express-safe-send

Do not send on already sent Express headers.

Install

npm install express-safe-send

The Problem

This solves a problem that arises rarely, but persistently, in production deployments. In express route handlers that have defined timeout handling, Node.js timers and timing behaviour can be slighlty undeterministic, because of the nature of Node and libraries that handle timeouts themselves not properly.

Think of a handler that has a 10s timeout defined in which you make a database call (e.g. to postgres) that itself has a timeout of less than 10 seconds set, because you assign it that safe budget. Node.js event loop delays e.g. through parsing of large incoming data, or time that is not accounted for in your database client, like connection delay, may add to your timeout budget, eventually exceeding it.

When the budget is exceeded your API consumer will already have received headers and assume a timeout. However, your handler might not have knowledge about that fact. A synchronous call (they all are) to express send functions, like res.status(200).json({ users: [] }) will then throw an exception, terminating your server hardly.

This plugin exposes a middleware for just ignoring sending, or extending express functions and adding optional callbacks to notify you about it.

Usage

const safeSend = require('express-safe-send')
 
const app = express()
 
app.use(safeSend())
app.use(timeout('10s'))
 
app.get('/user', function (req, res, next) {
  // make a very long running request, that might time out.
  Users.find({}).toArray(function(err, items) {
    if (err) return next(err)
 
    // if it (at all) returns the, ignore trying to send the data
    // as Node.js would throw in this process.
    res.status(200).safeJsonSend(items)
  })
})

API

safeSend([options, callback])

options

Type: Object

proxyMode

Type: boolean
Default: false

Override behaviour on the send and json functions via a JS proxy.

callback

Type: Function

optional callback for global notification

const safeSend = require('express-safe-send')
 
const app = express()
 
app.use(safeSend({ proxyMode: false }, (err, req, res, next) => {
  // an error noting the reason why sending was not possible
  console.log(err)
}))
 
// or
 
app.use(safeSend((err, req, res, next) => {
  // an error noting the reason why sending was not possible
  console.log(err)
}))

License

MIT © Robert Jefe Lindstaedt