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minor typo fix (#692)

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an CLB  -> a CLB
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Dan 2019-04-07 02:15:31 -07:00 • committed by Maish
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@ -1075,11 +1075,11 @@ Load Balancers
### Load Balancer Tips
- If you don’t have opinions on your load balancing up front, and don’t have complex load balancing needs like application-specific routing of requests, it’s reasonable just to use an CLB or ALB for load balancing instead.
- If you don’t have opinions on your load balancing up front, and don’t have complex load balancing needs like application-specific routing of requests, it’s reasonable just to use a CLB or ALB for load balancing instead.
- Even if you don’t want to think about load balancing at all, because your architecture is so simple (say, just one server), put a load balancer in front of it anyway. This gives you more flexibility when upgrading, since you won’t have to change any DNS settings that will be slow to propagate, and also it lets you do a few things like terminate SSL more easily.
- **CLBs and ALBs have many IPs:** Internally, an AWS load balancer is simply a collection of individual software load balancers hosted within EC2, with DNS load balancing traffic among them. The pool can contain many IPs, at least one per availability zone, and depending on traffic levels. They also support SSL termination, which is very convenient.
- **Scaling:** CLBs and ALBs can scale to very high throughput, but scaling up is not instantaneous. If you’re expecting to be hit with a lot of traffic suddenly, it can make sense to load test them so they scale up in advance. You can also [contact Amazon](http://aws.amazon.com/articles/1636185810492479) and have them “pre-warm” the load balancer.
- **Client IPs:** In general, if servers want to know true client IP addresses, load balancers must forward this information somehow. CLBs add the standard [X-Forwarded-For](https://en.wikipedia.org/wiki/X-Forwarded-For) header. When using an CLB as an HTTP load balancer, it’s possible to get the client’s IP address from this.
- **Client IPs:** In general, if servers want to know true client IP addresses, load balancers must forward this information somehow. CLBs add the standard [X-Forwarded-For](https://en.wikipedia.org/wiki/X-Forwarded-For) header. When using a CLB as an HTTP load balancer, it’s possible to get the client’s IP address from this.
- **Using load balancers when deploying:** One common pattern is to swap instances in the load balancer after spinning up a new stack with your latest version, keep old stack running for one or two hours, and either flip back to old stack in case of problems or tear it down.
- **Rotating Certificates while retaining ARN:** Rotating IAM Server Certificates can be difficult as the standard practice is to upload a new one then update all resources with the new ARN. You can however retain the same ARN using the `update-certificate` call with the following process:
1. Upload a new IAM Server Certificate with a unique name (e.g fuzzy.com.new)
@ -1091,10 +1091,10 @@ Load Balancers
### Load Balancer Gotchas and Limitations
- ❗CLBs and ALBs have **no fixed external IP** that all clients see. For most consumer apps this doesn’t matter, but enterprise customers of yours may want this. IPs will be different for each user, and will vary unpredictably for a single client over time (within the standard [EC2 IP ranges](http://docs.aws.amazon.com/general/latest/gr/aws-ip-ranges.html)). And similarly, never resolve an CLB name to an IP and put it as the value of an A record — it will work for a while, then break!
- ❗CLBs and ALBs have **no fixed external IP** that all clients see. For most consumer apps this doesn’t matter, but enterprise customers of yours may want this. IPs will be different for each user, and will vary unpredictably for a single client over time (within the standard [EC2 IP ranges](http://docs.aws.amazon.com/general/latest/gr/aws-ip-ranges.html)). And similarly, never resolve a CLB name to an IP and put it as the value of an A record — it will work for a while, then break!
- ❗Some web clients or reverse proxies cache DNS lookups for a long time, which is problematic for CLBs and ALBs, since they change their IPs. This means after a few minutes, hours, or days, your client will stop working, unless you disable DNS caching. Watch out for [Java’s settings](http://docs.oracle.com/javase/8/docs/api/java/net/InetAddress.html) and be sure to [adjust them properly](http://docs.aws.amazon.com/AWSSdkDocsJava/latest/DeveloperGuide/java-dg-jvm-ttl.html). Another example is nginx as a reverse proxy, which [normally resolves backends only at start-up](https://www.jethrocarr.com/2013/11/02/nginx-reverse-proxies-and-dns-resolution/) (although there is [a way to get around this](https://tenzer.dk/nginx-with-dynamic-upstreams/)).
- ❗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 CLB 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).
- 🔸As an operator of a service behind a CLB 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).
- ❗CLBs 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 load balancer 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.
- ❗CLB HTTPS listeners don't support Server Name Indication (SNI). If you need SNI, you can work around this limitation by either providing a certificate with Subject Alternative Names (SANs) or by using TCP listeners and terminating SSL at your backend.
@ -1118,7 +1118,7 @@ CLB
- In general, CLBs are not as “smart” as some load balancers, and don’t have fancy features or fine-grained control a traditional hardware load balancer would offer. For most common cases involving sessionless apps or cookie-based sessions over HTTP, or SSL termination, they work well.
- 🔸By default, CLBs will refuse to route traffic from a load balancer in one Availability Zone (AZ) to a backend instance in another. This [will cause 503s](http://docs.aws.amazon.com/elasticloadbalancing/latest/classic/ts-elb-error-message.html#ts-elb-errorcodes-http503) if the last instance in an AZ becomes unavailable, even if there are healthy instances in other zones. If you’re running fewer than two backend instances per AZ, you almost certainly want to [enable cross-zone load balancing](http://docs.aws.amazon.com/elasticloadbalancing/latest/classic/enable-disable-crosszone-lb.html#enable-cross-zone).
- 🔸Complex rules for directing traffic are not supported. For example, you can’t direct traffic based on a regular expression in the URL, like [HAProxy](http://www.haproxy.org/) offers.
- **Apex DNS names:** Once upon a time, you couldn’t assign an CLB to an apex DNS record (i.e. example.com instead of foo.example.com) because it needed to be an A record instead of a CNAME. This is now possible with a Route 53 alias record directly pointing to the load balancer.
- **Apex DNS names:** Once upon a time, you couldn’t assign a CLB to an apex DNS record (i.e. example.com instead of foo.example.com) because it needed to be an A record instead of a CNAME. This is now possible with a Route 53 alias record directly pointing to the load balancer.
- 🔸CLBs use [HTTP keep-alives](https://en.wikipedia.org/wiki/HTTP_persistent_connection) on the internal side. This can cause an unexpected side effect: Requests from different clients, each in their own TCP connection on the external side, can end up on the same TCP connection on the internal side. Never assume that multiple requests on the same TCP connection are from the same client!
- 🔸 Traffic between CLBs and back-end instances in the same subnet **will** have [Network ACL](http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_ACLs.html) rules evaluated (EC2 to EC2 traffic in the same subnet would not have Network ACL rules evaluated). If the default '0.0.0.0/0 ALLOW' rule is removed from the Network ACL applied to the subnet, a rule that allows traffic on both the health check port and any listener port must be added.
- As of December 2016, CLBs launched in VPCs do not support IPv6 addressing. CLBs launched in EC2-Classic support both IPv4 and IPv6 [with the "dualstack" DNS name](http://docs.aws.amazon.com/elasticloadbalancing/latest/classic/elb-internet-facing-load-balancers.html#internet-facing-ip-addresses).