• The app uses a server memory to remember the state
• When the server restarts, the app state is lost

• Does not use a server memory to remember the app state
• State transferred between a client and a service via HTTP metadata and resources' representations

• Service in a current state generates only valid transitions that it includes in the representation of the resource.
• Transition logic is realized at the server-side

• only "entry-level" links published to the World
• other links within documents can change without changing client's logic
• Hypertext represents the current user's view, i.e. rights or other context

• no need for a logic to construct the links
• Clients know to which states they can move via links

• Better implementation of scalability

• anytime there is a static resource
• even there is a dynamic resource
• with chances it updates often
• you can force clients to always revalidate

• client GETs the resource representation
• server controls how it should cache through Cache-Control header
• client revalidates the content via conditional GET

• controls over local and proxy caches
• private – no proxy should cache, only clients can
• public – any intermediary can cache (proxies and clients)
• no-cache – the response should not be cached. If it is cached, the content should always be revalidated.
• no-store – must not store persistently (this turns off caching)
• no-transform – no transformation of cached data; e.g. compressions
• max-age, s-maxage a time in seconds how long the cache is valid; s-maxage for proxies

• Content last modified date and a content entity tag

• Content revalidation (conditional GET)

                            > GET /orders HTTP/1.1
                            > ...
                            
                            < HTTP/1.1 200 OK
                            < Content-Type: application/xml
                            < Cache-Control: private, max-age=200
                            < Last-Modified: Sun, 7 Nov 2011, 09:40 CET
                            <
                            < ...data...
                            

• only client can cache, the cache is valid for 200 seconds.

• A client revalidates the cache after 200 seconds.

                            > GET /orders HTTP/1.1
                            > If-Modified-Since: Sun, 7 Nov 2011, 09:40 CET
                            
                            < HTTP/1.1 304 Not Modified
                            < Cache-Control: private, max-age=200
                            < Last-Modified: Sun, 7 Nov 2011, 09:40 CET
                            

• A hash such as MD5
• A sequence number that changes with any modification of the content

• Strong ETag: reflects the content bit by bit
• Weak ETag: reflects the content "semantically"
• The app defines the meaning of its weak tags

                            < HTTP/1.1 200 OK
                            < Cache-Control: private, max-age=200
                            < Last-Modified: Sun, 7 Nov 2011, 09:40 CET
                            < ETag: "4354a5f6423b43a54d"
                            
                            > GET /orders HTTP/1.1
                            > If-None-Match: "4354a5f6423b43a54d"
                            
                            < HTTP/1.1 304 Not Modified
                            < Cache-Control: private, max-age=200
                            < Last-Modified: Sun, 7 Nov 2011, 09:40 CET
                            < ETag: "4354a5f6423b43a54d"
                            

                        { 
                            "orders" : 
                                [
                                    { "id"        : 2245,
                                      "customer"  : "Tomas",
                                      "descr"     : "Stuff to build a house.",
                                      "items"     : [...] },
                                    { "id"        : 5546,
                                      "customer"  : "Peter",
                                      "descr"     : "Things to build a pipeline.",
                                      "items"     : [...] }
                                ] 
                        }
                        

• Any modification to an order's items is not significant for /orders:

                            var crypto = require("crypto");
                        
                            function computeWeakETag(orders) {
                                var content = "";
                                for (var i = 0; i < orders.length; i++)
                                    content += orders[i].id + orders[i].customer + orders[i].descr;
                                return crypto.createHash('md5').update(content).digest("hex");
                            }
                        

• POST /orders/{order-id} inserts a new item to an order
• Any changes to orders' items will not change the Weak ETag

• 1) a client GETs a resource GET /orders/5545
• 2) the client modifies the resource
• 3) the client updates the resource via PUT /orders/5545 HTTP/1.1
• What happens if another client updates the resource between 1) and 3) ?

• Conditional PUT
• Update the resource only if it has not changed since a specified date or a specified ETag matches the resource content
• If-Unmodified-Since and If-Match headers
• Response to conditional PUT:
• 200 OK if the PUT was successful
• 412 Precondition Failed if the resource was updated in the meantime.

• Conditional PUT must always use strong entity tags or date validation