NSURLConnection returns http response status code 200 even the underlying response is 304

NSURLConnection.md

Overview

NSURLConnection is really a complex and convenient class for handling low level connections. Usually, you use NSURLConnection class to establish an FTP/HTTP/HTTPS/FILE connections and get data from remote servers or local files. NSURLConnection aimed to be flexible and handy for some common tasks like redirect/authentication/cache. Also, before iOS 4.0, it adopted delegate design pattern, will send the request to remote server in separated thread automatically.

Let's talk about cache

1. NSURLConnection provide both on-disk and in-memory cache.
2. There are two types of cache: local cache and remote cache.
3. Setting cache policy when create a NSURLRequest instance is ONLY for local cache.
4. Currently only responses to HTTP and HTTPS are cached.

NSURLRequest has 2 class method

+ (id)requestWithURL:(NSURL *)theURL
+ (id)requestWithURL:(NSURL *)theURL cachePolicy:(NSURLRequestCachePolicy)cachePolicy timeoutInterval:(NSTimeInterval)timeoutInterval

the first method will set the cachePolicy to NSURLRequestUseProtocolCachePolicy and default timeoutInterval to 60.

Using NSURLRequestUseProtocolCachePolicy means a very complex scenario:

About using HTTP cache

From the original document URL Loading System Programming Guide, it says:

Cache Use Semantics for the http Protocol

The most complicated cache use situation is when a request uses the http protocol and has set the cache policy to NSURLRequestUseProtocolCachePolicy.

If an NSCachedURLResponse does not exist for the request, then the data is fetched from the originating source. If there is a cached response for the request, the URL loading system checks the response to determine if it specifies that the contents must be revalidated. If the contents must be revalidated a connection is made to the originating source to see if it has changed. If it has not changed, then the response is returned from the local cache. If it has changed, the data is fetched from the originating source.

If the cached response doesn’t specify that the contents must be revalidated, the maximum age or expiration specified in the response is examined. If the cached response is recent enough, then the response is returned from the local cache. If the response is determined to be stale, the originating source is checked for newer data. If newer data is available, the data is fetched from the originating source, otherwise it is returned from the cache.

RFC 2616, Section 13 (http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13) specifies the semantics involved in detail.

Now we know why

When dealing with http://www-origin.sanspo.com/rss/baseball/news/pro-sd.xml, first it will check the LOCAL cache, then REMOTE cache(may return http status 304), and send back with local cached response and data.