> How to choose between hardware and software load balancer
Although all solutions tend to look like they can do everything,
this is not the case. Basically, there are three aspects to
consider to choose a load balancer :
- performance
- reliability
- features
Performance is critical, because when the load balancer becomes
the bottleneck, nothing can be done. Reliability is very important
because the load balancer takes all the traffic, so its
reliability must be orders of magnitude above the servers's, in
terms of availability as well as in terms of processing
quality. Features are determinant to choose a solution, but not
critical. It depends on what tradeoffs are acceptable regarding
changes to the application server.
1. Session count
One of the most common questions when comparing hardware-based
to proxy-based load balancers is why is there such a gap between
their session count. While proxies generally announce thousands
of concurrent sessions, hardware speaks in millions. Obviously,
few people have 20000 Apache servers to drain 4 millions of
concurrent sessions ! In fact, it depends whether the load
balancer has to manage TCP or not. TCP requires that once a
session terminates, it stays in the table in TIME_WAIT state
long enough to catch late retransmits, which can be seen several
minutes after the session has been closed. After that delay, the
session is automatically removed. In practise, delays between 15
and 60 seconds are encountered depending on
implementations. This causes a real problem on systems
supporting high session rates, because all terminated sessions
have to be kept in the table. A 60 seconds delay would consume
1.5 million entries at 25000 sessions per second. Fortunately,
sessions in this state do not carry any data and are very
cheap. Since they are transparently handled by the OS, a proxy
never sees them and only announces how many active sessions it
supports. But when the load balancer has to manage TCP, it must
support very large session tables to store those sessions, and
announces the global limit.
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