Category: Go

How To: Interact with AWS S3 Using the Go SDK and not lose your mind

After these messages we will carry on with our regularly scheduled programming…

Yesterday ( during the scribbling of this article ) AWS suffered one of it’s worst outages in history in the us-east-1 region. A reminder to us all to be multi-region and more importantly multi-cloud. Please see my other articles on HA deployments using AWS and my perspective & caution on the path to centralization or singularity we appear to be on (though the outage may help people wake up).

Now back to your regularly scheduled program…


My team and I are building a CMDB for AWS, which provides us with everything happening in our AWS environment + OS level metadata + change history. There will be a separate article on the CMDB journey, but today I want to focus on a specific service in AWS called S3, which is their object store. S3 is a bit of a special snowflake when it comes to AWS services and because of that I ran into challenges structuring my code, because up until S3 (which was the last service I wrote code for) everything had been very similar, and easily modularized. We will get to more detail, but let’s start this article by covering how to use the Go SDK for AWS.

Dependencies

This article assumes you already program in Go and have Go installed on your machine. To get started you will need a couple additional items.

  1. Download and install the SDK here : https://github.com/aws/aws-sdk-go 
  2. This is the documentation for the SDK, you will need it, bookmark it : http://docs.aws.amazon.com/sdk-for-go/api/
  3. It is extremely helpful when working with the API’s to have aws-shell installed : https://github.com/awslabs/aws-shell
    • This enables you to interact with AWS API’s on the fly so you can understand the output of commands as you are searching for what you are trying to accomplish.

The Collector Structure

The collector is the component in my CMDB architecture that does all the work of collecting the metadata that we shove into our CMDB. The collector is  heavily threaded using go routines for performance. The basic structure looks like this.

  • Call a go routine for each service you want to collect
    • //pass in all accounts, regions (from config-file) and pre-established awsSessions to each account you are collecting
    • Inside of a services go routine, loop overs accounts & regions
      • Launch a go routine for each account & region
        • Inside of those go routines make your AWS API call(s), example DescribeInstances
        • Store the response (I loop through mine and store it in a map using the resource-id as the key)
        • Finally, kick off another go routine to write to our API and store the data.

Ok, so hopefully that seems straight forward as a basic structure…let’s get to why S3 through me for a loop.

S3 Challenges

It will be best if I show you what I tried first, basically I tried to marry my existing pattern to S3 and that certainly was a bad idea from the start. Here was the structure of the S3 part of the code.

  • The S3 go routine gets called from main.go
  • //all accounts, regions and AWS Sessions are past into the next go routine
    • Inside of the S3 go routine, loop over accounts & regions
      • Launch a go routine for each account & region
        • Inside of those go routines List S3 Buckets
          • For each S3 buckets returned
            • Call additional API’s such as GetBucketTagging()

Ok so what happened ? I got a lot of errors that’s what 🙂 Ones like this….

At first, I thought maybe my code wasn’t thread safe…but that didn’t make much sense given the other services had no issues like this.

So as I debugged my code, I began to realize the buckets list I was getting, wasn’t limited to the region I was passing in/ establishing a session for.

Naturally, I googled can I list buckets for a single region ?

https://github.com/aws/aws-sdk-java/issues/920 (even though this is the Java SDK it still applies)..

Ah ok, the BucketList being returned on an AWS Session established with a specific account and region, ignores the region. Because S3 Buckets are global to an account, thus all buckets under an account are returned in the ListBuckets() call. I knew S3 buckets were global per account, but failed to expect a matching behavior/output when a specific region is passed into the SDK/API.

Ok so how then can I distinguish where a bucket actually lives?

As spfink says above, I needed to run GetBucketLocation() per bucket. Thus my code structure started to look like this…

  • For each account, region
    • ListBuckets
      • For each bucket returned in that account, region
        • GetBucketLocation
        • If a LocationConstraint (region) is returned, set the new region (otherwise if response is null, do nothing)
        • Get tags for the bucket in account, region

With this code I was still getting errors about region, but why ?

Well I made the mistake of thinking a ‘null’ response from the API for LocationConstraint had no meaning (or meant query it from any region), wrong (null actually means us-east-1 see from my google below) thus the IF condition evaluated false and the existing region from the outer loop was used because GetBucketLocation() returned null and this resulted in many errors.

Here’s what the google turned up..

https://github.com/aws/aws-cli/issues/564

So let’s clarify my mistakes…

  1. The S3 ListBuckets call returns all buckets under an account globally.
    • It does not abide by a region configured in an API Session
    • Thus I/you should not loop over regions from a config file for the S3 service.
    • Instead I/you need to find a buckets ‘real’ location using GetBucketLocation
    • Then set the region for actions other than ListBuckets (which is global per account and ignores region passed).
  2. GetBucketLocation returning null, doesn’t mean the bucket is global or that you can interact with the bucket from endpoint you please…it actually means us-east-1 http://docs.aws.amazon.com/general/latest/gr/rande.html#s3_region

The Working Code

So in the end the working code for S3 looks like this…

  • collector/main.go fires off a bunch of go routines per service we are collecting for.
  • It passes in accounts, and regions from a config file.
  • For the S3 service/file under the ‘services’ package the entry point is a function called StoreS3Resources.

Everything in the code should be self explanatory from that point on. You will note a function call to ‘writeToCis’… CIS is the name of our internal CMDB project/service. Again, I will later be blogging about the entire system in detail once we open source the code. Please keep in mind this code is MVP, it will be changed a lot (optimization, modularized, bug fixes, etc) before & after we open source it, but for now he is the quick and dirty, but hopefully functional code 🙂 Use at your own risk !

A simple, concurrent webcrawler written in Go

I have been playing with the Go programming language on an off for about a year. I started learning Go, because I was running into lots of issues distributing my Python codes dependencies to production machines, with specific security constraints. Go solves this problem by allowing you to compile a single binary that can be easily copied to all of your systems and you can cross compile it easily for different platforms. In addition, Go has a really great & simple way of dealing with concurrency, not to mention it really is concurrent unlike my beloved Python (GIL), which is great for plenty of use cases, but sometimes you need real concurrency. Here is some code I wrote for a simple concurrent webcrawler.

Here is the code for the command line utility fetcher. Notice it imports another package, crawler.

I am not going go over the basics in this code, because that should be fairly self explanatory. What is important here is how we are implementing concurrency. Once the scripts validates you passed a string in (that is hopefully a URL – No input validation yet!) it starts by creating a channel via

After we initialized the channel, we need to split the sites passed in from the command line -url flag via comma in case there is more than 1 site to crawl. Then we loop through each site and kick off a go routine like so.

At this point, our go routine is executing code from the imported crawler package mentioned above. Calling the method Crawl. Let’s take a look at it now…

This is pretty straight forward. We start a timer, take the passed in URL and do an http.get …if that doesn’t error, the response Body is copied into nbytes, which is ultimately returned to the channel at the bottom of the function.

Once the code returns from crawler.Crawl to fetcher… it loops through each URL for channel output. This is very important. If you try placing a print inside of the same loop as your go routine you are going to change the behavior of your application to work in a serial/slower fashion because after each go routine it will wait for output. Putting the loop for channel output outside of the loop that launches the go routine enables them to all be launched one right after another, and then output is gathered after they have all been launched. This creates a very highly performant outcome. Here is an example of this script once it has been compiled.

20 sites in 1.75s seconds..that is not too shabby. The remainder of the fetcher code runs a go routine if only one site is passed..then returns an error if message if a url is not passed in on the command line, and finally outputs the time it took total to run for all sites. The go routine is not necessary in the case of running a single url, however, it doesn’t hurt and I like the consistency of how the code reads this way.

Hopefully you enjoyed this brief show of the Go programming language. If you decide to get into Go, I cannot recommend this book enough : https://www.amazon.com/Programming-Language-Addison-Wesley-Professional-Computing/dp/0134190440 . This book has a bit of a cult following due to one of the authors being https://en.wikipedia.org/wiki/Brian_Kernighan who co-authored what consider to be the best book on C ever written (I own it, it’s really good too). I bought other Go books before this one, and I have to say don’t waste your money, buy this one and it is all you will need.

The github code for the examples above can be found here : https://github.com/jasonriedel/fetcher

Godspeed, happy learning.