CounterThing example on main page does not work because observeProperty cannot keep up

[jspaith]

I copy/pasted the examples on the welcome readme.md for the counter thing and client.

Repro

• Install various NPM packages
• Copy the snippets into counterThing.js and counterClient.js
• Run node on both of these in separate terminals

Behavior -- there are only two notifications fired and the client hangs forever

~/developer/jspaith/scratch/wot/test1 >node ./counterClient.js
count: 2
count: 4

This suggested a timing issue where the client isn't receiving updates and is hanging because its waiting for 5 before terminating. It seems that the observeProperty is not keeping up. Brute force, but this updated script does work.

I'm not doing a PR directly to the home page because I don't know if the script needs this paradigm or something is off with underlying observeProperty.

// client.js
// Required steps to create a servient for a client
const { Servient } = require("@node-wot/core");
const { HttpClientFactory } = require("@node-wot/binding-http");

const servient = new Servient();
servient.addClientFactory(new HttpClientFactory(null));

servient.start().then(async (WoT) => {
    const td = await WoT.requestThingDescription("http://localhost:8080/counter");
    // Then from here on you can consume the thing
    let thing = await WoT.consume(td);

    // Read initial value
    let read = await thing.readProperty("count");
    console.log("initial count:", await read.value());

    for (let i = 0; i < 5; i++) {
        await thing.invokeAction("increment");
        read = await thing.readProperty("count");
        console.log("count:", await read.value());
    }

    // Clean shutdown
    servient.shutdown();
}).catch((err) => { console.error(err); });

After this change client works fine

~/developer/jspaith/scratch/wot/test1 >node ./counterClient.js
initial count: 0
count: 1
count: 2
count: 3
count: 4
count: 5
~/developer/jspaith/scratch/wot/test1 >

Environment:

• WSLv2 / Ubuntu 22
• Windows 11 host
• Node version v18.20.8
• npm version 10.8.2
• I'm using the readme.md associated with #13ea375d323a93646b44bc79386b37703e53f598 in this repo.

[danielpeintner]

I can reproduce the problem on Windows Command shell.
We need to look into what causes the "hanging" 🤔

[relu91]

I investigated a little bit. Easy things first:

• The example has some minor issues

   let thing = await WoT.consume(td);
    thing.observeProperty("count", async (data) => { console.log("count:", await data.value()); }); // <-- problem here
   //  await thing.observeProperty("count", async (data) => { console.log("count:", await data.value()); });  --> we have to wait until the connection is enstablished
    for (let i = 0; i < 5; i++) {

• client does not exit cleanly because:

   let thing = await WoT.consume(td);
    thing.observeProperty("count", async (data) => { console.log("count:", await data.value()); }); // <-- problem here
   // const subscription =  await thing.observeProperty("count", async (data) => { console.log("count:", await data.value()); });  --> we need to caputre the subscription
    for (let i = 0; i < 5; i++) {
    /* ... */
   }
   await subscription.stop()

• Still, you might lose some counter increments. That is because long polling is a poor protocol for very fast-changing data. Basically, we need to add a timeout to wait for the client to reconnect. We should probably stick to SSE for examples with HTTP.

const { setTimeout } = require("timers/promises"); // <- need to add this
\* .. *\
for (let i = 0; i < 5; i++) {
        await thing.invokeAction("increment");
        await setTimeout(50); // small amount I even tried with 0, and it works most of the time, 50 is more conservative. 
    }

The hard thing is that await servient.shutdown(); should also work (instead of using the subscription object and stopping it). However, it does not close the subscription as it only calls HTTPFactory destroy -> which does nothing. Probably this is a separate issue, and this one can be closed once the example is updated as above.