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More load balancer / autoscaler healthcheck tips

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Resolves #33.
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Thanos Baskous 2016-08-24 20:00:24 -07:00 committed by GitHub
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README.md
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@ -691,6 +691,7 @@ Auto Scaling
- Dynamic Auto Scaling is easiest to use with stateless, horizontally scalable services.
- Even if you are not using ASGs to dynamically increase or decrease instance counts, you should seriously consider maintaining all instances inside of ASGs – given a target instance count, the ASG will work to ensure that number of instances running is equal to that target, replacing instances for you if they die or are marked as being unhealthy. This results in consistent capacity and better stability for your service.
- By default, ASGs will kill instances that the EC2 instance manager considers to be unresponsive. It is possible for instances whose CPU is completely saturated for minutes at a time to appear to be unresponsive, causing an ASG with the default '[ReplaceUnhealthy](http://docs.aws.amazon.com/autoscaling/latest/userguide/as-suspend-resume-processes.html#process-types)' setting turned on to replace them. When instances that are managed by ASGs are expected to consistently run with very high CPU, consider deactivating this setting. If you do so, however, detecting and killing unhealthy nodes will become your responsibility.
- Autoscalers can be [configured to terminate](http://docs.aws.amazon.com/autoscaling/latest/userguide/healthcheck.html) instances that an ELB has marked as being unhealthy.
EBS
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@ -745,6 +746,7 @@ Load Balancers
- ❗It’s not unheard of for IPs to be recycled between customers without a long cool-off period. So as a client, if you cache an IP and are not using SSL (to verify the server), you might get not just errors, but responses from completely different services or companies!
- 🔸As an operator of a service behind an ELB or ALB, the latter phenomenon means you can also see puzzling or erroneous requests by clients of other companies. This is most common with clients using back-end APIs (since web browsers typically cache for a limited period).
- ❗ELBs and ALBs take time to scale up, it does not handle sudden spikes in traffic well. Therefore, if you anticipate a spike, you need to “pre-warm” the ELB by gradually sending an increasing amount of traffic.
- ❗Tune your healthchecks carefully — if you are too aggressive about deciding when to remove an instance and conservative about adding it back into the pool, the service that your load balancer is fronting may become inaccessible for seconds or minutes at a time. Be extra careful about this when an autoscaler is configured to terminate instances that are marked as being unhealthy by a managed load balancer.
### ELB Tips