How does a database-driven dynamic web application handle a request, using a flight search as the example?
The browser sends form data to the server, a server program (e.g. PHP) queries a database, and the server builds fresh HTML from the results and sends it back.
Imagine searching for a flight from Zurich to Barcelona. Three machines cooperate: your browser (frontend), the web server running the program (backend), and a separate database server holding the flight data. The round-trip looks like this:
- URL — browser requests the search page from the web server.
- Form — server returns the search form (an HTML page).
- Click — you fill in the fields and press "Search".
- Data — browser sends your form data to the server, along with the name of the program that should handle it.
- Query — the server program (e.g. PHP) queries the flight database.
- Data — the database returns the matching rows.
- Webpage — the server turns those rows into HTML and sends the finished page to your browser.
Why three tiers? Separating display (browser), logic (web server), and storage (database) lets each scale and be replaced independently.
Gotcha: the page need not be rebuilt whole. With AJAX the browser can fetch just the new data in the background and update part of the page, so the rest stays put — that is why modern apps feel smooth instead of flashing a full reload.