Web Farming with the
Network Load Balancing Service
in Windows Server 2003
By Rick Strahl
Last Update: June 4th, 2003
When a single Web Server machine isn’t enough to handle the
traffic on your Web site it’s time to look into building a Web Farm that uses multiple
machines on the network acting as a single server. In this article Rick looks
at the Windows Load Balancing Service and the new interface it sports in
Windows Server 2003, which makes creating a Web Farm quick and easy and – gasp
–even an affordable solution.
With the release of Windows Server 2003 Network Load
Balancing has become a much more visible tool as a part of the operating
system, providing a very usable and relatively easy to configure interface that
makes it easy to build a Web Farm. The Network Load Balancing Service has been
around in one incarnation or another since Windows NT SP4, but Windows Server
2003 is the first operating system that brings this service into the forefront
as a main component of the OS. A new Network Load Balancing Manager application
is now directly available from the Adminstrative Tasks menu and it’s powerful
enough to allow to configure the entire cluster from a single console. The
service is now available for all products in the Windows Server family
including the lower end Web Edition which means that you now have a much more
affordable solution to create Web Farms at your disposal. Just add servers
In this article I’ll review the basics of a Load Balancing
service and then show you how to set up configure a basic installation using
Web Farms for city folk – do you need it?
A Web Farm is a not so fancy term for a collection of
servers that act as a single Web server. The process behind the scenes maps a
‘virtual’ IP address to multiple machines. Software such as the Network Load
Balancing Service or hardware like a specialized router or Load Balancer then
deals with dishing up requests to the appropriate machine in the server pool.
Web Farms are an obvious choice if you’ve hit the limits of your single machine
hardware. But before jumping on the Web Farm band wagon (or is that a tractor?)
you should look closely at your hardware and application and be sure that you
can’t make it all run on a single machine first. Although the process of
creating a Web Farm isn’t difficult, administration of two or more servers and
keeping them properly synched is actually a lot more work than administering a
Upgrading your hardware is certainly one option available to
you. Today’s hardware is incredibly capable and should be sufficient to handle
all but the most demanding Web applications on a single box. Multiprocessor
machines with up to 16 processors on Windows make a pretty powerful platform to
run Web applications with, even if those high end machines are rather pricey. While
the Yahoo’s and Amazon’s won’t run on a single box (or on Windows for that
matter), a vast majority of applications are likely to be able to comfortably
serve millions of transactional hits a day from a single machine even with a
But Load Balancing can also provide benefits in the overload
scenario. For one, it’s generally cheaper to throw mid-level machines at a load
problem rather than buying one top of the line high end machine. Even with
server licenses involved multiple low end machines might provide a more cost
Load Balancing also provides something else that has nothing
to do with scalability: The ability to have failover support if something goes
wrong on one of the servers in the pool. Because a Web Farm are made up of
essentially identically configured servers, a failure on a single server will
not bring down the entire Web site. Other servers in the pool can continue to
process requests and pick up the slack. For many companies this feature of load
balancing is often important for peace of mind both in the knowledge that a
single point of failure on the Web Server is avoided as well as providing an in
place mechanism to grow the application should the need arise at a later point.
How does it work?
The concept behind Network Load Balancing is pretty simple: Each
server in a Load Balancing Cluster is configured with a 'virtual' IP address.
This IP address is configured on all the servers that are participating in the
load balancing 'cluster' (a loose term that's unrelated to the Microsoft
Cluster Service). Whenever a request is made on this virtual IP a network
driver on each of these machines intercepts the request for the IP address and
re-routes the request to one of the machines in the Load Balancing Cluster
based on rules that you can configure for each of the servers in the cluster. Microsoft
this process Network Load Balancing (NLB). Figure 1 shows how the process works
Figure 1 – A network load balancing cluster routes
requests to a single virtual IP to available servers in the load balancing
cluster. Note that each machine is self-sufficient and runs independent of the
others duplicating all of the resources on each server. The database sits on a
separate box(es) accessible by all servers.
Although a Web Farm is a common scenario for this service
keep in mind that any IP based service can be run off this service. For
example, you could use a mail server that is under heavy load and uses a
central datastore to share multiple machines in a cluster.
Network Load Balancing facilitates the process of creating a
Web Server Farm. A Web Server farm is a redundant cluster of several Web
servers serving a single IP address. The most common scenario is that each of
the servers is identically configured running the Web server and whatever local
Web applications running on the Web server as shown in Figure 1. Each machine
has its own copy of everything it needs to run the Web application which
includes the HTML files, any script pages (ASP, ASP.Net), any binary files
(such as compiled .Net assemblies, COM objects or DLLs loaded from the Web app)
and any support files such as configuration and local data files (if any). In
short the application should be fully self-contained on a single machine, except
for the data which is shared in a central location. Data typically resides in a
SQL backend of some sort somewhere on the network, but could also be files
shared in a directory for files from a file based database engine such as
Visual FoxPro or Access.
Each server in the cluster is fully self-contained, which
means it should be able to function without any other in the cluster with the
exception of the database (which is not part of the NLB cluster). This means
each server must be configured separately and run the Web server as well as any
Web server applications that are running. If you're running a static site, all
HTML files and images must be replicated across servers. If you’re using ASP or
ASP.Net, those ASP pages and all associated binaries and support files must
also be replicated. Source control programs like Visual SourceSafe can make
this process relatively painless by allowing you to deploy updated files of a
project (in Visual Studio.Net or FrontPage for example) to multiple locations
Short of the data everything else is running on all of the
machines in the NLB cluster. The key is redundancy in addition to load
balancing – if any machine in the cluster goes down, NLB will re-balance the
incoming requests to the still running servers in the cluster. The servers in
the cluster need to be able to communicate with each other to exchange
information about their current processor and network load and even more basic
checks to see if a server went down.
If you have COM components as part of your Web application
things get more complicated, since the COM objects must be installed and
configured on each of the servers. This isn't as simple as copying the file,
but also requires re-registering the components, plus potentially moving any
additional support files (DLLs, configuration files if needed, non-sql data
files etc.). In addition, if you're using In-Process components you'll have to
shut down the Web server to unload the components. You'll likely want to set up
some scripts or batch files to perform these tasks in an automated fashion
pulling update files from a central deployment server. You can use the Windows
Scripting Host (.vbs or .js files) along with the IIS Admin objects to automate
much of this process. This is often tricky and can be a major job especially if
you have a large number of cluster nodes and updates are frequent – strict
operational rules are often required to make this process reliable. Luckily if
you’re building applications with pure ASP.Net you won’t have these issues
since ASP.Net can update .Net binary files without any shutdowns by detecting
changes to the source files and shadow copying binary files to a different
directory for execution.
Make sure you cover your database!
Since multiple redundant machines are involved in a cluster
you'll want to have your data in a central location that can be accessed from
all the cluster machines. It's likely that you will use a full client/server
database like SQL Server in a Web farm environment, but you can also use file
based data access like Visual FoxPro or Jet (Access) tables if those tables are
kept in a central location accessed over a LAN connection.
In heavy load balancing scenarios running a SQL backend,
it’s important to understand that the database not your application code can easily
become your bottleneck! Without going into details here, you need to think
about what happens when you overload the database, which is essentially running
on a single box. Max out that box and you have problems that are much harder to
address than Web load balancing I am going to describe here. At that point you
need to think about splitting your databases so that some data can potentially
be written to other machines. For redundancy you can use the Microsoft Cluster
Service to provide the ability to monitor and sync a backup system that can
take over in case of failure of the primary server.
It’s possible that the database can become your weakest link
so if you’re after redundancy, make sure you also have a backup plan for your
database. If you’re using SQL Server you might want to use Replication to
create live shadows on a backup box for example. At the very least make sure
that frequent automated backup are performed especially if you’re not using a SQL
backend and running file based data engines like FoxPro or Jet.
Network Load Balancing is very efficient and can provide you
reasonably close to 1:1 performance improvement for each machine added into the
cluster – there is some overhead involved, but I didn't notice much in my
performance tests with Vs.Net Application Center Test Tool with each machine
adding 90-95% of its standalone performance to the cluster even in my
non-optimized network setup that I was using to conduct the tests.
You may notice that with this level of redundancy increasing
your load balancing capability becomes simply a matter of adding additional
machines to the cluster, which gives you practically unlimited application
scalability (database allowing) if you need it.
Setting up NLB
In order to utilize the Windows Server Network Load
Balancing features you will need two machines running Windows Server 2003. Each
machine needs to have at least one network card and at least one fixed IP
address. Although running with one adapter works well, for best performance
it’s recommended that you have two adapters in each machine – one mapped to the
real IP Address (Microsoft calls this the Dedicated IP) and one mapped to the
‘virtual’ IP Address (Microsoft calls this the Cluster IP). Be aware that NLB uses
some advanced networking features of network adapters, so it’s possible that
some low end adapters (especially those for non-server machines) may not support
the required NDIS protocols.
In addition you will also need one more machine for testing
(3 machines total). The test machine should be external as you can’t use a
machine from the pool to test – it will only fire request on the local machine
since the IP requests are not traveling over the network when you hit the
virtual IP address – it goes to the local machine.
I'm going to use two ‘servers’ here to demonstrate how to
set up and run NLB. Assume the IP addresses for these machines are 18.104.22.168
and 22.214.171.124. To create a virtual IP address (Cluster IP) you need
to pick an available IP Address on the same Class C network segment. In my
example here I’ll use 126.96.36.199.
Unlike previous versions of NLB the new version has a
central manager application that you can use to create a cluster from a single
machine. Gone are the hassles of having to manually configure each machine
manually – you can do it all from a single machine over the network which is a
To start setting up this cluster bring up the Network Load
Balancing Manager from the Administrative Tools menu. Figure 1 shows what the
cluster manager looks like.
Figure 1 – To set up a new NLB cluster bring up
the Network Load Balancing Manager and right click to createa a new cluster.
Right-click on the root node to add a new cluster. Next
configure the basic cluster configuration, which will consist of assigning the
Cluster or virtual IP address. Figure 2 shows what this dialog looks like
filled out for our test network.
Figure 2 – Configuring the Cluster IP. This is
the ‘virtual’ IP address
that will service all servers in the cluster. Note that you should set the
operation mode to Multicast if you are using a single adapter.
The IP Address is the virtual IP address for the cluster
that will be used to address this cluster. NLB will actually create a new IP address
on each machine in the cluster and bind it to the specified network adapter (in
the next step). Choose a subnet mask – make sure you use the same one for all
servers in the cluster. The Full Internet name is only for reference and is
used here primarily for displaying the name of the server. But if you have a
domain configured for the server you should use that domain name.
Cluster operation mode is very important. Unicast mode means
that NLB takes over the network card it is bound to and doesn’t allow any
additional network traffic through it. This is the reason why two adapters are
a good idea – one that NLB can take over and one that can still handle all
other network traffic directed at the dedicated IP address of the server. If
you’re using a single adapter you should probably select Multicast which allows
both the NLB traffic and the native IP traffic to move through the same network
adapter. Multicast is slower than Unicast as both kinds of traffic need to be
handled by the network adapter but it’s the only way to remotely configure all
machines centrally. You can run a single adapter in Unicast mode, but the
cluster manager will not be able to communicate with the server after it’s
configured. As a general rule use Unicast for two adapters, Multicast for a
single adapter. With my network cards I had to use IGMP mode in order to get
the cards to converge properly – you may have to experiment with both modes to
see what works best for you.
Leave the Allow Remote Control option unchecked. This allows
you to reconfigure the nodes and port rules remotely, although I found little
need to do so. Any changes made to the cluster are automatically propagated
down to the nodes anyway, so there’s little need to do this with the exception
of changing the processing priority. If you do want this functionality I
suggest you enable it after you have the cluster up and running.
The next dialog called Cluster IP Addresses allows you to
add additional virtual IP addresses. This might be useful if you have a Web server
that is hosting multiple Web sites each of which is tied to a specific IP
address. For our example here, we don’t need any and can just click next as
shown in Figure 3.
Figure 3 – If you need to add additional IP addresses to
be load balanced
you can add them here. This is needed only if you host multiple sites
on separate IP addresses and you need separate IPs for these.
Next we need to configure port rules. Port rules determine
which TCP/IP port is handled and how. Figure 3 shows the Port Rules dialog with
two port rules defined for Port 80 (http) and 443 (SSL). The default port
configuration set up by NLB handles all ports, but in this case that rule is
too broad. Port rules can’t overlap so if you create specific rules you either
have to create them for each port specifically or create ranges that fit your specific
Figure 4 – The Port Rules dialog shows all of the
port rules defined for
cluster. By default a rule for all ports – 0 – 65365 is defined. Here I’ve
Created to specific port rules for port 80 and 443.
To add a new port rule click on the Add button which brings
up the dialog shown in Figure 5. Here you can configure how the specific port
is handled. The key property is the Filtering Mode which determines the
affinity of requests. Affinity refers to how requests are routed to a specific
server. None means any server can service the incoming request. Single means
that a specific server has to handle every request from a given IP address.
Generally None is the preferred mode as it scales better in stateless
applications. There’s less overhead in NLB as it doesn’t have to route requests
in many cases. Single mode is useful for server connections that do require
state, such as SSL connections for HTTPS. Secure Server Certificates performs
much better with a persistant connection rather than having to create new
connections on each of the servers in the pool for requests. Figure 1 shows the
configuration for the standard Web Server port - port 80.
Figure 5 – Setting port rules lets you configure
how the cluster
responds to client requests. Affinity in particular determines
whether the same server must handle all requests from
a specific IP address (single) or Class C IP address range (Class C).
To set up the second rule for the SSL Port I added another
rule and then changed the port to 443 and changed the affinity to single.
Although you can’t do it from here, another important
setting is the priority for each machine for each port rule. You can set up
Machine 1 to take 80% of the traffic and the second 20% for example. Each rule
can be individually configured. We’ll see a little later why this is important
for our SSL scenario.
The rules set in this dialog are propagated to all the
cluster servers, which is significant, because the cluster port rules must be
configured identically on each of the cluster node servers. The configuration
tool manages this by remotely pushing the settings to each of the cluster nodes
Network Connections IP configuration settings. This is a big improvement over
previous versions where you manually had to make sure each machine’s port rules
matched and stayed matching.
Up to this point we have configured the cluster and the
common parameters for each node. Now we need to add individual nodes to the
cluster. Figure 6 shows the dialog that handles this step for the first node as
part of the configuration process.
Figure 6 – Adding a node by selecting the IP
address and picking a specifc
When you click Next you get to another dialog that lets you
configure the cluster node. The main feature to configure on this dialog is the
Priority which is a unique ID that identifies each node in the cluster. Each
node must have a unique ID and the lower the number the higher the priority.
Node 1 is the master which means that it typically receives requests and acts
as the routing manager although when load is high other machines will take
Figure 7 – Setting the node parameters involves
setting a priority for
the machine, which is a unique ID you select. The lower the number
the higher the priority – this machine acts as the master host.
Click finish and now we have one node in our cluster.
Actually, not quite so fast. Be patient, this process isn’t
instant. When you click finish the NLB manager actually goes out and configures
your network adapter for you. It creates a new IP address in your network
connections, enables the Network Load Balancing service on your network
adapter(s) you chose during setup and configures the setting we assigned on the
NLB property sheet.
You’ll see your network connection flash on and off a few
times during this configuration process on the machine you are configuring to
be a host. This is normal, but be patient until you see your network connection
back up and running.
If all goes well you should see your network connection back
up and running and see a new node in the NLB Manager sitting below the cluster
(see Figure 8 which shows both nodes). If everything is OK the Status should
say Converged. If it does node 1 is ready.
But we’re not quite done yet – we still need to add the
second node. To do so right-click on the cluster, after which you go through
the steps shown in Figure 7 and 8 one more time. Again be patient, this process
is not super fast – it takes about 20 seconds or so to get a response back from
a remote machine. Once you click finish the process of Converging can take a minute
Figure 8 – The final cluster with both nodes converged and ready to
I’ve had a few problems getting convergence to happen for
the first time. It helps to follow the steps here closely from start to finish
and if for whatever reason you end up removing nodes make sure you double check
your network settings first before re-adding nodes.
You can check what NLB did in the Network Connections for
your machine (Figure 9). Click on the Load Balancing section to see the
settings made there. Remember that the settings should match between machines
with the exception of IP Addresses assigned for each machine. You should also
see the new IP address added in the Internet Protocol settings’ Advanced page.
Figure 9 – All of the setting that NLB makes are
to the network adapter that the virtual IP is bound to.
You can click on the Network Load Balancing item to
configure the node settings as described earlier. The Virtual
IP also has been added in the Internet Protocol | Advanced
If things look Ok, make sure that the machines can ping each
other with their dedicated IPs. Figure 10 shows what you should see for one of
the machines and you should run this test on both of them:
Figure 10 – Checking whether the machines can see each other.
Use IPCONFIG to see adapter information and you should see
both your physical adapter and the virtual IP configured. Make sure that you
don’t get any errors that say that there’s a network IP address conflict. If
you do it means that the virtual IP is not virtual – ie. It’s entered but it’s
not bound to the NLB service. In that case remove the IP and then configure the
NLB first, then re-add the IP address. Alternately remove everything then try
adding it one more time through the NLB manager.
I’ve also found that it helps to configure remote machines
first, then configure the machine running the NLB Manager (if you are using it
in the cluster) last. This avoids network issues on the manager machine – plain
network access gets a little weird once you have NLB configured on a machine.
Again this is a great reason to use two adapters rather than one.
Putting it all together
Ok, so now we’re ready to try it out. For kicks I ran two simple
tests using the Application Center Test tool that comes with VS.Net Enterprise
Architects on my two machines: My office server (P4 2.2ghz) and my Dell Laptop
(also P4 2.2ghz).
For the first test I used only a single ASP.Net page that
reads some data from a local SQL Server using a business object. Both machines
have SQL Server installed locally and for this first test both are using their
own local data from it. I did this to test and see them run individually under
load, and then together with Load Balancing to compare the results. This is a
contrived example for sure, but it shows nicely what load balancing is capable
of doing for you in a best case scenario. Figure 10 shows the output for a
short query running both machines with Load Balancing.
Figure 11 – Using Application Center Test to stress test a simple page. The result here is from
combined machines – which running around 275 rps. Machine 1 and 2 individually
were running 136 and 158 rps respectively.
The script hits only the ASPX page – no images or other
static content was hit. I tested each of the machines individually changing the
IP Addresses to their dedicated IPs in the ACT script first and then together by
changing the script to use the virtual IP. The results for this short 5 minute
test are as follows:
Web Store Single Read
Requests per second
Office Server 188.8.131.52
Both of them Load Balanced 184.108.40.206
This is a ratio of 91% for the load balanced vs. the
machines individually which is excellent given that we are running with a
single adapter here.
The second test is a bit more realistic in that it runs
through the entire Web Store application site and uses a shared SQL Server on a
Web Store Full
Requests per second
Office Server 220.127.116.11
Both of them Load Balanced 18.104.22.168
Here the ratio is a bit worse: 77%, but the reason for this
drop off has little to do with the Load Balancing, but the fact that there are
some limits being hit on the SQL Server. Looking at the lock count with
performance monitor reveals that the site is hitting the SQL box pretty heavily
and the locking thresholds are causing requests to start slowing down
This application is not heavily SQL optimized and
performance could be improved to make these numbers higher both for individual
and combined tests. However, this test shows that load balancing can help
performance of an app, but that there may still be other limits that can slow
down the application as a whole. In short, beware of load issues beyond the Web
front ends that can bite you in terms of performance. Still even in this test
where an external limit was being approached we still got a significant gain
from using Load Balancing.
Port Rules revisited: SSL
Remember I configured my server for HTTPS operation by
configuring port 443 earlier? Actually only one of the servers has the
certificate installed, so I need to manage the port rules to drive all HTTPS
traffic to the SSL enabled server. This must be administered manually through
the Network Connections dialog by clicking on the Load Balancing Service and
then configuring the Port Rules. Notice that this dialog shown in Figure 12 has
a Load Weight option, which is set to 100 in the SSL enabled server and 0 in
Figure 12 – When editing the Port Rules in
you can configure the load weight for each server in percentages.
This effectively drives all SSL traffic to the machine that
has the certificate installed.
Load Balancing and your Web applications
Running an application on more than one machine introduces
potential challenges into the design and layout of the application. If you're
Web app is not 100% stateless you will run into potential problems with
resources required on specific machines. You'll want to think about this as you
design your Web applications rather than retrofitting at the last minute.
If you're using Active Server Pages, you'll have to know
that ASP's useful Session and Application objects will not work across multiple
machines. This means you either have to run the cluster with Single Affinity
to keep clients coming back to the same machine, or you have to come up with a
different session management scheme that stores session data in a more central
data store such as a database.
Thankfully ASP.Net has several ways around this problem by providing
different options for storing Session state using either a separate State
Service that can be accessed across machines or by using Session state stored
in a SQL Server database. You should always use session state in one of these
mechanisms because these mechanisms can survive Web application restarts which
can happen more frequently in ASP.Net due to changes in web.config or simply
from the Web Server (IIS 6) recycling an Application Pool.
Finally, load balancing can allow you to scale applications
with multiple machines relatively easily. To add more load handling
capabilities just add more machines. But remember that when you build
applications this way that your weakest link can bring down the entire load
balancing scheme. If your SQL backend which all of your cluster nodes are
accessing is maxed out, no amount of additional machines in the load balancing
cluster will improve performance. The SQL backend is your weakest link and the
only way to wring better performance out of it is to upgrade hardware or start
splitting databases into separate servers.
Pulling the plug
As mentioned earlier redundancy is one of the goals of a
load balanced installation and to test this out I decided to test a failure
scenario by pulling the network cable out of one of my servers. With both
cluster nodes running one of the clusters went dead and after 10 seconds all
requests ended up going to the still active cluster providing the anticipated
redundancy. A few requests on the client ended up failing – basically those
that had made it into the servers request queue. All others are silently moved
over to the other server in the pool.
In another test I decided to turn off the Web service, which
resulted expectedly in the network connection still being fed requests that now
started to fail. This is to be expected because NLB deals at the network
protocol level but doesn’t check for failure of the requests at the network
application level (Web Server). For this scenario you will need a smart
monitoring application that can tell that your Web services are not responding
on port 80 or even better not returning the results that you should be getting
The bottom line here is: The service works well for catching
fatal failures such as hardware crashes or network failures that cause the
network connection to a single machine to die. But application level failures
continue to be your responsibility to monitor and respond to.
Just add water… eh, machines
The Windows Server Network Load Balancing service finally
makes load balancing affordable and relatively easy to implement. It’s taken a
while to get here from two Windows versions back, but now that the tools are
integrated into Windows it’s relatively painless to scale out to other
machines. It’s good to know that the capabilities are built-in and that you can
tackle applications that may require more than a single machine.
Just remember to plan ahead. Just like anything the process
of taking an application and making it do something new, spreading apps over
multiple machine takes time and some planning to get right. Don’t wait until
you really, really can’t live without this feature – start planning for it
before you do. Finally make sure you know your bottlenecks in your Web
applications. A load balancing cluster is only as good as its weakest link. Pay
special attention to data access as that is likely to be the most critical
non-cluster component that can potentially snag scalability.
But isn’t that a position we all wish we were in? So much
traffic we can’t handle it? Well, hopefully you’ll get to try out this scenario
for real – real soon, so you (or your boss) can retire rich…
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