ETag and If-None-Match HTTP Headers are not working

14
votes

I have a file in my webserver and I am downloading that to my app everytime I access it because its possible that file content might be changed But If it is changed I would like to download that time only so bandwidth can be saved and fortunately that's what this ETag and If-None-Match header fields are for.

  • When I make a request first time ,I retrieve the ETag from the HTTP response headers

First time request to the file

  • In the subsequent requests to download that file I'd attach the Etag value for If-None-Match headerfield so that if there is no change then I'd get HTTP response status code 304 or else I'd get 200 if there is a change in the file.

    Second time request for same file

Note:

When I try the above steps in Advanced REST Client Application in chrome it works fine as it is supposed to be but when I try that in iOS I always get the response code 200 but it should have given me 304 for the subsequent requests.

Here is the sample code I use

var request1 =  NSMutableURLRequest(URL:NSURL(string: "http://10.12.1.101/Etag/ringtone1.mp3")!)
let Etagvalue="\"36170-52c1cc36d9b40\""
var session1 = NSURLSession.sharedSession()
request1.HTTPMethod = "GET"
var err: NSError?
request1.addValue(Etagvalue, forHTTPHeaderField: "If-None-Match")

var task = session1.dataTaskWithRequest(request1, completionHandler: {data, response, error -> Void in
       print("response: \(response)")

})

Here is the response

response: Optional( { URL: http://10.12.1.101/Etag/ringtone1.mp3 } { status code: 200, headers { "Accept-Ranges" = bytes; Connection = "Keep-Alive"; "Content-Length" = 221552; "Content-Type" = "audio/mpeg"; Date = "Wed, 24 Feb 2016 14:57:53 GMT"; Etag = "\"36170-52c1cc36d9b40\""; "Keep-Alive" = "timeout=5, max=100"; "Last-Modified" = "Fri, 19 Feb 2016 10:15:33 GMT"; Server = "Apache/2.4.16 (Unix) PHP/5.5.29"; } })

What am I doing wrong here ?

3
iosswiftnsurlsessionetagif-none-match
show us the request generated. e.g. using a proxy server (e.g. charles) – Daij-Djan
@Daij-Djan - All of a sudden it started working ! I couldn't guess why this inconsistency ! – Durai Amuthan.H
@Daij-Djan - is there any dependency for E-tag to work other than ..server side configuration ? My fingers are still crossed how come it started working ? – Durai Amuthan.H
don't think so.. well you gotta not cash locally but I don't think you do ;) -- so no :/ – Daij-Djan
@Daij-Djan - As I am not specifying anything about cache policy in my request the default NSURLRequestCachePolicy.UseProtocolCachePolicy will be the behaviour...so caching will happen. – Durai Amuthan.H

3 Answers

17
votes

I've encountered the same problem. I've discovered that it's because of cachePolicy. You need to set it as follows:

request.cachePolicy = .ReloadIgnoringLocalAndRemoteCacheData

And you will be OK

2
votes

CachePolicy naming convention is confusing and better yet it actually does not implement some of them...

This article explains them well. http://nshipster.com/nsurlcache/

Also, if you let the Cache Policy to use UseProtocolCachePolicy then your NSURLSession will receive Status code 200 with response generated from Cache.

0
votes

This issue is due to the cache policy 'useProtocolCachePolicy', 'returnCacheDataElseLoad' or 'returnCacheDataDontLoad'.

You can use any other policy then above one. Preferably 'reloadIgnoringLocalAndRemoteCacheData'.