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I am wondering how I can dynamically increase and decrease the number of varnish instances for a Magento2 installation as needed - with an aws autoscaling group, for example.

Configuring them from the client side is easy: I can just use a load balancer. But Magento2 also needs to be aware of each instance, and this seems to be a bit more tricky. Updating the configuration file would require an app:config:import, which is a bit of a hassle in a production multi-server environment.

The Magento documentation suggests this command:

magento setup:config:set --http-cache-hosts=192.0.2.100,192.0.2.155:6081

(https://devdocs.magento.com/guides/v2.2/config-guide/varnish/use-multiple-varnish-cache.html)

Is it an appropriate way to update the varnish servers continuously in an operational / live / production system?

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The best way that I've found to do this is to write a PHP script that utilizes the AWS PHP SDK to get a list of all the current instances of your auto-scaling group:

https://docs.aws.amazon.com/aws-sdk-php/v3/api/api-autoscaling-2011-01-01.html#describeautoscalinggroups

That PHP script can then use the command you mentioned on all the production instances. I don't believe there's any better way to do it than that, and has been used in production instances with success.

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  • Sounds good! - Do you know if the command can be run in a production system without any negative impact?
    – hey
    Commented Jan 17, 2019 at 1:10
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    Yes, absolutely. :) Of course, always test things on staging, it never hurts!
    – John Cymerman
    Commented Jan 17, 2019 at 19:36

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