= Read-Me-First =

== The Scope of this Document ==

Computer clusters can be used to provide highly available services or
resources. The redundancy of multiple machines is used to guard
against failures of many types.
      
This document will walk through the installation and setup of simple
clusters using the Fedora distribution, version 20.
      
The clusters described here will use Pacemaker and Corosync to provide
resource management and messaging. Required packages and modifications
to their configuration files are described along with the use of the
Pacemaker command line tool for generating the XML used for cluster
control.

Pacemaker is a central component and provides the resource management
required in these systems. This management includes detecting and
recovering from the failure of various nodes, resources and services
under its control.

When more in depth information is required and for real world usage,
please refer to the http://www.clusterlabs.org/doc/[Pacemaker Explained] manual.

== What Is Pacemaker? ==

Pacemaker is a cluster resource manager.

It achieves maximum availability for your cluster services
(aka. resources) by detecting and recovering from node and
resource-level failures by making use of the messaging and membership
capabilities provided by your preferred cluster infrastructure (either
http://www.corosync.org/[Corosync] or
http://linux-ha.org/wiki/Heartbeat[Heartbeat]).

Pacemaker's key features include:

 * Detection and recovery of node and service-level failures
 * Storage agnostic, no requirement for shared storage
 * Resource agnostic, anything that can be scripted can be clustered
 * Supports STONITH for ensuring data integrity
 * Supports large and small clusters
 * Supports both quorate and resource driven clusters
 * Supports practically any redundancy configuration
 * Automatically replicated configuration that can be updated from any node
 * Ability to specify cluster-wide service ordering, colocation and anti-colocation
 * Support for advanced service types
 ** Clones: for services which need to be active on multiple nodes
 ** Multi-state: for services with multiple modes (eg. master/slave, primary/secondary)
 * Unified, scriptable, cluster management tools.

== Pacemaker Architecture ==

At the highest level, the cluster is made up of three pieces:

 * Non-cluster aware components. These pieces
   include the resources themselves, scripts that start, stop and
   monitor them, and also a local daemon that masks the differences
   between the different standards these scripts implement.

 * Resource management. Pacemaker provides the brain that processes
   and reacts to events regarding the cluster.  These events include
   nodes joining or leaving the cluster; resource events caused by
   failures, maintenance, scheduled activities; and other
   administrative actions. Pacemaker will compute the ideal state of
   the cluster and plot a path to achieve it after any of these
   events. This may include moving resources, stopping nodes and even
   forcing them offline with remote power switches.

 * Low level infrastructure. Projects like Corosync, CMAN and
   Heartbeat provide reliable messaging, membership and quorum
   information about the cluster.

When combined with Corosync, Pacemaker also supports popular open
source cluster filesystems.
footnote:[
Even though Pacemaker also supports Heartbeat, the filesystems need to
use the stack for messaging and membership and Corosync seems to be
what they're standardizing on.

Technically it would be possible for them to support Heartbeat as
well, however there seems little interest in this.
]

Due to past standardization within the cluster filesystem community,
they make use of a common distributed lock manager which makes use of
Corosync for its messaging and membership capabilities (which nodes
are up/down) and Pacemaker for fencing services.

.The Pacemaker Stack
image::images/pcmk-stack.png["The Pacemaker stack",width="10cm",height="7.5cm",align="center"]

=== Internal Components ===

Pacemaker itself is composed of five key components:

 * CIB (aka. Cluster Information Base)
 * CRMd (aka. Cluster Resource Management daemon)
 * LRMd (aka. Local Resource Management daemon)
 * PEngine (aka. PE or Policy Engine)
 * STONITHd

.Internal Components
image::images/pcmk-internals.png["Subsystems of a Pacemaker cluster",align="center",scaledwidth="65%"]

The CIB uses XML to represent both the cluster's configuration and
current state of all resources in the cluster. The contents of the CIB
are automatically kept in sync across the entire cluster and are used
by the PEngine to compute the ideal state of the cluster and how it
should be achieved.

This list of instructions is then fed to the DC (Designated
Controller).  Pacemaker centralizes all cluster decision making by
electing one of the CRMd instances to act as a master. Should the
elected CRMd process, or the node it is on, fail... a new one is
quickly established.

The DC carries out the PEngine's instructions in the required order by
passing them to either the LRMd (Local Resource Management daemon) or
CRMd peers on other nodes via the cluster messaging infrastructure
(which in turn passes them on to their LRMd process).

The peer nodes all report the results of their operations back to the
DC and, based on the expected and actual results, will either execute
any actions that needed to wait for the previous one to complete, or
abort processing and ask the PEngine to recalculate the ideal cluster
state based on the unexpected results.

In some cases, it may be necessary to power off nodes in order to
protect shared data or complete resource recovery. For this Pacemaker
comes with STONITHd. 

STONITH is an acronym for Shoot-The-Other-Node-In-The-Head and is
usually implemented with a remote power switch.

In Pacemaker, STONITH devices are modeled as resources (and configured
in the CIB) to enable them to be easily monitored for failure, however
STONITHd takes care of understanding the STONITH topology such that
its clients simply request a node be fenced and it does the rest.

== Types of Pacemaker Clusters ==

Pacemaker makes no assumptions about your environment, this allows it
to support practically any
http://en.wikipedia.org/wiki/High-availability_cluster#Node_configurations[redundancy
configuration] including Active/Active, Active/Passive, N+1, N+M,
N-to-1 and N-to-N.

.Active/Passive Redundancy
image::images/pcmk-active-passive.png["Active/Passive Redundancy",width="10cm",height="7.5cm",align="center"]

Two-node Active/Passive clusters using Pacemaker and DRBD are a
cost-effective solution for many High Availability situations.

.Shared Failover
image::images/pcmk-shared-failover.png["Shared Failover",width="10cm",height="7.5cm",align="center"]

By supporting many nodes, Pacemaker can dramatically reduce hardware
costs by allowing several active/passive clusters to be combined and
share a common backup node

.N to N Redundancy
image::images/pcmk-active-active.png["N to N Redundancy",width="10cm",height="7.5cm",align="center"]

When shared storage is available, every node can potentially be used
for failover.  Pacemaker can even run multiple copies of services to
spread out the workload.
