Introduces Node.js continuation-local-storage and the AsyncListener API. How to use it today to solve problems that come from the event loop.

1. Continuation-Local-Storage and
the Magic of AsyncListener
Islam Sharabash
@ibashes
@DataHero

2. Agenda
• Problems caused by the event loop of Node.js
• Solving problems with AsyncListener API
• How AsyncListener works

3. Mo Node Mo Problems
Things that are hard with the event loop:
• Having a request ID propagate throughout code
• Short stack traces from errors in async functions
• Profiling async functions is painful

4. Mo Node Mo Problems
Things that are hard with the event loop:
• Having a request ID propagate throughout code
• Short stack traces from errors in async functions
• Profiling async functions is painful

5. Propagating Request ID
(naive)

6. Propagating Request ID
(naive)
var ctx = {rid: uuid.v4()};
users.save(ctx, attrs, function(error, userId) {
users.load(ctx, userId, function(error, user) {
log.info(‘user created’,
{rid: ctx.rid, userId: user.id});
mailer.sendWelcome(ctx, user, function(error) {
log.info(‘welcome email sent’,
{rid: ctx.rid, userId: user.id});
});
});
});

7. Propagating Request ID
(naive)
var ctx = {rid: uuid.v4()};
users.save(ctx, attrs, function(error, userId) {
users.load(ctx, userId, function(error, user) {
log.info(‘user created’,
{rid: ctx.rid, userId: user.id});
mailer.sendWelcome(ctx, user, function(error) {
log.info(‘welcome email sent’,
{rid: ctx.rid, userId: user.id});
});
});
});

8. Propagating Request ID
(naive)
var ctx = {rid: uuid.v4()};
users.save(ctx, attrs, function(error, userId) {
users.load(ctx, userId, function(error, user) {
log.info(‘user created’,
{rid: ctx.rid, userId: user.id});
mailer.sendWelcome(ctx, user, function(error) {
log.info(‘welcome email sent’,
{rid: ctx.rid, userId: user.id});
});
});
});

9. Mo Node Mo Problems
Things that are hard with the event loop:
• Having a request ID propagate throughout code
• Short stack traces from errors in async functions
• Profiling async functions is painful

10. Short Stack Traces
users.save(attrs, function(error, userId) {
if (error) { return callback(error); }
users.load(userId, function(error, user) {
if (error) { return callback(error); }
mailers.sendWelcome(user, function(error) {
if (error) { return callback(error); }
});
});
});

11. Short Stack Traces
users.save(attrs, function(error, userId) {
if (error) { return callback(error); }
users.load(userId, function(error, user) {
if (error) { return callback(error); }
mailers.sendWelcome(user, function(error) {
if (error) { return callback(error); }
});
});
});

12. Short Stack Traces
Error: ER_PARSE_ERROR You have an error in your SQL syntax...
at getError (lib/models/database.js:479:24)
at Query._callback (lib/models/database.js:110:34)
at Query.Sequence.end (node_modules/mysql/lib/protocol/sequences/Sequence.js:75:24)
at Query.ErrorPacket (node_modules/mysql/lib/protocol/sequences/Query.js:93:8)
at Protocol._parsePacket (node_modules/mysql/lib/protocol/Protocol.js:192:24)
at Parser.write (node_modules/mysql/lib/protocol/Parser.js:62:12)
at Socket.ondata (stream.js:51:26)
at Socket.emit (events.js:117:20)
at Socket. (_stream_readable.js:748:14)
at Socket.emit (events.js:92:17)
at emitReadable_ (_stream_readable.js:410:10)
at emitReadable (_stream_readable.js:406:5)
at readableAddChunk (_stream_readable.js:168:9)
at Socket.Readable.push (_stream_readable.js:130:10)
at TCP.onread (net.js:528:21)

13. Short Stack Traces
Error: ER_PARSE_ERROR You have an error in your SQL syntax...
at getError (lib/models/database.js:479:24)
at Query._callback (lib/models/database.js:110:34)
at Query.Sequence.end (node_modules/mysql/lib/protocol/sequences/Sequence.js:75:24)
at Query.ErrorPacket (node_modules/mysql/lib/protocol/sequences/Query.js:93:8)
at Protocol._parsePacket (node_modules/mysql/lib/protocol/Protocol.js:192:24)
at Parser.write (node_modules/mysql/lib/protocol/Parser.js:62:12)
at Socket.ondata (stream.js:51:26)
at Socket.emit (events.js:117:20)
at Socket. (_stream_readable.js:748:14)
at Socket.emit (events.js:92:17)
at emitReadable_ (_stream_readable.js:410:10)
at emitReadable (_stream_readable.js:406:5)
at readableAddChunk (_stream_readable.js:168:9)
at Socket.Readable.push (_stream_readable.js:130:10)
at TCP.onread (net.js:528:21)

14. Mo Node Mo Problems
Things that are hard with the event loop:
• Having a request ID propagate throughout code
• Short stack traces from errors in async functions
• Profiling async functions is painful

15. Profiling Async Functions
var asyncFunc = function(callback) {
setTimeout(callback, 1000);
};

16. Profiling Async Functions
var orig = asyncFunc;
asyncFunc = function() {
var start = new Date();
var args = Array.prototype.slice.call(arguments, 0);
var cb = args[args.length - 1];
args[args.length - 1] = function() {
console.log((new Date()) - start);
return cb.apply(this, arguments);
}
return orig.apply(this, args);
}

17. Profiling Async Functions
var orig = asyncFunc;
asyncFunc = function() {
var start = new Date();
var args = Array.prototype.slice.call(arguments, 0);
var cb = args[args.length - 1];
args[args.length - 1] = function() {
console.log((new Date()) - start);
return cb.apply(this, arguments);
}
return orig.apply(this, args);
}

18. Where Do These Problems
Come From?

19. Where Do These Problems
Come From?

20. Where Do These Problems
Come From?
• Asynchronicity and the Event Loop

21. Where Do These Problems
Come From?
• Asynchronicity and the Event Loop
process
.nextTick(myFunc);
Event Loop myFunc Executed

22. Where Do These Problems
Come From?
• Asynchronicity and the Event Loop
process
.nextTick(myFunc);
Event Loop myFunc Executed
State and stack lost

23. Where Do These Problems
Come From?
• Asynchronicity and the Event Loop
process
.nextTick(myFunc);
Event Loop myFunc Executed
State and stack lost

24. How do we solve it?
• AsyncListener API
process.createAsyncListener
process.addAsyncListener
process.removeAsyncListener
• Available in Node >0.11.9; Polyfill for Node <= 0.11.8

25. AsyncListener
process
.nextTick(myFunc);

26. AsyncListener
process
.nextTick(myFunc);
Event Loop

27. AsyncListener
process
.nextTick(myFunc);
Some time later…
Event Loop myFunc Executed

28. AsyncListener
process
.nextTick(myFunc);
Event Loop myFunc Executed myFunc returns
myFunc errors
Some time later…

29. AsyncListener
process
.nextTick(myFunc);
Event Loop myFunc Executed myFunc returns
myFunc errors
Some time later…
Create Callback

30. AsyncListener
process
.nextTick(myFunc);
Event Loop myFunc Executed myFunc returns
myFunc errors
Some time later…
Create Callback Before Callback

31. AsyncListener
process
.nextTick(myFunc);
Event Loop myFunc Executed myFunc returns
myFunc errors
Some time later…
Create Callback Before Callback After / Error Callback

32. Propagate Request IDs
How do we solve it?

33. Propagate Request IDs
How do we solve it?
• Don’t use AsyncListener Directly

34. Propagate Request IDs
How do we solve it?
• Don’t use AsyncListener Directly
• Use continuation-local-storage

35. Propagate Request IDs
How do we solve it?
• Don’t use AsyncListener Directly
• Use continuation-local-storage
• But what is it?

36. continuation-local-storage

37. continuation-local-storage
async function 1

38. continuation-local-storage
Bag Of Holding
(CLS makes this) async function 1

39. continuation-local-storage
Bag Of Holding
(CLS makes this) async function 1
async function 2

40. continuation-local-storage
Bag Of Holding
(CLS makes this) async function 1
async function 2
async function 3

41. continuation-local-storage
Bag Of Holding
(CLS makes this)
async function 1
async function 2
async function 3
Each async chain gets it’s own bag

42. continuation-local-storage
Bag Of Holding
(CLS makes this) async function 1
async function 2
async function 3
async function 1
async function 2
async function 3
Each async chain gets it’s own bag

43. Propagate Request IDs
Want
• Request ID in every log message
• Easy

44. Propagate Request IDs
Want
2013-12-20T18:12:20.037Z - Executing SQL BEGIN statement
2013-12-20T18:12:20.385Z - Values and label are different datasources
2013-12-20T18:12:20.530Z - Executing SQL COMMIT statement
2013-12-20T18:12:20.531Z - Picking filter implementation: DEFAULT

45. Propagate Request IDs
Want
2013-12-20T18:12:20.037Z - Executing SQL BEGIN statement -
3dd4ee56-e8e3-4270-bda7-90c5ee409782
2013-12-20T18:12:20.385Z - Values and label are different datasources -
02671cb7-d22b-4a3e-9bb8-282608999688
2013-12-20T18:12:20.530Z - Executing SQL COMMIT statement -
3dd4ee56-e8e3-4270-bda7-90c5ee409782
2013-12-20T18:12:20.531Z - Picking filter implementation: DEFAULT -
02671cb7-d22b-4a3e-9bb8-282608999688

46. Propagate Request IDs
How?
• Create continuation-local-storage namespace
• Middleware to create/assign Request ID
• Look up Request ID when logging

47. Propagate Request IDs
Create namespace

48. Propagate Request IDs
Create namespace
var createNamespace =
require(‘continuation-local-storage')
.createNamespace;
var namespace = createNamespace('com.datahero');

49. Propagate Request IDs
Create namespace
var createNamespace =
require(‘continuation-local-storage')
.createNamespace;
var namespace = createNamespace('com.datahero');
Once at top of application

50. Propagate Request IDs
Middleware

51. Propagate Request IDs
Middleware
var getNamespace =
require(‘continuation-local-storage')
.getNamespace;
var namespace = getNamespace('com.datahero'),
uuid = require('node-uuid');

52. Propagate Request IDs
Middleware
var getNamespace =
require(‘continuation-local-storage')
.getNamespace;
var namespace = getNamespace('com.datahero'),
uuid = require('node-uuid');
Just requiring… nothing special

53. Propagate Request IDs
Middleware
app.use(function(req, res, next) {
var rid = uuid.v4();
namespace.bindEmitter(req);
namespace.bindEmitter(res);
namespace.run(function() {
namespace.set(‘rid’, rid);
next();
});
});

54. Propagate Request IDs
Middleware
app.use(function(req, res, next) {
var rid = uuid.v4();
namespace.bindEmitter(req);
namespace.bindEmitter(res);
namespace.run(function() {
namespace.set(‘rid’, rid);
next();
});
});

55. Propagate Request IDs
Middleware
app.use(function(req, res, next) {
var rid = uuid.v4();
namespace.bindEmitter(req);
namespace.bindEmitter(res);
namespace.run(function() {
namespace.set(‘rid’, rid);
next();
});
});

56. Propagate Request IDs
Middleware
app.use(function(req, res, next) {
var rid = uuid.v4();
namespace.bindEmitter(req);
namespace.bindEmitter(res);
namespace.run(function() {
namespace.set(‘rid’, rid);
next();
});
});

57. Propagate Request IDs
Middleware
app.use(function(req, res, next) {
var rid = uuid.v4();
namespace.bindEmitter(req);
namespace.bindEmitter(res);
namespace.run(function() {
namespace.set(‘rid’, rid);
next();
});
});

58. Propagate Request IDs
Logger

59. Propagate Request IDs
Logger
var getNamespace =
require(‘continuation-local-storage')
.getNamespace;
var namespace = getNamespace(‘com.datahero');

60. Propagate Request IDs
Logger
Logger.debug = function(message) {
console.log(
(new Date()).toISOString() + ‘ - ‘ +
message + ‘ - ‘ +
namespace.get(‘rid')
);
};

61. Propagate Request IDs
Logger
Logger.debug = function(message) {
console.log(
(new Date()).toISOString() + ‘ - ‘ +
message + ‘ - ‘ +
namespace.get(‘rid')
);
};

62. Propagate Request IDs
Boom!

63. Short Stack Traces

64. Short Stack Traces
setImmediate(function start() {
process.nextTick(function howDeep() {
setImmediate(function isTheStack() {
throw new Error('Oh no!');
});
});
});

65. Short Stack Traces
setImmediate(function start() {
process.nextTick(function howDeep() {
setImmediate(function isTheStack() {
throw new Error('Oh no!');
});
});
});
Error: Oh no!
at Object.isTheStack [as _onImmediate] (short-stack.js:4:13)
at processImmediate [as _immediateCallback] (timers.js:336:15)

66. Short Stack Traces
setImmediate(function start() {
process.nextTick(function howDeep() {
setImmediate(function isTheStack() {
throw new Error('Oh no!');
});
});
});
Error: Oh no!
at Object.isTheStack [as _onImmediate] (short-stack.js:4:13)
at processImmediate [as _immediateCallback] (timers.js:336:15)

67. Short Stack Traces

68. Short Stack Traces
process
.nextTick(myFunc);
Event Loop myFunc Executed myFunc returns
myFunc errors
Save copy of stack Stitch stack together

69. Short Stack Traces
Use stackup
• https://www.npmjs.org/package/stackup

70. Short Stack Traces
Use stackup
• https://www.npmjs.org/package/stackup
npm install --save stackup

71. Short Stack Traces
Use stackup
• https://www.npmjs.org/package/stackup
npm install --save stackup
require(‘stackup’);

72. Profiling Async Code

73. Profiling Async Code
process
.nextTick(myFunc);
Event Loop myFunc Executed myFunc returns
myFunc errors
Record Time
Compute / Record
Time Difference

74. Profiling Async Code
Use async-profile
• https://www.npmjs.org/package/async-profile

75. Profiling Async Code
Use async-profile
• https://www.npmjs.org/package/async-profile
npm install --save async-profile

76. Profiling Async Code
Use async-profile
• https://www.npmjs.org/package/async-profile
var AsyncProfile =
require('async-profile')
function () {
…
new AsyncProfile();
doAsyncWork();
}

77. Summary
• Problems
• Can we solve them? Yes we can!
• Use continuation-local-storage, stackup, and
async-profile
• All built on AsyncListener
• Prepare for more tracing packages

78. We’re Hiring
Email me at islam@datahero.com

79. Islam Sharabash
@ibashes
islam@datahero.com

80. AsyncListener API
require(‘async-listener’);
var al = process.createAsyncListener({
create: function(data) {},
before: function(context, data) {},
after: function(context, data) {},
error: function(data, error) {}
}, data)
process.addAsyncListener(al);

81. Cause Of Short Stack Traces
function foo { bar(); }
function bar { baz(); }
function baz {
process.nextTick(bonk);
}

82. Cause Of Short Stack Traces
function foo { bar(); }
function bar { baz(); }
function baz {
process.nextTick(bonk);
}
Stack:
baz
bar
foo

83. Cause Of Short Stack Traces
function foo { bar(); }
function bar { baz(); }
function baz {
process.nextTick(bonk);
}
Stack:

84. Cause Of Short Stack Traces
function foo { bar(); }
function bar { baz(); }
function baz {
process.nextTick(bonk);
}
Stack:
— tick —
bonk