A cluster is a group of two or more servers
dedicated to running a specific application (or applications) and
connected to provide fault tolerance and load balancing. Clustering is
intended for organizations running applications that must be always
available, making any server downtime unacceptable. In a server
cluster, each computer is running the same critical applications so
that if one server fails, the others detect the failure and take over
at a moment’s notice. This is called failover.
When the failed node returns to service, the other nodes take notice
and the cluster begins to use the recovered node again. This is called failback.
Clustering capabilities are installed automatically in the Windows
Server 2003 operating system. In Microsoft Windows 2000 Server, you had
to install Microsoft Clustering Service as a separate module.
1. Clustering Types
Windows Server 2003 supports two types of clustering: server clusters and Network Load Balancing (NLB).
The difference between the two types of clustering is based on the
types of applications the servers must run and the nature of the data
they use.
Important
Server
clustering is intended to provide high availability for applications,
not data. Do not mistake server clustering for an alternative to data
availability technologies, such as RAID (redundant array of independent
disks) and regular system backups. |
Server Clusters
Server
clusters are designed for applications that have long-running in-memory
states or large, frequently changing data sets. These are called stateful applications,
and they include database servers such as Microsoft SQL Server, e-mail
and messaging servers such as Microsoft Exchange, and file and print
services. In a server cluster, all the computers (referred to as nodes)
are connected to a common data set, such as a shared SCSI bus or a
storage area network. Because all the nodes have access to the same
application data, any one of them can process a request from a client
at any time. You configure each node in a server cluster to be either
active or passive. An active nodereceives and processes requests from clients, while a passive node remains idle and functions as a fallback, should an active node fail.
For
example, a simple server cluster might consist of two computers running
both Windows Server 2003 and SQL Server and that are connected to the
same Network-Attached Storage (NAS) device, which contains the database
files. (See Figure 1.)
One of the computers is an active node, and one is a passive node. Most
of the time, the active node is functioning normally, running the
database server application, receiving requests from database clients,
and accessing the database files on the NAS device. However, if the
active node should suddenly fail, for whatever reason, the passive node
detects the failure, immediately goes active, and begins processing the
client requests, using the same database files on the NAS device.
See Also
The
obvious disadvantage of this two-node, active/passive design is that
one of the servers is being wasted most of the time, doing nothing but
functioning as a passive standby machine. Depending on the capabilities
of the application, you can also design a server cluster with multiple
active nodes that share the processing tasks among themselves. You
learn more about designing a server cluster later in this lesson. |
A
server cluster has its own name and Internet Protocol (IP) address,
separate from those of the individual computers in the cluster.
Therefore, when a server failure occurs, there is no apparent change in
functionality to the clients, which continue to send their requests to
the same destination. The passive node takes over the active role
almost instantaneously, so there is no appreciable delay in
performance. The server cluster ensures that the application is both
highly available and highly reliable because, despite a failure of one
of the servers in the cluster, clients experience few, if any,
unscheduled application outages.
Windows
Server 2003, Enterprise Edition, and Windows Server 2003, Datacenter
Edition, both support server clusters consisting of up to eight nodes.
This is an increase over the Windows 2000 operating system, which
supports only two nodes in the Advanced Server product and four nodes
in the Datacenter Server product. Neither Windows Server 2003, Standard
Edition, nor Windows 2000 Server supports server clusters at all.
Planning
Although
Windows Server 2003, Enterprise Edition, and Windows Server 2003,
Datacenter Edition, both support server clustering, you cannot create a
cluster with computers running both versions of the operating system.
All your cluster nodes must be running either Enterprise Edition or
Datacenter Edition. You can, however, run Windows 2000 Server in a
Windows Server 2003, Enterprise Edition, or Windows Server 2003,
Datacenter Edition, cluster. |
Network Load Balancing
Network
Load Balancing (NLB) is another type of clustering that provides high
availability and high reliability, with the addition of high
scalability as well. NLB is intended for applications with relatively
small data sets that rarely change (or which might even be read-only)
and that do not have long-running in-memory states. These are called stateless applications,
and they typically include Web, File Transfer Protocol (FTP), and
virtual private network (VPN) servers. Every client request to a
stateless application is a separate transaction, so it is possible to
distribute the requests among multiple servers to balance the
processing load.
Instead of being
connected to a single data source, as in a server cluster, the servers
in an NLB cluster all have identical cloned data sets and are all
active nodes. (See Figure 2.)
The clustering software distributes incoming client requests among the
nodes, each of which processes its requests independently, using its
own local data. If one or more of the nodes should fail, the others
take up the slack by processing some of the requests to the failed
server.
|
Network
Load Balancing is clearly not suitable for stateful applications such
as database and e-mail servers, because the cluster nodes do not share
the same data. If one server in an NLB cluster were to receive a new
record to add to the database, the other servers would not have access
to that record until the next database replication. It is possible to
replicate data between the servers in an NLB cluster, for example, to
prevent administrators from having to copy modified Web pages to each
server individually. However, this replication is an occasional event,
not an ongoing occurrence.
|
Network
Load Balancing provides scalability in addition to availability and
reliability because all you have to do when traffic increases is add
more servers to the cluster. Each server then has to process a smaller
number of incoming requests. Windows Server 2003, Web Edition, Windows
Server 2003, Standard Edition, Windows Server 2003, Enterprise Edition,
and Windows Server 2003, Datacenter Edition, all support NLB clusters
of up to 32 computers.
Off the Record
There is also a third type of clustering, known as component load balancing (CLB),
designed for middle-tier applications based on Component Object Model
(COM+) programming components. Balancing COM+ components among multiple
nodes provides many of the same availability and scalability benefits
as Network Load Balancing. The Windows Server 2003 operating systems do
not include support for CLB clustering, but it is included in the
Microsoft Windows 2000 Application Center product. |
Tip
Be
sure you understand the differences between a server cluster and a
Network Load Balancing cluster, including the hardware requirements,
the difference between stateful and stateless applications, and the
types of clusters supported by the various versions of Windows Server
2003. |
