load balancing is a technique to spread work between many computers, processes, disks or other resources in order to get optimal resource utilization and decrease computing time.
A load balancer can be used to increase the capacity of a server farm beyond that of a single server .
It can also allow the service to continue even in the face of server down time due to server failure or server maintenance.
A load balancer consists of a virtual server (also referred to as vserver or VIP ) which, in turn, consists of an IP address and port .
virtual server is bound to a number of physical services running on the physical servers in a server farm.
A client sends a request to the virtual server, which in turn selects a physical server in the server farm and directs this request to the selected physical server.
Different virtual servers can be configured for different sets of physical services, such as TCP and UDP services in general.
Application specific virtual server may exist to support HTTP, FTP, SSL, DNS, etc.
The load balancing methods manage the selection of an appropriate physical server in a server farm.
Persistence can be configured on a virtual server; once a server is selected, subsequent requests from the client are directed to the same server.
Persistence is sometimes necessary in applications where client state is maintained on the server, but the use of persistence can cause problems in failure and other situations.
A more common method of managing persistence is to store state information in a shared database, which can be accessed by all real servers, and to link this information to a client with a small token such as a cookie, which is sent in every client request.
Load balancers also perform server monitoring of services in a web server farm.
case of failure of a service, the load balancer continues to perform load balancing across the remaining services that are UP.
In case of failure of all the servers bound to a virtual server, requests may be sent to a backup virtual server (if configured) or optionally redirected to a configured URL.
In Global Server Load Balancing (GSLB) the load balancer distributes load to a geographically distributed set of server farms based on health, server load or proximity.
Load balancing methods:
Least response time
Domain name hashing
Source IP address
Destination IP address
Source IP - destination
Static proximity , used for GSLB
Web Server Load Balancing
One major issue for large Internet sites is how to handle the load of the large number of visitors they get.
This is routinely encountered as a scalability problem as a site grows.
There are several ways to accomplish load balancing
For example in WikiMedia load is balanced as:
Round robin DNS distributed page requests evenly to one of three Squid Cache servers
Squid cache servers used response time measurements to distribute page requests between seven web servers.
In addition, the Squid servers cached pages and delivered about 75% of all pages without ever asking a web server for help.
The PHP scripts which run the web servers distribute load to one of several database servers depending on the type of request, with updates going to a master database server and some database queries going to one or more slave database servers.
Server Load Balancing and redundancy
Alternative methods include use of Layer 4 Router
Linux virtual server, which is an advanced open source load balancing solution for network services.
Network Load Balancing Services, which is an advanced open source load balancing solution for network services.
Many sites are turning to the multi-homed scenario; having multiple connections to the Internet via multiple providers to provide a reliable and high throughput service.
Linux Virtual Server
Started in 1998, the Linux Virtual Server (LVS) project combines multiple physical servers into one virtual server, eliminating single points of failure (SPOF).
Built with off-the-shelf components, LVS is already in use in some of the highest-trafficked sites on the Web.
Requirements for LVS:
The service must scale: when the service workload increases, the system must scale up to meet the requirements.
The service must always be on and available, despite transient partial hardware and software failures.
The system must be cost-effective: the whole system must be economical to build and expand.
Although the whole system may be big in physical size, it should be easy to manage.
In LVS, a cluster of Linux servers appear as a single (virtual) server on a single IP address.
Client applications interact with the cluster as if it were a single, high-performance, and highly-available server.
Inside the virtual server, LVS directs incoming network connections to the different servers according to scheduling algorithms.
Scalability is achieved by transparently adding or removing nodes in the cluster.
High availability is provided by detecting node or daemon failures and reconfiguring the system accordingly, on-the-fly.
For transparency, scalability, availability and manageability, LVS is designed around a three-tier architecture, as illustrated in next figure
The load balancer, servers, and shared storage are usually connected by a high-speed network, such as 100 Mbps Ethernet or Gigabit Ethernet, so that the intranetwork does not become a bottleneck of the system as the cluster grows.
IPVS modifies the TCP/IP stack inside the Linux kernel to support IP load balancing technologies
Three ways to balance load
IPVS supports following three ways to balance loads:
Virtual Server via NAT (VS/NAT)
Virtual Server via Tunneling (VS/TUN)
Virtual Server via Direct Routing (VS/DR)
Virtual Server via NAT (VS/NAT)
When a user accesses a virtual service provided by the server cluster, a request packet destined for the virtual IP address (the IP address to accept requests for virtual service) arrives at the load balancer.
The load balancer examines the packet's destination address and port number. If they match a virtual service in the virtual server rule table, a real server is selected from the cluster by a scheduling algorithm and the connection is added to hash table that records connections. Then, the destination address and the port of the packet are rewritten to those of the selected server, and the packet is forwarded to the server. When an incoming packet belongs to an established connection, the connection can be found in the hash table and the packet is rewritten and forwarded to the right server.
The request is processed by one of the physical servers.
When response packets come back, the load balancer rewrites the source address and port of the packets to those of the virtual service. When a connection terminates or timeouts, the connection record is removed from the hash table.
A reply is sent back to the user.
An example of Virtual Server via Nat
Packet rewriting flow
The incoming packet for web service:
The load balancer will choose a real server and rewritten forwards the packet to it:
Replies get back to the load balancer:
The packet is rewritten and forwarded back to the client
VS-NAT advantages and disadvantages
Real servers can run any OS that supports TCP/IP
Only an IP address is needed for the load balancer, real servers can use private IP address
The maximum number of server nodes is limited, because both request and response packers are rewritten by the load balancer. When the number of server nodes increase up to 20, the load balancer will probably become a new bottleneck
Virtual Server via IP Tunneling (VS/TUN)
IP tunneling (also called IP encapsulation ) is a technique to encapsulate IP datagrams within IP datagrams, which allows datagrams destined for one IP address to be wrapped and redirected to another IP address.
This technique can also be used to build a virtual server: the load balancer tunnels the request packets to the different servers, the servers process the requests, and return the results to the clients directly. Thus, the service appears as a virtual service on a single IP address.
VS-TUN advantages and disadvantages
Real servers send response packets to client directly, which can follow different network routes
Real servers can be in different networks, LAN/WAN
Greatly increasing the scalability of Virtual Server
Real server must support IP tunneling protocol
Virtual Server via Direct Routing (VS/DR)
The load balancer and the real servers must have one of their interfaces physically linked by an uninterrupted segment of LAN such as an Ethernet switch.
The virtual IP address is shared by real servers and the load balancer.
Each real server has a non-ARPing, loopback alias interface configured with the virtual IP address, and the load balancer has an interface configured with the virtual IP address to accept incoming packets.
The workflow of VS/DR is similar to that of VS/NAT or VS/TUN. In VS/DR, the load balancer directly routes a packet to the selected server (the load balancer simply changes the MAC address of the data frame to that of the server and retransmits it on the LAN).
When the server receives the forwarded packet, the server determines that the packet is for the address on its loopback alias interface, processes the request, and finally returns the result directly to the user.
VS-DR advantages and disadvantages
Real servers send response packets to clients directly, which can follow different network routes
No tunneling overhead
Servers must have non-arp alias interface
The load balancer and server must have one of their interfaces in the same LAN segment
Comparison Note: those numbers are estimated based on the assumption that load balancer and backend servers have the same hardware configuration.
The LocalNode feature
In a virtual server of only a few nodes(2,3 or more), it is a resource waste if the load balancer is only used to direct packets.
The LocalNode feature enable that the load balancer not only can redirect packets, but also can process some packets locally