In a typical load balancer scenario if any backend server is getting crashed or disconnected

Aniket Jagadale 41 Reputation points
2021-11-16T09:34:47.03+00:00

In a typical load balancer scenario what will happen if one of the backend server in the backend pool is active and serving an instance goes down or gets disconnected, will the LB move towards another backend server and what will happen to the application instance

Azure Load Balancer
Azure Load Balancer
An Azure service that delivers high availability and network performance to applications.
476 questions
0 comments No comments
{count} votes

Accepted answer
  1. GitaraniSharma-MSFT 49,806 Reputation points Microsoft Employee
    2021-11-17T10:49:29.863+00:00

    Hello @Aniket Jagadale ,

    Welcome to Microsoft Q&A Platform. Thank you for reaching out & hope you are doing well.

    When using Azure Load Balancer, you need to specify health probes in the load-balancing rules to allow Load Balancer to detect the backend endpoint status. The health probes can be used to detect the failure of an application on a backend endpoint.

    There are 3 types of health probes: TCP, HTTP & HTTPS probes
    NOTE : Basic SKU doesn't support HTTPS probe type but Standard load balancer supports all 3 probe types.

    For the backend servers to participate in the load balancer set, they must pass the probe check. The health probe will determine if an instance is healthy and can receive traffic.

    Health probe behavior:

    Probe up behavior:
    TCP, HTTP, and HTTPS health probes are considered healthy and mark the backend endpoint as healthy when:
    -The health probe is successful once after the VM boots.
    -Any backend endpoint which has achieved a healthy state is eligible for receiving new flows.

    Probe down behavior for TCP connections:
    -New TCP connections will succeed to remaining healthy backend endpoint.
    -If a backend endpoint's health probe fails, established TCP connections to this backend endpoint continue.
    -If all probes for all instances in a backend pool fail, no new flows will be sent to the backend pool. Standard Load Balancer will permit established TCP flows to continue. Basic Load Balancer will terminate all existing TCP flows to the backend pool.

    So, if one of the backend server in the backend pool is active and serving an instance goes down or gets disconnected, the load balancer stops sending new connections to that unhealthy instance and will send the new connections to remaining healthy backend endpoints. However, the existing connections on that disconnected instance are not affected by this probe failure. The existing connection continues until the application:
    -Ends the flow
    -Idle timeout occurs
    -The VM shuts down

    For more information, please refer:
    https://learn.microsoft.com/en-us/azure/load-balancer/load-balancer-custom-probe-overview
    https://learn.microsoft.com/en-us/azure/load-balancer/components#health-probes

    Kindly let us know if the above helps or you need further assistance on this issue.

    ----------------------------------------------------------------------------------------------------------------

    Please "Accept the answer" if the information helped you. This will help us and others in the community as well.

    0 comments No comments

0 additional answers

Sort by: Most helpful

Your answer

Answers can be marked as Accepted Answers by the question author, which helps users to know the answer solved the author's problem.