TCP server example - shared data hashmap

[Ibens3C]

When i connect from different clients to the TCP server (from example code)
and read/write some holding registers, each server instance has it's own data
hashmap.
Is there a way to use the same Arc<Mutex<HashMap<u16, u16>>> for
all server instances ?
Finally i would like to set the values for the data HashMap in a seperate thread...
Thanx a lot

[cdbennett]

Sure, you can do that. In the tcp-server.rs example, you just need to change server_context() so that it creates an instance of the server state, and then pass that into the new_service that's created.

This way, each client connection will create a new service (I think that's how tokio-modbus is designed) but all the services share a "state" value that's wrapped in an Arc<Mutex>. I'd probably combine the holding registers and input registers HashMaps together into a single "state" struct, and the Mutex and Arc would wrap the entire state.

If that doesn't make sense, I could revise the example to demonstrate.

[cdbennett]

And, if you want to set the value in a separate thread or async task, you just need to pass an Arc<Mutex<ServerState>> to that task and it can access the registers. For instance, we could maybe create a task that updates a timestamp register by incrementing it once per second.

[uklotzde]

Thank you @cdbennett for providing these helpful hints and proposals. These should be straightforward to implement.

I am closing this issue, because I don't see anything that the development team could do here.

@Ibens3C If you have managed to accomplish your task and want to improve the examples please share your knowledge by opening a pull/merge request.

[vladimirvrabely]

Hi, I'm bit late to the discussion, but I have a very similar question.

Basically, I would like to change the tcp-server.rs example and add one more async task in select! which would periodically update input registers in ExampleService. How can I do it?

@cdbennett, you have suggested passing Arc<Mutex<ServerState>>. Can you explain in more detail? Does ServerState refers to a new struct or to ExampleService? Should I define it early in the main function and pass it to both update task and server_context? What about new_service closure?

[vladimirvrabely]

Ah, I got your your suggestion. Thanks!

For the reference, I defined

type Word = u16;

struct DataStore {
    input_registers: HashMap<Address, Word>,
    holding_registers: HashMap<Address, Word>,
}

struct ExampleService {
    data_store: Arc<Mutex<DataStore>>,
}

I added one more argument data_store: &Arc<Mutex<DataStore>> to server_context and ExampleService::new and fixed things accordingly. In particular

 let new_service = |_socket_addr| Ok(Some(ExampleService::new(data_store.clone())));

The main looks like this

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let socket_addr = "127.0.0.1:5502".parse().unwrap();
    let data_store = Arc::new(Mutex::new(DataStore {
        input_registers: HashMap::from([(0, 1234), (1, 5678)]),
        holding_registers: HashMap::from([(0, 10), (1, 20), (2, 30), (3, 40)]),
    }));

    tokio::select! {
        _ = server_context(socket_addr, &data_store) => unreachable!(),
        _ = state_context(&data_store) => unreachable!(),
        _ = client_context(socket_addr) => println!("Exiting"),
    }

    Ok(())
}

where state_context is

async fn state_context(data_store: &Arc<Mutex<DataStore>>) {
    let data_store = data_store.clone();
    loop {
        {
            let input_registers = &mut data_store.lock().unwrap().input_registers;
            *input_registers.get_mut(&0).unwrap() += 1;
        }

        tokio::time::sleep(Duration::from_millis(100)).await;
    }
}

and client_context makes repated request to input registers

// Read again
tokio::time::sleep(Duration::from_secs(1)).await;
let response = ctx.read_input_registers(0x00, 2).await.unwrap();
println!("CLIENT: The result is '{response:?}'");