Beginnings in Golang and AWS – Part VI – Events, Lambda and Transcribe (cont’d)


In today’s post, we’re going to be looking at the code within the handler’s function. As part of this, we’ll be covering using structs and JSON together, logging to CloudWatch, marshaling, processing of S3 event data, and how to start a Transcribe job. It’s a bit of a longer post today as we’ll go through the entire code in the function.

The Story So Far…

At this point, our handler has been triggered by a file being placed in our S3 bucket to which there is an event subscription for CreateObject (more about this in the next blog). We’ve received the event information, which is placed in our variable S3Event, a struct. We have the information we need for further processing of S3Event and can proceed immediately with it. However, it’s worthwhile spending a couple of minutes looking at how Go processes the information received to place it into the variable.

A Bit About JSON & Go

Go does not have a native parsing mechanism for JSON data that allows dynamic generation of a struct based on the content (think PowerShell’s ConvertFrom-JSON cmdlet for example). Instead, (unless you want to go into the murky world of reflection and creating maps) you are expected to have some degree of awareness of the schema of data being received. Go still handles the conversion process, but it looks to you for information on how to map content. This is done in struct definitions simply by indicating the location of the data to map to, of the format json:"location".

In our situation, the definition of an S3Event for example (an array of type S3EventRecord), maps to the “Records” section of the event data (see below). When information is nested (i.e. a subsection of another), we just make sure our struct matches this. An example of this is the EventVersion string, contained in the S3EventRecord struct, which is mapped to “eventVersion” in the JSON file.

An extract of the struct configuration and JSON data is below. You can examine the complete definition of an S3Event from the aws-lambda-go sdk, here:

Here’s the first two levels of our struct…

…and an extract from the S3Data JSON

Notice how they match up. The parser uses this information to populate the struct properties.

One aspect that is quite nice about the mapping process is that providing the top level outline structure matches the data being received, the entire substructure does not need to be present. You have complete control over which properties should be mandatory or not.

Naturally, it makes sense to have your struct defined to represent the “full” content schema of the JSON data, but content being received into this struct does not need to be as complete. This can help when JSON content may contain less fields, yet still come from the same event source.

If you’d like to spend some more time looking how a Go struct can be designed from a JSON source, I’d recommend taking a look at:

Code Description

Let’s move onto the code itself now. Here’s our entire function as a reminder, before we burrow down into what it’s doing.

As mentioned previously, we’re going to use Cloudwatch for logging, using the aptly named log package. The first logging we’ll do is that of the S3Event data.

Initially, we need to marshal the data. Marshaling takes an interface (our struct in this case) and returns a JSON encoding of it.

With this done, we use the string function, which in turn converts our byte data to a string.

We then log this information to CloudWatch. A nice touch of CloudWatch is that it picks up that the string data is JSON and formats it nicely for us. You’ll see this firsthand in the final part of this series.

We then need to iterate through each record entry in the event data. We use for to do this, assigning the record variable on each interaction from s3Event.Records. We do not need to set an initial value in the declaration, hence the “_”.

Inside the loop, we set s3 to the value of the s3 branch, and from this we log the key referred to in the event. We will use this later as a parameter for our Transcribe job.

Any time we want to perform operations with another AWS service, a session needs to be created. We define the parameters that the session will use (a region of eu-west-1, and using my own development profile), then create it using transcribeservice.New, a function from the package.

Next, a check is made to ensure that a successful session was established. This is easily verified by ensuring that a non nil value was returned to our variable, transcriber. If a nil value was returned, we exit the function. We log the result irrespective to CloudWatch.

Now we want to get our parameters set before starting a transcription job.

  • A random job name needs be created, so the GUID function we created earlier is used to populate the variable jobname.
  • We set mediafileuri, using string expansion with the the bucket name and key name that we got from the S3EventData
  • Mediaformat is set to mp4
  • Lastly, we set a language code of en-US for the languagecode variable.


We define StrucMedia, which is of type transcribeservice.Media. One thing to be mentioned here is that we pass in a pointer to mediafileuri, not a string. This is because the MediaFileUri definition in the transcribeservice.Media struct specifies via *string that it expects to receive a pointer.

As such, our definition is as below.


Then, we invoke the StartTranscriptionJob function. This takes as its parameter a pointer to a StartTranscriptionJobInput struct, whose properties we set within it.  Lastly, a completion message is logged to CloudWatch.


In this post, we’ve covered the code within our lambda function and in doing so have covered how structs and json interoperate, logging to CloudWatch, marshaling, processing of S3 event data and finally how to create a Transcribe job.

We’re nearly there. In the next blog, we’ll run through the entire process of getting our code in S3, creating the lambda function, creating the event subscription, and triggering our function.

thanks for reading! Feedback always welcome. 🙂




Leave a Reply

Your email address will not be published. Required fields are marked *