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re-publishing data from multiple topics

asked 2013-04-13 16:19:01 -0600

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I've written a node which takes data from node A via a topic, which uses custom messages, does some manipulation with it, then publishes it to another topic. However the node also listens to a topic from node B, which is not always active. I'm not sure how I go about getting the node to use data from node B when it is active, but using node A otherwise?

I've tried a few different ways and come across different issues. Firstly I was thinking of somehow being able to switch to listen to the higher priority topic B, when the node is initiated.

I also tried to play with some if statements in the callback, but realised the callback can only deal with 1 set of custom messages at once.

Note: this is using python

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answered 2013-04-15 17:19:09 -0600

joq gravatar image

A crude solution:

  • Keep track of when you last received a message from node B.

  • When a message arrives from node A, ignore it if the last node B message was more recent than some timeout threshold. Otherwise, handle it normally.

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+1 from me. Using some kind of timeout is the only solution i can think of for this problem. You may also read this answer regarding a somewhat similar question:

Ben_S gravatar image Ben_S  ( 2013-04-16 04:34:53 -0600 )edit

Since your messages are custom, you could send a "good-bye" message from node B, after which the subscriber would switch back to handling node A messages immediately.

joq gravatar image joq  ( 2013-04-16 05:12:26 -0600 )edit

Cheers, guys. Yeah I was just thinking that joq, but I was thinking the opposite way around, i.e. node B has a "hello" message, or an "on" message that is sent with each data set. I'm not quite sure how to implement this though.

luke91 gravatar image luke91  ( 2013-04-17 00:06:35 -0600 )edit

Edit: managed to work it out, think I've gone about it the long way but it works!

luke91 gravatar image luke91  ( 2013-04-17 00:10:26 -0600 )edit

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Asked: 2013-04-13 16:19:01 -0600

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Last updated: Apr 15 '13