nim-libp2p/examples/tutorial_2_customproto.md

# Custom protocol in libp2p

In the [previous tutorial](tutorial_1_connect.md), we've looked at how to create a simple ping program using the `nim-libp2p`.

We'll now look at how to create a custom protocol inside the libp2p

Let's create a `part2.nim`, and import our dependencies:
```nim
import chronos
import stew/byteutils

import libp2p
```
This is similar to the first tutorial, except we don't need to import the `Ping` protocol.

Next, we'll declare our custom protocol
```nim
const TestCodec = "/test/proto/1.0.0"

type TestProto = ref object of LPProtocol
```

We've set a [protocol ID](https://docs.libp2p.io/concepts/protocols/#protocol-ids), and created a custom `LPProtocol`. In a more complex protocol, we could use this structure to store interesting variables.

A protocol generally has two part: and handling/server part, and a dialing/client part.
Theses two parts can be identical, but in our trivial protocol, the server will wait for a message from the client, and the client will send a message, so we have to handle the two cases separately.

Let's start with the server part:
```nim
proc new(T: typedesc[TestProto]): T =
  # every incoming connections will in be handled in this closure
  proc handle(conn: Connection, proto: string) {.async, gcsafe.} =
    # Read up to 1024 bytes from this connection, and transform them into
    # a string
    echo "Got from remote - ", string.fromBytes(await conn.readLp(1024))
    # We must close the connections ourselves when we're done with it
    await conn.close()

  return T(codecs: @[TestCodec], handler: handle)
```
This is a constructor for our `TestProto`, that will specify our `codecs` and a `handler`, which will be called for each incoming peer asking for this protocol.
In our handle, we simply read a message from the connection and `echo` it.

We can now create our client part:
```nim
proc hello(p: TestProto, conn: Connection) {.async.} =
  await conn.writeLp("Hello p2p!")
```
Again, pretty straight-forward, we just send a message on the connection.

We can now create our main procedure:
```nim
proc main() {.async, gcsafe.} =
  let
    rng = newRng()
    testProto = TestProto.new()
    switch1 = newStandardSwitch(rng=rng)
    switch2 = newStandardSwitch(rng=rng)

  switch1.mount(testProto)

  await switch1.start()
  await switch2.start()

  let conn = await switch2.dial(switch1.peerInfo.peerId, switch1.peerInfo.addrs, TestCodec)

  await testProto.hello(conn)

  # We must close the connection ourselves when we're done with it
  await conn.close()

  await allFutures(switch1.stop(), switch2.stop()) # close connections and shutdown all transports
```

This is very similar to the first tutorial's `main`, the only noteworthy difference is that we use `newStandardSwitch`, which is similar to the `createSwitch` of the first tutorial, but is bundled directly in libp2p

We can now wrap our program by calling our main proc:
```nim
waitFor(main())
```

And that's it!