Azure Load Balancer and Sticky Connection (Source IP Affinity)

In this post we will talk about Azure Load Balancer and how we can have stick connection to our Azure Resources when we have multiple instances.

Background
When we are using Azure VMs, Web Roles or Worker Roles and having multiple instances of the same type, all our requests will fly through a Azure Load Balancer. By default, Azure Load Balancer is not offering sticky sessions based on Source IP and Destination IP (see the last part of blog post).
This means that multiple requests that are coming from the same client are not guaranteed to end up to the same instance when port is changing. Based on different counters, like CPU level for example, Azure Load Balancer can decide to redirect a request to another instance.
There are cases when we need sticky connection and all the requests to end up to the same instance on Azure side. A good example is a relay server, where ports are changing for each request. In this case we need to be sure that all requests from the same source end up on the same instance.
Another example is for cases when we have a web application and we want all the requests from the same client to end up on the same instance, even when the requests are made using different ports (80, 8080, 443, ...)

In the above example we can observe that two different request from the same client, where the source port is different can end up on different instances.

Azure Solution
Azure Load Balancer supports this features, offering us the possibility to have sticky connections. This custom configuration of Azure Load Balancer is called Source IP. It allow us to bind a client to a specific instance from Azure. In this case, when we multiple instance in the backend, we will know that requests from the same client will end up on the same instance.

The bind between client and instance is formed by two types of tuples:

Client IP, Destination IP
Client IP, Destination IP, Protocol

Type of instances supported

In this moment there 3 types of instances that are support

Web Roles
Worker Roles
Virtual Machines

, and load balancer endpoints.

How to configure

The configuration is very similar for all types of instances. We need to set the load balancer distribution to 'sourceIP'.

Web/Worker Roles

This can be done from the .csdef file, in the moment when we configure the input endpoint. See below example:

<WorkerRole name="foo" vmsize="small" enableNativeCodeExecution="false">
  <Endpoints>
    <InputEndpoint ... loadBalancerDistribution="sourceIP" />
  </Endpoints>
</WorkerRole>

Load Balancer Endpoint
This can be done using Power Shell, by setting the LoadBalancerDistribution attribute to 'sourceIP'

What we should be aware when using default Azure Load Balancer configuration
By default, Azure Load Balancer is using a tuple formed from 5 elements

(source IP, source port, destination IP, destination port, protocol type)

This means that we have a stick connection already, but the source port and destination port are used also to identify the instance. If we have multiple requests, with different port source or destination ports and we want to keep them on the same instance, that we should use Source IP Affinity.

In conclusion we should keep in mind what kind of sticky connection we need and what part of a request will define our sticky connection (IP, ports, protocol).

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