S3 Multi-File upload with Terraform

23 Apr 2017

Hosting a static website with S3 is really easy, especially from terraform:

First off, we want a public readable S3 bucket policy, but we want to apply this only to one specific bucket. To achive that we can use Terraform’s template_file data block to merge in a value:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "PublicReadGetObject",
      "Effect": "Allow",
      "Principal": "*",
      "Action": [
        "s3:GetObject"
      ],
      "Resource": [
        "arn:aws:s3:::${bucket_name}/*"
      ]
    }
  ]
}

As you can see the interpolation syntax is pretty much the same as how you use variables in terraform itself. Next we define a template_file to do the transformation. As the bucket name is going to be used many times, we extract that into a variable block also:

variable "bucket" {
  default = "examplebucket"
}

data "template_file" "s3_public_policy" {
  template = "${file("policies/s3-public.json")}"
  vars {
    bucket_name = "${var.bucket}"
  }
}

Next we want to create the S3 bucket and set it to be a static website, which we can do using the website sub block. For added usefulness, we will also define an output to show the website url on the command line:

resource "aws_s3_bucket" "static_site" {
  bucket = "${var.bucket}"
  acl = "public-read"
  policy = "${data.template_file.s3_public_policy.rendered}"

  website {
    index_document = "index.html"
  }
}

output "url" {
  value = "${aws_s3_bucket.static_site.bucket}.s3-website-${var.region}.amazonaws.com"
}

Single File Upload

If you just want one file in the website (say the index.html file), then you can add the following block. Just make sure the key property matches the index_document name in the aws_s3_bucket block.

resource "aws_s3_bucket_object" "index" {
  bucket = "${aws_s3_bucket.static_site.bucket}"
  key = "index.html"
  source = "src/index.html"
  content_type = "text/html"
  etag = "${md5(file("src/index.html"))}"
}

Multi File Upload

Most websites need more than one file to be useful, and while we could write out an aws_s3_bucket_object block for every file, that seems like a lot of effort. Other options include manually uploading the files to S3, or using the aws cli to do it. While both methods work, they’re error prone - you need to specify the content_type for each file for them to load properly, and you can’t change this property once a file is uploaded.

To get around this, I add one more variable to my main terraform file, and generate a second file with all the aws_s3_bucket_object blocks in I need.

The added variable is a lookup for mime types:

variable "mime_types" {
  default = {
    htm = "text/html"
    html = "text/html"
    css = "text/css"
    js = "application/javascript"
    map = "application/javascript"
    json = "application/json"
  }
}

I then create a shell script which will write a new file containing a aws_s3_bucket_object block for each file in the src directory:

#! /bin/sh

SRC="src/"
TF_FILE="files.tf"
COUNT=0

cat > $TF_FILE ''

find $SRC -iname '*.*' | while read path; do

    cat >> $TF_FILE << EOM

resource "aws_s3_bucket_object" "file_$COUNT" {
  bucket = "\${aws_s3_bucket.static_site.bucket}"
  key = "${path#$SRC}"
  source = "$path"
  content_type = "\${lookup(var.mime_types, "${path##*.}")}"
  etag = "\${md5(file("$path"))}"
}
EOM

    COUNT=$(expr $COUNT + 1)

done

Now when I want to publish a static site, I just have to make sure I run ./files.sh once before my terraform plan and terraform apply calls.

Caveats

This technique has one major drawback: it doesn’t work well with updating an existing S3 bucket. It won’t remove files which are no longer in the terraform files, and can’t detect file moves.

However, if you’re happy with a call to terraform destroy before applying, this will work fine. I use it for a number of test sites which I don’t tend to leave online very long, and for scripted aws infrastructure that I give out to other people so they can run their own copy.

code, aws, terraform, s3

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