docs: update services description for concept guide

This patch updates the API concept guide about services.

blueprint complete-todo-in-api-concept-doc

Change-Id: I9196cb6b700687d2a42d4bf32ecb02a7300a9475

api-guide/source/general_info.rst

@@ -118,6 +118,82 @@ Administrator Concepts
 Come APIs are largely focused on administration of Nova, and generally focus
 on compute hosts rather than servers.
 
+-  **Services**
+
+   Services are provide by Nova components. Normally, the Nova component runs
+   as a process on the controller/compute node to provide the service. These
+   services may be end-user facing, such as the the OpenStack Compute REST API
+   service, but most just work with other Nova services. The status of each
+   service is monitored by Nova, and if it is not responding normally, Nova
+   will update its status so that requests are not sent to that service
+   anymore. The service can also be controlled by an Administrator in order to
+   run maintenance or upgrades, or in response to changing workloads.
+
+   - **nova-osapi_compute**
+
+     This service provides the OpenStack Compute REST API to end users.
+
+   - **nova-metadata**
+
+     This service provides the OpenStack Metadata API to servers. The metadata
+     is used to configure the running servers.
+
+   - **nova-scheduler**
+
+     This service provides compute request scheduling by tracking available
+     resources, and finding the host that can best fulfill the request.
+
+   - **nova-conductor**
+
+     This service provides database access for the other services, and handles
+     internal version compatibility when different services are running
+     different versions of code. The conductor service also handles
+     long-running requests.
+
+   - **nova-compute**
+
+     This service runs on every compute node, and communicates with a
+     hypervisor for managing compute resources on that node.
+
+   - **nova-network**
+
+     This service handles networking of virtual servers. It is no longer under
+     active development, and is being replaced by Neutron.
+
+   - **nova-ec2(deprecated)**
+
+     This service provides AWS EC2 API compatibility.
+
+   - **nova-consoleauth**
+
+     This service provides authorization for consoles.
+
+   - **nova-cert**
+
+     This service handles the management of X509 certificates.
+
+-  **Services Actions**
+
+   - **enable, disable, disable-log-reason**
+
+     The service can be disabled to indicate the service didn't provided
+     service anymore. This is used by admin to stop service for maintenance.
+     For example, when Administrator wants to maintain a specific compute node,
+     Administrator can disable nova-compute service on that compute node. Then
+     nova won't dispatch any new compute request to that compute node anymore.
+     Administrator also can add note for disable reason.
+
+   - **forced-down**
+
+     This action allows you set the state of service down immediately. Actually
+     Nova only provides the health monitor of service status, there isn't any
+     guarantee about health status of other parts of infrastructure, like the
+     health status of data network, storage network and other hardwares. The
+     more complete health monitor of infrastructure is provided by external
+     system normally. An external health monitor system can marks the service
+     down for notifying the fault.
+     `(This action is enabled in Microversion 2.11)`
+
 -  **Hosts**
 
    TODO