Is it possible to share global variables between nodes?

edit

it seems to me - no. Nodes - are different processes.

Maybe you can use the parameter server... "Nodes [...] communicate with one another using streaming topics, RPC services, and the Parameter Server."

EDIT: In order to mantain modularity and independent nodes, you can modify (increment?) a parameter '/pub_counter' everytime you publish on a topic. On the other side you can check this parameter within your callback function.

You could use a shared memory buffer if you really want to have a shared global state. Or, you could publish the number of messages published on a separate topic. Anyone interested could subscribe to that topic. In the callback, you would copy the number from the message into a local variable, and this should (most of the time) correspond to the actual number of sent messages, from which you can calculate the number in the queue.

Maybe you could create a service call where the publisher can ask the subscriber how many items in your queue, but i am not sure how to check the size of the message buffer

Perhaps the nodes could put their information in shared memory or mailboxes for message passing?

I have heard that global variables are bad and you better use "var" before the references. But is this a case where global variables are good?...

The Node-architecture is imho not very suitable for a global variable. To keep track of a FIFO-load level you would typically go for a handshake protocol, like the proposal with the ack.

If you really want to have a shared memory my proposal would be to create an additional Node, let's call it "RAM", that offers a suitable set of services and topics to read/write to it. This way you have global storage for all your Nodes.