Extend Mongo documentation

docs/src/docs/asciidoc/index.adoc

@@ -1,5 +1,5 @@
 = Spring Session
-Rob Winch, Vedran Pavić
+Rob Winch, Vedran Pavić, Jakub Kubrynski
 :doctype: book
 :indexdoc-tests: {docs-test-dir}docs/IndexDocTests.java
 :websocketdoc-test-dir: {docs-test-dir}docs/websocket/
@@ -305,6 +305,56 @@ You can read the basic steps for integration below, but you are encouraged to fo
 
 include::guides/mongo.adoc[tags=config,leveloffset=+3]
 
+==== Session serialization mechanisms
+
+To be able to persist session objects in MongoDB we need to provide the serialization/deserialization mechanism. Depending on your classpath Spring Session will choose one of two build-in converters:
+
+* `JacksonMongoSessionConverter` when `ObjectMapper` class is available, or
+* `JdkMongoSessionConverter` otherwise.
+
+===== JacksonMongoSessionConverter
+
+This mechanism uses Jackson to serialize session objects to/from JSON. Converter is registered automatically by Spring Session always when the `ObjectMapper` class is available (usually provided by the `com.fasterxml.jackson.core:jackson-databind` dependency) and user hasn't registered own converter.
+
+If you have to include custom Jackson's modules you can do it by registering converter by hand:
+
+[source, java]
+----
+@Configuration
+@EnableMongoHttpSession
+public class MySessionConfiguration {
+
+	@Bean
+	public AbstractMongoSessionConverter mongoSessionConverter() {
+		return new JacksonMongoSessionConverter(getJacksonModules());
+	}
+
+	public Iterable<Module> getJacksonModules() {
+		return Collections.<Module>singletonList(new MyJacksonModule());
+	}
+
+}
+----
+
+==== JdkMongoSessionConverter
+
+This converter uses standard Java serialization to persist session attributes map to MongoDB in a binary form. However standard session elements like id, access time, etc are still written as a plain Mongo objects and can be read and queried without additional effort.
+You can enforce use of this converter by registering it manually:
+
+[source, java]
+----
+@Bean
+public AbstractMongoSessionConverter mongoSessionConverter() {
+	return new JdkMongoSessionConverter();
+}
+----
+
+==== Using custom converters
+
+You can create your own session converter by extending `AbstractMongoSessionConverter` class. At least you have to implement `getQueryForIndex` and two `convert` methods.
+First is used to return query operating on a particular session attribute value, which usually depends on internal session collection schema provided by your mechanism.
+Two other methods are used to convert `MongoExpiringSession` from/to Mongo's `DBObject`.
+
 [[httpsession-how]]
 === How HttpSession Integration Works