‘AWS 503 service temporarily unavailable’ error occurs due to load balancer or due to the registered instances.

Here at Bobcares, we have seen several such AWS related errors as part of our AWS Support Services for AWS users, and online service providers.

Today we’ll take a look at the causes for this error and see how to fix it.

Why ‘AWS 503 service temporarily unavailable’ error occurs

Now let’s take a look at the causes for this error to occur.

1. If there are no registered instances.
2. Also, no healthy instances
3. There is an insufficient capacity in the load balancer to handle the request.
4. The surge queue is full.

How we resolve the error ‘AWS 503 service temporarily unavailable’

Now let’s see the suggestions our Support Engineers provide to our customers to resolve this error message.

1. Make sure to register at least one instance in every Availability Zone that your load balancer is configured to respond in. Verify it by checking HealthyHostCount metrics in CloudWatch.

In case, if there is no instance registered in each Availability Zone then enable cross-zone load balancing. Here are the steps to enable it.

• First, open the Amazon EC2 console. Here is its link: https://console.aws.amazon.com/ec2/
• From the left navigation pane, click on ‘Load Balancers’ under LOAD BALANCING.
• Here, select your load balancer.
• After that, on the Description tab, choose Change cross-zone load balancing setting.
• Then on the Configure Cross-Zone Load Balancing page, select Enable.
• Finally, click on Save.

2. As above, make sure that you have a healthy instance in every Availability Zone. Verify this by checking the HealthyHostCount metric.

3. In case, if there is no capacity in the load balancer, then it is a temporary issue. So this error must not appear more than a few hours.

4. Fixing surge queue is also necessary. First of all, ensure that there is enough capacity to handle the request rate. You can verify this by checking the SpilloverCount metric.

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Conclusion

In short, this error indicates a performance issue that occurs due to load balancer or due to the registered instances. Today, we saw the solution to this error.