Azure Cloud Services (classic) Definition LoadBalancerProbe Schema

The load balancer probe is a customer defined health probe of UDP endpoints and endpoints in role instances. The LoadBalancerProbe isn't a standalone element; it's combined with the web role or worker role in a service definition file. More than one role can use a LoadBalancerProbe.

The default extension for the service definition file is .csdef.

The function of a load balancer probe

The Azure Load Balancer is responsible for routing incoming traffic to your role instances. The load balancer determines which instances can receive traffic by regularly probing each instance in order to determine the health of that instance. The load balancer probes every instance multiple times per minute. There are two different options for providing instance health to the load balancer - the default load balancer probe, or a custom load balancer probe, which is implemented by defining the LoadBalancerProbe in the .csdef file.

The default load balancer probe utilizes the Guest Agent inside the virtual machine, which listens and responds with an HTTP 200 OK response only when the instance is in the Ready state (like when the instance isn't in the Busy, Recycling, Stopping, etc. states). If the Guest Agent fails to respond with HTTP 200 OK, the Azure Load Balancer marks the instance as unresponsive and stops sending traffic to that instance. The Azure Load Balancer continues to ping the instance, and if the Guest Agent responds with an HTTP 200, the Azure Load Balancer sends traffic to that instance again. When using a web role your website code typically runs in w3wp.exe, which the Azure fabric and guest agent don't monitor. Failures in w3wp.exe (for example, HTTP 500 responses) aren't reported to the guest agent, and the load balancer doesn't know to take that instance out of rotation.

The custom load balancer probe overrides the default guest agent probe and allows you to create your own custom logic to determine the health of the role instance. The load balancer regularly probes your endpoint (every 15 seconds, by default). The instance is considered in rotation if it responds with a TCP ACK or HTTP 200 within the timeout period (default of 31 seconds). This process can be useful to implement your own logic to remove instances from load balancer rotation (for example, returning a non-200 status if the instance is above 90% CPU). For web roles using w3wp.exe, this setup also means you get automatic monitoring of your website, since failures in your website code return a non-200 status to the load balancer probe. If you don't define a LoadBalancerProbe in the .csdef file, then the default load balancer behavior (as previously described) is used.

If you use a custom load balancer probe, you must ensure that your logic takes into consideration the RoleEnvironment.OnStop method. When you use the default load balancer probe, the instance is taken out of rotation before OnStop is called, but a custom load balancer probe can continue to return a 200 OK during the OnStop event. If you're using the OnStop event to clean up the cache, stop the service, or otherwise make changes that can affect the runtime behavior of your service, then you need to ensure your custom load balancer probe logic removes the instance from rotation.

Basic service definition schema for a load balancer probe

The basic format of a service definition file containing a load balancer probe is as follows.

<ServiceDefinition …>
   <LoadBalancerProbes>
      <LoadBalancerProbe name="<load-balancer-probe-name>" protocol="[http|tcp]" path="<uri-for-checking-health-status-of-vm>" port="<port-number>" intervalInSeconds="<interval-in-seconds>" timeoutInSeconds="<timeout-in-seconds>"/>
   </LoadBalancerProbes>
</ServiceDefinition>

Schema elements

The LoadBalancerProbes element of the service definition file includes the following elements:

• LoadBalancerProbes Element
• LoadBalancerProbe Element

LoadBalancerProbes Element

The LoadBalancerProbes element describes the collection of load balancer probes. This element is the parent element of the LoadBalancerProbe Element.

LoadBalancerProbe Element

The LoadBalancerProbe element defines the health probe for a model. You can define multiple load balancer probes.

The following table describes the attributes of the LoadBalancerProbe element:

See Also

Cloud Service (classic) Definition Schema