Handling dead nodes in ROS
By default ROS doesn’t indicate what is the state of a particular node. If a node exits abnormally (for example when an exception is raised, or a computer running the node is power cycled) ROS master will still hold a reference to this node. Fortunately there are some tools that can help you check/clean the node status.
Couple of moths ago I noticed that ROS master in my setup was holding references to the nodes run on a remote machine, even after it has been shut down. Not being able to trust the list of ROS nodes was far from ideal in my case hence I started looking for a solution. I asked a question on answers.ros.org and was pointed to similar issues: rosnode zombies, abnormal exited rosnode still be seen on Master?.
Here are some approaches that I’ve found out about / I’ve been pointed towards:
Brute force cleanup
According to the docs cleanup will purge registration of any nodes that can’t be contacted immediately. Depending on your use case this might be suboptimal, if the network interface on a remote machine is down for a second or you cannot contact a node for any other reason you are risking purging a relatively happy node.
A bond package
Via ROS wiki: A bond allows two processes, A and B, to know when the other has terminated, either cleanly or by crashing. The bond remains connected until it is either broken explicitly or until a heartbeat times out.
In practice I’ve seen it applied to nodelets. I believe that out of the box 1-1 relationship is supported but maybe, with some clever hacking, it could be extended.
Ping - node alive
rosnode ping allows you to, you guessed it, ping a node (or all of them). The nice thing about rosnode ping is that it’s non-destructive (compared to rosnode cleanup). I really like the node_alive package that nicely wraps a call rosnode ping -a and outputs all the node statuses (either alive, removed, not started or dead) as a DiagnosticArray message.
There is one potentially big issue with rosnode ping -a - it has a timeout of 3 seconds per node (can go up to 30 seconds before a bugfix is applied). This brings the following points:
- You can’t use node_alive for any critical code that isn’t designed to handle the delays in some way
- If you are anticipating to lose many nodes - beware of the delay caused by timeouts (if 5 nodes are out then you are most likely looking at receiving an update from node_alive every 15 seconds + the time it takes to ping the healthy nodes)
Well architected system
Assuming you have full control over the nodes you use, you can embed a watchdog node in your system that will monitor the state of all the crucial nodes and act accordingly in case you establish any one of them timed out.