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Encyclopedia > Load balancing (computing)

In computer networking, load balancing is a technique (usually performed by load balancers) to spread work between many computers, processes, hard disks or other resources in order to get optimal resource utilization and decrease computing time. This article or section is in need of attention from an expert on the subject. ...



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 typical server farm. ... In information technology, a server is an application or device that performs services for connected clients as part of a client-server architecture. ...

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. This virtual server is bound to a number of physical services running on the physical servers in a server farm. These physical services contain the physical server's IP address and port. 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. Load balancers are sometimes referred to as "directors"; while originally a marketing name chosen by various companies, it also reflects the load balancer's role in managing connections between clients and servers. A virtual IP address (VIP) is an IP address that is not connected to a specific computer or network interface card (NIC) on a computer. ... An IP address (Internet Protocol address) is a unique address that certain electronic devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. ... It has been suggested that this article or section be merged into Computer port (software). ... A virtual private server (also referred to as virtual dedicated server or virtual server, and abbreviated VPS or VDS) is a method of partitioning one physical server computer into multiple servers that each has the appearance and capabilities of running on its own dedicated machine. ...

Different virtual servers can be configured for different sets of physical services, such as TCP and UDP services in general. Protocol- or application-specific virtual servers that may be supported include HTTP, FTP, SSL, SSL BRIDGE, SSL TCP, NNTP, SIP, and DNS. The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite. ... User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. ... This article concerns communication between pairs of electronic devices. ... HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ... This article is about the File Transfer Protocol standardised by the IETF. For other file transfer protocols, see File transfer protocol (disambiguation). ... Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, instant messaging and other data transfers. ... The Network News Transfer Protocol or NNTP is an Internet application protocol used primarily for reading and posting Usenet articles, as well as transferring news among news servers. ... The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. ... On the Internet, the Domain Name Server (DNS) associates various sorts of information with so-called domain names; most importantly, it serves as the phone book for the Internet by translating human-readable computer hostnames, e. ...

The load balancing methods (listed below) manages the selection of an appropriate physical server in a server farm.

Load balancers also perform server monitoring of services in a web server farm. In 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. For example, a page on a local or remote server which provides information on the site maintenance or outage. The inside/front of a Dell PowerEdge web server The term Web server can mean one of two things: A computer program that is responsible for accepting HTTP requests from clients, which are known as Web browsers, and serving them HTTP responses along with optional data contents, which usually are...

Among the server types that may be load balanced are:

In Global Server Load Balancing (GSLB) (also known as Global Traffic Management) the load balancer distributes load to a geographically distributed set of server farms based on health, server load or proximity. A typical server farm. ... For other uses, see cache (disambiguation). ... This article is about the network security device. ... An Intrusion Detection System or IDS is a software tool used to detect unauthorised access to a computer system or network. ... Secure Sockets Layer (SSL) and Transport Layer Security (TLS), its successor, are cryptographic protocols which provide secure communications on the Internet. ... Internet services hosted in a single facility or geographic area are subject to downtime due to general facility or network issues that lead to the entire site being unavailable, and will result in high latency for clients whose network and/or geographic location is far from the server farm. ... National motto: El Pais De La Eterna Primavera The Country of The Eternal Spring Official language Spanish (Official) (23 indigenous languages have been officially recognised, but all transactions are in Spanish) Capital Guatemala City President Óscar Berger Area  - Total  - % water Ranked 103rd 108,890 km² 0. ...

Alternative to BGP *[[1]]

Load balancer features

  • SSL Offload and Acceleration: SSL applications can be a heavy burden on the resources of a Web Server, especially on the CPU and the end users may see a slow response (or at the very least the servers are spending a lot of cycles doing things they weren't designed to do). To resolve these kinds of issues, a Load Balancer capable of handling SSL Offloading in specialized hardware may be used. When Load Balancers are taking the SSL connections, the burden on the Web Servers is reduced and performance will not degrade for the end users.
  • Distributed Denial of Service Attack (DDoS) Protection: load balancers can provide features such as SYN cookies and delayed-binding (the back-end servers don't see the client until it finishes its TCP handshake) to mitigate SYN flood attacks and generally offload work from the servers to a more efficient platform.
  • HTTP Compression: reduces amount of data to be transferred for HTTP objects by utilizing gzip compression available in all modern web browsers
  • TCP Offload: different vendors use different terms for this, but the idea is that normally each HTTP request from each client is a different TCP connection. This feature utilizes HTTP/1.1 to consolidate multiple HTTP requests from multiple clients into a single TCP socket to the back-end servers.
  • TCP Buffering: the load balancer can buffer responses from the server and spoon-feed the data out to slow clients, allowing the server to move on to other tasks.
  • HTTP Caching: the load balancer can store static content so that some requests can be handled without contacting the web servers.
  • Content Filtering: some load balancers can arbitrarily modify traffic on the way through.
  • HTTP Security: some load balancers can hide HTTP error pages, remove server identification headers from HTTP responses, and encrypt cookies so end users can't manipulate them.
  • Priority Queuing: also known as rate shaping, the ability to give different priority to different traffic.
  • Content Aware Switching: most load balancers can send requests to different servers based on the URL being requested.
  • Global Server Load Balancing: directing traffic to the best datacenter on a global or national basis
  • Link Load Balancing: monitor and load balance traffic between multiple ISP links
  • Client Authentication: authenticate users against a variety of authentication sources before allowing them access to a website.
  • Web Application Firewall: enforces a layer-7 security policy to secure HTTP and HTTPS websites.
  • Web Acceleration: sometimes this just entails HTTP caching and compression but some vendors provide additional acceleration features.
  • Spam Filtering: at least one load balancer allows the use of an IP reputation database to refuse mail from known spammers even before sending the messages to other spam filters being load balanced.
  • Programmatic Traffic Manipulation: at least one load balancer allows the use of a scripting language to allow custom load balancing methods, arbitrary traffic manipulations, and more.

SYN Cookies are the key element of a technique used to guard against SYN flood attacks. ... A normal connection between a user (Alice) and a server. ... HTTP compression is a capability built into both web servers and web browsers, to make better use of available bandwidth. ... Internet services, like a web site, hosted in a single facility or geographic area are subject to downtime due to general facility or network issues that lead to the entire site being unavailable, and will result in high latency for clients whose network and/or geographic location is far from... A mail filter is a piece of software which takes an input of an email message. ...


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, and in other cases it can simply provide better performance (as one server may have data cached related to a particular user while other servers may not).

But reliance on persistence can cause problems if the persistence fails. Further, persistence mechanisms can vary across load balancer implementations. An alternative 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. Of course this can be less efficient than persistence provided by a load balancer.

The easiest and most widely supported persistence method is source-address affinity. What this means is when the load balancer makes a load balancing decision, it creates a record in memory that records which server that remote IP address was sent to. Future requests from the same IP address will be sent to the same server. A timeout is usually specified so these entries do not need to be stored forever. The problem with this persistence method is that some users (notably AOL users) access the internet through a cluster of proxy servers which means their request address may change from request to request. In addition your typical home internet user has a dynamic IP address assigned by their ISP using DHCP and it is possible for their IP address to change before they finish a web transaction. Finally, for internal corporate applications, many users might come from a few source addresses (i.e. internal NAT addresses) which limits the granularity of the load balancing decisions using this persistence method (huge groups of users may come from one IP address and thus be sent to the same server, even if it is overloaded). For other uses, see AOL (disambiguation). ... ISP may mean: Internet service provider, an organization that offers users access to the Internet and related services. ... DHCP in the context of computing can stand for: Dynamic Host Configuration Protocol - one of the protocols in the TCP/IP networking suite Decentralized Hospital Computer Program of the United States Department of Veterans Affairs This article consisting of a 4-letter acronym or initialism is a disambiguation page — a...

The best load balancers out there will support multiple persistence methods so that no one failure will break server persistence. For HTTP traffic on load balancers that support it, it is common to use cookie persistence as the primary persistence method as this is the most robust persistence solution for this type of traffic. High-end load balancers are able to insert the cookie into the HTTP responses automatically and the browser will re-send this cookie on the next request allowing the load balancer to read the cookie and use it to send the request to the same real server. In case a browser does not allow or support cookies, a fallback persistence method is usually used such as source-address affinity.


The following methods are possibilities that are supported by some load balancers for deciding which real server to relay a client request to. If persistence is being used then this will only be used for the first request from a new client, after that the persistence method will override these methods. This means that with persistence it is possible for things to be less than evenly balanced.

  • Least connections: a request is sent to the server with the fewest active connections at the moment
  • Round-robin: requests are sent to servers in a sequential and circular pattern -- server 1, server2, server3, ..., serverN, server1, ...
  • Fastest: server responsiveness is dynamically measured and requests are sent to the server with the fastest current response time.

In addition ratios can usually be assigned to servers so that some servers get a greater share than others, usually used if the servers have different capabilities. Advanced methods like Fastest make ratios unnecessary since it measures actual server performance. Connectivity is the property of a device such as a PC, peripheral, PDA, mobile phone, robot, home appliance, or car that enables it to be connected, generally to a PC or another device without the need of a PC - autonomously. ...

Finally some load balancers support the idea of priority activation -- when the number of available servers drops below a certain number, other standby servers can be brought online (possibly serving as backups for a number of different applications).

Web server methods

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; an example of a site using the approach is the Wikimedia Foundation and its projects. In June 2004 the load was balanced using a combination of: It has been suggested that this article or section be merged with Scale (computing). ... For the wiki software used and developed by the Wikimedia Foundation, see MediaWiki. ... 2004 : January - February - March - April - May - June - July - August - September - October - November - December See also: June 2004 in sports Deaths in June • 28 Anthony Buckeridge • 26 Naomi Shemer • 26 Yash Johar • 22 Bob Bemer • 22 Thomas Gold • 22 Francisco Ortiz Franco • 16 Thanom Kittikachorn • 10 Ray Charles • 5 Ronald Reagan...

  • 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.

Alternative methods include use of layer 4 routers, and for Linux, the Linux Virtual Server, which is an advanced open source load balancing solution for network services. Other load balancing reverse proxies for UNIX systems include XLB, HAProxy, Balance, Pen and Pound. With the appropriate modules, the Apache, Lighttpd and Nginx web servers can also act as a reverse proxy. Fastream IQ Reverse Proxy is a scalable and robust reverse proxy for Windows 2000/XP/2003/Vista. Network Load Balancing Services is a Microsoft proprietary clustering and load balancing implementation. Round robin DNS is a technique in which load balancing is performed by a DNS server instead of a strictly dedicated machine. ... Squid is a proxy server and web cache daemon. ... For other uses, see cache (disambiguation). ... For other uses, see PHP (disambiguation). ... A database server is a computer program that provides database services to other computer programs or computers, as defined by the client-server model; the term may also refer to a computer dedicated to running such a program. ... A typical router simply sends incoming packets onto the appropriate IP address on its network. ... Linux Virtual Server (LVS) is an advanced load balancing solution for Linux systems. ... A reverse proxy is a proxy server that is installed within the neighborhood of one or more servers. ... Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®, sometimes also written as or ® with small caps) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... Balance is a simple but powerful generic tcp proxy with round robin load balancing and failover mechanisms. ... Pound is a lightweight open source reverse proxy program suitable to be used as a web server load balancing solution. ... The Apache HTTP Server, commonly referred to simply as Apache, is a web server notable for playing a key role in the initial growth of the World Wide Web. ... lighttpd (pronounced lighty) is a web server which is designed to be secure, fast, standards-compliant, and flexible while being optimized for speed-critical environments. ... nginx (pronounced engine X) is a lightweight web server/reverse proxy and mail (IMAP/POP3) proxy. ... Network Load Balancing Services (NLBS) is a proprietary Microsoft implementation of clustering and load balancing that is intended to provide high availability and high reliability, as well as high scalability. ...

Networking and redundancy

When providing an online service, it is important to always have access to the Internet. You cannot limit yourself to choose one provider and being at their mercy. They may fail or you may overload the circuit with peak traffic during the day, resulting in slower network speed. Unavailability when a site is down and slow connections can cause clients to go elsewhere, perhaps to your competitors for better service.

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. Multi-homed networks are increasing in popularity because they provide networks with better reliability and performance to the end-user. Better reliability results from the fact that the network is protected in case one of the Internet links or access routers fails. Performance increases due to the fact that the network's bandwidth to the Internet is the sum of the different pipes available through the different access links.


Server monitoring checks the state of a server by periodic probing of the specified destination. Based on the response, it takes appropriate action. Monitors - also known as keep-alives - specify the types of request sent to the server and the expected response from the server. The load balancing system sends periodic requests to the server. The response from the servers must be received not later than configured response timeout. If the configured number of probes fail, the server is marked "DOWN" and the next probe is sent after the configured down time. The destination of the probe may be different from the server's IP address and port. A load balancer may support multiple monitors. When a service is bound to multiple monitors, the status of the service is arrived based on the results sent by all monitors. Look up monitor in Wiktionary, the free dictionary. ...

Monitors can be simple like pinging a server or checking to see if a certain port is open, or they can be advanced and application-specific such as performing an FTP directory listing or requesting an HTTP page and matching a string in the response. The better load balancers support external monitors that can be written in a variety of programming languages so that in-depth evaluation of the server's proper operation can be performed.


For the end-user CPE (as opposed to ISPs, public hosts or other telco infrastructure), load balancing can be used to provide resilience or redundant backup in the case of failure of one part of WAN access. For example, a company with an ADSL line might have a secondary ADSL line, or any other kind of Internet feed. Internet traffic may be evenly distributed across both connections, giving more total net bandwidth, or traffic policies may determine specific types of data use a specific connection. Where one connection fails, all traffic can automatically route via the other connection. This is also known as Link Load Balancing. Customer premises (or provided) equipment (CPE): Terminal and associated equipment and inside wiring located at a subscribers premises and connected with a carriers communication channel(s) at the demarcation point (demarc) . In other words, CPE is an acronym meaning customer premises equipment. ...


  • Tony Bourke: Server Load Balancing, O'Reilly, ISBN 0-596-00050-2
  • Chandra Kopparapu: Load Balancing Servers, Firewalls & Caches, Wiley, ISBN 0-471-41550-2
  • Robert J. Shimonski: Windows Server 2003 Clustering & Load Balancing, Osborne McGraw-Hill, ISBN 0-07-222622-6
  • Jeremy Zawodny, Derek J. Balling: High Performance MySQL, O'Reilly, ISBN 0-596-00306-4
  • Matthew Syme, Philip Goldie: Optimizing Network Performance with Content Switching: Server, Firewall and Cache Load Balancing'', Prentice Hall PTR, ISBN 0-13-101468-4

Chandra Kopparapu is the Vice President - Sales (Asia Pacific) of Foundry Networks. ... Jeremy Zawodny Jeremy Zawodny is currently an employee of Yahoo! in the platform engineering group. ...


A10 Networks is a networking and security vendor that was founded in 2004 by Lee Chen, the former co-founder of Foundry Networks and Centillion Networks. ... Astaro protects corporate networks against Internet threats such as system intruders, viruses, spam, and spyware. ... Barracuda Networks, Inc. ... CAI Networks, Inc. ... Cisco may refer to: Cisco Systems, a computer networking company Cisco IOS, an internet router operating system CISCO Security Private Limited, a security company in Singapore Commercial and Industrial Security Corporation, a statutory board in Singapore Abbreviation for San Francisco, California Cisco (wine) The Cisco Kid, a fictional character created... Citrix Systems (NASDAQ: CTXS) is a U.S. high technology software company, based in Fort Lauderdale, Florida, with subsidiary operations in the UK and Australia. ... Crescendo Networks is an Israeli company dealing in the Application Front End Acceleration market. ... F5 Networks, Inc. ... Foundry Networks is a network system vendor selling high-end managed ethernet switches. ... Juniper Networks NASDAQ: JNPR is a telecommunications equipment company. ... Northern Telecommunications Networks, commonly known as Nortel, is a telecommunications equipment manufacturer headquartered in Canada. ... Radware (NASDAQ: RDWR), engages in the development, manufacture, and sale of intelligent application switching solutions worldwide. ... Zeus Technology, Ltd. ...

See also

This article does not cite any references or sources. ... This article or section does not adequately cite its references or sources. ... This article or section does not adequately cite its references or sources. ...

External links

Wikimedia Commons has media related to:
  • XLB Open Source HTTP (Layer-7) Loadbalancer
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  • Dynamic Load Balancing and Scheduling
  • Load Sharing and Balancing
  • Load Balancing in Signaling Transfer Points
  • What Is Network Load Balancing?
  • Article "Load Balancing for Small Businesses" by Network Computing
  • Article "BalanceNG: A simple approach to load balancing" by Anze Vidmar
  • Article "Taking a load off: Load balancing with balance" by Costa Walcott
  • Article "Simple webserver load balancing with Pound" by Steve Kemp
  • Article "Whitepaper on software load balancing" by Proto Co Networking
  • Article "DNS Load Balancing" by Mark Henry
  • Article "Making applications scalable with Load Balancing" by Willy Tarreau
  • How does CORBA support load-balancing?
  • A Practical Path Towards the Ultimate Microsoft SQL Server Scalability and Availability by Justin Shih



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