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One can add to ALB multiple listeners and rules that can check for hostname and forward requests to appropriate target groups. So, I'm not sure what could be the point in using more than 1 Application Load Balancer, if I can just add a new rule?

3
  • That would depend on whether you want one? Without knowing what you're trying to accomplish, all I can tell you is to search your feelings.
    – Spooler
    Jul 24, 2018 at 14:30
  • The thing is that I don't know what to wish for because I don't know what actually I can accomplish by having more than 1 load balancer? Is there any case when I need more than 1 application load balancer? This is a hypothetical question asked from curiosity.
    – Defozo
    Jul 24, 2018 at 15:03
  • The goal here is not to pay for resources that I don't need.
    – Defozo
    Jul 24, 2018 at 15:04

1 Answer 1

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The ALB has a limit on how many hosts it can support. If your services does not fit within these limits, you'll need to use more than 1 ALB. https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-limits.html

The most prohibitive one is probably the certificate limit, which will restrict the maximum number of HTTPS hosts to 25.

You also the security group limit, which is 5 per ALB. If you were to set up your hosts serving your domains within 5 SG, it would work, but not best practices.

If you only need HTTP, you can get away with 100 domains, per

Rules per load balancer (not counting default rules): 100

And even if your service falls within all these, you have a limit of 1000 targets per ALB, which would be a pain to manage during a spike.

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  • 1
    IMHO certificate limit isn't the most restrictive one since you can use wildcard certificates. Generally speaking, I see the limits are rather high and one should make really thoughtful decision about spinning up another ALB. Moreover, please bear in mind that these limits are subject to increase simply by filling up limit increase form: console.aws.amazon.com/support/home#/case/…. Please correct me if I'm wrong.
    – Defozo
    Jul 27, 2018 at 11:04
  • 1
    In the case of wildcards, yes, not that restrictive. But that can differ from setup to setup. I have never requested limit increases for an ALB and can't say if those are artificial limits or actual hardware limits that AWS cannot address.
    – Dumitru
    Jul 27, 2018 at 12:21

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