Dr. Zahid Anwar

Simplified Architecture of Linux Cluster Simplified Architecture of a Single Computer Simplified architecture of an enterprise cluster Load BalancerCluster NodesShared StoragePrint Server

No Single Point of Failure An enterprise cluster should always have the following characteristic: “Any computer within the cluster, or any computer the cluster depends upon for normal operation, can be rebooted without rebooting the entire cluster.” e.g. by building high- availability server pairs

Clustering Terminology  When a program runs  When Process runs on a Linux System  A demon and the effects it produces  A service when combined with its operating environment (config files, data, network)  When a resource moves from one computer to another  A proper failover configuration has no single point of failure. Process Service Fail-Over High Availability

Types of Clusters Originally, "clusters" and "high-performance computing" were synonymous. Today, the meaning of the word "cluster" has expanded beyond high- performance to include high-availability (HA) clusters and load-balancing (LB) clusters

Types of Clusters  High-availability clusters,  also called failover clusters,  used in mission-critical applications.  The key to high availability is redundancy.  Load-balancing cluster  provide better performance by dividing the work  This might be accomplished using a simple round- robin algorithm.  For example, Round-Robin DNS

Terminology  Parallel computing  Tightly coupled sets of computation.  E.g. Several pieces of data are being processed simultaneously in the same CPU  Homogenous collection of computers  Distributed computing  Computing that spans multiple machines or multiple locations.  Heterogeneous collection  Cluster Computing  A form of Distributed Computing  Generally restricted to computers on the same subnetwork or LAN.  Grid computing  Frequently describes computers working together across a WAN or the Internet.  Much larger scale,  tend to be used more asynchronously,  and have much greater access, authorization, accounting, and security concerns.  Peer-to-Peer  Data or file-sharing (Napster, Gnutella, or Kazaa) 

Building a HA Cluster using Heartbeat  Heartbeat: ability to failover a resource from one computer to another  Functioning  Tell Heartbeat which computer owns a particular resource (define primary and backup server  Heartbeat daemon on backup server listens to the "heartbeats" coming from the primary server.heartbeats  If backup server does not hear the primary's heartbeat, it initiates a failover and takes ownership of the resource.

The Physical Paths of Heartbeat  Normally Heartbeat configured to work over a separate physical connection between two servers.  Separate physical connection can be either a  serial cable or  another Ethernet network connection (via a crossover cable or mini hub).  Adds extra traffic to your network

Heartbeat Control Messages  3 basic kinds  Heartbeat (status msgs)  Typically 150 bytes  broadcast, unicast, or multicast  Cluster Transition msgs  relatively rare  contains conversation b/w daemons to move resources  ip-request : to release the resource of ownership  ip-request-resp :shuts off the service and no longer owns the resource.  On receiving ip-request-resp, it starts up the service and offers it to client  Retransmission Requests  Rexmit-a request for a retransmission of a heartbeat control message when one of the servers notices that it is receiving heartbeat control messages out of sequence.

Secondary IP Addresses (Virtual Ips)  Method for adding multiple IP addresses to the same physical network card.  When you use Heartbeat to offer services it is done using secondary IP addresses

Lab Exercise  Set up a 2-node cluster  Configure a highly-available web server

Load Balancing using Ultra Monkey (LVS)  Linux Virtual Server (LVS) enables TCP/UDP connections to be load balanced  Mechanism of connection control is referred to Layer 4 Switching. Layer 3 IP address/port information is used.  The host that LVS runs on is referred to as the Linux-Director (specialized router)  Packets received for a virtual service by linux-director, routed by a scheduling algo  subsequent packets for the same connection sent to the same real server  Advantage of load balancer over round robin DNS  directs requests to less load nodes  accounts for sessions. (e.g. forum software, shopping carts)