Consul: Conditional tag in query

[jhmartin]

It would be useful to be able to query DNS / HTTP with a 'conditional' tag. The use case I am thinking of is that I have a service in AWS that is in multiple availability zones. I want clients in us-east-1a to prefer services that are also in us-east-1a, so I've added that as a tag to the service. However if the 1a service fails, I'd like it to fall back to the other zones.

[armon]

This makes sense, let me think about it more generically.

[keithchambers]

+1

[edurdias]

That could be solved using a external load balancer, correct? I was thinking what would be the criteria to redirect to a new endpoint, especially if you have multiple options (us-west-1a, us-west-2a, us-central-1).

[slackpad]

This can be done at the datacenter level with prepared queries - https://www.consul.io/docs/agent/http/query.html. There's still no ability to do it by tag, though.

[daveadams]

Yes, we very much need this feature. An additional parameter in prepared queries allowing specification of a priority tag or tags would probably address this need (but would also require the DNS prepared-query API _not_ to randomly shuffle the results) without having to extend the DNS syntax.

[nanoz]

Hi ! Any news on this update ?

Having the same use case as @jhmartin, I can tell the cost savings of this simple change would be HUGE 😃

[i0rek]

Consul now supports specifying tags in prepared queries: https://www.consul.io/api/query.html#tags. Thanks for reporting and thanks for your patience!

[daveadams]

I'm confused, why was this closed, @i0rek ? Tags can be specified, but the docs you link imply that the tags are then required. This issue is asking for a preferred, but not required tag. So, for example, _if_ a service with tag "x" exists, then use that, otherwise, fall back to something else.

The original example is still very relevant: if I tag my services with what AZ they are located in, then I can have my consumers query for my service preferring tag us-east-1a. But if us-east-1a isn't available, it is okay to return other values.

In short, there needs to be an additional way to specify "preferred tags" in addition to "required tags" and "excluded tags".

[daveadams]

If that is too specific, it would also be nice to have a way to failover to another query, instead of just to another datacenter.

[i0rek]

Thank you for clarifying @daveadams! I can see how that would be useful in your scenario!
Having that said, right now we are very busy with other things and the only help I can offer is that I would be open to review a pull request that implements this functionality.

Thanks,
Hans

[i0rek]

After giving it more thought and coming back to this a couple of times, we decided that this is not something we will implement anytime soon. Instead of adding features to prepared queries, we are proposing that you look into https://www.consul.io/docs/connect/l7-traffic-management.html. I am not sure it covers your use case 100% but it definitely is the way to create to create rules for advanced routing.

I am closing this feature request, because we are not going to have time to work on it any time soon. Thank you everybody for reporting and clarifying.