How to get the Peer count in an Off Chain Worker?

Question

I have an Off Chain Worker that validates some data in IPFS and then calls a function on chain to set the status to valid or invalid. That's all working fine.

However, I do not need all nodes to validate all of the data. To avoid wasting node resources I would like to rotate the work of validation between nodes. For example, imagine my network has 5 nodes, on block 1 node 1 runs the validation, on block 2 node 2, on block 3 node 3 etc.

Given that it doesn't matter if there are occasional blocks when 2 nodes run validation or blocks when no node does, my idea has been to use a random number generator in the off chain worker for each node to decide whether to do validation or not. The offchain worker would do something like this:

let peer_count = no. of peers
let random_int = random integer between 0 and peer_count
if random_int == 0 {Carry out the valdiation}
else { quit }

So in my example with 5 nodes, each node will do the validation, on average, once every 5 blocks.

Here's my question - how do I get the no. of peers?

Answer 1

There is no way to get the number of peers of your node. Doing that would also only work as long as all nodes are connected to each other, which isn't working in a really decentralized network. So, you will need to do something different. You will not be able to prevent that multiple nodes are validating the same data. From your short description it actually sounds like you want/need multiple nodes that validate the data. How do you want to ensure that when only one node is validating the data, it isn't lying? That it actually verified the data correctly.

Now to a possible solution. You can use the random_seed function to give you a random seed.

let random_seed = sp_io::offchain::random_seed();

let number = u32::decode(&mut &random_seed[..]).expect("The seed is 32 byte and `u32` can be decoded from any 4 bytes; qed");

if number % 100 < 50 {
    do_the_validation();
} 

Be aware that for you real implementation you should ensure that always multiple nodes are validating the data and sending the result to the runtime.