+ * Provides a registry for Remote Procedure Call (RPC) service implementations. The RPCs are
+ * defined in YANG models.
+ * <p>
+ * There are 2 types of RPCs:
+ * <ul>
+ * <li>Global</li>
+ * <li>Routed</li>
+ * </ul>
+ *
+ * <h2>Global RPC</h2>
+ * <p>
+ * An RPC is global if there is intended to be only 1 registered implementation. A global RPC is not
+ * explicitly declared as such, essentially any RPC that is not defined to be routed is considered global.
+ * <p>
+ * Global RPCs are registered using the
+ * {@link #addRpcImplementation(Class, RpcService)} method.
+ *
+ * <h2>Routed RPC</h2>
+ * <p>
+ * MD-SAL supports routing of RPC between multiple implementations where the appropriate
+ * implementation is selected at run time based on the content of the RPC message as described in
+ * YANG model.
+ * <p>
+ * RPC routing is based on:
+ * <ul>
+ * <li><b>Route identifier</b> -
+ * An {@link org.opendaylight.yangtools.yang.binding.InstanceIdentifier InstanceIdentifier} value
+ * which is part of the RPC input. This value is used to select the correct
+ * implementation at run time.</li>
+ * <li><b>Context Type</b> - A YANG-defined construct which constrains the subset of
+ * valid route identifiers for a particular RPC.</li>
+ * </ul>
+ *
+ * <h3>Context type</h3>
+ * <p>
+ * A context type is modeled in YANG using a combination of a YANG <code>identity</code>
+ * and Opendaylight specific extensions from <code>yang-ext</code> module. These extensions are:
+ * <ul>
+ * <li><b>context-instance</b> - This is used in the data tree part of a YANG model to
+ * define a context type that associates nodes with a specified context <code>identity</code>.
+ * Instance identifiers that reference these nodes are valid route identifiers for RPCs that
+ * reference this context type.</li>
+ * <li><b>context-reference</b> - This is used in RPC input to mark a leaf of type
+ * <code>instance-identifier</code> as a reference to the particular context type defined by the
+ * specified context <code>identity</code>. The value of this
+ * leaf is used by the RPC broker at run time to route the RPC request to the correct implementation.
+ * Note that <code>context-reference</code> may only be used on leaf elements of type
+ * <code>instance-identifier</code> or a type derived from <code>instance-identifier</code>.</li>
+ * </ul>
+ *
+ *
+ * <h3>Routed RPC example</h3>
+ * <p>
+ * <h5>1. Defining a Context Type</h5>
+ * <p>
+ * The following snippet declares a simple YANG <code>identity</code> named <code>example-context</code>:
+ *
+ * <pre>
+ * module example {
+ * ...
+ * identity example-context {
+ * description "Identity used to define an example-context type";
+ * }
+ * ...
+ * }
+ * </pre>
+ * <p>
+ * We then use the declared identity to define a context type by using it in combination
+ * with the <code>context-instance</code> YANG extension. We'll associate the context type
+ * with a list element in the data tree. This defines the set of nodes whose instance
+ * identifiers are valid for the <code>example-context</code> context type.
+ * <p>
+ * The following YANG snippet imports the <code>yang-ext</code> module and defines the list
+ * element named <code>item</code> inside a container named <code>foo</code>:
+ *
+ * <pre>
+ * module foo {
+ * ...
+ * import yang-ext {prefix ext;}
+ * ...
+ * container foo {
+ * list item {
+ * key "id";
+ * leaf id {type string;}
+ * ext:context-instance "example-context";
+ * }
+ * }
+ * ...
+ * }
+ * </pre>
+ * <p>
+ * The statement <code>ext:context-instance "example-context";</code> inside the list element
+ * declares that any instance identifier referencing <code>item</code> in the data
+ * tree is valid for <code>example-context</code>. For example, the following instance
+ * identifier:
+ * <pre>
+ * InstanceIdentifier.create(Foo.class).child(Item.class,new ItemKey("Foo"))
+ * </pre>
+ * is valid for <code>example-context</code>. However the following:
+ * <pre>
+ * InstanceIdentifier.create(Example.class)
+ * </pre>
+ * is not valid.
+ * <p>
+ * So using an <code>identity</code> in combination with <code>context-instance</code> we
+ * have effectively defined a context type that can be referenced in a YANG RPC input.
+ *
+ * <h5>2. Defining an RPC to use the Context Type</h5>
+ * <p>
+ * To define an RPC to be routed based on the context type we need to add an input leaf element
+ * that references the context type which will hold an instance identifier value to be
+ * used to route the RPC.
+ * <p>
+ * The following snippet defines an RPC named <code>show-item</code> with 2 leaf elements
+ * as input: <code>item</code> of type <code>instance-identifier</code> and <code>description</code>:
+ *
+ * <pre>
+ * module foo {
+ * ...
+ * import yang-ext {prefix ext;}
+ * ...
+ * rpc show-item {
+ * input {
+ * leaf item {
+ * type instance-identifier;
+ * ext:context-reference example-context;
+ * }
+ * leaf description {
+ * type "string";
+ * }
+ * }
+ * }
+ * }
+ * </pre>
+ * <p>
+ * We mark the <code>item</code> leaf with a <code>context-reference</code> statement that
+ * references the <code>example-context</code> context type. RPC calls will then be routed
+ * based on the instance identifier value contained in <code>item</code>. Only instance
+ * identifiers that point to a <code>foo/item</code> node are valid as input.
+ * <p>
+ * The generated RPC Service interface for the module is: