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High Level

Nodelets are designed to provide a way to run multiple algorithms on a single machine, in a single process, without incurring copy costs when passing messages intraprocess. roscpp has optimizations to do zero copy pointer passing between publish and subscribe calls within the same node. To do this nodelets allow dynamic loading of classes into the same node, however they provide simple separate namespaces such that the nodelet acts like a seperate node, despite being in the same process. This has been extended further in that it is dynamically loadable at runtime using pluginlib.

Applications

• high throughput data flows can be composed of many nodelets and then loaded into the same process to avoid copying and network traffic.

Design Goals

• use the existing C++ ROS interfaces.
• allow zero copy passing of data between nodelets
• dynamically load as plugins to break build time dependencies
• location transparent except for performance improvements
• writing code in a node or a nodelet will be minimally different.

Technique

• Define a base class nodelet::Nodelet which will be used for dynamic loading. All nodelets will inherit from this base class, and be dynamically loadable using pluginlib.
• It will provide the namespace, remapping arguments and parameters automatically, like they were a first class node.
• There will be a nodelet_manager process into which one or more nodelets can be loaded. Any communications between them can use the zero copy roscpp publish call with a boost shared pointer.

Basic Usage

nodelet usage:
nodelet load pkg/Type manager - Launch a nodelet of type pkg/Type on manager manager
nodelet standalone pkg/Type   - Launch a nodelet of type pkg/Type in a standalone node
nodelet unload name manager   - Unload a nodelet a nodelet by name from manager
nodelet manager               - Launch a nodelet manager node

For command line and launch file examples see this tutorial Running a nodelet

API

Nodelet Base Class:

nodelet::Nodelet

Public methods:

   1 Nodelet() //Default constructor used when dynamically loaded
   2 void init (const std::string& name, const ros::M_string& remapping_args, const std::vector<std::string>& my_argv); // This method is how a nodelet should be started.  The arguments are what is required from the manager to start the nodelet.  This will initialize the nodelet base class and then call the subclass's onInit() method.
   3 

Protected members and methods for use in subclass:

   1 std::string             getName() //Get the name of the nodelet
   2 ros::NodeHandle&        getNodeHandle () // Get the node handle (provides this nodelets custom remappings and name)
   3 ros::NodeHandle&        getPrivateNodeHandle () // Get the private node handle (provides this nodelets custom remappings in its private namespace)
   4 ros::NodeHandle&        getMTNodeHandle () // Get the node handle with the Multi Threaded callback queue. (provides this nodelets custom remappings and name)
   5 ros::NodeHandle&        getMTPrivateNodeHandle () // Get the private node handle with the Multi Threaded callback queue. (provides this nodelets custom remappings in its private namespace)
   6 ros::CallbackQueue&     getMTCallbackQueue () // Get the callback queue (threadpool available from the manager)
   7 std::vector<std::string> getMyArgv() // Get command line arguments to the nodelet stripped of ROS and nodelet specific args.  
   8 

Initialization method used to start ROS API in subclass:

   1 virtual void onInit () = 0 //Virtual and must be overridden by subclass. All initialization of the ROS infrastructure must be put into this function.
   2 

NODELET ROSCONSOLE MACROS

These are nodelet aware wrappers around rosconsole macros. They include verbosity levels DEBUG, INFO, WARN, ERROR, and FATAL. These macros will only compile inside nodelet methods.

They operate by setting up a named logger in the name of the nodelet running so that you can differentiate between the output of two of the same type nodelets running on the same manager. They also have the advantage that you can turn one specific nodelet into debug, instead of all nodelets of a specific type.

   1 #include "nodelet/nodelet.h"
   2 
   3 //... inside a nodelet method
   4 NODELET_DEBUG("My debug statement")
   5 NODELET_DEBUG_STREAM("my debug statement " << (double) 1.0)
   6 NODELET_DEBUG_COND( 1 == 1, "my debug_statement")
   7 NODELET_DEBUG_STREAM_COND( 1 == 1, "my debug statement " << (double) 1.0)

Publishing from a Nodelet

If you want the no-copy pub/sub to work you must publish your messages as shared_ptrs. See roscpp/Overview/Publishers and Subscribers#Intraprocess_Publishing for more details.

Threading Model

A nodelet manager has a pool of threads which is shared across all nodelets run within the manager. This is set by the parameter "num_worker_threads".

There are two possible threading APIs to use in code running in nodelets. The default threading model has a single thread for all callbacks. There is a multithreaded API as well.

onInit

This method is called on init and should not block or do significant work.

Single Threaded API

Using the methods getNodeHandle() and getPrivateNodeHandle() will guarantee that all callbacks arrive serially.

Multi Threaded API

Using the methods getMTNodeHandle() and getMTPrivateNodeHandle() callbacks will be distributed over the thread pool of the manager.

Additional Threads

It is valid operation for a nodelet to create its own threads for operation. These threads should be cleaned up properly in the destructor.

Thread Sharing

All nodelets share the thread pool of the manager. If nodelets are blocking threads they may prevent other nodelets from getting callbacks. Make sure the manager is configured with enough threads to prevent blocking. Note: Even the single threaded Node Handles can consume 1 thread of the pool per nodelet.

Helper tools

List all declared nodelets

Following commands are helpful to list all nodelets available on your system found in ROS_PACKAGE_PATH. Note that it's NOT the list of currently running nodelet nor nodelet managers.

rosrun nodelet declared_nodelets

Or list the nodelet xml files for pluginlib by:

rospack plugins --attrib=plugin nodelet

Report a Bug

File a Ticket