On Domains and Connections with node.js

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The reason for this question was to ensure that in actionHero exceptions thrown after a call to the api.cache methods would still be caught by the domain they should have been in. Here is the commit which force-binds callbacks from the redis client back to the domain they should have been in. This is needed due to the fact that connection-pooled clients (which were created before the domain) will always revert back to their original scope.

The Question:

I’ve been having trouble lately with domains in node.js, in that I have found a few occasions where what is ‘in scope’ confuses me. Here’s a collection of tests to illustrate my I set up the test to have a domain which I will run each test in, and I expect all of the tests to throw an error and to be caught by the domain’s on(‘error’) event. I chose to use a redis client here (because it’s common), but I do not think that this is a problem with the awesome redis package, and I’ve observed similar behavior with the mysql / seq All of the tests work except #4, which throws an out-of-domain exception and causes the script to crash

1var domain = require("domain");
2var redis = require("redis");
3var eventEmitter = require("events").EventEmitter;
4
5var tests = [];
6var testCounter = 0;
7var runTest = function () {
8  if (tests.length > testCounter) {
9    tests[testCounter]();
10  } else {
11    console.log("all done!");
12    process.exit();
13  }
14};
15
16var myDomain = new domain.create();
17myDomain.on("error", function (err) {
18  console.log("Yo, I just saved you from the error: " + err);
19  testCounter++;
20  runTest();
21});
22
23// PASSING
24tests[0] = function () {
25  myDomain.run(function () {
26    throw new Error("A simple error");
27  });
28};
29
30// PASSING
31tests[1] = function () {
32  myDomain.run(function () {
33    setTimeout(function () {
34      process.nextTick(function () {
35        var E = new eventEmitter();
36        E.on("thing", function () {
37          throw new Error("A deeply nested error");
38        });
39        setTimeout(function () {
40          E.emit("thing");
41        }, 100);
42      });
43    }, 100);
44  });
45};
46
47// PASSING
48var Emm = new eventEmitter();
49Emm.on("thing", function () {
50  throw new Error("Emmited Error defined outside of scope");
51});
52tests[2] = function () {
53  myDomain.run(function () {
54    setTimeout(function () {
55      Emm.emit("thing");
56    }, 100);
57  });
58};
59
60// PASSING
61tests[3] = function () {
62  myDomain.run(function () {
63    clientA = redis.createClient();
64    clientA.hget("hash", "key", function (err, data) {
65      throw new Error("An error after redis (A)");
66    });
67  });
68};
69
70// PASSING
71tests[4] = function () {
72  clientB = redis.createClient();
73  myDomain.run(function () {
74    clientB.hget("hash", "key", function (err, data) {
75      throw new Error("An error after redis (B)");
76    });
77  });
78};
79
80// FAILING
81clientC = redis.createClient();
82tests[5] = function () {
83  myDomain.run(function () {
84    clientC.hget("hash", "key", function (err, data) {
85      throw new Error("An error after redis (C)");
86    });
87  });
88};
89
90// start it up
91runTest();

• Test #0 is a simple throw in the scope of running domian. Everything works great.
• Test #1 is a very convoluted throw in the scope of the domain involving timeouts, process.nextTick, and event emitters (I was trying to break things). Node preforms like a boss, and the eventual throw is caught.
• Test #2 defines the emitter outside of the domain, but invokes the event from within it. Still the error is caught.
• Test #3 creates the redis client within the scope of the domain. The error is caught and all is well
• Test #4 creates the redis client OUTSIDE of the domain’s scope, but within a containing function. This error is caught by the domain
• Test #5 fails, and the redis client here is created before the tests begin to run.

In all of these cases, I would have hoped for the domain to catch the exceptions. I also would have also expected tests #4 and #5 to behave the same way regardless of whether or n Can anyone help me to explain why test 5 fails? domain can have their exceptions caught by it.