Load Balancing FAQ

Layer-2 Load Balancing (bonding)

Layer-2 load balancing, aka link aggregation, port aggregation, ether channel, or gigabit ether channel port bundling is to bond two or more links into a single, higher-bandwidth logical link. Aggregated links also provide redundancy and fault tolerance if each of the aggregated links follows a different physical path. Link aggregation may be used to improve access to public networks by aggregating modem links or digital lines. Link aggregation may also be used in the enterprise network to build multi-gigabit backbone links between gigabit ethernet switches. See also NIC teaming or Link Aggregation Control Protocol (LACP).

The Linux kernel has the Linux bonding driver, which can aggregate multiple links for higher throughput or fault tolerance.

Our Opinion: The Linux Bonding driver works really well in master/slave mode without any changes to your infrastructure. If you have a trunk configured on your switches then you can use full 802.3ad LACP.

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. The load balancer distributes network connections from clients who know a single IP address for a service, to a set of servers that actually perform the work. Since connection must be established between client and server in connection-oriented transport before sending the request content, the load balancer usually selects a server without looking at the content of the request.

IPVS / LVS is an implementation of layer-4 load balancing for the Linux kernel, and has been ported to FreeBSD recently. Loadbalancer.org, Kemp Technologies & Barracuda et al. use IPVS extensively in their hardware load balancers.

Layer-4 load balancing can also be used to balance traffic at multiple Internet access links, in order to increase Internet access speed. See DSL load balancing for more information. SmoothWall, FatPipe, Xrio et al. provide appliances to do this.

Our Opinion: IPVS aka. LVS is awesome, a fast reliable open source load balancing solution best combines with HA-Linux (Heartbeat), Keepalive or Ultramonkey / Ldirectord.

Layer-7 Load Balancing (reverse proxy)

Layer-7 load balancing, also known as application-level load balancing, is to parse requests in application layer and distribute requests to servers based on different types of request contents, so that it can provide quality of service requirements for different types of contents and improve overall cluster performance. The overhead of parsing requests in application layer is high, thus its scalability is limited, compared to layer-4 load balancing.

KTCPVS is an implementation of layer-7 load balancing for the Linux kernel. With the appropriate modules, the Apache, Lighttpd and nginx web servers can also provide layer-7 load balancing as a reverse proxy.

Lots of commercial vendors use Layer 7 load balancing for cookie insertion etc. Barracuda do cookie insertion OK... Loadbalancer.org and Kemp do a nice extra which is Terminal Server RDP cookies....BUT for real flexibility F5 and Citrix netscaler dominate the Layer 7 Load Balancing market, F5 like to call it ADC Application Delivery Controller or ADN Application Delivery Network... we prefer the honest term of proxy or reverse proxy but that's not so sexy is it?

Our Opinion: KTCPVS doesn't seem as mature as HAProxy and it looks like the best features of kernel splicing etc. are being integrated into HAProxy as well. Exceliance and Loadbalancer.org are working with the community to ensure RDP cookies, source IP persistence and keepalive are integrated into the open source HAProxy solution so that it can give the big boys a run for their money.

SSL Termination

Hardware SSL acceleration or offload

Persistence / Sticky / Affinity

Persistence is a feature that is required by many web applications. Once a user has interacted with a particular server all subsequent requests are sent to the same server thus persisting to that particular server. It is normally required when the session state is stored locally to the web server as opposed to a database.

Source IP based persistence: This is a simple and fast method, and these days you are very unlikely to get a client IP change during  a session so it is perfectly safe to use. The old FUD about super or mega proxies changing the client IP address is old news and hasn't been an issue on the Internet for years. However, if you have several large offices accessing a load balanced service from a large proxy i.e. office with 1,000 users represented by a single public IP address you will get very un-even load balancing.

Cookie based: Layer 7 devices can take advantage of setting a load balancer cookie with the persistence information. This works well as long as the clients accept cookies, but obviously can only work with HTTP traffic. Some vendors (Loadbalancer.org, Kemp, F5 & Citrix) can also do RDP cookies.

Server health checking

Ping: This is the most simple method, however it is not very reliable as the server can be up whilst the web service could be down. Also ICMP pings are often blocked by firewalls.

TCP connect:  This is a more sophisticated method which can check if a service is up and running like a service on port 80 for web. i.e. try and open a telnet connection to that port on the real server.

HTTP GET HEADER: This will make a HTTP GET request to the web server and typically check for a header response such as 200 OK.

HTTP GET CONTENTS:  (negotiate or regex) This will make a HTTP GET and check the actual content body for a correct response. Can be useful to check a dynamic web page that returns 'OK' only if some application health checks work i.e. backend database query validates.

DNS Load Balancing

DNS load balancing is to distribute requests to different servers though resolving the domain name to different IP addresses of servers. When a DNS request comes to the DNS server to resolve the domain name, it gives out one of the server IP addresses based on scheduling strategies, such as simple round-robin scheduling or geographical scheduling. This redirects the request to one of the servers in a server group. Once the domain is resolved to one of the servers in specified time-to-live, subsequent requests from the clients using the same local caching DNS server are sent to the same server.

More information is on the DNS Load Balancing page. PowerDNS link here?

Our Opinion:  Much maligned this can be a great way to properly load balance a site... What do you think Google uses!?.. OK they probably do a bit of GSLB and health checking as well but at the end of the day I'm sure they use a GSLB based DNS solution with an LVS backend.... but who am I to guess?

Checkout UltraDNS.com and AutoFailover.com...

Link Load Balancing

Link load balancing is to balance traffic among multiple links from different ISPs or one ISP for better scalability and availability of Internet connectivity, and also cost saving.

See Link Load Balancing for more information. SmoothWall, FatPipe, Xrio et al provide appliances to do this.

Load Balancing Optimization / Compression

WAN Load Balancing Optimization / Compression

Database Load Balancing

Database load balancing is to balance database access requests among a cluster of database servers, in order to achieve database scalability and high availability.

Our Opinion: Really easy to do this with read only replicas.. kind of tricky for writeable databases (Requires Oracle RAC / MySQL Cluster or similar)...

Or middleware like conexant or MySQL load balancing proxy.

SIP Load Balancing

SIP Load Balancing is to load balance SIP related services, in order to achieve performance scalability and high availability of the services.

Our Opinion: Pretty easy at Layer 4 as long as the servers support a backend persistent data store... if they don't you may need SIP caller ID packet inspection from an F5, Zeus or Citrix type appliance.

Update: We've just been working with Horms to get SIP caller ID packet inspection for LVS in the Linux Kernel...ClusterScale appliances now support SIP load balancing for the telco market with NEBS AC & DC options.

Other options: OpenSIPS is more than a SIP proxy/router as it includes application-level functionalitiy.

Computing Load Balancing

Computing load balancing is to split a computational task across many different nodes in the cluster, so that the whole cluster system can provide increased performance. This kind of cluster systems is also known as High-performance cluster, which is most commonly used in scientific computing.

Beowulf type HPCs etc... loads of stuff in Linux Magazine about this kind of specialist area......

Free BSD stuff CARP,  PF and hoststated

Load Balancing Appliance vendors