yihuang / kademlia

A haskell library implementing the Kademlia DHT

kademlia

kademlia is a haskell libraray implementing the Kademlia distributed hash table.

It aims to be flexible and handle all the low-level work, in order to allow for easy integration of the DHT-technology into applications.

The library is built after the original Kademlia Paper.

For more information, take a look at the libraries hackage page.

Implementation note (to understand some parts of code)

Flow to find nodes by key:

lookupNode => runLookup go inst nid : create chan replyChan, launch go

go:

1. send signal
2. expect registration (reg, replyChan, timeoutTid) for this (signal, nodeId)
3. wait for reply on reg, continue if needed (we handle nodes we know now and check whether we want to query more neighbors)

To wait for reply on (reg, replyChan, timeoutTid) we listen to replyChan

How incoming requests are dispatched, workers:

1. Network.Kademlia.Networking.startRcvProcess:

     read `(signal, nodeId)` from UDP socket, combine it to reply `reply` (pure operation)
     write `reply` to `timeoutChan`
   

2. Network.Kademlia.Instance.receivingProcess:

    read `reply` from `timeoutChan`
    (`notResponse` || `expectedReply`):
        check either:
          * there was a registration `reg` for our `reply`
          * `reply` is a request (not response)
        if call `receivingProcessDo` which calls `dispatch`
   

   Network.Kademlia.ReplyQueue.dispatch:

        * we check whether there is registration corresponding to `reply`
          + if there is registrartion `(reg, replyChan, timeoutTid)` then we
                kill timeout thread by it's id `timeoutTid`
                write `reply` to `replyChan` // futher it would be handled by thread which sent `signal` and was waiting for reply
          + if there is no registration // note, that this may happen only for requests
                write `reply` to `defaultChan`
   

3. Network.Kademlia.Instance.backgraoundProcess:

    read `reply` from `defaultChan` //note that only requests can get here
    `handleCommand` for `reply`