A network load
balancer is an appliance device that
is used to split network load across
multiple servers.
Why use a hardware load balancer?
The main reasons
for using a load balancer are:
How does Hardware load balancing
work?
The basic principle is that
network traffic is sent to a
shared IP in many cases called a
virtual IP (VIP), or listening
IP. This VIP is an address that
it attached to the load
balancer. Once the load balancer
receives a request on this VIP
it will need to make a decision
on where to send it.
This decision is normally
controlled by a “load balancing
method/ strategy”, a “Server
health check” or, in the case of
a next generation device in
addition a rule set.
The request is then sent to the
appropriate server and the
server will produce a response
(hopefully). Depending on the
type of device, the response
will be sent either back to the
load balancer, in the case of a
Layer 7 device, or more
typically with a layer 4 device
directly back to the end user
(normally via its default
gateway).
In the case of a proxy based
load balancer, the request from
the web server can be returned
to the load balancer and
manipulated before being sent
back to the user. This
manipulation could involve
content substitution,
compression. Indeed some top end
devices offer full scripting
capability.
Depending on the capability of
the load balancer, in many cases
it is desirable for the same
user to be sent back to the same
web server. This is generally
referred to as persistence.
More info - Load Balancing
jetNEXUS Load Balancer Product Range
What are load balancing
methods/strategies/ schedules?
Typically a load balancing
method or strategy is used to
decide how the load balancer
chooses where to send the
request. There are many
strategies available depending
on the vendor, however a few
common ones are listed below:
Round
robin: The most simple
method, each server takes a
turn.
Least
number of connections:
The load balancer will keep
track of the number of
connections a server has and
send the next request to the
server with the least
connections.
Weighted: Typically servers are allocated
a percentage capability as one
server could be twice as
powerful as another. Weighted
methods are useful if the load
balancer does not know the real
and actual performance of the
server.
Fastest
response time: This
method is normally only
available on more advanced
products. The request will be
sent to the fastest responding
server.
Server
agent: A client is
installed on the server that
communicates with the load
balancer. This is sometimes
required when you are using a
basic load balancer that has
direct server return. I.e. it
does not know how many actual
connections the server has or
how well it is responding as it
does not get the responses from
the servers.
Methods such as server agent and
weighted try to guess what the
performance should be like for
the next request whilst methods
such as fastest response time
actually know what the server is
doing in real time.
More info - Load Balancing
jetNEXUS Load Balancer Product Range
What is server health checking?
Server health checking is the
ability of the load balancer to
run a test against the servers
to determine if they are
providing service.
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.
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.
Simple
HTTP GET: This will make
a HTTP GET request to the web
server and typically check for a
header response such as 200 OK.
Full HTTP
GET: This will make a
HTTP GET and check the actual
content body for a correct
response. This feature is only
available on some of the more
advanced products but is the
superior method for web apps as
its will check that the actual
application is available.
More info - Load Balancing
jetNEXUS Load Balancer Product Range
What is persistence?
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.
IP based
persistence: This is a
simple method but is not very
effective due to the “super
proxies” i.e. a user could
connect to the site via a range
of external IPs thus breaking
the persistence.
Cookie
based: Layer 7 devices
can take advantage of setting a
load balancer cookie with the
persistence information. This is
a more reliable method and does
not suffer from the super proxy
problem.
There are also a number of
vendor specific methods that use
header information and other
parameters found in the content
such as SSL session ID to track
uniqueness.
More info - Load Balancing
jetNEXUS Load Balancer Product Range
What is SSL acceleration and off
load?
SSL acceleration or SSL
offload is the ability for a
load balancer to establish a
secure tunnel with the client
thus in most cases replacing the
requirement for the web server
to perform SSL.
In order for the load balancer
to perform this function it must
be configured with an SSL
certificate either self
generated or signed by a
certificate authority. By
default SSL is initiated to the
server on port 443, as such, the
load balancer should be
configured to either terminate
this or simply pass it straight
through to the web server.
Some load balancers offer the
facility to import and export
certificates from common web
servers such as MS IIS and
Apache.
More info - SSL Offload
jetNEXUS Load Balancer Product Range
Why Offload SSL?
SSl (Secure Sockets Layer)
can be a very CPU intensive
operation thus reducing the
speed and capacity of the web
server. In addition there can be
cost saving and management
simplifications of having all
the certificates stored in one
place. And yes one certificate
can be used for many web
servers.
More info - SSL Offload
jetNEXUS Load Balancer Product Range
What is a next generation load
balancer or ADC (Application
Delivery Controller)?
An ADC can be described as a
next generation load balancer.
Typically they provide features
to improve capacity whilst
speeding up system performance.
They tend to offer a richer
feature set including advanced
load balancing, content caching,
content compression, connection
management, connection Pooling ,SSL,
advanced routing, highly
configurable server health
monitoring and content
manipulation.
These devices tend to run at the
Layer 7 level.
Why choose Next Generation load Balancing?
What is web acceleration?
Web acceleration typically
consists of a number of methods
including HTTP compression and
connection management/ pooling.
Compression can dramatically
reduce the size of a typical web
page by 70-90%.
This means that over a real
internet connection i.e. with
some latency, the page will be
compressed, delivered and
uncompressed much faster than a
page without compression. Over a
modem or contended ADSl this can
be 3-4x faster. Many commercial
web sites use compression to
speed up the delivery of the
site to end users. Other
effects of compression include a
reduction in the amount of data
sent over the network that could
lead to reduced bandwidth bills!
As the pages are served faster,
session times are reduced and
in most cases this will lead to
an increase in server capacity.
More info - Web Acceleration
jetNEXUS Load Balancer Product Range
Layer 4 vs Layer 7
Layer 4 devices are still
commonly available although
their market share has been
reducing significantly as layer
7 devices and ADC’s are becoming
more powerful and cost
effective. Some Layer 7 devices
can run at over 3.2gig! This
together with n=n scalability, a
feature which is often found
with layer 7 devices, means that
it is very unlikely there are
any web sites in the world that
can’t take advantage of this
technology.
Although layer 4 devices offer
little functionality compared to
layer 7 devices and ADC’s, they
have traditionally been cheaper.
This is no longer the case with
many of the new ADC/l7 load
balancing products.
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