DistinctBy in LINQ

The operator DistinctBy is lacking in LINQ, but we can achieve it with the GroupBy operator.

using System;
using System.Collections.Generic;
using System.Linq;

namespace GroupingLinq
{
    
  

        public static class EnumerableExtensions
        {

            /// <summary>
            /// Returns a ienumerable which is distinct by a given property key selector. If a custom equality 
            /// comparer is to be used, pass this in as the comparer. By setting the comparer default to null,
            /// the default comparer is used. 
            /// </summary>
            /// <typeparam name="T">The item type in the ienumerable</typeparam>
            /// <typeparam name="TKey">The type of the key selector (property to disinct elements by)</typeparam>
            /// <param name="coll">The source ienumerable</param>
            /// <param name="keySelector">The key selector, use a member expression in a lambda expression</param>
            /// <param name="comparer">Custom comparer to use, pass in null here to specify that default comparer is used,
            /// however, this is default set to null and not required parameter</param>
            /// <returns></returns>
            public static IEnumerable<T> DistinctBy<T, TKey>(this IEnumerable<T> coll, Func<T, TKey> keySelector,
                IEqualityComparer<TKey> comparer = null)
            {
                if (coll == null)
                    throw new ArgumentNullException("coll");
                if (keySelector == null)
                    throw new ArgumentNullException("keySelector");

                var result = coll.GroupBy(keySelector, comparer).Select(g => g.First()).ToList();
                return new List<T>(result).AsEnumerable();
            }

        }
    

}

The following entertaining example which is a console application shows how we can use this operator:

class Program
    {

        static void Main(string[] args)
        {

            var cars = new []
            {
                new Car {Model = "Audi", Make = "A4", Color = "Black"},
                new Car {Model = "Audi", Make = "A8", Color = "Red"},
                new Car {Model = "Audi", Make = "TT", Color = "Black"},
                new Car {Model = "Volvo", Make = "XC90", Color = "Black"},
                new Car {Model = "Volvo", Make = "S90", Color = "Black"},
                new Car {Model = "Ferrari", Make = "F500", Color = "Yellow"},
                new Car {Model = "Ferrari", Make = "F500", Color = "Red"},
                new Car {Model = "Lada", Make = "Limousine", Color = "Rusty"}
            };

            var groupedCars = cars.DistinctBy(c => new {c.Model, c.Color});


            foreach (var gc in groupedCars)
            {
                Console.WriteLine(gc.ToString()); 
            }

            Console.WriteLine("Press any key to continue ...");
            Console.ReadKey(); 
        }




        // Define other methods and classes here

    }

The sample output is then:

Model: Audi, Make: A4, Color: Black
Model: Audi, Make: A8, Color: Red
Model: Volvo, Make: XC90, Color: Black
Model: Ferrari, Make: F500, Color: Yellow
Model: Ferrari, Make: F500, Color: Red
Model: Lada, Make: Limousine, Color: Rusty
Press any key to continue ...

In the example we used DistinctBy some Cars based on their properties Model and Color. We want in other words unique combinations of Model AND Color. I.e. we can have multiple Cars of the same Model but WITH different Color OR we can have multiple cars with same Color but then the Model but be different. Uniqueness Criterion: (Model, Color) We did not get the item "Audi TT Black" because we already got a black Audi. We did not get the "Volvo S90 Black" because we already got a black Volvo. We got both the Ferrari F500 because they got different colors. And sadly, we are stuck with the "Lada Limousine Rusty" since it was the only combination of Model and Color.

ASP.NET Cache and Redis

ASP.NET Cache can utilize Redis for caching page data, but also additional kinds of data such as images and arbitrary data. This is documented in the MSDN article "ASP.NET Output Cache Provider for Azure Redis Cache": https://msdn.microsoft.com/en-us/library/azure/dn798898.aspx First off, we create a new ASP.NET Solution such as a MVC solution and select after creating it Manage Nuget Packages through right clicking the Solution Explorer.




We can also add this Nuget package using Package Manager Console and type:

Install-Package Install-Package Microsoft.Web.RedisOutputCacheProvider

Note, this will add a <caching> element into Web.Config that you now can change. The following sets up the RedisOutputCacheProvider to a Redis Server running on localhost, not using SSL and using port 6379:

      <add name="MyRedisOutputCache" type="Microsoft.Web.Redis.RedisOutputCacheProvider" host="localhost" accessKey="" applicationName="RedisCache" port="6379" ssl="false" />
   

We can monitor the activity on the local Redis server by starting a redis client and type the command:

c:\Redis>redis-cli
redis 127.0.0.1:6379> monitor
OK

Note: If your Redis Server is not running yet, start it by running redis-server

c:\Redis>redis-server
[10092] 01 Oct 15:05:45.635 # Warning: no config file specified, using the default config. In order to specify a config file use redis-server /path/to/redis.conf
                _._
           _.-``__ ''-._
      _.-``    `.  `_.  ''-._           Redis 2.6.12 (00000000/0) 64 bit
  .-`` .-```.  ```\/    _.,_ ''-._
 (    '      ,       .-`  | `,    )     Running in stand alone mode
 |`-._`-...-` __...-.``-._|'` _.-'|     Port: 6379
 |    `-._   `._    /     _.-'    |     PID: 10092
  `-._    `-._  `-./  _.-'    _.-'
 |`-._`-._    `-.__.-'    _.-'_.-'|
 |    `-._`-._        _.-'_.-'    |           http://redis.io
  `-._    `-._`-.__.-'_.-'    _.-'
 |`-._`-._    `-.__.-'    _.-'_.-'|
 |    `-._`-._        _.-'_.-'    |
  `-._    `-._`-.__.-'_.-'    _.-'
      `-._    `-.__.-'    _.-'
          `-._        _.-'
              `-.__.-'

[10092] 01 Oct 15:05:45.656 # Server started, Redis version 2.6.12
[10092] 01 Oct 15:05:45.659 * DB loaded from disk: 0.001 seconds
[10092] 01 Oct 15:05:45.660 * The server is now ready to accept connections on port 6379

Tips of how to run Redis as a Windows Service is available in the Code Project article "Running Redis as a Windows Service": http://www.codeproject.com/Articles/715967/Running-Redis-as-a-Windows-Service Note: If you install Redis on a Linux system, it is possible to set up Redis as a service / daemon quite easily. Check the INSTALLATION notes. Already now you will be able to cache page data quite easily, that is the HTML and javascript/CSS content of the page. To do this, just add the [OutputCache] action attribute on the MVC controller action you want to cache a page:

     [OutputCache(Duration=5)]
        public ActionResult Index()
        {
            return View();
        }

In the example above, the page data is cached with a duration of five seconds. We need to do additional measures for caching image data in Redis, this is also possible. First we define a controller action to handle the parsing of image paths and do necessary actions to Redis caching:

using System.Linq;
using System.Web.Mvc;
using RedisCacheTest.Models;

namespace RedisCacheTest.Controllers
{
    public class ImagesController : Controller
    {
       
        [OutputCache(Duration=5)]
        public ImageResult ShowImage(string id)
        {
            if (string.IsNullOrEmpty(id))
                return null; 

            string[] fileComponents = id.Split('-');
            if (!fileComponents.Any() || fileComponents.Count() != 2)
                return null;

            ImageFileExtension imageFileExtension = ImageFileExtensionPatternProvider.GetImageFileExtension()
                .FirstOrDefault(x => x.Extension == fileComponents[1]);
            if (imageFileExtension == null)
                return null; 

            ImageResult result = new ImageResult(Url.Content("~/Content/Images/" + fileComponents[0] + "." + imageFileExtension.Extension), imageFileExtension.ContentType);
            return result; 
        }

         

    }
}


namespace RedisCacheTest.Models
{

    public class ImageFileExtension
    {

        public string Extension { get; set; }

        public string ContentType { get; set; }


    }

}

namespace RedisCacheTest.Models
{

    public static class ImageFileExtensionPatternProvider
    {

        public static ImageFileExtension[] GetImageFileExtension()
        {
            return new ImageFileExtension[]
            {
                new ImageFileExtension { Extension = "gif", ContentType = "image/gif" },
                new ImageFileExtension { Extension = "png", ContentType = "image/png"  },
                new ImageFileExtension { Extension = "jpeg", ContentType = "image/jpeg"  },
                new ImageFileExtension { Extension = "jpg", ContentType = "image/jpeg" },
                new ImageFileExtension { Extension = "tif", ContentType = "image/tif" },
                new ImageFileExtension { Extension = "tiff", ContentType = "image/tif" },
                new ImageFileExtension { Extension = "bmp", ContentType = "image/bmp" },
            };
        }

    }
}

As one can see from the MVC controller action we expect images to reside in the Content/Images folder of the MVC solution and we use the Url.Content method. Our <IMG> tag will use a specific syntax for paths in the SRC attribute (shown later in the article). Note that a key point here is that we have a controller action that uses also the [OutputCache] directive. We have defined some familiar image file extension and associated ContentType values. In addition we return a new kind of ActionResult, ImageResult. This is defined next:

using System;
using System.IO;
using System.Web.Mvc;

namespace RedisCacheTest
{
    
    public class ImageResult : ActionResult 
    {

        public string ContentType { get; set; }

        public byte[] ImageBytes { get; set;  }


        public string SourceFileName { get; set; }

        public ImageResult(string sourceFileName, string contentType)
        {
            SourceFileName = sourceFileName;
            ContentType = contentType; 
        }

        public ImageResult(byte[] sourceStream, String contentType)
        {
            ImageBytes = sourceStream;
            ContentType = contentType; 
        }

        public override void ExecuteResult(ControllerContext context)
        {
            var response = context.HttpContext.Response;
            response.Clear();
            response.ContentType = ContentType;

            if (ImageBytes != null)
            {
                var stream = new MemoryStream(ImageBytes);
                stream.WriteTo(response.OutputStream);
                stream.Dispose();
            }
            else
            {
                response.TransmitFile(SourceFileName); 
            }
        }

    }
}

The class ImageResult inherits from ActionResult and will return image data in its override of ExecuteResult. We grab hold of the response from the ControllerContext.HttpContext.Response property and use methods on this response object to either TransmitFile or write directly to the stream. Next off, we need a simple MVC page to test out the Redis Cache of both page data and image data:

@using System.Globalization
@{
    ViewBag.Title = "Home Page";
}

<style type="text/css">

img {
    border: 1px solid black;
}

</style>


<h4>CROATIA Images (Cached 5 seconds)</h4>


<h5>Last update:</h5>
@Html.Label(DateTime.Now.ToString(CultureInfo.InvariantCulture))



@{
    var rnd = new Random(); 
}

@foreach (var number in Enumerable.Range(1, 1000))
{
    var imageIndex = rnd.Next(1, 7);
    var filename = Url.Action("ShowImage", "Images", new { id = Url.Encode("Croatia" + imageIndex + "-jpg") });

    <img src="@filename" width="150" />     
}

We use the syntax for images to do the action /Images/ShowImage and provide as a parameter (id) the name of the file minus its extension and then dash its extension type, in this case "-jpg". The MVC controller action will then use output caching also for image data since we have tagged the MVC controller action with [OutputCache] action filter. Here is an image of the cached web page, which will be refreshed every five seconds with some nice images from Croatia:



Of course, now our Redis server instance got more caching up to do since it also caches image data:


The contents of the Redis server can be interrogated by typing the KEYS command:

keys * 

We can even be more specific of using Redis to cache image data for example for an expiration of 60 seconds and directly use RedisNativeClient to take control of which keys to use for saving the image data. The following adjusted ImageResult action result will directly take care of loading the image data into a byte array and store it to Redis, then retrieve the byte array if the same key is already defined. I set here an expiration of 60 seconds. This makes it possible to reflect changes in images for example every 60 seconds, since the key is then set to expired. We can see that Redis is a great technology to be used to both cache HTML, javascript, CSS and image data in Redis, making it possible to quickly load media-intensive web pages and web sites. Best of all, we offload the ASP.NET application server running say a ASP.NET web site or MVC web site and retrieve data from Redis.

using System;
using System.Configuration;
using System.Diagnostics;
using System.IO;
using System.Web.Mvc;
using ServiceStack.Redis;
using System.Web;
using System.Web.Configuration;

namespace RedisCacheTest
{
    
    public class ImageResult : ActionResult 
    {

        
        public string ContentType { get; set; }

        public byte[] ImageBytes { get; set;  }


        public string SourceFileName { get; set; }

        public ImageResult(string sourceFileName, string contentType)
        {
            SourceFileName = sourceFileName;
            ContentType = contentType; 
        }

        public ImageResult(byte[] sourceStream, String contentType)
        {
            ImageBytes = sourceStream;
            ContentType = contentType; 
        }

        public override void ExecuteResult(ControllerContext context)
        {
            var response = context.HttpContext.Response;
            response.Clear();
            response.ContentType = ContentType;

            CheckImageDataPreload();

            if (ImageBytes != null)
            {
                WriteImageDataToStream(response);
            }
            else
            {
                byte[] imageData = UpdateImagePreloadData(context);
                if (imageData != null)
                {
                    ImageBytes = imageData;
                    WriteImageDataToStream(response);
                }
                else
                {
                    response.TransmitFile(SourceFileName);
                    response.Flush();
                }
            }
        }

        private void WriteImageDataToStream(HttpResponseBase response)
        {
            var stream = new MemoryStream(ImageBytes);
            stream.WriteTo(response.OutputStream);
            stream.Dispose();
        }

        private byte[] UpdateImagePreloadData(ControllerContext context)
        {
            try
            {
                var imageFileName = context.HttpContext.Server.MapPath(SourceFileName);
                byte[] imageFileContents = File.ReadAllBytes(imageFileName);

                using (var redisClient = new RedisNativeClient())
                {
                    redisClient.Set(GetRedisImageKey(), imageFileContents);
                    redisClient.Expire(GetRedisImageKey(), 60);
                    return imageFileContents;
                }
            }
            catch (Exception err)
            {
                Debug.WriteLine(err.Message);
            }
            return null; 
        }

        private void CheckImageDataPreload()
        {
            try
            {
                using (var redisClient = new RedisNativeClient())
                {
                    string redisImageKey = GetRedisImageKey();
                    var imageData = redisClient.Get(redisImageKey);
                    if (imageData != null)
                    {
                        ImageBytes = imageData;
                    }
                }
            }
            catch (Exception err)
            {
                Debug.WriteLine(err.Message);
            }
        }

        private string GetRedisImageKey()
        {
            return RedisCacheConfiguratedApplicationName + "/IMAGEBANK" + SourceFileName;
        }

        private static readonly string RedisCacheConfiguratedApplicationName = GetRedisApplicationName();

        private static string GetRedisApplicationName()
        {
            var section = ConfigurationManager.GetSection("system.web/caching/outputCache") as OutputCacheSection;
            if (section != null)
            {
                if (section.Providers != null && section.Providers.Count > 0)
                {
                    ProviderSettings providerSettings = section.Providers[0];
                    if (providerSettings.Parameters["applicationName"] != null)
                    {

                        string applicationName = providerSettings.Parameters["applicationName"];
                        return applicationName;
                    }
                }
            }
            return "RedisApplication";
        } 


    }
}

In fact, we can now remove our [OutputCache] action attribute of the ShowImage action, since we now use directly Redis through RedisNativeClient to persist the image data to Redis. Actually I have seen a benefit of this method by auto-expiring images. With Redis you usually want to cache the data which many users request. ASP.NET Cache will actually accumulate its content into the Redis cache irrespective of the last time the key in the Redis cache was entered. By using the ImageResult ActionResult here that works directly with Redis, we can auto expire image data, reducing memory pressure on Redis server. We can configure Redis to limit the maximum amount of memory that Redis can use by setting the maxmemory setting. The following redis.conf file which is a Redis configuration file for 2.6 has been adjusted to have 300 MB max memory limit:

# Redis configuration file example

# Note on units: when memory size is needed, it is possible to specify
# it in the usual form of 1k 5GB 4M and so forth:
#
# 1k => 1000 bytes
# 1kb => 1024 bytes
# 1m => 1000000 bytes
# 1mb => 1024*1024 bytes
# 1g => 1000000000 bytes
# 1gb => 1024*1024*1024 bytes
#
# units are case insensitive so 1GB 1Gb 1gB are all the same.

# By default Redis does not run as a daemon. Use 'yes' if you need it.
# Note that Redis will write a pid file in /var/run/redis.pid when daemonized.
daemonize no

# When running daemonized, Redis writes a pid file in /var/run/redis.pid by
# default. You can specify a custom pid file location here.
pidfile /var/run/redis.pid

# Accept connections on the specified port, default is 6379.
# If port 0 is specified Redis will not listen on a TCP socket.
port 6379

# If you want you can bind a single interface, if the bind option is not
# specified all the interfaces will listen for incoming connections.
#
# bind 127.0.0.1

# Specify the path for the unix socket that will be used to listen for
# incoming connections. There is no default, so Redis will not listen
# on a unix socket when not specified.
#
# unixsocket /tmp/redis.sock
# unixsocketperm 755

# Close the connection after a client is idle for N seconds (0 to disable)
timeout 0

# TCP keepalive.
#
# If non-zero, use SO_KEEPALIVE to send TCP ACKs to clients in absence
# of communication. This is useful for two reasons:
#
# 1) Detect dead peers.
# 2) Take the connection alive from the point of view of network
#    equipment in the middle.
#
# On Linux, the specified value (in seconds) is the period used to send ACKs.
# Note that to close the connection the double of the time is needed.
# On other kernels the period depends on the kernel configuration.
#
# A reasonable value for this option is 60 seconds.
tcp-keepalive 0

# Specify the server verbosity level.
# This can be one of:
# debug (a lot of information, useful for development/testing)
# verbose (many rarely useful info, but not a mess like the debug level)
# notice (moderately verbose, what you want in production probably)
# warning (only very important / critical messages are logged)
loglevel notice

# Specify the log file name. Also 'stdout' can be used to force
# Redis to log on the standard output. Note that if you use standard
# output for logging but daemonize, logs will be sent to /dev/null
logfile stdout

# To enable logging to the system logger, just set 'syslog-enabled' to yes,
# and optionally update the other syslog parameters to suit your needs.
# syslog-enabled no

# Specify the syslog identity.
# syslog-ident redis

# Specify the syslog facility. Must be USER or between LOCAL0-LOCAL7.
# syslog-facility local0

# Set the number of databases. The default database is DB 0, you can select
# a different one on a per-connection basis using SELECT  where
# dbid is a number between 0 and 'databases'-1
databases 16

################################ SNAPSHOTTING  #################################
#
# Save the DB on disk:
#
#   save  
#
#   Will save the DB if both the given number of seconds and the given
#   number of write operations against the DB occurred.
#
#   In the example below the behaviour will be to save:
#   after 900 sec (15 min) if at least 1 key changed
#   after 300 sec (5 min) if at least 10 keys changed
#   after 60 sec if at least 10000 keys changed
#
#   Note: you can disable saving at all commenting all the "save" lines.
#
#   It is also possible to remove all the previously configured save
#   points by adding a save directive with a single empty string argument
#   like in the following example:
#
#   save ""

save 900 1
save 300 10
save 60 10000

# By default Redis will stop accepting writes if RDB snapshots are enabled
# (at least one save point) and the latest background save failed.
# This will make the user aware (in an hard way) that data is not persisting
# on disk properly, otherwise chances are that no one will notice and some
# distater will happen.
#
# If the background saving process will start working again Redis will
# automatically allow writes again.
#
# However if you have setup your proper monitoring of the Redis server
# and persistence, you may want to disable this feature so that Redis will
# continue to work as usually even if there are problems with disk,
# permissions, and so forth.
stop-writes-on-bgsave-error yes

# Compress string objects using LZF when dump .rdb databases?
# For default that's set to 'yes' as it's almost always a win.
# If you want to save some CPU in the saving child set it to 'no' but
# the dataset will likely be bigger if you have compressible values or keys.
rdbcompression yes

# Since version 5 of RDB a CRC64 checksum is placed at the end of the file.
# This makes the format more resistant to corruption but there is a performance
# hit to pay (around 10%) when saving and loading RDB files, so you can disable it
# for maximum performances.
#
# RDB files created with checksum disabled have a checksum of zero that will
# tell the loading code to skip the check.
rdbchecksum yes

# The filename where to dump the DB
dbfilename dump.rdb

# The working directory.
#
# The DB will be written inside this directory, with the filename specified
# above using the 'dbfilename' configuration directive.
# 
# The Append Only File will also be created inside this directory.
# 
# Note that you must specify a directory here, not a file name.
dir ./

################################# REPLICATION #################################

# Master-Slave replication. Use slaveof to make a Redis instance a copy of
# another Redis server. Note that the configuration is local to the slave
# so for example it is possible to configure the slave to save the DB with a
# different interval, or to listen to another port, and so on.
#
# slaveof  

# If the master is password protected (using the "requirepass" configuration
# directive below) it is possible to tell the slave to authenticate before
# starting the replication synchronization process, otherwise the master will
# refuse the slave request.
#
# masterauth 

# When a slave loses its connection with the master, or when the replication
# is still in progress, the slave can act in two different ways:
#
# 1) if slave-serve-stale-data is set to 'yes' (the default) the slave will
#    still reply to client requests, possibly with out of date data, or the
#    data set may just be empty if this is the first synchronization.
#
# 2) if slave-serve-stale-data is set to 'no' the slave will reply with
#    an error "SYNC with master in progress" to all the kind of commands
#    but to INFO and SLAVEOF.
#
slave-serve-stale-data yes

# You can configure a slave instance to accept writes or not. Writing against
# a slave instance may be useful to store some ephemeral data (because data
# written on a slave will be easily deleted after resync with the master) but
# may also cause problems if clients are writing to it because of a
# misconfiguration.
#
# Since Redis 2.6 by default slaves are read-only.
#
# Note: read only slaves are not designed to be exposed to untrusted clients
# on the internet. It's just a protection layer against misuse of the instance.
# Still a read only slave exports by default all the administrative commands
# such as CONFIG, DEBUG, and so forth. To a limited extend you can improve
# security of read only slaves using 'rename-command' to shadow all the
# administrative / dangerous commands.
slave-read-only yes

# Slaves send PINGs to server in a predefined interval. It's possible to change
# this interval with the repl_ping_slave_period option. The default value is 10
# seconds.
#
# repl-ping-slave-period 10

# The following option sets a timeout for both Bulk transfer I/O timeout and
# master data or ping response timeout. The default value is 60 seconds.
#
# It is important to make sure that this value is greater than the value
# specified for repl-ping-slave-period otherwise a timeout will be detected
# every time there is low traffic between the master and the slave.
#
# repl-timeout 60

# Disable TCP_NODELAY on the slave socket after SYNC?
#
# If you select "yes" Redis will use a smaller number of TCP packets and
# less bandwidth to send data to slaves. But this can add a delay for
# the data to appear on the slave side, up to 40 milliseconds with
# Linux kernels using a default configuration.
#
# If you select "no" the delay for data to appear on the slave side will
# be reduced but more bandwidth will be used for replication.
#
# By default we optimize for low latency, but in very high traffic conditions
# or when the master and slaves are many hops away, turning this to "yes" may
# be a good idea.
repl-disable-tcp-nodelay no

# The slave priority is an integer number published by Redis in the INFO output.
# It is used by Redis Sentinel in order to select a slave to promote into a
# master if the master is no longer working correctly.
#
# A slave with a low priority number is considered better for promotion, so
# for instance if there are three slaves with priority 10, 100, 25 Sentinel will
# pick the one wtih priority 10, that is the lowest.
#
# However a special priority of 0 marks the slave as not able to perform the
# role of master, so a slave with priority of 0 will never be selected by
# Redis Sentinel for promotion.
#
# By default the priority is 100.
slave-priority 100

################################## SECURITY ###################################

# Require clients to issue AUTH  before processing any other
# commands.  This might be useful in environments in which you do not trust
# others with access to the host running redis-server.
#
# This should stay commented out for backward compatibility and because most
# people do not need auth (e.g. they run their own servers).
# 
# Warning: since Redis is pretty fast an outside user can try up to
# 150k passwords per second against a good box. This means that you should
# use a very strong password otherwise it will be very easy to break.
#
# requirepass foobared

# Command renaming.
#
# It is possible to change the name of dangerous commands in a shared
# environment. For instance the CONFIG command may be renamed into something
# hard to guess so that it will still be available for internal-use tools
# but not available for general clients.
#
# Example:
#
# rename-command CONFIG b840fc02d524045429941cc15f59e41cb7be6c52
#
# It is also possible to completely kill a command by renaming it into
# an empty string:
#
# rename-command CONFIG ""
#
# Please note that changing the name of commands that are logged into the
# AOF file or transmitted to slaves may cause problems.

################################### LIMITS ####################################

# Set the max number of connected clients at the same time. By default
# this limit is set to 10000 clients, however if the Redis server is not
# able to configure the process file limit to allow for the specified limit
# the max number of allowed clients is set to the current file limit
# minus 32 (as Redis reserves a few file descriptors for internal uses).
#
# Once the limit is reached Redis will close all the new connections sending
# an error 'max number of clients reached'.
#
# maxclients 10000

# Don't use more memory than the specified amount of bytes.
# When the memory limit is reached Redis will try to remove keys
# accordingly to the eviction policy selected (see maxmemmory-policy).
#
# If Redis can't remove keys according to the policy, or if the policy is
# set to 'noeviction', Redis will start to reply with errors to commands
# that would use more memory, like SET, LPUSH, and so on, and will continue
# to reply to read-only commands like GET.
#
# This option is usually useful when using Redis as an LRU cache, or to set
# an hard memory limit for an instance (using the 'noeviction' policy).
#
# WARNING: If you have slaves attached to an instance with maxmemory on,
# the size of the output buffers needed to feed the slaves are subtracted
# from the used memory count, so that network problems / resyncs will
# not trigger a loop where keys are evicted, and in turn the output
# buffer of slaves is full with DELs of keys evicted triggering the deletion
# of more keys, and so forth until the database is completely emptied.
#
# In short... if you have slaves attached it is suggested that you set a lower
# limit for maxmemory so that there is some free RAM on the system for slave
# output buffers (but this is not needed if the policy is 'noeviction').
#
# maxmemory 

maxmemory 300mb

# MAXMEMORY POLICY: how Redis will select what to remove when maxmemory
# is reached. You can select among five behaviors:
# 
# volatile-lru -> remove the key with an expire set using an LRU algorithm
# allkeys-lru -> remove any key accordingly to the LRU algorithm
# volatile-random -> remove a random key with an expire set
# allkeys-random -> remove a random key, any key
# volatile-ttl -> remove the key with the nearest expire time (minor TTL)
# noeviction -> don't expire at all, just return an error on write operations
# 
# Note: with any of the above policies, Redis will return an error on write
#       operations, when there are not suitable keys for eviction.
#
#       At the date of writing this commands are: set setnx setex append
#       incr decr rpush lpush rpushx lpushx linsert lset rpoplpush sadd
#       sinter sinterstore sunion sunionstore sdiff sdiffstore zadd zincrby
#       zunionstore zinterstore hset hsetnx hmset hincrby incrby decrby
#       getset mset msetnx exec sort
#
# The default is:
#
# maxmemory-policy volatile-lru

# LRU and minimal TTL algorithms are not precise algorithms but approximated
# algorithms (in order to save memory), so you can select as well the sample
# size to check. For instance for default Redis will check three keys and
# pick the one that was used less recently, you can change the sample size
# using the following configuration directive.
#
# maxmemory-samples 3

############################## APPEND ONLY MODE ###############################

# By default Redis asynchronously dumps the dataset on disk. This mode is
# good enough in many applications, but an issue with the Redis process or
# a power outage may result into a few minutes of writes lost (depending on
# the configured save points).
#
# The Append Only File is an alternative persistence mode that provides
# much better durability. For instance using the default data fsync policy
# (see later in the config file) Redis can lose just one second of writes in a
# dramatic event like a server power outage, or a single write if something
# wrong with the Redis process itself happens, but the operating system is
# still running correctly.
#
# AOF and RDB persistence can be enabled at the same time without problems.
# If the AOF is enabled on startup Redis will load the AOF, that is the file
# with the better durability guarantees.
#
# Please check http://redis.io/topics/persistence for more information.

appendonly no

# The name of the append only file (default: "appendonly.aof")
# appendfilename appendonly.aof

# The fsync() call tells the Operating System to actually write data on disk
# instead to wait for more data in the output buffer. Some OS will really flush 
# data on disk, some other OS will just try to do it ASAP.
#
# Redis supports three different modes:
#
# no: don't fsync, just let the OS flush the data when it wants. Faster.
# always: fsync after every write to the append only log . Slow, Safest.
# everysec: fsync only one time every second. Compromise.
#
# The default is "everysec", as that's usually the right compromise between
# speed and data safety. It's up to you to understand if you can relax this to
# "no" that will let the operating system flush the output buffer when
# it wants, for better performances (but if you can live with the idea of
# some data loss consider the default persistence mode that's snapshotting),
# or on the contrary, use "always" that's very slow but a bit safer than
# everysec.
#
# More details please check the following article:
# http://antirez.com/post/redis-persistence-demystified.html
#
# If unsure, use "everysec".

# appendfsync always
appendfsync everysec
# appendfsync no

# When the AOF fsync policy is set to always or everysec, and a background
# saving process (a background save or AOF log background rewriting) is
# performing a lot of I/O against the disk, in some Linux configurations
# Redis may block too long on the fsync() call. Note that there is no fix for
# this currently, as even performing fsync in a different thread will block
# our synchronous write(2) call.
#
# In order to mitigate this problem it's possible to use the following option
# that will prevent fsync() from being called in the main process while a
# BGSAVE or BGREWRITEAOF is in progress.
#
# This means that while another child is saving, the durability of Redis is
# the same as "appendfsync none". In practical terms, this means that it is
# possible to lose up to 30 seconds of log in the worst scenario (with the
# default Linux settings).
# 
# If you have latency problems turn this to "yes". Otherwise leave it as
# "no" that is the safest pick from the point of view of durability.
no-appendfsync-on-rewrite no

# Automatic rewrite of the append only file.
# Redis is able to automatically rewrite the log file implicitly calling
# BGREWRITEAOF when the AOF log size grows by the specified percentage.
# 
# This is how it works: Redis remembers the size of the AOF file after the
# latest rewrite (if no rewrite has happened since the restart, the size of
# the AOF at startup is used).
#
# This base size is compared to the current size. If the current size is
# bigger than the specified percentage, the rewrite is triggered. Also
# you need to specify a minimal size for the AOF file to be rewritten, this
# is useful to avoid rewriting the AOF file even if the percentage increase
# is reached but it is still pretty small.
#
# Specify a percentage of zero in order to disable the automatic AOF
# rewrite feature.

auto-aof-rewrite-percentage 100
auto-aof-rewrite-min-size 64mb

################################ LUA SCRIPTING  ###############################

# Max execution time of a Lua script in milliseconds.
#
# If the maximum execution time is reached Redis will log that a script is
# still in execution after the maximum allowed time and will start to
# reply to queries with an error.
#
# When a long running script exceed the maximum execution time only the
# SCRIPT KILL and SHUTDOWN NOSAVE commands are available. The first can be
# used to stop a script that did not yet called write commands. The second
# is the only way to shut down the server in the case a write commands was
# already issue by the script but the user don't want to wait for the natural
# termination of the script.
#
# Set it to 0 or a negative value for unlimited execution without warnings.
lua-time-limit 5000

################################## SLOW LOG ###################################

# The Redis Slow Log is a system to log queries that exceeded a specified
# execution time. The execution time does not include the I/O operations
# like talking with the client, sending the reply and so forth,
# but just the time needed to actually execute the command (this is the only
# stage of command execution where the thread is blocked and can not serve
# other requests in the meantime).
# 
# You can configure the slow log with two parameters: one tells Redis
# what is the execution time, in microseconds, to exceed in order for the
# command to get logged, and the other parameter is the length of the
# slow log. When a new command is logged the oldest one is removed from the
# queue of logged commands.

# The following time is expressed in microseconds, so 1000000 is equivalent
# to one second. Note that a negative number disables the slow log, while
# a value of zero forces the logging of every command.
slowlog-log-slower-than 10000

# There is no limit to this length. Just be aware that it will consume memory.
# You can reclaim memory used by the slow log with SLOWLOG RESET.
slowlog-max-len 128

############################### ADVANCED CONFIG ###############################

# Hashes are encoded using a memory efficient data structure when they have a
# small number of entries, and the biggest entry does not exceed a given
# threshold. These thresholds can be configured using the following directives.
hash-max-ziplist-entries 512
hash-max-ziplist-value 64

# Similarly to hashes, small lists are also encoded in a special way in order
# to save a lot of space. The special representation is only used when
# you are under the following limits:
list-max-ziplist-entries 512
list-max-ziplist-value 64

# Sets have a special encoding in just one case: when a set is composed
# of just strings that happens to be integers in radix 10 in the range
# of 64 bit signed integers.
# The following configuration setting sets the limit in the size of the
# set in order to use this special memory saving encoding.
set-max-intset-entries 512

# Similarly to hashes and lists, sorted sets are also specially encoded in
# order to save a lot of space. This encoding is only used when the length and
# elements of a sorted set are below the following limits:
zset-max-ziplist-entries 128
zset-max-ziplist-value 64

# Active rehashing uses 1 millisecond every 100 milliseconds of CPU time in
# order to help rehashing the main Redis hash table (the one mapping top-level
# keys to values). The hash table implementation Redis uses (see dict.c)
# performs a lazy rehashing: the more operation you run into an hash table
# that is rehashing, the more rehashing "steps" are performed, so if the
# server is idle the rehashing is never complete and some more memory is used
# by the hash table.
# 
# The default is to use this millisecond 10 times every second in order to
# active rehashing the main dictionaries, freeing memory when possible.
#
# If unsure:
# use "activerehashing no" if you have hard latency requirements and it is
# not a good thing in your environment that Redis can reply form time to time
# to queries with 2 milliseconds delay.
#
# use "activerehashing yes" if you don't have such hard requirements but
# want to free memory asap when possible.
activerehashing yes

# The client output buffer limits can be used to force disconnection of clients
# that are not reading data from the server fast enough for some reason (a
# common reason is that a Pub/Sub client can't consume messages as fast as the
# publisher can produce them).
#
# The limit can be set differently for the three different classes of clients:
#
# normal -> normal clients
# slave  -> slave clients and MONITOR clients
# pubsub -> clients subcribed to at least one pubsub channel or pattern
#
# The syntax of every client-output-buffer-limit directive is the following:
#
# client-output-buffer-limit    
#
# A client is immediately disconnected once the hard limit is reached, or if
# the soft limit is reached and remains reached for the specified number of
# seconds (continuously).
# So for instance if the hard limit is 32 megabytes and the soft limit is
# 16 megabytes / 10 seconds, the client will get disconnected immediately
# if the size of the output buffers reach 32 megabytes, but will also get
# disconnected if the client reaches 16 megabytes and continuously overcomes
# the limit for 10 seconds.
#
# By default normal clients are not limited because they don't receive data
# without asking (in a push way), but just after a request, so only
# asynchronous clients may create a scenario where data is requested faster
# than it can read.
#
# Instead there is a default limit for pubsub and slave clients, since
# subscribers and slaves receive data in a push fashion.
#
# Both the hard or the soft limit can be disabled by setting them to zero.
client-output-buffer-limit normal 0 0 0
client-output-buffer-limit slave 256mb 64mb 60
client-output-buffer-limit pubsub 32mb 8mb 60

# Redis calls an internal function to perform many background tasks, like
# closing connections of clients in timeot, purging expired keys that are
# never requested, and so forth.
#
# Not all tasks are perforemd with the same frequency, but Redis checks for
# tasks to perform accordingly to the specified "hz" value.
#
# By default "hz" is set to 10. Raising the value will use more CPU when
# Redis is idle, but at the same time will make Redis more responsive when
# there are many keys expiring at the same time, and timeouts may be
# handled with more precision.
#
# The range is between 1 and 500, however a value over 100 is usually not
# a good idea. Most users should use the default of 10 and raise this up to
# 100 only in environments where very low latency is required.
hz 10

# When a child rewrites the AOF file, if the following option is enabled
# the file will be fsync-ed every 32 MB of data generated. This is useful
# in order to commit the file to the disk more incrementally and avoid
# big latency spikes.
#aof-rewrite-incremental-fsync yes

################################## INCLUDES ###################################

# Include one or more other config files here.  This is useful if you
# have a standard template that goes to all Redis server but also need
# to customize a few per-server settings.  Include files can include
# other files, so use this wisely.
#
# include /path/to/local.conf
# include /path/to/other.conf

We can now check our maxmemory setting by using the following Redis command:

config get maxmemory

To sum up, we now have a powerful way of caching page content and image data to Redis!

Download Visual Studio 2013 solution discussed in this article

Redis Cache VS 2013 Solution [ZIP] | 27,5 MB Tested with Redis 2.6 Windows binaries (64-bits edition): https://github.com/MSOpenTech/redis/tree/2.6/bin/release

C# and UTC timestamps

DateTime values are usually stored in a local datetime format using DateTime.Now This is sufficient if your system or application only runs in a specific time zone, but storing DateTime values can be erroneous since clients can adjust their time zones. If you save the timestamps as DateTime.Now, the value will be displayed wrong for users with a different time zone. Instead, use DateTime.UtcNow After saving the time stamp as UTC to a database for example, the value must be retrieved again and displayed. This is done with the method SpecifyKind(), passing in the datetime value and setting DateTimeKind to Utc. Finally the code must use the Utc Time packed value and use the method ToLocalTime to finally get the correct timestamp. The consistent use of Utc-time for time stamps and using the SpecifyKind method together with ToLocalTime method will display the time stamp correct on all clients, regardless of their time zone. After all, if you use DateTime.UtcNow and not Datetime.Now, the value displayed will be offset regarding of your DateTimeOffSet value (+/- hours). For users in UTC+00:00 time zone, no adjustment needs to be specified, but then other users in other time zones will not be able to show the time stamp correct. To sum up:

1. Use DateTime.UtcNow consistently for time stamps. Do not mix DateTime.Now and DateTime.UtcNow
2. "Pack" the DateTime value using the SpecifyKind() method.
3. Display the packed datetime value using ToLocalTime method.

//Define a static class to hold the extension method GetUtcDateTimePacked()
public static class DateTimeExtensions {
 
        /// 

        /// Packs the DateTime value packed into a new UtcTime 
        /// 
        public static DateTime GetUtcDateTimePacked(this DateTime dt)
        {
            DateTime convertedDate = DateTime.SpecifyKind(dt, DateTimeKind.Utc);
            return convertedDate;
        } 
		
}

void Main()
{	
	var sampleDt = DateTime.UtcNow; 
	Console.WriteLine(sampleDt.GetUtcDateTimePacked()); 
	Console.WriteLine(sampleDt.ToLocalTime()); 
}

The output given a UtcNow time of 28.09.2015 18:27:09 in a time zone with UTC +02:00 offset is then:
28.09.2015 16:27:09
28.09.2015 18:27:09
Note that we need to pack the datetime value and then use the ToLocalTime method. There you are - you can now use UTC DateTime values and display time stamps accross clients with different time zones. Now go code some more.