# Long Polling
This example illustrates how to configure Web Integration Service and different
client applications to subscribe to Square Topics using Long Polling.

Long Polling emulates a server push mechanism over HTTP by configuring client
and server applications to perform long-lasting HTTP requests that do not
trigger an immediate response.

In the context of _Web Integration Service_, long polling enables client
applications to register ``on_data_available()`` functions that only get called
when new data becomes available in the remote DDS _DataReader_. To achieve this
behavior, the _Web Integration Service_ client in this example leverages a set
of query parameters supported by the GET operation that triggers a DDS
``take()`` or a ``read()`` operation on the remote DDS _DataReader_. These
query parameters configure a set of DDS *WaitSets* on the server side that
block the ``read()`` or ``take()`` operation until certain conditions are met.
In this specific example, we use two query parameters that control the way
samples are read:
* ``maxWait = 30``, which configures the internal *WaitSet* of _Web Integration
Service_ to wait for up to 30 seconds for the read conditions to be met. In
this case, the only read condition is set by the value of ``sampleStateMask``.
If samples become available before ``maxWait``, the *WaitSet* wakes up and _Web
Integration Service_ generates an HTTP response right away containing the
sequence of read samples. Otherwise, after 30 seconds, _Web Integration
Service_ times out and generates a response containing an empty sequence of
samples.
* ``sampleStateMask = NOT_READ``, which configures the *WaitSet* to wait until
new samples (i.e., samples that have not been read) become available. This
query parameter can be skipped when performing a ``take()`` operation.

Instructions on how to run _Web Integration Service_ and the client
applications for each specific language are available under:

* [js/README.md](js/README.md)
* [python/README.md](python/README.md)

![Alt text](../../resources/img/long_polling.png "Long Polling Example Screenshot")