standalone_complexed_nodelet

Why needed?

nodelet is a good framework to decrease overhead of network communication between ROS nodes.

roscpp has a feature to communicate topics by passing reference pointers if publisher and subscriber are in the same process. By taking advantage of this feature, nodelet can communicate without network overhead each other.

However, nodelet has a problem in system architecture of server-client model. server is a manager and all the actual computation run in this process. client just sends a request to manager to load some classes in the manager process by dynamic loading.

The issue occurs in bond between server and client. If server is killded, client should be killed as well. If a client is killed, the class which is requested by the client loaded in server process should be unloded. nodelet implements this functionality by bond package. server publishes heatbeat topic in fixed rate and client subscribes the topic. If client detects stoll of the topic, client regards manager is dead and kill itself and request unloading to the manager.

This mechanism works well if nodelet system is relatively small but if we run a lot of nodelet, it does not work because communication via ROS topics is not reliable and somtimes it has a lot of latency.

This “unreliable bond mechanism” issue occurs a lot and standalone_complexed_nodelet can solve it. standalone_complexed_nodelet is a nodelet manager but it does not require client to send request. standalone_complexed_nodelet knows nodelet classes via ros parameter instead of server-client model. Limitation of standalone_complexed_nodelet is that it does not support dynamic loading/unloading of nodelet classes but we know we do not need the feature from our experiences.

We recommend to use standalone_complexed_nodelet instead of normal nodelet.

APIs

standalone_complexed_nodelet read definition of nodelet classes from ros parameter. The parameter are ~nodelets, ~nodelets_0, ..., ~nodelets_99. You can extend the suffix numbers by ~candidate_num parameter. default is 100. These multiple parameters (~nodelets_0, nodelets_1, ...) are useful to include multiple launch files and run one standalone_complexed_nodelet.

nodelet definition should follow next format:

nodelets:
  - name: <name of nodelet, required>
    type: <type of nodelet, required>
    if: <the defnition is enabled only if this field is true, optional>
    unless: <the defnition is enabled only if this field is false, optional>
    remappings: <field to lead remapping definition, optional>
      - from: <ros name to be remapped>
        to: <ros name to remap to>
      - from:
        to:
        ...
  - name:
    type:
    remappings:
    ...

Examples

• simple.launch