Selection Strategies: Balancing vs Stickiness

Balancing strategies:

leastConns

random

roundRobin

shuffle

Sticky strategies:

client IP

USER

HOST

What's better for FTP, SFTP connections? RoundRobin or LeastConns? And

why? The HAproxy docs for the 'balance' keyword say that 'leastconn' is

best for long-lived connections:

leastconn The server with the lowest number of connections receives the

connection. Round-robin is performed within groups of servers

of the same load to ensure that all servers will be used. Use

of this algorithm is recommended where very long sessions are

expected, such as LDAP, SQL, TSE, etc... but is not very well

suited for protocols using short sessions such as HTTP. This

algorithm is dynamic, which means that server weights may be

adjusted on the fly for slow starts for instance.

(from http://haproxy.1wt.eu/download/1.4/doc/configuration.txt)

Is this true? Corroboration or statistics showing the relationship?

Dangers of LeastConns

Consider that you are bringing up a new server, or restarting a server.

Being new, it will have no connections -- and the load balancer sees

this. Thus in a short period of time, ALL new connections may end up

slamming that server. If the server can handle this type of workload,

then you are good. If, on the other hand, the server performs warm-up

tasks (e.g. filling caches, performing discovery of necessary downstream

resources, etc), then this is quite undesirable.

This is why HAproxy has a "warm up" period.

Links/References

http://www.timgalyean.com/2011/03/load-balancing-with-haproxy-and-apache/

http://blog.loadbalancer.org/load-balancing-windows-terminal-server-haproxy-and-rdp-cookies/

http://blogs.citrix.com/2010/09/02/load-balancing-least-connections/

http://www.brocade.com/support/Product_Manuals/ServerIron_SLBGuide/health.4.19.html#49842

https://kb.wisc.edu/ns/page.php?id=13201

http://serverfault.com/questions/457506/ha-proxy-roundrobin-vs-leastconn

http://blog.exceliance.fr/2011/08/03/layer-7-load-balancing-proxy-mode/

https://devcentral.f5.com/articles/intro-to-load-balancing-for-developers-ndash-the-algorithms#.UsPAPY3TNe4

Nice description of methods, and quite a variety of them.

https://devcentral.f5.com/articles/intro-to-load-balancing-for-developers-ndash-how-they-work#.UsT6cI3TNe4

Reuse the JPG/pic there?

https://devcentral.f5.com/articles/intro-load-balancing-for-developers-ndash-the-architect-rsquos-view#.UsT6dI3TNe4

https://devcentral.f5.com/articles/advanced-load-balancing-for-developers-ndash-adcs-what-rsquos-the-difference#.UsT6XY3TNe4

The 'Programmability' bit is a good point. mod_proxy might need some

ftpdctl actions implemented for altering things on-the-fly.

https://devcentral.f5.com/articles/advanced-load-balancers-for-developers-adcs-the-code#.UsT6Y43TNe4

http://www.onjava.com/pub/a/onjava/2001/09/26/load.html

Nice discussion of DNS round robin

http://www.javaworld.com/article/2077921/architecture-scalability/server-load-balancing-architectures--part-1--transport-level-load-balancing.html

http://www.javaworld.com/article/2077922/architecture-scalability/server-load-balancing-architectures--part-2--application-level-load-balanci.html

http://www.infoq.com/articles/scalability-principles

http://www.infoq.com/articles/ebay-scalability-best-practices

Note Best Practice #4, and how that pertains to Black Pearl (think

intra-service messaging/notifications)

Aside: Software Architect Role

http://www.codingthearchitecture.com/2008/02/11/the_key_difference_between_developer_and_architect_roles.html

http://www.infoq.com/presentations/Role-of-the-Architect

Need to watch this!