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+Dealing with Stale Elements
+===========================
+.. py:currentmodule:: marionette
+
+Marionette does not keep a live representation of the DOM saved. All it can do
+is send commands to the Marionette server which queries the DOM on the client's
+behalf. References to elements are also not passed from server to client. A
+unique id is generated for each element that gets referenced and a mapping of
+id to element object is stored on the server. When commands such as
+:func:`~HTMLElement.click()` are run, the client sends the element's id along
+with the command. The server looks up the proper DOM element in its reference
+table and executes the command on it.
+
+In practice this means that the DOM can change state and Marionette will never
+know until it sends another query. For example, look at the following HTML::
+
+ <head>
+ <script type=text/javascript>
+ function addDiv() {
+ var div = document.createElement("div");
+ document.getElementById("container").appendChild(div);
+ }
+ </script>
+ </head>
+
+ <body>
+ <div id="container">
+ </div>
+ <input id="button" type=button onclick="addDiv();">
+ </body>
+
+Care needs to be taken as the DOM is being modified after the page has loaded.
+The following code has a race condition::
+
+ button = client.find_element('id', 'button')
+ button.click()
+ assert len(client.find_elements('css selector', '#container div')) > 0
+
+
+Explicit Waiting and Expected Conditions
+----------------------------------------
+
+To avoid the above scenario, manual synchronisation is needed. Waits are used
+to pause program execution until a given condition is true. This is a useful
+technique to employ when documents load new content or change after
+``Document.readyState``'s value changes to "complete".
+
+The :class:`Wait` helper class provided by Marionette avoids some of the
+caveats of ``time.sleep(n)``. It will return immediately once the provided
+condition evaluates to true.
+
+To avoid the race condition in the above example, one could do::
+
+ button = client.find_element('id', 'button')
+ button.click()
+
+ def find_divs():
+ return client.find_elements('css selector', '#container div')
+
+ divs = Wait(client).until(find_divs)
+ assert len(divs) > 0
+
+This avoids the race condition. Because finding elements is a common condition
+to wait for, it is built in to Marionette. Instead of the above, you could
+write::
+
+ button = client.find_element('id', 'button')
+ button.click()
+ assert len(Wait(client).until(expected.elements_present('css selector', '#container div'))) > 0
+
+For a full list of built-in conditions, see :mod:`~marionette.expected`.