicate in a DDS network.

For further information about each entity, please refer to the DDS specification or visit the Fast DDS Documentation.

1.1.1. DomainParticipant

DomainParticipant is the main entity in the DDS protocol. It represents a collection of DataReaders/DataWriters, and manage the whole DDS Discovery of other DomainParticipants and DataReaders/DataWriters within the DDS Domain to which it belongs. Refer to DomainParticipant Fast DDS Documentation for a more detailed explanation of the DomainParticipant entity in DDS.

Each DomainParticipant can only communicate under one Domain (see Logical Entities) and so it exists a direct connection between each DomainParticipant and the Domain in which it works. From image fig_entities_diagram it can be seen that DomainParticipant entities are contained in a Process, this is because a system process (so-called Process entity) executes an application using Fast DDS that instantiates DomainParticipants. The same applies to DataReaders/DataWriters instantiated by a DomainParticipant belonging to a specific Process. Therefore, a Process can contain as many DDS entities as the Fast DDS application running in that Process has instantiated.

1.1.2. DataWriter

DataWriter is the DDS entity in charge of publishing data. Each DataWriter is directly contained in a single DomainParticipant. In addition, since a DataWriter is associated to the Topic under which it publishes, it is possible to define a direct containment relationship of a DataWriter in a Topic. Thus, the Topic will contain all the DataWriters that are publishing under it.

Therefore, each DataWriter is directly connected with the DomainParticipant it belongs, and with the Topic under which it publishes. Also, a DataWriter is associated with one or multiple Locators that would represent the physical communication channel this DataWriter is using to send data.

1.1.3. DataReader

DataReader holds the subscribe function of the communication. As for the DataWriter, each DataReader is directly contained in a single DomainParticipant. In addition, since a DataReader is associated to the Topic to which it is subscribed, it is possible to define a direct containment relationship of a DataReader in a Topic. Thus, the Topic will contain all the DataReaders that are subscribed under it. Therefore, each DataReader is directly connected with the DomainParticipant to which it belongs, and with the Topic to which it is subscribed.

1.1.4. Locator

Locator represents the physical address and port that a DataReader/DataWriter uses to send or/and receive data. This entity is related with the physical division of the entities, as a Locator belongs to a unique Host (see section Physical Entities). However, the monitor treats this entity as a DDS Entity in order to simplify the entities’ connection and improve comprehensibility.

A Locator is connected with one or multiple DataReader/DataWriter, and so it is related with a Host by relating each of these DataReader/DataWriter with a DomainParticipant and each DomainParticipant with its Host.

1.2. Logical Entities

1.2.1. Domain

Domain represents a logical abstraction in DDS protocol that divides the DDS network into partitions, making each Domain completely independent and unaware of others. This logical partition depends on the chosen discovery protocol.

In case of using Simple Discovery Protocol (discovery protocol by default in Fast DDS) as the default discovery mechanism, the Domain will be represented by a number (domain ID), and every DDS entity in that Domain will discover the rest of entities deployed on the same Domain. In case of using Discovery Server as discovery protocol, the partition will be made by the Discovery Server or Discovery Servers Net to which the monitor connects. Please refer to Fast DDS documentation for more information about this feature. Each entity connected to a Discovery Server on the same network will know all other entities with which it needs to communicate.

This entity inside the monitor is related with the DomainParticipants that communicate under this same Domain, and the Topics created in this Domain.

1.2.2. Topic

Topic is an abstract DDS entity that represents the channel of communications between publishers and subscribers. Every DataReader subscribed to a Topic will receive the publications of every DataWriter publishing under the same Topic.

This entity inside the monitor is directly connected with the Domain it belongs to, and with the DataReaders/DataWriters communicating under this Topic.

1.3. Physical Entities

1.3.1. Host

Host makes reference to the physical (or logical i.e. Docker) machine where one or more DDS Processes are running. This entity is connected directly with the Users running in this Host.

1.3.2. User

User makes reference to the different users that could run in a Host.

This entity is connected directly with the Host it belongs to, and the running Processes that are being executed within this User.

1.3.3. Process

Process represents each process running an application using Fast DDS. Be aware that it is possible to run more than one DomainParticipant in the same Process, and they do not require to be related with each other, not even under the same Domain.

This entity is connected directly with the User to which it belongs, and the Participants running within it.