<template name="concepts">

<h1 id="concepts">Concepts</h1>

We've written our fair share of single-page JavaScript applications by hand.
Writing an entire application in one language (JavaScript) with one
data format (JSON) is a real joy.  Meteor is everything we wanted
when writing those apps.

{{> structure }}
{{> data }}
{{> reactivity }}
{{> livehtml }}
{{> templates }}
{{> packages_concept }}
{{> deploying }}
</template>

<template name="structure">
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<h2 id="structuringyourapp">Structuring your application</h2>

A Meteor application is a mix of JavaScript that runs inside a client web
browser, JavaScript that runs on the Meteor server inside a
[Node.js](http://nodejs.org/) container, and all the supporting HTML fragments,
CSS rules, and static assets.  Meteor automates the packaging and transmission
of these different components.  And, it is quite flexible about how you choose
to structure those components in your file tree.

The only server asset is JavaScript.  Meteor gathers all your JavaScript
files, excluding anything under the `client`
and `public` subdirectories, and loads them into a Node.js
server instance inside a fiber.  In Meteor, your server code runs in
a single thread per request, not in the asynchronous callback style
typical of Node.  We find the linear execution model a better fit for
the typical server code in a Meteor application.

There are more assets to consider on the client side.  Meteor
gathers all JavaScript files in your tree with the exception of
the `server` and `public` subdirectories for the
client.  It minifies this bundle and serves it to each new client.
You're free to use a single JavaScript file for your entire application, or
create a nested tree of separate files, or anything in between.

Files outside the `client`, `server` and `tests` subdirectories are loaded on
both the client and the server!  That's the place for model definitions and
other functions.  Meteor provides the variables [`isClient` and
`isServer`](#meteor_isclient) so that your code can alter its behavior depending
on whether it's running on the client or the server. (Files in directories named
`tests` are not loaded anywhere.)

Any sensitive code that you don't want served to the client, such as code
containing passwords or authentication mechanisms, should be
kept in the `server` directory.

CSS files are gathered together as well: the client will get a bundle with all
the CSS in your tree (excluding the `server`
and `public` subdirectories).

In development mode, JavaScript and CSS files are sent individually to make
debugging easier.

HTML files in a Meteor application are treated quite a bit differently
from a server-side framework.  Meteor scans all the HTML files in your
directory for three top-level elements: `<head>`, `<body>`, and
`<template>`.  The head and body sections are separately concatenated
into a single head and body, which are transmitted to the client on
initial page load.

Template sections, on the other hand, are converted into JavaScript
functions, available under the `Template` namespace.  It's
a really convenient way to ship HTML templates to the client.
See the [templates](#templates) section for more.

Lastly, the Meteor server will serve any files under the `public`
directory, just like in a Rails or Django project.  This is the place
for images, `favicon.ico`, `robots.txt`, and anything else.

It is best to write your application in such a way that it is
insensitive to the order in which files are loaded, for example by
using [Meteor.startup](#meteor_startup), or by moving load order
sensitive code into Smart Packages, which can explicitly control both
the load order of their contents and their load order with respect to
other packages. However sometimes load order dependencies in your
application are unavoidable. The JavaScript and CSS files in an
application are loaded according to these rules:

* Files in the `lib` directory at the root of your application are
  loaded first.

* Files that match `main.*` are loaded after everything else.

* Files in subdirectories are loaded before files in parent
  directories, so that files in the deepest subdirectory are loaded
  first (after `lib`), and files in the root directory are loaded last
  (other than `main.*`).

* Within a directory, files are loaded in alphabetical order by
  filename.

These rules stack, so that within `lib`, for example, files are still
loaded in alphabetical order; and if there are multiple files named
`main.js`, the ones in subdirectories are loaded earlier.

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<template name="data">
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<h2 id="dataandsecurity">Data and security</h2>

Meteor makes writing distributed client code as simple as talking to a
local database.  It's a clean, simple, and secure approach that obviates
the need to implement individual RPC endpoints, manually cache data on
the client to avoid slow roundtrips to the server, and carefully
orchestrate invalidation messages to every client as data changes.

In Meteor, the client and server share the same database API.  The same
exact application code &mdash; like validators and computed properties &mdash; can
often run in both places.  But while code running on the server has
direct access to the database, code running on the client does *not*.
This distinction is the basis for Meteor's data security model.

{{#note}}
By default, a new Meteor app includes the `autopublish` and `insecure`
packages, which together mimic the effect of each client having full
read/write access to the server's database.  These are useful
prototyping tools, but typically not appropriate for production
applications.  When you're ready, just remove the packages.
{{/note}}

Every Meteor client includes an in-memory database cache.  To manage the
client cache, the server *publishes* sets of JSON documents, and the
client *subscribes* to those sets.  As documents in a set change, the
server patches each client's cache.

Each document set is defined by a publish function on the server.  The
publish function runs each time a new client subscribes to a document
set.  The data in a document set can come from anywhere, but the common
case is to publish a database query.

    // server: publish all room documents
    Meteor.publish("all-rooms", function () {
      return Rooms.find(); // everything
    );

    // server: publish all messages for a given room
    Meteor.publish("messages", function (roomId) {
      return Messages.find({room: roomId});
    });

    // server: publish the set of parties the logged-in user can see.
    Meteor.publish("parties", function () {
      return Parties.find({$or: [{"public": true},
                                 {invited: this.userId},
                                 {owner: this.userId}]});
    });

Publish functions can provide different results to each client.  In the
last example, a logged in user can only see `Party` documents that
are public, that the user owns, or that the user has been invited to.

Once subscribed, the client uses its cache as a fast local database,
dramatically simplifying client code.  Reads never require a costly
round trip to the server.  And they're limited to the contents of the
cache: a query for every document in a collection on a client will only
return documents the server is publishing to that client.

    // client: start a parties subscription
    Meteor.subscribe("parties");

    // client: return array of Parties this client can read
    return Parties.find().fetch(); // synchronous!

Sophisticated clients can turn subscriptions on and off to control how
much data is kept in the cache and manage network traffic.  When a
subscription is turned off, all its documents are removed from the cache
unless the same document is also provided by another active
subscription.

When the client *changes* one or more documents, it sends a message to
the server requesting the change.  The server checks the proposed change
against a set of allow/deny rules you write as JavaScript functions.
The server only accepts the change if all the rules pass.

    // server: don't allow client to insert a party
    Parties.allow({
      insert: function (userId, party) {
        return false;
      }
    });

    // client: this will fail
    var party = { ... };
    Parties.insert(party);

If the server accepts the change, it applies the change to the database
and automatically propagates the change to other clients subscribed to
the affected documents.  If not, the update fails, the server's database
remains untouched, and no other client sees the update.

Meteor has a cute trick, though.  When a client issues a write to the
server, it also updates its local cache immediately, without waiting for
the server's response.  This means the screen will redraw right away.
If the server accepted the update &mdash; what ought to happen most of the
time in a properly behaving client &mdash; then the client got a jump on the
change and didn't have to wait for the round trip to update its own
screen.  If the server rejects the change, Meteor patches up the
client's cache with the server's result.

Putting it all together, these techniques accomplish latency
compensation.  Clients hold a fresh copy of the data they need, and
never need to wait for a roundtrip to the server. And when clients
modify data, those modifications can run locally without waiting for the
confirmation from the server, while still giving the server final say
over the requested change.

Meteor includes [Meteor Accounts](#accounts_api), a state-of-the-art
authentication system. It features secure password login using the [Secure
Remote Password
protocol](http://en.wikipedia.org/wiki/Secure_Remote_Password_protocol), and
integration with external services including Facebook, GitHub, Google, Twitter,
and Weibo. Meteor Accounts defines a [`Meteor.users`](#meteor_users) collection
where developers can store application-specific user data.

Meteor also includes pre-built forms for common tasks like login, signup,
password change, and password reset emails. You can add [Accounts
UI](#accountsui) to your app with just one line of code. The `accounts-ui` smart
package even provides a configuration wizard that walks you through the steps to
set up the external login services you're using in your app.

{{#note}}
The current release of Meteor supports MongoDB, the popular document
database, and the examples in this section use the
        [MongoDB API](http://www.mongodb.org/display/DOCS/Manual).  Future
releases will include support for other databases.
{{/note}}

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</template>

<template name="reactivity">
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<h2 id="reactivity">Reactivity</h2>

Meteor embraces the concept of [reactive
programming](http://en.wikipedia.org/wiki/Reactive_programming). This means that
you can write your code in a simple imperative style, and the result will be
automatically recalculated whenever data changes that your code depends on.

    Meteor.autorun(function () {
      Meteor.subscribe("messages", Session.get("currentRoomId"));
    });

This example (taken from a chat room client) sets up a data
subscription based on the session variable `currentRoomId`.
If the value of `Session.get("currentRoomId")` changes for any reason, the
function will be automatically re-run, setting up a new subscription that
replaces the old one.

This automatic recomputation is achieved by a cooperation
between `Session` and `Meteor.autorun`.
Methods like `Meteor.autorun` establish a "reactive
context" inside of which data dependencies are tracked, and they are
prepared to re-run their function argument as necessary.  Data
providers like `Session`, on the other hand, make note of
the context they are called from and what data was requested, and they
are prepared to send an invalidation signal when the data changes.

This simple pattern (reactive context + reactive data source) has wide
applicability.  Above, the programmer is saved from writing
unsubscribe/resubscribe calls and making sure they are called at the
right time.  In general, Meteor can eliminate whole classes of data
propagation code which would otherwise clog up your application with
error-prone logic.

These Meteor functions run your code in a reactive context:

* [Templates](#templates)
* [`Meteor.render`](#meteor_render) and [`Meteor.renderList`](#meteor_renderlist)
* [`Meteor.autorun`](#meteor_autorun)

And the reactive data sources that can trigger changes are:

* [`Session`](#session) variables
* Database queries on [Collections](#find)
* [`Meteor.status`](#meteor_status)
* [`Meteor.user`](#meteor_user)
* [`Meteor.userId`](#meteor_userid)
* [`Meteor.loggingIn`](#meteor_loggingin)

In addition, calling [`Meteor.subscribe`](#meteor_subscribe) in a reactive
context sets up a subscription which is automatically stopped when the context
is invalidated, as shown above.

Meteor's
[implementation](https://github.com/meteor/meteor/blob/master/packages/deps/deps.js)
of reactivity is short and sweet, about 50 lines of code.  You can hook into it
yourself to add new reactive contexts or data sources, using the
[`Meteor.deps`](#meteor_deps) module.

{{/better_markdown}}
</template>

<template name="livehtml">
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<h2 id="livehtml">Live HTML</h2>

HTML templating is central to web applications. With Meteor's live
page update technology, you can render your HTML _reactively_, meaning
that it will update automatically to track changes in the data used to
generate it.

This optional feature works with any HTML templating library, or even
with HTML you generate manually from Javascript. Here's an example:

    var fragment = Meteor.render(
      function () {
        var name = Session.get("name") || "Anonymous";
        return "<div>Hello, " + name + "</div>";
      });
    document.body.appendChild(fragment);

    Session.set("name", "Bob"); // page updates automatically!

[`Meteor.render`](#meteor_render) takes a rendering function, that is, a
function that returns some HTML as a string. It returns an auto-updating
`DocumentFragment`. When there is a change to data used by the rendering
function, it is re-run. The DOM nodes in the `DocumentFragment` then
update themselves in-place, no matter where they were inserted on the
page. It's completely automatic. [`Meteor.render`](#meteor_render) uses
[reactive contexts](#reactivity) to discover what data is used by the
rendering function.

Most of the time, though, you won't call these functions directly
&mdash; you'll just use your favorite templating package, such as
Handlebars or Jade. The `render` and `renderList` functions are intended
for people that are implementing new templating systems.

Meteor normally batches up any needed updates and executes them only
when your code isn't running. That way, you can be sure that the DOM
won't change out from underneath you. Sometimes you want the opposite
behavior. For example, if you've just inserted a record in the
database, you might want to force the DOM to update so you can find
the new elements using a library like jQuery. In that case, call
[`Meteor.flush`](#meteor_flush) to bring the DOM up to date
immediately.

When live-updating DOM elements are taken off the screen, they are automatically
cleaned up &mdash; their callbacks are torn down, any associated database
queries are stopped, and they stop updating. For this reason, you never have to
worry about the [zombie
templates](http://lostechies.com/derickbailey/2011/09/15/zombies-run-managing-page-transitions-in-backbone-apps/)
that plague hand-written update logic. To protect your elements from cleanup,
just make sure that they on-screen before your code returns to the event loop,
or before any call you make to [`Meteor.flush`](#meteor_flush).

Another thorny problem in hand-written applications is element
preservation. Suppose the user is typing text into an `<input>`
element, and then the area of the page that includes that element is
redrawn. The user could be in for a bumpy ride, as the focus, the
cursor position, the partially entered text, and the accented
character input state will be lost when the `<input>` is recreated.

This is another problem that Meteor solves for you. You can specify
elements to preserve when templates are re-rendered with the
[`preserve`](#template_preserve) directive on the template. Meteor will
preserve these elements even when their enclosing template is
rerendered, but will still update their children and copy over any
attribute changes.

{{/better_markdown}}
</template>

<template name="templates">
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<h2 id="templates">Templates</h2>

Meteor makes it easy to use your favorite HTML templating language,
such as Handlebars or Jade, along with Meteor's live page update
technology. Just write your template as you normally would, and Meteor
will take care of making it update in realtime.

To use this feature, create a file in your project with the `.html`
extension. In the file, make a `<template>` tag and give it a
`name` attribute. Put the template contents inside the tag. Meteor
will precompile the template, ship it down to the client, and make it
available as a function on the global `Template` object.

{{#note}}
Today, the only templating system that has been packaged for Meteor is
Handlebars. Let us know what templating systems you'd like to use with
Meteor. Meanwhile, see the [Handlebars
documentation](http://www.handlebarsjs.com/) and [Meteor Handlebars
extensions](https://github.com/meteor/meteor/wiki/Handlebars).
{{/note}}

A template with a `name` of `hello` is rendered by calling the
function `Template.hello`, passing any data for the template:

    <!-- in myapp.html -->
    <template name="hello">
      <div class="greeting">Hello there, {{dstache}}first}} {{dstache}}last}}!</div>
    </{{! }}template>

    // in the JavaScript console
    > Template.hello({first: "Alyssa", last: "Hacker"});
     => "<div class="greeting">Hello there, Alyssa Hacker!</div>"

This returns a string. To use the template along with the [`Live
HTML`](#livehtml) system, and get DOM elements that update
automatically in place, use [`Meteor.render`](#meteor_render):

    Meteor.render(function () {
      return Template.hello({first: "Alyssa", last: "Hacker"});
    })
      => automatically updating DOM elements

The easiest way to get data into templates is by defining helper
functions in JavaScript. Just add the helper functions directly on the
`Template.[template name]` object. For example, in this template:

    <template name="players">
      {{dstache}}#each topScorers}}
        <div>{{dstache}}name}}</div>
      {{dstache}}/each}}
    </{{! }}template>

instead of passing in `topScorers` as data when we call the
template function, we could define a function on `Template.players`:

    Template.players.topScorers = function () {
      return Users.find({score: {$gt: 100}}, {sort: {score: -1}});
    };

In this case, the data is coming from a database query. When the
database cursor is passed to `#each`, it will wire up all of the
machinery to efficiently add and move DOM nodes as new results enter
the query.

Helpers can take arguments, and they receive the current template data
in `this`:

    // in a JavaScript file
    Template.players.leagueIs = function (league) {
      return this.league === league;
    };

    <!-- in a HTML file -->
    <template name="players">
      {{dstache}}#each topScorers}}
        {{dstache}}#if leagueIs "junior"}}
          <div>Junior: {{dstache}}name}}</div>
        {{dstache}}/if}}
        {{dstache}}#if leagueIs "senior"}}
          <div>Senior: {{dstache}}name}}</div>
        {{dstache}}/if}}
      {{dstache}}/each}}
    </{{! }}template>

{{#note}}
Handlebars note: `{{dstache}}#if leagueIs "junior"}}` is
allowed because of a Meteor extension that allows nesting a helper
in a block helper. (Both `if` and `leagueIs` are
technically helpers, and stock Handlebars would not invoke
`leagueIs` here.)
{{/note}}

Helpers can also be used to pass in constant data.

    // Works fine with {{dstache}}#each sections}}
    Template.report.sections = ["Situation", "Complication", "Resolution"];

Finally, you can use an `events` declaration on a template function to set up a
table of event handlers. The format is documented at [Event
Maps](#eventmaps). The `this` argument to the event handler will be
the data context of the element that triggered the event.

    <!-- myapp.html -->
    <template name="scores">
      {{dstache}}#each player}}
        {{dstache}}> playerScore}}
      {{dstache}}/each}}
    </{{! }}template>

    <template name="playerScore">
      <div>{{dstache}}name}}: {{dstache}}score}}
        <span class="givePoints">Give points</span>
      </div>
    </{{! }}template>

    <!-- myapp.js -->
    Template.playerScore.events({
      'click .givePoints': function () {
        Users.update({_id: this._id}, {$inc: {score: 2}});
      }
    });

Putting it all together, here's an example of how you can inject
arbitrary data into your templates, and have them update automatically
whenever that data changes. See [Live HTML](#livehtml) for further
discussion.

    <!-- in myapp.html -->
    <template name="forecast">
      <div>It'll be {{dstache}}prediction}} tonight</div>
    </{{! }}template>

    <!-- in myapp.js -->
    // JavaScript: reactive helper function
    Template.forecast.prediction = function () {
      return Session.get("weather");
    };

    <!-- in the console -->
    > Session.set("weather", "cloudy");
    > document.body.appendChild(Meteor.render(Template.forecast));
    In DOM:  <div>It'll be cloudy tonight</div>

    > Session.set("weather", "cool and dry");
    In DOM:  <div>It'll be cool and dry tonight</div>

{{/better_markdown}}
</template>

<template name="packages_concept">
{{#better_markdown}}

  <h2 id="smartpackages">Smart packages</h2>

Meteor has an unusually powerful package system. All of the
functionality you've read about so far is implemented as standard
Meteor packages.

Meteor packages are intelligent: the packages are themselves
JavaScript programs. They can inject code into the client or the
server, or hook new functions into the bundler, so they can extend the
Meteor environment in arbitrary ways. Some examples of packages are:

* The [coffeescript](#coffeescript) package extends the
  bundler, automatically compiling any `.coffee` files in
  your tree.  Once added, you can write your application in CoffeeScript
  instead of JavaScript.

* The [jQuery](#jquery) and [Backbone](#backbone) packages are examples of using
  Meteor to prepackage client JavaScript libraries. You could get
  the same result by copying the JavaScript files into your tree, but
  it's faster to add a package.

* The [underscore](#underscore) package extends both the
  client and server environments.  Many of the core Meteor features,
  including Minimongo, the Session object, and reactive Handlebars
  templates, are implemented as internal packages automatically
  included with every Meteor application.

You can see a list of available packages
with [`meteor list`](#meteorlist), add packages to your project
with [`meteor add`](#meteoradd), and remove them
with [`meteor remove`](#meteorremove).

See the [Package List](#packages) section for a description
of the existing packages.

{{#warning}}
The package API is rapidly changing and isn't documented, so you can't
make your own packages just yet. Coming soon.
{{/warning}}

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</template>





<template name="deploying">
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<h2 id="deploying">Deploying</h2>

Meteor is a full application server.  We include everything you need
to deploy your application on the internet: you just provide the JavaScript,
HTML, and CSS.

<h3 class="nosection">Running on Meteor's infrastructure</h3>

The easiest way to deploy your application is to use `meteor
deploy`.  We provide it because it's what, personally, we've always
wanted: an easy way to take an app idea, flesh it out over a weekend,
and put it out there for the world to use, with nothing getting in the
way of creativity.

    $ meteor deploy myapp.meteor.com

Your application is now available at myapp.meteor.com.  If
this is the first time deploying to this hostname, Meteor creates a
fresh empty database for your application.  If you want to deploy an
update, Meteor will preserve the existing data and just refresh the
code.

You can also deploy to your own domain.  Just set up the hostname you
want to use as a CNAME to `origin.meteor.com`,
then deploy to that name.

    $ meteor deploy www.myapp.com

We provide this as a free service so you can try Meteor.  It is also
helpful for quickly putting up internal betas, demos, and so on.

<h3 class="nosection">Running on your own infrastructure</h3>

You can run also your application on your own infrastructure, or any
other hosting provider like Heroku.

To get started, run

    $ meteor bundle myapp.tgz

This command will generate a fully-contained Node.js application in
the form of a tarball.  To run this application, you need to provide
Node.js 0.8 and a MongoDB server.  You can then run the application by
invoking node, specifying the HTTP port for the application to listen
on, and the MongoDB endpoint.  If you don't already have a MongoDB
server, we can recommend our friends at [MongoHQ](http://mongohq.com).

    $ PORT=3000 MONGO_URL=mongodb://localhost:27017/myapp node bundle/main.js

Other packages may require other environment variables (for example, the `email`
package requires a `MAIL_URL` environment variable).

{{#warning}}
For now, bundles will only run on the platform that the bundle was
created on.  To run on a different platform, you'll need to rebuild
the native packages included in the bundle.  To do that, make sure you
have `npm` available, and run the following:

    $ cd bundle/server/node_modules
    $ rm -r fibers
    $ npm install fibers@1.0.0
{{/warning}}

{{/better_markdown}}
</template>
