Conditional GET; 3-tier architecture components

Conditional GET

A conditional GET lets a cache (browser or proxy) check whether its stored copy of an object is still current, so the server need not resend the object if it has not changed.

Operation

1. The first time, the server sends the object with a header Last-Modified: Wed, 9 Sep 2020 09:23:24 (or an ETag). The cache stores the object with this date.
2. Later, when the user requests it again, the cache sends a GET with If-Modified-Since: Wed, 9 Sep 2020 09:23:24 (or If-None-Match: <etag>).
3. If unchanged, the server replies 304 Not Modified with an empty body. The cache uses its local copy.
4. If changed, the server replies 200 OK with the new object.

 Cache                               Server
   |-- GET obj, If-Modified-Since -->|
   |<-- 304 Not Modified (no body) --|   (unchanged)
   |<-- 200 OK + new object ---------|   (changed)

Role: saves bandwidth and server load, reduces response time, and still guarantees the user gets an up-to-date page.

Components of 3-tier client/server architecture

The application is divided into three logical layers, which may run on different machines.

1. Presentation tier (client): user interface such as a browser or GUI; accepts input and displays results.
2. Application / business logic tier (middle tier): web/application server that processes requests, applies business rules and talks to the database (PHP, Java servlets, ASP.NET).
3. Data tier (database server): stores and manages data (MySQL, Oracle); handles queries and transactions.

 [Browser] <--HTTP--> [App/Web server] <--SQL--> [Database]
 Presentation         Logic                      Data

Advantages: better scalability, easier maintenance, security (clients never reach the database directly), and independent upgrading of each tier.