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Nodes
The communication in Gazebo Transport follows a pure distributed architecture. There is no central process, broker or similar central structure. Instead, all the nodes in the network can act as publishers, subscribers, provide services and request services.
A publisher is a node that produces information and a subscriber is a node that consumes information. There are two ways to communicate in Gazebo Transport. First, we could use a publish/subscribe approach, where a node advertises a topic, and then publishes periodic updates. On the other side, one or more nodes subscribe to the same topic registering a function that will be executed each time a new message is received. An alternative communication paradigm is based on service calls (Services), covered in another tutorial. A service call is a remote service that a node offers to the rest of the nodes. A node can request a service in a similar way a local function is executed.
Topics
A topic is simply a name for grouping a specific set of messages or a particular service. Imagine that you have a camera and want to periodically publish its images. Your node could advertise a topic called /image, and then publish a new message on this topic every time a new image is available. Other nodes will subscribe to the same topic and will receive the messages containing the image. A node could also offer an echo service in the topic /echo. Any node interested in this service will request a service call on topic /echo. The service call will accept arguments and will return a result. In our echo service example (Services), the result will be similar to the input parameter passed to the service.
There are some rules to follow when selecting a topic name. It should be any alphanumeric name followed by zero or more slashes. For example: /image, head_position, /robot1/joints/HeadPitch are examples of valid topic names. The next table summarizes the allowed and not allowed topic rules. The following symbols are not allowed as part of the topic name: @, :=, ~.
| Topic name | Validity | Comment |
|---|---|---|
| /topicA | Valid | |
| /topicA/ | Valid | Equivalent to /topicA |
| topicA | Valid | |
| /a/b | Valid | |
| Invalid | Empty string is invalid | |
| my topic | Invalid | Contains white space |
| //image | Invalid | Contains two consecutive // |
| / | Invalid | / topic is not allowed |
| ~myTopic | Invalid | Symbol ~ not allowed |
@myTopic | Invalid | Symbol @ not allowed |
| myTopic:= | Invalid | Symbol := not allowed |
Topic scope
A topic can be optionally advertised with a scope. A scope allows you to set the visibility of a topic. The available scopes are Process, Host, and All. A Process scope means that the advertised topic will only be visible to the nodes within the same process as the advertiser. A topic with a Host scope restricts the visibility of a topic to nodes located on the same machine as the advertiser. Finally, by specifying a scope with an All value, you're allowing your topic to be visible by any node.
Partition and namespaces
When you create your node you can specify some options to customize its behavior. Among those options is setting a partition name and a namespace.
A partition is used to isolate a set of topics or services within a group of nodes that share the same partition name, e.g.: Node1 advertises topic /foo and Node2 advertises /foo too. If we don't use a partition, a node subscribed to /foo will receive the messages published from Node1 and Node2. Alternatively, we could specify p1 as a partition for Node1 and p2 as a partition for Node2. When we create the node for our subscriber, if we specify p1 as a partition name, we'll receive the messages published only by Node1. If we use p2, we'll only receive the messages published by Node2. If we don't set a partition name, we won't receive any messages from Node1 or Node2.
A partition name is any alphanumeric string with a few exceptions. The symbol / is allowed as part of a partition name but just / is not allowed. The symbols @, ~, := or white spaces are not allowed as part of a partition name. Two or more consecutive slashes (//) are not allowed.
The default partition name is created using a combination of your hostname, followed by : and your username, e.g.: bb8:caguero . It's also possible to use the environment variable GZ_PARTITION for setting a custom partition name.
A namespace is considered a prefix that might be potentially applied to some of the topic/services advertised in a node.
E.g.: Node1 sets a namespace ns1 and advertises the topics t1, t2 and /t3. /t3 is considered an absolute topic (starts with /) meaning it won't be affected by a namespace. However, t1 and t2 will be advertised as /ns1/t1 and /ns1/t2.
A namespace is any alphanumeric string with a few exceptions. The symbol / is allowed as part of a namespace but just / is not allowed. The symbols @, ~ or white spaces are not allowed as part of a namespace. Two or more consecutive slashes (//) are not allowed. If topic name or namespace is invalid then the fully qualified topic name is invalid too.
| Namespace | Topic name | Fully qualified topic | Validity | Comment |
|---|---|---|---|---|
| ns1 | /topicA | /topicA | Valid | Absolute topic |
| /topicA | /topicA | Valid | Absolute topic | |
| ns1 | topicA | /ns1/topicA | Valid | |
| topicA | /topicA | Valid | ||
| ns1 | topic A | Invalid | Topic contains white space | |
| topic A | Invalid | Topic contains white space | ||
| my ns | topicA | Invalid | Namespace contains white space | |
| //ns | topicA | Invalid | Namespace contains two consecutive // | |
| / | topicA | Invalid | / namespace is not allowed | |
| ~myns | topicA | Invalid | Symbol ~ not allowed |