Mike-Ward.Net

Asset Bundling in NancyFx – Part 2

20 Feb 2014

Last time I wrote a “naïve” implementation of asset bundling for NancyFx. While effective it was inefficient. This time around I’ll add cache support and, “304, Not Modified” handling.

Here’s the code.

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using Nancy;
using Nancy.Helpers;
using Nancy.Responses;

namespace SendExplorer.utilities
{
    public static class Bundle
    {
        private static readonly ConcurrentDictionary<int, assetbundle> BundleCache = 
            new ConcurrentDictionary<int, assetbundle>();

        public static Response Styles(this IResponseFormatter formatter, IEnumerable<string> files, 
		IRootPathProvider rootPathProvider)
        {
            return formatter.GetBundle(files, rootPathProvider, "text/css");
        }

        public static Response Scripts(this IResponseFormatter formatter, IEnumerable<string> files, 
		IRootPathProvider rootPathProvider)
        {
            return formatter.GetBundle(files, rootPathProvider, "application/x-javascript");
        }

        public static Response GetBundle(this IResponseFormatter formatter, IEnumerable<string> files,
            IRootPathProvider rootPathProvider, string contentType)
        {
            var hash = BundleHash(files, rootPathProvider);
            if (BundleCache.Keys.Contains(hash) == false)
            {
                var text = ConcatTextFiles(files, rootPathProvider);
                var bytes = Encoding.UTF8.GetBytes(text);
                var lastWriteTimeUtc = DateTime.Now;
                var etag = string.Concat("\"", lastWriteTimeUtc.Ticks.ToString("x"), "\"");
                var assetBundle = new AssetBundle 
			{ LastUpdate = lastWriteTimeUtc, ETag = etag, Bytes = bytes };
                BundleCache.TryAdd(hash, assetBundle);
            }
            var bundle = BundleCache[hash];

            if (CacheHelpers.ReturnNotModified(bundle.ETag, bundle.LastUpdate, formatter.Context))
            {
                var response = new Response
                {
                    StatusCode = HttpStatusCode.NotModified,
                    ContentType = null,
                    Contents = Response.NoBody
                };
                return response;
            }

            return ResponseFromBundle(bundle, contentType);
        }

        private static int BundleHash(IEnumerable<string> files, IRootPathProvider rootPathProvider)
        {
            return files
                .Select(file => FullPath(file, rootPathProvider))
                .Select(file => new FileInfo(file).LastWriteTimeUtc.GetHashCode() ^ file.GetHashCode())
                .Aggregate((a, b) => a ^ b);
        }

        public static Response ResponseFromBundle(AssetBundle assetBundle, string contentType)
        {
            var stream = new MemoryStream(assetBundle.Bytes);
            var response = new StreamResponse(() => stream, contentType);
            response.Headers["ETag"] = assetBundle.ETag;
            response.Headers["Last-Modified"] = assetBundle.LastUpdate.ToString("R");
            return response;
        }

        public static string ConcatTextFiles(IEnumerable<string> files, 
            IRootPathProvider rootPathProvider)
        {
            return string.Join(Environment.NewLine, 
		files.Select(p => ReadTextFile(p, rootPathProvider)));
        }

        private static string ReadTextFile(string path, IRootPathProvider rootPathProvider)
        {
            return File.ReadAllText(FullPath(path, rootPathProvider));
        }

        private static string FullPath(string path, IRootPathProvider rootPathProvider)
        {
            return (Path.Combine(rootPathProvider.GetRootPath(), path));
        }

        public class AssetBundle
        {
            public DateTime LastUpdate { get; set; }
            public string ETag { get; set; }
            public byte[] Bytes { get; set; }
        }
    }
}

https://gist.github.com/mike-ward/9100604

Code highlights:

Assets are bundled once and cached for later reference.
ETag and Last-Modified headers are added and checked (using some Nancy helper methods). If the nothing has changed, a 304 Not Modified response is returned.
Of particular interest is the BundleHash method. It builds a hash based on the names and timestamps of the assets. Change one of the asset timestamps and the hash should change forcing a new bundle to get generated.

And here’s how to use it:

using Nancy;
using SendExplorer.utilities;

namespace SendExplorer.Modules
{
    public class SiteModule : NancyModule
    {
        public SiteModule(IRootPathProvider rootPathProvider)
        {
            Get["/"] = p => View["home"];
            Get["/about"] = p => View["about"];

            Get["/styles"] = p => Response.Styles(
		new[] {"css/pure-min.css", "css/styles.css"}, 
		rootPathProvider);

            Get["/scripts"] = p => Response.Scripts(
		new[] {"js/third-party/angular.min.js", "js/app/app.js"}, 
		rootPathProvider);
        }
    }
}

And here’s a test to show how the 304 Not Modified response works:

[TestMethod]
public void BundlingReturnsNotModifiedOnSecondAccess()
{
    var browser = new Browser(with => with.Module<sitemodule>());
    var actual = browser.Get("/scripts");
    actual.StatusCode.Should().Be(HttpStatusCode.OK);
    var lastModified = actual.Headers["Last-Modified"];
    actual = browser.Get("/scripts", with =>
    {
        with.Header("If-None-Match", "*");
        with.Header("If-Modified-Since", lastModified);
    });
    actual.StatusCode.Should().Be(HttpStatusCode.NotModified);
    actual.Body.Should().BeEmpty();
}

The final piece of the puzzle is to add compression. This is a harder nut-to-crack in that the type of asset determines the tool used to compress it.

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Asset Bundling in NancyFx – Part 2

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