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  # Continuation-Local Storage
  
  Continuation-local storage works like thread-local storage in threaded
  programming, but is based on chains of Node-style callbacks instead of threads.
  The standard Node convention of functions calling functions is very similar to
  something called ["continuation-passing style"][cps] in functional programming,
  and the name comes from the way this module allows you to set and get values
  that are scoped to the lifetime of these chains of function calls.
  
  Suppose you're writing a module that fetches a user and adds it to a session
  before calling a function passed in by a user to continue execution:
  
  ```javascript
  // setup.js
  
  var createNamespace = require('continuation-local-storage').createNamespace;
  var session = createNamespace('my session');
  
  var db = require('./lib/db.js');
  
  function start(options, next) {
    db.fetchUserById(options.id, function (error, user) {
      if (error) return next(error);
  
      session.set('user', user);
  
      next();
    });
  }
  ```
  
  Later on in the process of turning that user's data into an HTML page, you call
  another function (maybe defined in another module entirely) that wants to fetch
  the value you set earlier:
  
  ```javascript
  // send_response.js
  
  var getNamespace = require('continuation-local-storage').getNamespace;
  var session = getNamespace('my session');
  
  var render = require('./lib/render.js')
  
  function finish(response) {
    var user = session.get('user');
    render({user: user}).pipe(response);
  }
  ```
  
  When you set values in continuation-local storage, those values are accessible
  until all functions called from the original function – synchronously or
  asynchronously – have finished executing. This includes callbacks passed to
  `process.nextTick` and the [timer functions][] ([setImmediate][],
  [setTimeout][], and [setInterval][]), as well as callbacks passed to
  asynchronous functions that call native functions (such as those exported from
  the `fs`, `dns`, `zlib` and `crypto` modules).
  
  A simple rule of thumb is anywhere where you might have set a property on the
  `request` or `response` objects in an HTTP handler, you can (and should) now
  use continuation-local storage. This API is designed to allow you extend the
  scope of a variable across a sequence of function calls, but with values
  specific to each sequence of calls.
  
  Values are grouped into namespaces, created with `createNamespace()`. Sets of
  function calls are grouped together by calling them within the function passed
  to `.run()` on the namespace object. Calls to `.run()` can be nested, and each
  nested context this creates has its own copy of the set of values from the
  parent context. When a function is making multiple asynchronous calls, this
  allows each child call to get, set, and pass along its own context without
  overwriting the parent's.
  
  A simple, annotated example of how this nesting behaves:
  
  ```javascript
  var createNamespace = require('continuation-local-storage').createNamespace;
  
  var writer = createNamespace('writer');
  writer.run(function () {
    writer.set('value', 0);
  
    requestHandler();
  });
  
  function requestHandler() {
    writer.run(function(outer) {
      // writer.get('value') returns 0
      // outer.value is 0
      writer.set('value', 1);
      // writer.get('value') returns 1
      // outer.value is 1
      process.nextTick(function() {
        // writer.get('value') returns 1
        // outer.value is 1
        writer.run(function(inner) {
          // writer.get('value') returns 1
          // outer.value is 1
          // inner.value is 1
          writer.set('value', 2);
          // writer.get('value') returns 2
          // outer.value is 1
          // inner.value is 2
        });
      });
    });
  
    setTimeout(function() {
      // runs with the default context, because nested contexts have ended
      console.log(writer.get('value')); // prints 0
    }, 1000);
  }
  ```
  
  ## cls.createNamespace(name)
  
  * return: {Namespace}
  
  Each application wanting to use continuation-local values should create its own
  namespace. Reading from (or, more significantly, writing to) namespaces that
  don't belong to you is a faux pas.
  
  ## cls.getNamespace(name)
  
  * return: {Namespace}
  
  Look up an existing namespace.
  
  ## cls.destroyNamespace(name)
  
  Dispose of an existing namespace. WARNING: be sure to dispose of any references
  to destroyed namespaces in your old code, as contexts associated with them will
  no longer be propagated.
  
  ## cls.reset()
  
  Completely reset all continuation-local storage namespaces. WARNING: while this
  will stop the propagation of values in any existing namespaces, if there are
  remaining references to those namespaces in code, the associated storage will
  still be reachable, even though the associated state is no longer being updated.
  Make sure you clean up any references to destroyed namespaces yourself.
  
  ## process.namespaces
  
  * return: dictionary of {Namespace} objects
  
  Continuation-local storage has a performance cost, and so it isn't enabled
  until the module is loaded for the first time. Once the module is loaded, the
  current set of namespaces is available in `process.namespaces`, so library code
  that wants to use continuation-local storage only when it's active should test
  for the existence of `process.namespaces`.
  
  ## Class: Namespace
  
  Application-specific namespaces group values local to the set of functions
  whose calls originate from a callback passed to `namespace.run()` or
  `namespace.bind()`.
  
  ### namespace.active
  
  * return: the currently active context on a namespace
  
  ### namespace.set(key, value)
  
  * return: `value`
  
  Set a value on the current continuation context. Must be set within an active
  continuation chain started with `namespace.run()` or `namespace.bind()`.
  
  ### namespace.get(key)
  
  * return: the requested value, or `undefined`
  
  Look up a value on the current continuation context. Recursively searches from
  the innermost to outermost nested continuation context for a value associated
  with a given key. Must be set within an active continuation chain started with
  `namespace.run()` or `namespace.bind()`.
  
  ### namespace.run(callback)
  
  * return: the context associated with that callback
  
  Create a new context on which values can be set or read. Run all the functions
  that are called (either directly, or indirectly through asynchronous functions
  that take callbacks themselves) from the provided callback within the scope of
  that namespace. The new context is passed as an argument to the callback
  when it's called.
  
  ### namespace.runAndReturn(callback)
  
  * return: the return value of the callback
  
  Create a new context on which values can be set or read. Run all the functions
  that are called (either directly, or indirectly through asynchronous functions
  that take callbacks themselves) from the provided callback within the scope of
  that namespace. The new context is passed as an argument to the callback
  when it's called.
  
  Same as `namespace.run()` but returns the return value of the callback rather
  than the context.
  
  ### namespace.bind(callback, [context])
  
  * return: a callback wrapped up in a context closure
  
  Bind a function to the specified namespace. Works analogously to
  `Function.bind()` or `domain.bind()`. If context is omitted, it will default to
  the currently active context in the namespace, or create a new context if none
  is currently defined.
  
  ### namespace.bindEmitter(emitter)
  
  Bind an EventEmitter to a namespace. Operates similarly to `domain.add`, with a
  less generic name and the additional caveat that unlike domains, namespaces
  never implicitly bind EventEmitters to themselves when they're created within
  the context of an active namespace.
  
  The most likely time you'd want to use this is when you're using Express or
  Connect and want to make sure your middleware execution plays nice with CLS, or
  are doing other things with HTTP listeners:
  
  ```javascript
  http.createServer(function (req, res) {
    writer.bindEmitter(req);
    writer.bindEmitter(res);
  
    // do other stuff, some of which is asynchronous
  });
  ```
  
  ### namespace.createContext()
  
  * return: a context cloned from the currently active context
  
  Use this with `namespace.bind()`, if you want to have a fresh context at invocation time,
  as opposed to binding time:
  
  ```javascript
  function doSomething(p) {
    console.log("%s = %s", p, ns.get(p));
  }
  
  function bindLater(callback) {
    return writer.bind(callback, writer.createContext());
  }
  
  setInterval(function () {
    var bound = bindLater(doSomething);
    bound('test');
  }, 100);
  ```
  
  ## context
  
  A context is a plain object created using the enclosing context as its prototype.
  
  # copyright & license
  
  See [LICENSE](https://github.com/othiym23/node-continuation-local-storage/blob/master/LICENSE)
  for the details of the BSD 2-clause "simplified" license used by
  `continuation-local-storage`. This package was developed in 2012-2013 (and is
  maintained now) by Forrest L Norvell, [@othiym23](https://github.com/othiym23),
  with considerable help from Timothy Caswell,
  [@creationix](https://github.com/creationix), working for The Node Firm. This
  work was underwritten by New Relic for use in their Node.js instrumentation
  agent, so maybe give that a look if you have some Node.js
  performance-monitoring needs.
  
  [timer functions]: https://nodejs.org/api/timers.html
  [setImmediate]:    https://nodejs.org/api/timers.html#timers_setimmediate_callback_arg
  [setTimeout]:      https://nodejs.org/api/timers.html#timers_settimeout_callback_delay_arg
  [setInterval]:     https://nodejs.org/api/timers.html#timers_setinterval_callback_delay_arg
  [cps]:             http://en.wikipedia.org/wiki/Continuation-passing_style