DOCS - Add documentation on Corda Services / Service classes (#5079) (#5096)

(cherry picked from commit a9a0f422af)

docs/source/api-service-classes.rst

@@ -0,0 +1,100 @@
+.. highlight:: kotlin
+.. raw:: html
+
+   <script type="text/javascript" src="_static/jquery.js"></script>
+   <script type="text/javascript" src="_static/codesets.js"></script>
+
+API: Service Classes
+====================
+
+Service classes are long-lived instances that can trigger or be triggered by flows from within a node. A Service class is limited to a
+single instance per node. During startup, the node handles the creation of the service.
+
+Services allow related, reusable, functions to be separated into their own class where their functionality is
+grouped together. These functions can then be called from other services or flows.
+
+Creating a Service
+------------------
+
+To define a Service class:
+
+    * Add the ``CordaService`` annotation
+    * Add a constructor with a single parameter of ``AppServiceHub``
+    * Extend ``SingletonSerializeAsToken``
+
+Below is an empty implementation of a Service class:
+
+.. container:: codeset
+
+   .. sourcecode:: kotlin
+
+        @CordaService
+        class MyCordaService(private val serviceHub: AppServiceHub) : SingletonSerializeAsToken() {
+
+            init {
+                // code ran at service creation / node startup
+            }
+
+            // public api of service
+        }
+
+   .. sourcecode:: java
+
+        @CordaService
+        public class MyCordaService extends SingletonSerializeAsToken {
+
+            private AppServiceHub serviceHub;
+
+            public MyCordaService(AppServiceHub serviceHub) {
+                this.serviceHub = serviceHub;
+                // code ran at service creation / node startup
+            }
+
+            // public api of service
+        }
+
+The ``AppServiceHub`` provides the ``ServiceHub`` functionality to the Service class, with the extra ability to start flows. Starting flows
+from ``AppServiceHub`` is explained further in :ref:`Starting Flows from a Service <starting_flows_from_a_service>`.
+
+Code can be run during node startup when the class is being initialised. To do so, place the code into the ``init`` block or constructor.
+This is useful when a service needs to establish a connection to an external database or setup observables via ``ServiceHub.trackBy`` during
+its startup. These can then persist during the service's lifetime.
+
+Retrieving a Service
+--------------------
+
+A Service class can be retrieved by calling ``ServiceHub.cordaService`` which returns the single instance of the class passed into the function:
+
+.. container:: codeset
+
+   .. sourcecode:: kotlin
+
+        val service: MyCordaService = serviceHub.cordaService(MyCordaService::class.java)
+
+   .. sourcecode:: java
+
+       MyCordaService service = serviceHub.cordaService(MyCordaService.class);
+
+.. warning:: ``ServiceHub.cordaService`` should not be called during initialisation of a flow and should instead be called in line where
+             needed or set after the flow's ``call`` function has been triggered.
+
+.. _starting_flows_from_a_service:
+
+Starting Flows from a Service
+-----------------------------
+
+Starting flows via a service can lead to deadlock within the node's flow worker queue, which will prevent new flows from
+starting. To avoid this, the rules bellow should be followed:
+
+    * When called from a running flow, the service must invoke the new flow from another thread. The existing flow cannot await the
+      execution of the new flow.
+    * When ``ServiceHub.trackBy`` is placed inside the service, flows started inside the observable must be placed onto another thread.
+    * Flows started by other means, do not require any special treatment.
+
+.. note:: It is possible to avoid deadlock without following these rules depending on the number of flows running within the node. But, if the
+          number of flows violating these rules reaches the flow worker queue size, then the node will deadlock. It is best practice to
+          abide by these rules to remove this possibility.
+
+
+
+

docs/source/corda-api.rst

@@ -15,6 +15,7 @@ The following are the core APIs that are used in the development of CorDapps:
    api-flows
    api-identity
    api-service-hub
+   api-service-classes
    api-rpc
    api-core-types
    api-testing