REST & Caching

1. REST & Caching Scaling like the internets

2. HTTP and REST • REST is an architectural style • Draws and relies heavily on HTTP • HTTP just scales

3. HTTP and Caching • Conditional GET • Caching intermediaries • Web Server Caching • Data Caching SOAP Because SOAP uses POST and POST is unsafe to cache *Except Data Caching

4. Server-side Caching • HttpContext.Current.Cache • Velocity • IIS kernel and user mode caching

5. DEMO IIS Caching

6. Conditional GET (304 Not Modified) • This may be the most important status code of them all • This one of the ways the web scales • REST piggybacks on this • Saves bandwidth, as well as processing power on both server and client • Works like this • Server sends resource • Headers include “extra” information • Client asks for resource again (GET) • Sends the “extra” information (condition) • If request meets condition, server returns 304 • If request doesn’t, server returns the resource again

7. Last Modified • One way to implement conditional GET • Server sets Last-Modified header with timestamp • Client uses If-Modified-Since header value • If resource hasn’t changed, server returns a 304

8. ETag • Another way to implement conditional GET • Server sets an opaque value for the ETag header • Generally implementation is to take a hash value of some or all of the bytes of the resource • Client sends If-None-Match header

9. Generating ETag ETag is opaque to user agent: meaningful to service string GenerateETag(User u) { byte[] bytes = Encoding.UTF8.GetBytes(u.UserId + u.LastModified.ToString()); byte[] hash = MD5.Create().ComputeHash(bytes); string etag = Convert.ToBase64String(hash); return etag; } void SetETag(string etag) { OutgoingWebResponseContext ctx = WebOperationContext.Current.OutgoingResponse; ctx.ETag = etag; }

10. DEMO Conditional GET

11. Summary • REST is an architectural style for building client-server applications • REST relies on HTTP • HTTP scales because of caching

12. References • HTTP http://www.w3.org/Protocols/ • Conditional GET http://ftp.ics.uci.edu/pub/websoft/caching.html • ETags http://bitworking.org/news/150/REST-Tip-Deep-etags-give-you-more-benefits • REST Starter Kit - HttpClient

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