Offload Your Static Content to Amazon S3

Offload Your Static Content to Amazon S3

The first question that comes to mind is: Why do I want to offload my static content to Amazon S3 when I’m already serving my static content via content delivery network (CDN)? Amazon S3 is not a CDN and by definition it’ll have a poorer performance because it’ll serve the static content from one particular region instead from the nearest point of presence (PoP) to the user.

That’s true, but by offloading your static content to Amazon s3 you will not ditch the CDN you’re using. You will just point the CDN to pull your resources from the Amazon s3 bucket instead of directly from your server. By doing that you’ll enable a layer between your CDN and your server that will literally cost pennies.

Your site we’ll be no longer mirrored to your CDN, there will be no more requests from your CDN to your server, your static content we’ll be arguable more accessible because it’s not sitting on a live server, but in a static storage. Plus it’s very fun to do this.

In this tutorial:

  1. Prerequisites
  2. Create a sync script
  3. Setup a cron job
  4. Update your CDN


First, you need to accomplish two very important tasks:

  1. Set up your AWS, and
  2. Install and configure the AWS CLI on your server.

Create a sync script

Now since you’ve created an s3 bucket and IAM user at AWS, and installed and configured AWS CLI on your server, create/open a shell script file:

nano ~/.local/bin/s3-sync

Paste the following:


# find out the aws path with 'which aws' in your terminal

# your s3 bucket

# get into the website's root folder
cd /var/www/

echo "Syncing htdocs (static files) with $bucket..."

# sync images(jpg, jpeg, png, gif, svg), css and js to $bucket
$aws s3 sync . $bucket --exclude "*" --include "*.jpg" \
--include "*.jpeg" --include "*.png" --include "*.gif" \
--include "*.svg" --include "*.css" --include "*.js" \
--cache-control "public, max-age=31536000" \
--storage-class INTELLIGENT_TIERING \
--acl public-read --delete --quiet

# sync json, ico and xml to $bucket with less max-age
$aws s3 sync . $bucket --exclude "*" --include "*.json" \
--include "*.ico" --include ".xml" \
--cache-control "public, max-age=86400" \
--storage-class INTELLIGENT_TIERING \
--acl public-read --delete --quiet

now=$(date + "%D - %T")
echo "Static files synced. Current time: $now.\n"

Exit CTRL + X, save y.

/usr/local/bin/aws is the path to aws. Find out the path to aws on your server with this command:

which aws

/var/www/ is your website’s root directory.
--exclude "*" - exclude everything.
--include ".jpg" - include all files with jpg extension, for example.
--acl public-read - set the files to be public.
--delete - delete files in the bucket that do not exist at origin (your server).
--quiet - do all this quietly without output.

Make the script executable only by you:

chmod 700 ~/.local/bin/s3-sync

Test the script:


Go over to your bucket at Amazon s3 and see if your website’s static assets are copied there. If so, you’re golden.

Setup a cron job

The script above is very heavy. In a solidly populated website, with thousands of files, it will take a long time to finish (around 40 - 50 seconds). And it will spike the server’s CPU usage if the server is somewhat underpowered. So it doesn’t make sense to run this script frequently but only when there are changes in the website’s static content. But how to detect these changes?

We can run a small python detection script on small intervals via cron and only when there are changes in the static content of the website we can launch the big script that communicates with s3.

nano ~/.local/bin/detect-changes

Below is the content of the script. We will not go into it, there are comments inside which explain what the script is trying to do. It basically creates a dictionary of static files as keys and their last modified times as values and compares that dictionary against the latest one stored in a json file.

If there are changes (new, deleted or modified static files) it stores/overwrites the dictionary in the json file which later serves as the latest directory snapshot to compare any new state to it.

#! /usr/bin/env python3

import os
import json

def compute_dir_index(path, extensions):
    """ path: path to a website's root directory.
        extensions: allowed file extensions.
        Returns a dictionary with 'file: last modified time'. """
    # traverse the website's dir
    for root, dirs, filenames in os.walk(path):
        # loop through the files in the current directory
        for f in filenames:
            # if a file ends with a desired extension
            if f.endswith(extensions):
                # get the file path relative to the website's dir
                file_path=os.path.relpath(os.path.join(root, f), path)
                # get the last modified time of that file
                mtime=os.path.getmtime(os.path.join(path, file_path))
                # put them in the index

    # return a dict of files as keys and
    # last modified time as their values
    return index

if __name__ == "__main__":
    # file extensions to track (basically all the static files)
    extensions = ('.jpg', '.jpeg', '.png', '.gif', '.svg',
                  '.css', '.js', '.json', '.ico', '.xml')

    # path to the website's root directory
    path = '/var/www/'

    # compute the new index
    new_index = compute_dir_index(path, extensions)

    # the old index json file
    json_file = '/var/www/'

    # try to read the old json file
        with open(json_file, 'r') as f:
    # if there's no such file the old_index is an empty dict
    except IOError:
        old_index = {}

    # if there's a difference
    if new_index != old_index:
        # run the s3-sync script
        # save/overwrite the json file with the new index
        with open(json_file, 'w') as f:
            json.dump(new_index, f, indent=4)

Now we need to create a cronjob.
Open crontab:

crontab -e

Paste this cronjob:

# Run the script every 5 minutes
* /5 * * * * /home/user/.local/bin/detect-changes >> /tmp/offload.log

Exit Ctrl + x, save y.

As you can see we’ll dump the logs to /tmp/offload.log.

Update your CDN

Now, go to your CDN and configure it to pull from the following URL instead of from your server, which is the endpoint of the bucket:

s3-us-west-1 is the region of the bucket and you need to identify yours.

However, if you use CloudFront as your CDN you don’t need to declare the objects in the bucket public, meaning you can omit --acl public-read above in the syncing script and you also don’t need the endpoint URL.

Instead you can allow ONLY your designated CloudFront distribution to access that bucket by choosing the bucket as the CloudFront distribution’s origin, restrict bucket access from there and grant to the distribution read permissions on the bucket which will automatically write a bucket policy for this purpose if you choose so.

If you go over to your bucket now you’ll see a policy that looks like this:

    "Sid": "1",
    "Effect": "Allow",
    "Principal": {
        "AWS": "arn:aws:iam::cloudfront:user/CloudFront \
                Origin Access Identity EAF5XXXXXXXXX"
    "Action": "s3:GetObject",
    "Resource": "arn:aws:s3:::AWSDOC-EXAMPLE-BUCKET/*"

The above method won’t work with static websites hosted on an s3 bucket. In that case you’ll still need to declare the endpoint ULR of the bucket over to the CloudFront distribution and make the objects public either via --acl public-read on every upload / command or by making the whole bucket public.

Wrap up

We’re done. One downside that I can think of is the fact that when you upload a static file to your website, it will be available at the s3 bucket 5 minutes later(you can always make the intervals smaller). So you’ll need to wait for a few minutes before hitting ‘publish post’ so all your newest static assets become available at the CDN.

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