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IIS (Internet Information Services) · Operations

Load Balancing with ARR

When one server is not enough, Application Request Routing turns IIS into a load balancer that spreads traffic across several web servers.

Advanced 5 min Lesson 32 of 33Covers IIS 10 on Windows Server 2019/2022/2025 and Windows 10/11.
Before you start:URL RewriteApplication Pool Recycling
0/33 in IIS
On this page · 5 min · Advanced

On this page

  1. In simple words
  2. Learning objectives
  3. Explanation
  4. Code example
  5. Step by step
  6. Real-world use
  7. Common mistakes
  8. Best practices
  9. Remember this
  10. Practice exercise
  11. Mini quiz
  12. Related topics
  13. My notes
Reading time
5 min
Difficulty
Advanced
Updated
Oct 30, 2026

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Interview questions

In simple words#

ARR is a restaurant host who seats each arriving group at whichever kitchen (server) is least busy, and stops seating people at a kitchen whose stove has broken (health check failed).

Server farm
Group of back-end servers behind ARR.
Health test
URL ARR checks to know a server is healthy.
Affinity (sticky sessions)
Same client always goes to the same server.

Learning objectives#

  • Install ARR and create a server farm
  • Configure health checks
  • Choose a load balancing algorithm
  • Make apps work behind a load balancer

Explanation#

Install Application Request Routing 3.0 (with URL Rewrite) on a front-end IIS server. Create a Server Farm and add back-end servers.

Health Test: ARR requests a URL (e.g. /health) every few seconds and removes unhealthy servers.

Algorithms: weighted round robin, least requests, least response time, hashing. Client affinity (cookie) pins users to one server — avoid it by keeping apps stateless.

Apps behind ARR must share session storage and Data Protection keys, and read the client IP from X-Forwarded-For.

Code example: Server farm with health check (applicationHost.config)#

XMLServer farm with health check (applicationHost.config) Illustrative
<webFarms>
  <webFarm name="ShopFarm" enabled="true">
    <server address="web01.internal" enabled="true" />
    <server address="web02.internal" enabled="true" />
    <applicationRequestRouting>
      <healthCheck url="http://shop.example.com/health" interval="00:00:10" responseMatch="Healthy" />
      <loadBalancing algorithm="LeastRequests" />
      <affinity useCookie="false" />
    </applicationRequestRouting>
  </webFarm>
</webFarms>


<!-- URL Rewrite rule on the front-end site routes requests to the farm -->
<!-- <action type="Rewrite" url="http://ShopFarm/{R:0}" /> -->
Expected output
Requests are spread to the server with the fewest active requests; if web02’s /health stops returning “Healthy”, all traffic goes to web01 until it recovers.

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

  1. Two back-end servers form the farm.
  2. Every 10 seconds ARR checks /health on each server.
  3. LeastRequests sends traffic to the less busy server.
  4. Affinity is off, so the app must be stateless.

Real-world use case#

An e-commerce site survives a server patch window: one server is drained and updated while the other keeps serving customers.

Common mistakes#

In-memory sessions with no affinity

Users lose their cart when they switch servers. Use a shared session store.

Data Protection keys per server

Cookies issued by one server fail on another. Share the key ring.

Best practices#

  • Stateless apps
  • Meaningful /health endpoints
  • Shared Data Protection keys
  • Drain servers before maintenance

Remember this#

ARR = IIS as a load balancer.

Health checks remove broken servers.

Stateless apps scale; sticky sessions are a crutch.

Practice exercise#

Your task: Which ASP.NET Core feature lets the app see the real client IP behind ARR?

Show solution
C# Illustrative
app.UseForwardedHeaders(new ForwardedHeadersOptions
{
    ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto
});

Mini quiz#

Load Balancing with ARR quizMultiple choice Question 1 of 1

What does an ARR health test do?

Score so far: 0/0

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My notes#

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