User manual

For example, node A could ask node B to read his ﬁrst analog input channel but tell node C what the answer

was. (Imagine for instance that node C has a graphical LCD display that the other nodes lack.)

rpc(node_b_address, "callout", node_c_address, "tellSensorReading", "readAdc", 0)

As another example, node A could ask node B to ﬁnd out how well all the other nodes one hop away could hear

node B, but send the answers to node A by doing something like:

rpc(b_addr, "mcastRpc", 1, 1, "callout", a_addr, "tell_link_quality", "getLq")

Translang the above line of SNAPpy script into English:

Node A is asking node B to send a 1-hop mulcast that asks all nodes in mulcast group number 1 that hear it to

invoke their getLq() funcons (this answering the queson “how well did they hear node B’s mulcast

transmission?”) and send all of the results to node A.

Node A’s script would have a snippet like:

def tell_link_quality(lq):

who = rpcSourceAddr()

# do something with the address and the reported link quality

Clever use of mcastRpc(), rpc(), callback(), and callout() can let you create applicaons like “Network Topology

Explorers”.

Additional Reminder

You cannot invoke a funcon that a node does not have.

In the case of mcastRpc(), the unit will silently ignore the funcon it does not know how to call.

If sent via one of the unicast mechanisms (rpc(), callback() or callout()) the RPC packet will be acknowledged, but

then ignored.

SNAP® Network Operang System 37