# Introduction to Appium

Appium is an open-source tool you can use to automate mobile native,
mobile web, and mobile hybrid applications on iOS and Android platforms.
"Mobile native apps" are those written using the iOS or Android SDKs. "Mobile
web apps" are web apps accessed using a mobile browser (Appium supports
Safari on iOS and Chrome on Android). "Mobile hybrid apps" have a native
wrapper around a "webview"--a native control that enables interaction with
web content. Projects like [Phonegap](http://phonegap.com/), for example,
make it easy to build apps using web technologies that are then bundled into
a native wrapper---these are hybrid apps.

Importantly, Appium is "cross-platform": it allows you to write tests against
multiple platforms (iOS, Android), using the same API. This enables a large
or total amount of code reuse between iOS and Android testsuites.

For specific information about what it means for Appium to "support" its
platforms, version, and automation modalities, please see the [platform support doc](platform-support).

## Appium Philosophy

Appium was designed to meet mobile automation needs according to a certain
philosophy. The key points of this philosophy can be stated as 4 requirements:

1. You shouldn't have to recompile your app or modify it in any way in order
   to automate it.
2. You shouldn't be locked into a specific language or framework to write and
   run your tests.
3. A mobile automation framework shouldn't reinvent the wheel when it comes
   to automation APIs.
4. A mobile automation framework should be open source,
   in spirit and practice as well as in name!

## Appium Design

So how does the structure of the Appium project live out this philosophy? We
meet requirement #1 by using vendor-provided automation frameworks under the
hood. That way, we don't need to compile in any Appium-specific or
third-party code or frameworks to your app. This means you're testing the
same app you're shipping. The vendor-provided frameworks we use are:

* iOS: Apple's [UIAutomation](https://developer.apple.com/library/ios/documentation/DeveloperTools/Reference/UIAutomationRef/_index.html)
* Android 4.2+: Google's [UiAutomator](http://developer.android.com/tools/help/uiautomator/index.html)
* Android 2.3+: Google's [Instrumentation](http://developer.android.com/reference/android/app/Instrumentation.html). (Instrumentation support is provided by bundling a separate project, [Selendroid](http://selendroid.io))

We meet requirement #2 by wrapping the vendor-provided frameworks in one API,
the [WebDriver](http://docs.seleniumhq.org/projects/webdriver/) API.
WebDriver (aka "Selenium WebDriver") specifies a client-server protocol
(known as the [JSON Wire Protocol](https://code.google.com/p/selenium/wiki/JsonWireProtocol)).
Given this client-server architecture, a client written in any language can
be used to send the appropriate HTTP requests to the server. There are
already clients written in every popular programming language. This also
means that you're free to use whatever test runner and test framework you
want; the client libraries are simply HTTP clients and can be mixed into your
code any way you please. In other words, Appium & WebDriver clients are not
technically "test frameworks"--they are "automation libraries". You can
manage your test environment any way you like!

We meet requirement #3 in the same way: WebDriver has become the de facto
standard for automating web browsers, and is a [W3C Working Draft](https://dvcs.w3.org/hg/webdriver/raw-file/tip/webdriver-spec.html).
Why do something totally different for mobile? Instead we have [extended the protocol](https://code.google.com/p/selenium/source/browse/spec-draft.md?repo=mobile)
with extra API methods useful for mobile automation.

It should be obvious that requirement #4 is a given---you're reading this
because [Appium is open source](https://github.com/appium/appium).

## Appium Concepts

*Client/Server Architecture*
Appium is at its heart a webserver that exposes a REST API. It receives
connections from a client, listens for commands, executes those commands on a
mobile device, and responds with an HTTP response representing the result of
the command execution. The fact that we have a client/server architecture
opens up a lot of possibilities: we can write our test code in any language
that has a client. We can put the server on a different machine than our
tests are running on. We can write test code and rely on a cloud service
like [Sauce Labs](https://saucelabs.com/mobile) to receive and interpret the
commands.

*Session*
Automation is always performed in the context of a session. Clients initiate
a session with a server in ways specific to each library,
but they all end up sending a `POST /session` request to the server,
with a JSON object called the 'desired capabilities' object. At this point
the server will start up the automation session and respond with a session ID
which can be used in sending further commands.

*Desired Capabilities*
Desired capabilities are sets of keys and values (i.e.,
a map or hash) sent to the Appium server to tell the server what kind of
automation session we're interested in starting up. There are also various
capabilities which can modify the behavior of the server during automation.
For example, we might set the `platformName` capability to `iOS` to tell
Appium that we want an iOS session, rather than an Android one. Or we might
set the `safariAllowPopups` capability to `true` in order to ensure that,
during a Safari automation session, we're allowed to use JavaScript to open
up new windows. See the [capabilities doc](caps) for the complete list of
capabilities available for Appium.

*Appium Server*
Appium is a server written with Node.js. It can be built and installed from
source or directly from NPM.

*Appium.app, Appium.exe*
There exist GUI wrappers around the Appium server that can be downloaded.
These come bundled with everything required to run the Appium server,
so you don't need to worry about Node. They also come with an Inspector,
which enables you to check out the hierarchy of your app. This can come in
very handy when writing tests!

## Getting Started

Congratulations! You are now armed with enough knowledge to begin using
Appium. Why not head back to the [getting started doc](getting-started) for
more detailed requirements and instructions?