CHAPTER 7CLUSTERING SERVERS

CLUSTERING TYPES

There are 2 types of clustering ; Server clusters Network Load Balancing (NLB)

The difference between the two types is based on the type of applications the servers must run & the nature of the data they use.

SERVER CLUSTERS (STATEFULL APPLICATIONS) Designed for application that have long running in

memory states or large frequently changing data sets.

Includes database servers such as SQL server, email & massaging servers; microsoft exchange

In server cluster all the computers(nodes)are connected to a common data set such as shared SCSI bus / storage area network.

Since all nodes have access to same application, any of them can process a request from client

Active nodes receives & processes requests from clients while passive stays idle as fallback(instantaneously) should an active node fail.

NETWORK LOAD BALANCING (STATELESS APPLICATION)

Provides high availability & reliability with the addition of high scalability as well.

Intended for applications for small data sets that rarely change & do not have long running in memory states.

Include Web, file transfer protocol, and VPN servers. Every client request is a separate transaction so its

possible to distribute the requests among multiple servers to balance the processing loads.

NBL cluster all have identical cloned data sets and are all active nodes, should one or more fail, the others will take up the tasks.

Scalability is due to the possibility to add more servers to the cluster.

DESIGNING A CLUSTERING SOLUTION

Access how much availability, reliability & scalability you need.

Why we need clustering solution; Software failures – application malfunction,

interference with other applications, failure applying upgrades, conflict with newly installed program, virus or malicious code.\

Hardware failures – harddrives, cooling fan, power supplies & other components have limited life span

Site failure – geographically dispersed cluster, with servers are in different building or cities.

ESTIMATING AVAILABILITY REQUIREMENTS Degree of availability required depends on

variety of factors; Nature of application running Size Location Distribution of user base Role of application in your organization.

Ex; if the server is said 99% available, that means the application would be unavailable for up to 87.6 hours during a year! 99.9% = 8.76 hours a year

To achieve high availability, apart from clustering solution, you may have to install fault tolerant hardware, create hardware testing plan & establish operational policies.

SCALING CLUSTERS & NUMBER OF CLUSTERS

Both server clusters and NLB are scalable clustering solutions = you can improve the performance of the cluster as the need of your organization grow.

2 basic methods are; Scaling up – improving individual server performance

by modifying the computers hardware configuration. Scaling out – adding servers to an existing cluster.

Servers in a cluster can run multiple applications, so; You may combine multiple applications in a single cluster Create a separate cluster for each application.

UNDERSTANDING NBL

Nbl cluster consists up to 32 servers reffered to as hosts.

Each hosts runs a duplicate copy of the application you want the cluster to provide to clients.

NBL works by creating on each host a virtual network adapter that represents the cluster as a single entity.

NBL NETWORK DESIGN

Internet

Internet

Internet router

Internet router

Node

Firewall

Node Node Node

Firewall

Switch

PLANNING A SERVER CLUSTER HARDWARE CONFIGURATION

For maximum availability having two network interface adapters in each computer is preferable;

One providing connection to client network One connecting to a network dedicated to

communications between the servers in the cluster. Each server must have a separate connection to the

shared storage device. 2 types of storage connection; Using SCSI – bus architecture used to connect storage

devices & other peripherals to personal computers. Using Fiber Channel – high speed serial networking

technology that was originally conceived as general purpose networking solution.

SELECTING A QUORUM MODEL

Every node in a server maintains a copy of the cluster database in its registry.

When a cluster goes offline, the database is no longer updated & the status changes. When the node comes back online, its obtains the current copy of the database to rejoin the cluster, & gets the copy from the cluster’s quorum resources.

Selecting the location for the quorum is a crucial part of creating cluster. 3 quorum models are; Single-node cluster – a cluster that consists of only 1

server. Single-quorum device cluster – the cluster uses a single

quorum resource Majority node set cluster – separate copy of quorum is

stored in each cluster node.

CONFIGURING FAILOVER POLICIES

You may implement a number of different failover policies that control which application nodes an application uses & when. Failover pairs Hot standby server N+I Failover ring Random