Adding Conditional Get Support to the WCF REST Starter kit
Some weeks ago, I discussed how important "Conditional Get" can be for some scenarios, specially when we want to make a better use of the network traffic.
The WCF REST Starter kit introduced a new extension "WebCache", implemented as an operation behavior, to automatically add caching support to any "Get" operation in a service contract. Using this new feature is a simple as annotating an existing service operation with a "WebCache" attribute, as it is shown below:
[WebCache(CacheProfileName = "CacheFor1Min")]
[WebGet(UriTemplate = "customer/{id}")]
[OperationContract]
public Customer GetCustomer(string id)
The underline implementation of this new behavior relies on the existing ASP.NET web cache. For more details about this new feature, Jesus Rodriguez has written a very good summary here. I also mentioned how to enable Sql dependencies with this feature in this post.
Unfortunately, this implementation does not support Conditional Get out of the box, which means that the client always receive a complete response, no matter if the response was served directly from the cache or not. For instance, if one operation returns a feed with one hundred items, and the client application does not have a way to specify when was the last time it got access to that feed, it will have to download the complete feed all the times.
Although the ASP.NET does provide support for conditional gets through the public methods "SetLastModified" (for date times) and "SetETag" (for entity representations), the web cache behavior does not make use of them.
If we just modified the Web cache behavior to include the following line at the end, the additional LastModified header will be included in the response
if (!string.IsNullOrEmpty(cacheProfile.VaryByCustom))
{
HttpContext.Current.Response.Cache.SetVaryByCustom(cacheProfile.VaryByCustom);
}
HttpContext.Current.Response.Cache.SetLastModified(DateTime.Now); //Line Added for Conditional Get support
It is a simple as that. After enabling some trace to the service, we will see
Server ASP.NET Development Server/9.0.0.0
Date Mon, 08 Dec 2008 15:41:54 GMT
X-AspNet-Version 2.0.50727
Cache-Control public, max-age=44
Expires Mon, 08 Dec 2008 15:42:42 GMT
Last-Modified Mon, 08 Dec 2008 15:41:43 GMT
Vary *
Content-Type application/xml; charset=utf-8
Content-Length 195
Connection Close
However, setting DateTime.Now for every operation is not efficient for every scenario because it will be tied to the item lifetime in the cache. For instance, if our service operation is returning a feed, we might want to use the feed's last update time.
It will be much better if have a configurable way to set up this date for every entry in the cache. For that reason, I came up with a solution based on a simple strategy pattern through a pluggeable interface "ILastModified",
public interface ILastModified
{
DateTime? GetLastModifiedForOutput(object[] outputs, object returnValue);
}
The method "GetLastModifiedForOutput" basically determines the "LastModified" date for the given outputs parameters and return value of the service operation. Given this simple interface, we could have default implementations for feeds or the current date time.
public class SyndicationLastModified : ILastModified
{
public DateTime? GetLastModifiedForOutput(object[] outputs, object returnValue)
{
if(returnValue is SyndicationFeedFormatter)
{
var formatter = (SyndicationFeedFormatter)returnValue;
return formatter.Feed.LastUpdatedTime.UtcDateTime;
}
return null;
}
}
public class CurrentDateLastModified : ILastModified
{
public DateTime? GetLastModifiedForOutput(object[] outputs, object returnValue)
{
return DateTime.Now;
}
}
And finally configure these classes as part of the WebCache attribute.
[WebCache(CacheProfileName = "CacheFor1Min", LastModifiedType=typeof(CurrentDateLastModified))]
[WebGet(UriTemplate = "customer/{id}")]
[OperationContract]
public Customer GetCustomer(string id)
{
if (id == "foo")
return new Customer { Id = id, FullName = "John Foo" };
else if (id == "bar")
return new Customer { Id = id, FullName = "John Bar" };
return new Customer { Id = id, FullName = "Unknow" };
}
If no LastModifiedType is specified as argument for the "WebCache" attribute, the default behavior is used, resulting in a complete response message served to the client all the times. Otherwise, the server will evaluate the "If-Modified-Since" header first to know if the valid response for that request should be "serving all the content" vs returning a "Http 304 - Not Modified" status code.