Update Ironic User Guide post landing for 41af7d6b

This patch is a post landing for 41af7d6b741a5a24e2cd212ab543b0b5be105551
Fixing:
Update logical architecture diagram
Brief description of Ironic service
Bullet points

Change-Id: I177041fa7db035d95a93e6345faa64177dbf888e

doc/source/deploy/user-guide.rst

@@ -27,15 +27,15 @@ Why Provision Bare Metal
      Here are a few use-cases for bare metal (physical server) provisioning in cloud; there are doubtless many more interesting ones.
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-        High-performance computing clusters
+        1.  High-performance computing clusters
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-        Computing tasks that require access to hardware devices which can't be virtualized
+        2.  Computing tasks that require access to hardware devices which can't be virtualized
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-        Database hosting (some databases run poorly in a hypervisor)
+        3.  Database hosting (some databases run poorly in a hypervisor)
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-        Single tenant, dedicated hardware for performance, security, dependability and other regulatory requirements
+        4.  Single tenant, dedicated hardware for performance, security, dependability and other regulatory requirements
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-        Or, rapidly deploying a cloud infrastructure
+        5.  Or, rapidly deploying a cloud infrastructure
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 Conceptual Architecture
@@ -51,25 +51,41 @@ Conceptual Architecture
 Logical Architecture
 =====================
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-     The Ironic service is composed of the following components,
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-     A RESTful API service, by which operators and other services may interact with the managed bare metal servers.
-|
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-     A Conductor service, which does the bulk of the work. Functionality is exposed via the API service.
-     The Conductor and API services communicate via RPC.
-|
-|
-     A Message Queue
-|
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-     A Database for storing the state of the Conductor and Drivers.
+     To successfully deploy the Ironic service in cloud, the administrator users need to understand the logical architecture.
+     The below diagram shows the basic components that form the Ironic service, the relation of Ironic service with other
+     Openstack services and the logical flow of a boot instance request resulting in the provisioning of a physical server.
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 .. figure:: ../images/logical_architecture.gif
    :alt: Logical Architecture
+|
+     The Ironic service is composed of the following components,
+|
+|
+     1.  A RESTful API service, by which operators and other services may interact with the managed bare metal servers.
+|
+|
+     2.  A Conductor service, which does the bulk of the work. Functionality is exposed via the API service.
+         The Conductor and API services communicate via RPC.
+|
+|
+     3.  A Message Queue
+|
+|
+     4.  A Database for storing the state of the Conductor and Drivers.
+|
+|
+     As in Figure 1.2. Logical Architecture, a user request to boot an instance is passed to the Nova Compute service
+     via Nova API and Nova Scheduler. The Compute service hands over this request to the Ironic service, which comprises
+     of the Ironic API, the Ironic Conductor, many Drivers to support heterogeneous hardware, Database etc. The request
+     passes from the Ironic API, to the Conductor and the Drivers to successfully provision a physical server to the user.
+
+|
+     Just as Nova Compute service talks to various Openstack services like Glance, Neutron, Swift etc to provision a
+     virtual machine instance, here the Ironic service talks to the same Openstack services for image, network and other
+     resource needs to provision a bare metal instance.
+
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 Key Technologies for Bare Metal Hosting
@@ -78,7 +94,7 @@ Key Technologies for Bare Metal Hosting
 PXE
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-Preboot Execution Environment (PXE) is part of the Wired for Management (WfM) specification developed by Intel and Microsoft.
+        Preboot Execution Environment (PXE) is part of the Wired for Management (WfM) specification developed by Intel and Microsoft.
 The PXE enables system's BIOS and network interface card (NIC) to bootstrap a computer from the network in place of a disk. Bootstrapping is the process by which a system loads the OS into local memory so that it can be executed by the processor.
 This capability of allowing a system to boot over a network simplifies server deployment and server management for administrators.
 
@@ -86,14 +102,14 @@ This capability of allowing a system to boot over a network simplifies server de
 DHCP
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-Dynamic Host Configuration Protocol (DHCP) is a standardized networking protocol used on Internet Protocol (IP) networks for dynamically distributing network configuration parameters, such as IP addresses for interfaces and services.
+        Dynamic Host Configuration Protocol (DHCP) is a standardized networking protocol used on Internet Protocol (IP) networks for dynamically distributing network configuration parameters, such as IP addresses for interfaces and services.
 Using PXE, the BIOS uses DHCP to obtain an IP address for the network interface and to locate the server that stores the network bootstrap program (NBP).
 
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 NBP
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-Network Bootstrap Program (NBP) is equivalent to GRUB (GRand Unified Bootloader) or LILO (LInux LOader) - loaders which are traditionally used in local booting. Like the boot program in a hard drive environment, the NBP is responsible for loading the OS kernel into memory so that the OS can be bootstrapped over a network.
+        Network Bootstrap Program (NBP) is equivalent to GRUB (GRand Unified Bootloader) or LILO (LInux LOader) - loaders which are traditionally used in local booting. Like the boot program in a hard drive environment, the NBP is responsible for loading the OS kernel into memory so that the OS can be bootstrapped over a network.
 
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 TFTP
@@ -106,6 +122,6 @@ In a PXE environment, TFTP is used to download NBP over the network using inform
 IPMI
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-Intelligent Platform Management Interface (IPMI) is a standardized computer system interface used by system administrators for out-of-band management of computer systems and monitoring of their operation.
+        Intelligent Platform Management Interface (IPMI) is a standardized computer system interface used by system administrators for out-of-band management of computer systems and monitoring of their operation.
 It is a method to manage systems that may be unresponsive or powered off by using only a network connection to the hardware rather than to an operating system.