Building a Dynamic Email Personalization System with Resend, LaunchDarkly, and SQLite

In this guide, we'll build a dynamic email marketing system that personalizes emails for different user segments.

We'll use Resend to send emails, LaunchDarkly to control which users receive customized content using feature flags, and SQLite to store and manage user data. We'll also use python-dotenv to securely manage our environment variables, such as API keys.

💡 What is Resend?

Resend is a simple email API that makes it easy to send emails from your app. Also, even though we’re using Resend and SQLite in this guide, you can easily swap them out for other tools like SendGrid, Amazon SES, or Firebase if those fit your setup better.

What You'll Learn

How to use LaunchDarkly to manage feature flags and personalize email content.
How to send customized emails at scale using Resend.
How to pull user data from a database and use it to guide decisions in LaunchDarkly.
How to tweak your email strategy in real-time with LaunchDarkly's flexible rules.

You can find the entire project code at this GitHub repo.

What We're Building

We're building a system that sends personalized emails to two types of users: premium and regular. We'll use LaunchDarkly feature flags to decide what content each user gets, and you won’t have to change any code to update these rules.

We’ll set up a feature flag called premium-content in LaunchDarkly, which will check each user’s subscription status and purchase count to decide what kind of email to send. Then, Resend will handle sending the personalized emails.

Premium users get premium content.
Regular users get regular content.
If a user has made no purchases, they’ll get the premium content, no matter their status, to encourage them to buy.

The best part? You can update these rules anytime in LaunchDarkly without touching the code.

Let’s get started!

Prerequisites

Basic understanding of Python and web development.
A LaunchDarkly account (sign up for a free one here).
A Resend API key for sending emails.
The pre-populated SQLite database with user data. Download the sample users.db file from this GitHub repository. Optional: download a SQLite browser to update DB data.
We'll walk you through installing the necessary Python packages -resend, launchdarkly-server-sdk, python-dotenv, so don't worry if you don't have them yet!

💡 Why SQLite? We’re using SQLite in this guide because it’s lightweight, easy to set up, and doesn’t require server configuration. This makes it a good fit for learning. You can apply the same principles to more robust databases like PostgreSQL or MySQL in production environments.

Step 1: Setting Up the Database

Before we can start sending personalized emails, we need to store user data. We’ll use a simple SQLite database called users.db to track important details like email addresses, subscription status, last login, and purchase history.

Using the Provided users.db

To make things easy, you can download a pre-populated users.db file from this GitHub repository. This database includes sample users, and you can modify it as needed.

💡 Tip: You can edit the database directly using any SQLite browser like DB Browser for SQLite. It’s an easy-to-use, open-source tool that lets you view, edit, and manage your SQLite databases.

Step 2: Creating Feature Flags in LaunchDarkly

Before we integrate LaunchDarkly into our code, we need to create the feature flags that will control the personalized email content.

Creating the premium-contentFlag

Log in to your LaunchDarkly account.
Navigate to your project and environment where you want to create the feature flags.
Click on the Flags tab in the sidebar.
Click Create flag.

Screenshot of create flag button.

Flag name: "Premium Content"
Flag key: "premium-content"
Description: "Controls whether premium content is shown to users. Allows differentiation between premium and regular users."

Create flag configuration screen.

In the Configuration section: Choose Custom. Set the Flag type to boolean. Check "Is this flag temporary?": No
Under Variations, define the following: True: Premium content is enabled. False: Premium content is disabled.
Leave the Default variations as: Serve when targeting is ON: Premium content is enabled. Serve when targeting is OFF: Premium content is disabled.
Click Create flag.

Dynamic email personalization - flag variation config.

Turn the Flag On

Once the flag is added, it is turned off by default. Turn on the flag by toggling the switch at the top of the page. Follow the prompt to add a comment explaining these changes, then click "Review and save."

Premium User Content flag toggled ON.

Add Custom Rules for subscription_statusand purchase_count

Now that we've created the flag, let's define rules that target premium users based on their subscription_status and purchase_count.

Scroll down to the Rules section.
Click on + Add rule and select Build a custom rule.

Add a flag targeting rule for Premium User Content.

Rule 1: Target Premium Subscription Users

Rule Name: Type "Is Premium Subscription User".
Context Kind: User.
Attribute: Type subscription_statusand click Add subscription_status.
Operator: is one of.
Values: Type premium and click Add premium.
Serve: Premium content is enabled.

💡 This rule essentially translates to: "If the user’s subscription_status is premium, they will receive premium content."

Rule 2: Target Users with Zero Purchase Count

Rule Name: Type "Has Zero Purchase Count".
Context Kind: User.
Attribute: Type purchase_count and click Add purchase_count.
Operator: is one of.
Values: Type 0 and click Add 0.
Serve: Premium content is enabled.

This rule is useful if you want to encourage users who haven’t made any purchases yet by serving them premium content to entice them to make a purchase.

The two rules should look like this:

Dynamic email personalization - flag rule configuration.

Update Default Rule Behavior

Finally, update the Default Rule to serve "Premium content is disabled." This ensures that regular content is shown to all users by default unless their subscription status is set to premium, or their purchase count is zero.

Click on "Review and save" to apply the changes.

The rules for the Premium User Content flag should now look like this:

Flag rules for Premium User Content.

Now that we’ve set up the feature flags in LaunchDarkly, it's time to integrate them into your Python project. This will allow us to dynamically control email content based on user data without changing the code every time.

Get Your LaunchDarkly SDK Key

To begin, you’ll need your SDK key from LaunchDarkly to authenticate your app:

Login to LaunchDarkly, and navigate to your Projects.
Click on the project you're working on.
In the Environments tab, copy the SDK key.

You’ll find two keys: one for Test and one for Production. Since we’re testing, let’s use the Test key.

How to copy your LaunchDarkly SDK key from the "test" environment.

Setting Up Environment Variables

To securely manage sensitive information like API keys, we’ll use environment variables to securely store your LaunchDarkly SDK key and Resend API key.

Create an .env file in your project directory and add the following:

LAUNCHDARKLY_API_KEY=your-launchdarkly-sdk-key
RESEND_API_KEY=your-resend-api-key

Install Dependencies

To work with LaunchDarkly and Resend in our Python app, we’ll need to install some packages.

First, create a requirements.txt file that lists the dependencies:

resend
launchdarkly-server-sdk
python-dotenv

Pro Tip: Consider using a virtual environment to isolate dependencies:

python -m venv venv
source venv/bin/activate  # On Windows: venv\Scripts\activate

Then, run the following command to install all the necessary packages:

pip install -r requirements.txt

These packages will handle environment variables (python-dotenv), feature flags (launchdarkly-server-sdk), and email (resend) functionality.

Setup LaunchDarkly SDK

Now let’s integrate the LaunchDarkly SDK into our project so we can dynamically control the email content based on user data. Here’s the script that will connect to LaunchDarkly, evaluate the feature flags, and determine which content should be sent to each user.

import os
import ldclient
from ldclient.config import Config
from ldclient.context import Context
from dotenv import load_dotenv

# Load environment variables from .env file
load_dotenv()

def evaluate_flag(sdk_key, feature_flag_key, user_email, subscription_status, purchase_count):
    """
    This function evaluates the feature flag using user-specific data like email, 
    subscription status

, and purchase count. It also provides the reason for why 
    the feature flag evaluated to True or False.
    """
    
    # Initialize LaunchDarkly client
    ldclient.set_config(Config(sdk_key))

    # Create a context with user email, subscription status, and purchase count
    context = Context.builder(user_email) \
        .kind('user') \
        .set("email", user_email) \
        .set("subscription_status", subscription_status) \
        .set("purchase_count", purchase_count) \
        .build()

    # Get detailed evaluation of the feature flag
    flag_detail = ldclient.get().variation_detail(feature_flag_key, context, False)
    flag_status = flag_detail.value
    flag_reason = flag_detail.reason

    # Print the flag evaluation result and the reason why
    print(f"LaunchDarkly Feature flag '{feature_flag_key}' for user '{user_email}' evaluated to: {flag_status}")
    print(f"Reason: {flag_reason}")

    # Close the LaunchDarkly client after evaluation
    ldclient.get().close()

    return flag_detail

# Example usage
if __name__ == "__main__":
    sdk_key = os.getenv("LAUNCHDARKLY_API_KEY")
    feature_flag_key = "premium-content"
    user_email = "user1@example.com"  # Test user email
    subscription_status = "premium"  # Example subscription status
    purchase_count = 3  # Example purchase count

    # Get the flag status and reason for evaluation
    flag_detail = evaluate_flag(sdk_key, feature_flag_key, user_email, subscription_status, purchase_count)

Expected Output

When you run this script (python launchdarkly_setup.py), LaunchDarkly checks whether the user should receive premium content by evaluating their subscription_status and purchase_countagainst the rules you created. The output will indicate whether premium content is enabled or not, along with a reason based on the matched rules.

If the conditions match, you’ll see something like:

LaunchDarkly Feature flag 'premium-content' for user 'user1@example.com' evaluated to: True
Reason: {'kind': 'RULE_MATCH', 'ruleIndex': 0, 'ruleId': '38bd9f73-9ed5-4eed-8159-c4c76aa5e8e8'}

If the user doesn’t meet the conditions, you’ll see:

LaunchDarkly Feature flag 'premium-content' for user 'user1@example.com' evaluated to: False
Reason: {'kind': 'FALLTHROUGH'}

Step 4: Setting Up Resend to Send Personalized Emails

Now that we have our feature flags in place, we'll use Resend to send personalized emails based on those flags. We'll determine which content to send (premium or regular) by checking the user's subscription status and evaluating the flags we set up in LaunchDarkly.

Get the Resend API Key

Navigate to https://resend.com/api-keys, and create a new API key. To send emails, you'll also need to verify a domain. For a detailed quickstart with Resend, check out their Python quickstart guide.

Screenshot of the dialog for creating a new Resend API key.

Update the .env with API Credentials

In your .env file, add the RESEND_API_KEY, and RESEND_EMAIL_FROM address that you'll use to send the emails. Here’s what your .env file should look like:

LAUNCHDARKLY_API_KEY=your-launchdarkly-api-key
RESEND_API_KEY=your-resend-api-key
RESEND_EMAIL_FROM=your-email@example.com

With your API key and RESEND_EMAIL_FROM set up, it’s time to send personalized emails using Resend, based on the feature flags evaluated by LaunchDarkly.

File: send_emails.py

This script fetches user data, evaluates feature flags, and sends personalized emails.

💡 Before running the script, update a user's email in users.db with your own to test the email delivery. You can do this by opening the users.db file in any SQLite browser like DB Browser for SQLite and editing the email field. If you prefer SQL queries, you can update the email directly like this:

UPDATE users SET email = 'your-email@example.com' WHERE id = 1;

Once updated, run python send_emails.py in your terminal to receive the test email.

#send_emails.py

import sqlite3
import resend
from launchdarkly_setup import evaluate_flag
import os
from dotenv import load_dotenv
import datetime

# Load environment variables from .env file
load_dotenv()

# Set the Resend API key
resend.api_key = os.getenv('RESEND_API_KEY')

# Get the email sender address from environment variables
email_from = os.getenv('RESEND_EMAIL_FROM')

def fetch_users_from_db():
    """Connect to the SQLite database and fetch user data."""
    conn = sqlite3.connect('users.db')
    cursor = conn.cursor()

    # Fetch all users from the database
    cursor.execute('SELECT email, subscription_status, last_login_date, purchase_count FROM users')
    users = cursor.fetchall()

    # Close the database connection
    conn.close()
    return users

def get_email_content(show_premium_content_flag):
    """Determine the email content based on the feature flag evaluation."""

    if show_premium_content_flag:
        subject = "✨ Exclusive Offer Just for You, Premium Member! ✨"
        body = '''
        <h1>Hey, Premium Member! 🌟</h1>
        <p>As a valued premium member, we're thrilled to bring you an exclusive offer. Enjoy <strong>20% off</strong> on your next purchase!</p>
        <p>Use the code <strong>PREMIUM20</strong> at checkout to claim your discount. 🎁</p>
        <p>Thank you for being a part of our premium family. We appreciate you! 💖</p>
        '''
    else:
        subject = "👀 Peek Inside – We’ve Got Something New! 👀"
        body = '''
        <h1>Hello, Wonderful You! 🌟</h1>
        <p>Check out the latest updates. 🧐</p>
        '''
    return subject, body

def send_personalized_email(email_to, show_premium_content_flag):
    """Send a personalized email based on the user's subscription status and feature flag."""
    
    # Get the subject and body based on the flag evaluation
    subject, body = get_email_content(show_premium_content_flag)

    # Send the email using the Resend SDK
    try:
        response = resend.Emails.send({
            "from": email_from,
            "to": email_to,
            "subject": subject,
            "html": body
        })
        print(f"Email sent to {email_to} with subject: {subject}. Response: {response}")
    except Exception as e:
        print(f"Failed to send email to {email_to}. Error: {e}")

def process_and_send_emails():
    """Fetch user data, evaluate feature flags, and send the appropriate type of email."""
    users = fetch_users_from_db()

    for user in users:
        email_to, subscription_status, last_login_date, purchase_count = user
        print(f"Database: User '{email_to}' has a '{subscription_status}' subscription.")

        # Evaluate the feature flag using the user's subscription status and purchase count
        feature_flag_key = 'premium-content'
        flag_detail = evaluate_flag(
            os.getenv("LAUNCHDARKLY_API_KEY"), 
            feature_flag_key, 
            email_to, 
            subscription_status, 
            purchase_count
        )

        show_premium_content_flag = flag_detail.value

        # Send personalized email
        send_personalized_email(email_to, show_premium_content_flag)

# Run the process to determine email type and send emails
if __name__ == "__main__":
    process_and_send_emails()

Expected Output

When the script (launchdarkly_setup.py) is run, it will evaluate the feature flag for a user based on their subscription_status and purchase_count. The script will output the feature flag's evaluation (True or False), as well as the reason for the evaluation (such as whether it matched a rule or was based on a default condition).

You can test with multiple email addresses by adding them to your database or edit user attributes such as subscription status or purchase count to simulate different scenarios. This way, you can test all possible cases without needing multiple email addresses.

Scenario 1: User with a Premium Subscription

For a user with a premium subscription, LaunchDarkly would evaluate the flag premium-content to True because it will match our first rule (index 0) - Is Premium Subscription User.

Database: User 'premium-user@example.com' has a 'premium' subscription.
LaunchDarkly Feature flag 'premium-content' for user 'premium-user@example.com' evaluated to: True
Reason: {'kind': 'RULE_MATCH', 'ruleIndex': 0, 'ruleId': '38bd9f73-9ed5-4eed-8159-c4c76aa5e8e8'}
Email sent to hello@ajot.me with subject: ✨ Exclusive Offer Just for You, Premium Member! ✨. Response: {'id': 'e8f562cf-c9ff-477c-a35f-1fee13f76af8'}

Scenario 2: Regular User with Zero Purchase Count

For a user with a regular subscription and 0 purchases, LaunchDarkly will evaluate that they should receive premium content (based on the rule we created for users with zero purchase count - rule index 1 - Has Zero Purchase Count).

Database: User 'regular-user@example.com' has a 'regular' subscription.
LaunchDarkly Feature flag 'premium-content' for user 'regular-user@example.com' evaluated to: True
Reason: {'kind': 'RULE_MATCH', 'ruleIndex': 1, 'ruleId': 'b5c0ca81-39b5-4e78-9e40-85bdae1649a6'}
Email sent to regularuser@example.com with subject: ✨ Exclusive Offer Just for You, Premium Member! ✨. Response: {'id': 'e8747fcd-d132-4e7c-b442-9cb69a4bfd1b'}

Both these users will receive an email with Premium Content similar to this:

Personalized email for premium members.

Scenario 3: Regular User with More Than 0 Purchase Count

For a user with a regular subscription and more than 0 purchases, LaunchDarkly will evaluate the feature flag to False, meaning the user should receive regular content:

Database: User 'anotheruser@example.com' has a 'regular' subscription.
LaunchDarkly Feature flag 'premium-content' for user 'anotheruser@example.com' evaluated to: False
Reason: {'kind': 'FALLTHROUGH'}
Email sent to anotheruser@example.com with subject: 👀 Peek Inside – We’ve Got Something New! 👀. Response: {'id': 'f8a672cf-1234-5678-b442-1abc23d45efg'}

They will receive an email similar to this:

non-premium personalized email example.

Conclusion: Recap and What’s Next?

You’ve set up a system that dynamically personalizes emails based on user data, with LaunchDarkly controlling what content gets sent and Resend handling the delivery. This lets you adjust email content in real-time without touching your code, giving you flexibility to target premium users, new buyers, or any other segment you define.

Recap

Dynamically personalized email content using LaunchDarkly feature flags.
Sent tailored emails with Resend.
Stored and managed user data with SQLite.

What’s Next?

Enhance personalization: Fine-tune and experiment with more detailed feature flag rules to create even more targeted campaigns, such as by region or engagement level.
Test and iterate: Run A/B tests with LaunchDarkly’s rollouts to boost engagement.

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Building a Dynamic Email Personalization System with Resend, LaunchDarkly, and SQLite

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💡 What is Resend?

What You'll Learn

What We're Building

Prerequisites

Step 1: Setting Up the Database

Using the Provided users.db

Step 2: Creating Feature Flags in LaunchDarkly

Rule 1: Target Premium Subscription Users

Rule 2: Target Users with Zero Purchase Count

Update Default Rule Behavior

Get Your LaunchDarkly SDK Key

Setting Up Environment Variables

Install Dependencies

Setup LaunchDarkly SDK

Step 4: Setting Up Resend to Send Personalized Emails

Get the Resend API Key

Update the .env with API Credentials

Expected Output

Conclusion: Recap and What’s Next?

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