LOAD BALANCING Dawid Królak

Intro  Problem  Model OSI  Layer 2  Layer 4-7  Reverse Proxy

Problem How to maintain large amount of queries, requests? Buy a new faster, batter hardware? Buy faster links? Reduce service? Use load balancing methods!

Advantages  Use hardware that is available but unused at this time.  Use cheaper hardware.  First properly configured easier to upgrade, maintain.  Scalability.  It`s all about speed.

Disadvantages  Difficult to configure.  Need more IT knowledge.

Levels of Load Balancing  Layer 2 OSI Model (Link aggregation)  Layer 4 OSI Model (TCP/UDP)  Layer 7 OSI Model (Reverse Proxy)

Model OSI Application Presentation Session Transport Network Data Link Physical

Data link – link aggregation  Many physical links can be seen as one logical.  Defined by IEEE 802.3ad standard.

Caution! Standard IEEE 802.3ad is not defining Load Balancing! To load balancing we need to use algorithms! But without link aggregation we can`t talk about layer 2 load balancing.

Link Aggregation Features:  Load Sharing (using available links)  Automatic configuration  Deterministic (predictable, controllable)  Frame order (frame order is preserved)

Limiations  We need to use additional algorithms to use load balancing  Works only between two devices.  Same speed on links (should be).

Additional info Some algorhithms can be found at:  alancing+algorithms alancing+algorithms

Layer 4 load balancing  Layer-4 load balancing is to distribute requests to the servers at transport layer, such as TCP, UDP and SCTP transport protocol.  We know one address, load balancer is switching us to one of many servers.  Pure 4-layer load balancing is being displacing by layer4-7 load balancing.

Layer4-7 load balancing Problems:  One service can be run on multiple logical machines.  Logical machine can be built from many physical machines. Main role of load balancing is to distribute requests to multiple logic machines/services.

Layer 4-7 load balancing Advantages:  Scalability  We can scale that part of system that we need.  Nowadays many servers are supporting this. (database, www, webservices, dns)

Layer 4-7 load balancing

Reverse proxy  Reverse proxy is special server proxy type.  Typically set at front of web servers.  This proxy can distribute connections to a different address.  Fast and easy to configure in popular webservers (apache, lighttp)

Load Balancing layer  Load balancing 2 and 4-7 can be used at the same time.  Load balancer can distribute requests to other load balancer  Load balancing is used in cloud computing.  Load balancer and services can be runned at one physical and logical machine, like casual PC /but there is no sense to do it/  It`s better to set some load balancers instead of buying new better faster and more expensive hardware.

Bibliography

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