Refactor email sending for non-blocking performance and add request timing

• Dockerfile: bump Gunicorn workers to 4 for concurrent request handling
• server.py:

- Load SMTP credentials once at startup
- Wrap send_confirmation_email in a daemon Thread for “fire-and-forget” delivery
- Replace print() with structured app.logger.error() on failure
- Add before_request/after_request hooks to log per-request durations
- Use context manager for SMTP_SSL connections (auto-close)
- Simplify route JSON responses and HTTP methods declarations
- Retain existing DB logic but recommend low DB connect_timeouts
- Ensure Flask binds on 0.0.0.0 in development
These changes eliminate request blocking on email I/O, expose request latencies in logs, and improve overall concurrency.

Dockerfile

@@ -14,5 +14,4 @@ ENV FLASK_APP=server.py
 
 EXPOSE 5000
 
-CMD ["gunicorn", "--bind", "0.0.0.0:5000", "server:app"]
-
+CMD ["gunicorn", "--bind", "0.0.0.0:5000", "--workers", "4", "server:app"]

server.py

@@ -1,59 +1,84 @@
 import os
+import time
+from threading import Thread
 import smtplib
 from email.mime.text import MIMEText
 from flask import Flask, render_template, request, jsonify
-from database import get_connection, init_db, add_email, remove_email
 from dotenv import load_dotenv
-from collections import namedtuple
+from database import init_db, get_connection, add_email, remove_email
 
 load_dotenv()
+
+SMTP_SERVER   = os.getenv('SMTP_SERVER')
+SMTP_PORT     = int(os.getenv('SMTP_PORT', 465))
+SMTP_USER     = os.getenv('SMTP_USER')
+SMTP_PASSWORD = os.getenv('SMTP_PASSWORD')
+
 app = Flask(__name__)
 init_db()
 
-def send_confirmation_email(email):
-    SMTP_SERVER = os.getenv('SMTP_SERVER')
-    SMTP_PORT = int(os.getenv('SMTP_PORT', 465))
-    SMTP_USER = os.getenv('SMTP_USER')
-    SMTP_PASSWORD = os.getenv('SMTP_PASSWORD')
+@app.before_request
+def start_timer():
+    request._start_time = time.time()
 
-    unsubscribe_link = f"{request.url_root}unsubscribe?email={email}"
-
-    subject = 'Thanks for subscribing!'
+@app.after_request
+def log_request(response):
+    elapsed = time.time() - getattr(request, '_start_time', time.time())
+    app.logger.info(f"{request.method} {request.path} completed in {elapsed:.3f}s")
+    return response
 
+def send_confirmation_email(to_address: str, unsubscribe_link: str):
+    """
+    Sends the HTML confirmation email to `to_address`.
+    This runs inside its own SMTP_SSL connection (timeout=10s).
+    """
+    subject = "Thanks for subscribing!"
     html_body = render_template(
-        'confirmation_email.html',
+        "confirmation_email.html",
         unsubscribe_link=unsubscribe_link
     )
 
-    msg = MIMEText(html_body, 'html', 'utf-8')  # Specify HTML
-    msg['Subject'] = subject
-    msg['From'] = SMTP_USER
-    msg['To'] = email
+    msg = MIMEText(html_body, "html", "utf-8")
+    msg["Subject"] = subject
+    msg["From"]    = SMTP_USER
+    msg["To"]      = to_address
 
     try:
-        server = smtplib.SMTP_SSL(SMTP_SERVER, SMTP_PORT, timeout=10)
+        with smtplib.SMTP_SSL(SMTP_SERVER, SMTP_PORT, timeout=10) as server:
             server.login(SMTP_USER, SMTP_PASSWORD)
-        server.sendmail(SMTP_USER, email, msg.as_string())
-        server.quit()
+            server.sendmail(SMTP_USER, [to_address], msg.as_string())
     except Exception as e:
-        print(f"Failed to send email to {email}: {e}")
+        app.logger.error(f"Failed to send email to {to_address}: {e}")
 
-@app.route("/")
+def send_confirmation_async(email, unsubscribe_link):
+    """
+    Wrapper for threading.Thread target.
+    """
+    send_confirmation_email(email, unsubscribe_link)
+
+@app.route("/", methods=["GET"])
 def index():
     return render_template("index.html")
 
 @app.route("/subscribe", methods=["POST"])
 def subscribe():
-    data = request.get_json()
-    email = data.get('email')
+    data = request.get_json() or {}
+    email = data.get("email")
     if not email:
-        return jsonify({"error": "No email provided"}), 400
+        return jsonify(error="No email provided"), 400
 
     if add_email(email):
-        send_confirmation_email(email)
-        return jsonify({"message": "Email has been added"}), 201
-    else:
-        return jsonify({"error": "Email already exists"}), 400
+        unsubscribe_link = f"{request.url_root}unsubscribe?email={email}"
+
+        Thread(
+            target=send_confirmation_async,
+            args=(email, unsubscribe_link),
+            daemon=True
+        ).start()
+
+        return jsonify(message="Email has been added"), 201
+
+    return jsonify(error="Email already exists"), 400
 
 @app.route("/unsubscribe", methods=["GET"])
 def unsubscribe():
@@ -63,42 +88,57 @@ def unsubscribe():
 
     if remove_email(email):
         return f"The email {email} has been unsubscribed.", 200
-    else:
     return f"Email {email} was not found or has already been unsubscribed.", 400
 
-@app.route("/newsletters")
+
+@app.route("/newsletters", methods=["GET"])
 def newsletters():
+    """
+    List all newsletters (newest first).
+    """
     conn   = get_connection()
     cursor = conn.cursor()
-    cursor.execute("SELECT id, subject, body, sent_at FROM newsletters ORDER BY sent_at DESC")
-    results = cursor.fetchall()
-    newsletters = [
-        {"id": rec[0], "subject": rec[1], "body": rec[2], "sent_at": rec[3]} 
-        for rec in results
-    ]
+    cursor.execute(
+        "SELECT id, subject, body, sent_at "
+      + "FROM newsletters ORDER BY sent_at DESC"
+    )
+    rows = cursor.fetchall()
     cursor.close()
     conn.close()
+
+    newsletters = [
+        {"id": r[0], "subject": r[1], "body": r[2], "sent_at": r[3]}
+        for r in rows
+    ]
     return render_template("newsletters.html", newsletters=newsletters)
 
-@app.route("/newsletter/<int:newsletter_id>")
+
+@app.route("/newsletter/<int:newsletter_id>", methods=["GET"])
 def newsletter_detail(newsletter_id):
+    """
+    Show a single newsletter by its ID.
+    """
     conn   = get_connection()
     cursor = conn.cursor()
-    cursor.execute("SELECT id, subject, body, sent_at FROM newsletters WHERE id = %s", (newsletter_id,))
-    record = cursor.fetchone()
+    cursor.execute(
+        "SELECT id, subject, body, sent_at "
+      + "FROM newsletters WHERE id = %s",
+        (newsletter_id,)
+    )
+    row = cursor.fetchone()
     cursor.close()
     conn.close()
 
-    if record is None:
+    if not row:
         return "Newsletter not found.", 404
 
     newsletter = {
-        "id": record[0],
-        "subject": record[1],
-        "body": record[2],
-        "sent_at": record[3]
+        "id":      row[0],
+        "subject": row[1],
+        "body":    row[2],
+        "sent_at": row[3]
     }
     return render_template("newsletter_detail.html", newsletter=newsletter)
 
 if __name__ == "__main__":
-    app.run(debug=True)
+    app.run(host="0.0.0.0", debug=True)