nym/documentation/docs/pages/developers/rust/tcpproxy/examples/multiconn.mdx

# Multi Connection Example

This example starts off several Tcp connections on a loop to a remote endpoint: in this case the `TcpListener` behind the `NymProxyServer` instance on the echo server found in
[`nym/tools/echo-server/`](https://github.com/nymtech/nym/tree/develop/tools/echo-server). It pipes a few messages to it, logs the replies, and keeps track of the number of replies received per connection.

> You can find this code [here](https://github.com/nymtech/nym/blob/develop/sdk/rust/nym-sdk/examples/tcp_proxy_multistream.rs)

```rust
use nym_sdk::mixnet::Recipient;
use nym_sdk::tcp_proxy;
use rand::rngs::SmallRng;
use rand::Rng;
use rand::SeedableRng;
use serde::{Deserialize, Serialize};
use std::env;
use tokio::io::AsyncWriteExt;
use tokio::net::TcpStream;
use tokio::signal;
use tokio_stream::StreamExt;
use tokio_util::codec;

#[derive(Serialize, Deserialize, Debug)]
struct ExampleMessage {
    message_id: i8,
    message_bytes: Vec<u8>,
    tcp_conn: i8,
}

// To run:
// - run the echo server with `cargo run`
// - run this example with `cargo run --example tcp_proxy_multistream -- <ECHO_SERVER_NYM_ADDRESS> <ENV_FILE_PATH> <CLIENT_PORT>` e.g.
// cargo run --example tcp_proxy_multistream -- DMHyxo8n6sKWHHTVvjRVDxDSMX8gYXRU1AQ6UpwsrWiB.6STYCWGWyRxqn2juWdgjMkAMsT9EaAzPpLWq5zkS68MB@CJG5zTcmoLijmDrtAiLV9PZHxNz8LQu6hmgA89V2RxxL ../../../envs/canary.env 8080
#[tokio::main]
async fn main() -> anyhow::Result<()> {
    let server_address = env::args().nth(1).expect("Server address not provided");
    let server: Recipient =
        Recipient::try_from_base58_string(&server_address).expect("Invalid server address");

    // Comment this out to just see println! statements from this example.
    // Nym client logging is very informative but quite verbose.
    // The Message Decay related logging gives you an ideas of the internals of the proxy message ordering: you need to switch
    // to DEBUG to see the contents of the msg buffer, sphinx packet chunking, etc.
    tracing_subscriber::fmt()
        .with_max_level(tracing::Level::INFO)
        .init();

    let env_path = env::args().nth(2).expect("Env file not specified");
    let env = env_path.to_string();

    let listen_port = env::args().nth(3).expect("Port not specified");

    // Within the TcpProxyClient, individual client shutdown is triggered by the timeout.
    let proxy_client =
        tcp_proxy::NymProxyClient::new(server, "127.0.0.1", &listen_port, 45, Some(env)).await?;

    tokio::spawn(async move {
        proxy_client.run().await?;
        Ok::<(), anyhow::Error>(())
    });

    println!("waiting for everything to be set up..");
    tokio::time::sleep(tokio::time::Duration::from_secs(5)).await;
    println!("done. sending bytes");

    // In the info traces you will see the different session IDs being set up, one for each TcpStream.
    for i in 0..4 {
        let conn_id = i;
        println!("Starting TCP connection {}", conn_id);
        let local_tcp_addr = format!("127.0.0.1:{}", listen_port.clone());
        tokio::spawn(async move {
            // Now the client and server proxies are running we can create and pipe traffic to/from
            // a socket on the same port as our ProxyClient instance as if we were just communicating
            // between a client and host via a normal TcpStream - albeit with a decent amount of additional latency.
            //
            // The assumption regarding integration is that you know what you're sending, and will do proper
            // framing before and after, know what data types you're expecting; the proxies are just piping bytes
            // back and forth using tokio's `Bytecodec` under the hood.

            let stream = TcpStream::connect(local_tcp_addr).await?;
            let (read, mut write) = stream.into_split();

            // Lets just send a bunch of messages to the server with variable delays between them, with a message and tcp connection ids to keep track of ordering on the server side (for illustrative purposes **only**; keeping track of anonymous replies is handled by the proxy under the hood with Single Use Reply Blocks (SURBs); for this illustration we want some kind of app-level message id, but irl most of the time you'll probably be parsing on e.g. the incoming response type instead)
            tokio::spawn(async move {
                for i in 0..4 {
                    let mut rng = SmallRng::from_entropy();
                    let delay: f64 = rng.gen_range(2.5..5.0);
                    tokio::time::sleep(tokio::time::Duration::from_secs_f64(delay)).await;
                    let random_bytes = gen_bytes_fixed(i as usize);
                    let msg = ExampleMessage {
                        message_id: i,
                        message_bytes: random_bytes,
                        tcp_conn: conn_id,
                    };
                    let serialised = bincode::serialize(&msg)?;
                    write
                        .write_all(&serialised)
                        .await
                        .expect("couldn't write to stream");
                    println!(
                        ">> client sent {}: {} bytes on conn {}",
                        &i,
                        msg.message_bytes.len(),
                        &conn_id
                    );
                }
                Ok::<(), anyhow::Error>(())
            });

            tokio::spawn(async move {
                let mut reply_counter = 0;
                let codec = codec::BytesCodec::new();
                let mut framed_read = codec::FramedRead::new(read, codec);
                while let Some(Ok(bytes)) = framed_read.next().await {
                    match bincode::deserialize::<ExampleMessage>(&bytes) {
                        Ok(msg) => {
                            println!(
                                "<< client received {}: {} bytes on conn {}",
                                msg.message_id,
                                msg.message_bytes.len(),
                                msg.tcp_conn
                            );
                            reply_counter += 1;
                            println!(
                                "tcp connection {} replies received {}/4",
                                msg.tcp_conn, reply_counter
                            );
                        }
                        Err(e) => {
                            println!("<< client received something that wasn't an example message of {} bytes. error: {}", bytes.len(), e);
                        }
                    }
                }
            });
            Ok::<(), anyhow::Error>(())
        });
        let mut rng = SmallRng::from_entropy();
        let delay: f64 = rng.gen_range(4.5..7.0);
        tokio::time::sleep(tokio::time::Duration::from_secs_f64(delay)).await;
    }

    // Once timeout is passed, you can either wait for graceful shutdown or just hard stop it.
    signal::ctrl_c().await?;
    println!("CTRL+C received, shutting down");
    Ok(())
}

// emulate a series of small messages followed by a closing larger one
fn gen_bytes_fixed(i: usize) -> Vec<u8> {
    let amounts = [10, 15, 50, 1000];
    let len = amounts[i];
    let mut rng = rand::thread_rng();
    (0..len).map(|_| rng.gen::<u8>()).collect()
}
```