Breaking the async boundary

James Allardice

The event loop

Used by JS engines to schedule tasks
Most callbacks (e.g. setTimeout) are tasks
Tasks are scheduled to run sequentially
Different tasks have different stack frames

The problem

startTimer();
a();
setTimeout(b, 0);
c();
stopTimer();

How long does it take the following program, including `b`, to execute?

startTimer();
a();
setTimeout(b, 0); // asynchronous 
c();
stopTimer();

startTimer();                                // 1
a();
setTimeout(b, 0); // asynchronous 
c();
stopTimer();

startTimer();                                // 1
a();                                         // 2
setTimeout(b, 0); // asynchronous 
c();
stopTimer();

startTimer();                                // 1
a();                                         // 2
setTimeout(b, 0); // asynchronous 
c();                                         // 3
stopTimer();

startTimer();                                // 1
a();                                         // 2
setTimeout(b, 0); // asynchronous 
c();                                         // 3
stopTimer();                                 // 4

startTimer();                                // 1
a();                                         // 2
setTimeout(b, 0); // asynchronous            // 5
c();                                         // 3
stopTimer();                                 // 4

The problem

How can we associate data with the HTTP request in the following program?

// Assume `app` is an Express instance
app.use((req, res, next) => {
  
  // Generate a unique identifier for this request.
  const requestId = generateId();
  next();
});

// Assume `app` is an Express instance
app.use((req, res, next) => {
  
  // Generate a unique identifier for this request.
  const requestId = generateId();

  // Attach the identifier to the request object.
  req.id = requestId;

  next();
});

The problem

How can we capture a full stack trace when the following program dies?

Solutions

Domains
Async Listener
Continuation Local Storage (CLS)
AsyncWrap
Zones

Domains

Introduced in 0.8 but now deprecated (not yet removed)
"Handle multiple different IO operations as a single group"

https://nodejs.org/api/domain.html

Async Listener

Pull request to Node core never merged
Polyfill exists
Provides a mechanism to run functions just before and after a new task begins and ends

https://github.com/othiym23/async-listener

Continuation Local Storage

Built on Async Listener
Associate values with a namespace
Namespaces are groups of functions, synchronous or otherwise
Retrieve value from a namespace in any function in that group

https://github.com/othiym23/node-continuation-local-storage

AsyncWrap

Undocumented API
Available via process.binding('async_wrap')

Zones

Stage 0 ECMAScript proposal
Inspired by Dart
Numerous existing implementations (Angular zone.js, StrongLoop zone, can-zone)
All tasks scheduled within a zone will execute within that context
Provides hooks into the event loop

https://github.com/domenic/zones

Zones

// Requires Node 0.12 or earlier.

require('zone').enable();

zone
  .create(function () {

    startTimer();

    a();
    setTimeout(b, 0);
    c();
  })
  .setCallback(function () {
    stopTimer();
  });

Using StrongLoop zone which is no longer maintained and requires Node.js 0.12 or earlier.

Thank you for listening!

Any questions?