Top 3 AWS Gotchas

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I’ve been using AWS for a few years now and over the years I noticed there some questions that keep popping up. I was confused by these issues at first and as they look like they are tripping everybody up at some point I decided to compile of a small list of common gotchas. I’ll update this or post another when if I come across more of these.

1. The S3 folder delusion

When you AWS console you can create folders to group objects but this is just a delusion deliberately created by AWS to simplify the usage. In reality, S3 has a flat structure and all the objects are on the same level. Here’s the excerpt from AWS documentation that states this fact:

In Amazon S3, buckets and objects are the primary resources, where objects are stored in buckets. Amazon S3 has a flat structure with no hierarchy like you would see in a typical file system. However, for the sake of organizational simplicity, the Amazon S3 console supports the folder concept as a means of grouping objects. Amazon S3 does this by using key name prefixes for objects.

So essentially AWS is just smart enough to recognize the standard folder notation we’ve been using for ages to make this things easier for us.

2. Reserved instance confusion

Reserved instances cost less but require some resource planning and paying some money up-front. Although there is now an option to buy reserved instances with no upfront payment they generally shine on long-term commitments with heavy usage (always-on machines). The confusing bit you don’t reserve actual instances. Unfortunately management console doesn’t do a great job in bridging that gap and when you buy a reserved instance you don’t even know which running instance it covers.

Basically you just buy a subscription for 1 or 3 years and you pay less for any machine that meets that criteria. For instance, say you reserved 1 Linux t1.small instance for 12 months and you are running 2 t1.small Linux instances at the moment. You will pay reserved instance prices for one of them and on-demand price for the other. From a financial point of view it doesn’t matter which one is which. If you shut down one of those instances, again regardless of the instance, you still pay for reserved instance price as it matches your reserved instance criteria.

So that’s all there is to it really. Reserved instance is just about billing and has nothing to do with the actual running instances.

3. Public/Elastic IP uncertainty

There are 3 types of IP addresses in AWS:

Private IPs are internal IP that every instance are assigned. They remain the same throughout the lifespan of the instance and as the name implies they are not addressable from the Internet.

Public IPs are optional. They remain the same as long as the instance is running but they are likely to change after a reboot. So they are not reliable for web-accessible applications.

Elastic IPs are basically static IPs that never change. By default AWS gives up to 5 EIPs. If you need more you have to contact their support. They come free of charge as long as they are associated with a running instance. It costs a small amount if you just want to keep them around without using them though.

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