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+# Introduction
+
+The basic idea of a filter is to pre-process incoming data and post-process outgoing data.
+Filters are part of a filter chain, thus following the [chain of responsability design pattern](http://en.wikipedia.org/wiki/Chain-of-responsibility_pattern).
+
+Each filter decides to call the next filter or not.
+
+# Levels
+
+In EWF, there are two levels of filters.
+
+## WSF_FILTER
+
+Typical examples of such filters are: logging, compression, routing (WSF_ROUTING_FILTER), ...
+
+## WSF_FILTER_HANDLER
+
+Handler that can also play the role of a filter.
+
+Typical examples of such filters are: authentication, ...
+
+# References
+
+Filters (also called middelwares) in other environments:
+* in Python: http://www.wsgi.org/en/latest/libraries.html
+* in Node.js: http://expressjs.com/guide.html#middleware
+* in Apache: http://httpd.apache.org/docs/2.2/en/filter.html
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