Simple middleware to limit concurrency for Koa v1

Need to restrict how much traffic your application can take at any given point in time? With koa-limit-connections you can easily define your limits both statically or dynamically.

Install the module to your project

$ npm install --save koa-limit-connections

Add it to your middleware stack. It's highly recommended that it's included near the top before any downstream computation occurs to prevent unnecessary load on your application. It's a fail fast approach.

const limitConnections = require("koa-limit-connections");
const Koa = require('koa');
const app = new Koa();

app.use(limitConnections({
    // Override default options
}));

// response
app.use(ctx => {
  ctx.body = 'Hello Koa';
});

app.listen(3000);

number - Default 100

Use any value > 0 to set your threshold.

app.use(limitConnections({
    max: 10
}));

function - Has Koa context

Allows you to update the max threshold dynamically as long as it's parseable and value exceeds 0 else falls back to max

app.use(limitConnections({
    max: 20,
    dynamicMax: function () {
        // get request header
        return this.get("limit-connections");
    }
}));

function([connections][, max]) - Has Koa context

Called on initial request.

app.use(limitConnections({
    onConnection: function () {
        // Do stuff ...
    }
}));

function([connections][, max]) - Has Koa context

Called when threshold is met preventing the rest of the middelware stack being called. Use this as an opportunity to customize your response. By default it sets response status to 503 and response body to Too Busy

app.use(limitConnections({
    onMax: function () {
        this.body = "<h1>Application is too busy to handle your request</h1>";
    }
}));

boolean - Default true

Control whether you want to catch downstream exceptions.

app.use(limitConnections({
    catchExceptions: false
}));

function([exception]) - Has Koa context

Added as an additional security measure koa-limit-connections catches unhandled exceptions to ensure that connections are decremented even in the case of exceptions caused from downstream middleware or other application faults.

app.use(limitConnections({
    onException: function (exception) {
        // Handle exception ...
    }
}));