Akavache and ETags - http://cam.macfar.land/blog/akavache-and-etags/

AkavacheExtensions.cs

public static class AkavacheExtensions
{
    public static IObservable<string> GetOrFetchWithETag(this IBlobCache cache, string url)
    {
        var result =
            // Get from cache
            cache.GetObject<string>(url)

            // Cached values are true
            .Select(x => Tuple.Create(x, true))

            // Turn exceptions into false
            .Catch(Observable.Return(Tuple.Create("", false)))

            // If true, return an observable with the result, else an empty observable.
            .SelectMany(x => x.Item2 ? Observable.Return(x.Item1) : Observable.Empty<string>());

        var fetch =
            // Get the ETag from cache
            cache.GetObject<string>("etag-" + url)

            // Exceptions => Blank ETag
            .Catch(Observable.Return(""))

            // Call our web method
            .SelectMany(etag => GetFromWeb(url, etag)

                // Invalidate the old and add the new etag to the cache
                .SelectMany(x => cache.InvalidateObject<string>("etag-" + url).Select(_ => x))
                .SelectMany(x => cache.InsertObject("etag-" + url, x.Item1).Select(_ => x))

                // Invalidate the old and add the new data to the cache
                .SelectMany(x => cache.InvalidateObject<string>(url).Select(_ => x))
                .SelectMany(x => cache.InsertObject(url, x.Item2).Select(_ => x)))

            // Select the data from the tuple
            .Select(x => x.Item2);

        return result
            .Concat(fetch)
            .Replay()
            .RefCount();
    }

    private static HttpClient CreateWebClient()
    {
        var handler = new HttpClientHandler();
        if (handler.SupportsAutomaticDecompression)
        {
            handler.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;
        }
        var client = new HttpClient(handler);
        return client;
    }

    private static IObservable<Tuple<string, string>> GetFromWeb(string url, string etag)
    {
        return Observable.Create<Tuple<string, string>>(async observer =>
        {
            using (var client = CreateWebClient())
            {
                var request = new HttpRequestMessage
                {
                    Method = HttpMethod.Get,
                    RequestUri = new Uri(url)
                };

                if (!string.IsNullOrEmpty(etag))
                {
                    request.Headers.TryAddWithoutValidation("If-None-Match", etag);
                }

                var response = await client.SendAsync(request)
                    .ConfigureAwait(false);

                if (!response.IsSuccessStatusCode &&
                    response.StatusCode != HttpStatusCode.NotModified)
                {
                    observer.OnError(new HttpRequestException(
                        "Status code: " + response.StatusCode));
                }
                else if (response.IsSuccessStatusCode)
                {
                    var data = await response.Content.ReadAsStringAsync()
                        .ConfigureAwait(false);

                    observer.OnNext(Tuple.Create(response.Headers.ETag.Tag, data));
                }
            }
            observer.OnCompleted();
        });
    }
}