Last year I wrote an article about the right way to set up a Redis connection using StackExchange’s Redis client library. A lot of people found this useful, but at the same time the article went into a lot of detail in order to explain the dangers of doing this wrong. Also, there is a connection string format that’s a lot more concise.

So here’s how you set up a Redis connection using StackExchange.Redis, really quickly. If you need to just try this out quickly, you can grab a copy of Redis for Windows (just remember that this is not supported for production environments).

First, install the NuGet package for the Redis client library:

Install-Package StackExchange.Redis

Then, figure out what connection you need, and build a lazy factory for it (ideally the connection string should come from a configuration file):

        private static Lazy<ConnectionMultiplexer> conn
            = new Lazy<ConnectionMultiplexer>(
                () => ConnectionMultiplexer.Connect(
                    "localhost:6379,abortConnect=false,syncTimeout=3000"));

My original article goes into detail on why this lazy construct is necessary, but it is mainly because it guarantees thread safety, so the ConnectionMultiplexer will be created only once when it is needed (which is how it’s intended to be used).

You build up a connection string using a comma-separated sequence of configuration parameters (as an alternative to ConfigurationOptions in code, which the original article used). This is more concise but also makes configuration a lot easier.

At the very least, you should have one or more endpoints that you’ll connect to (6379 is the default port in case you leave it out), abortConnect=false to automatically reconnect in case a disconnection occurs (see my original article for details on this), and a reasonable syncTimeout in case some of your Redis operations take long.

The default for syncTimeout is 1 second (i.e. 1000, because the value in milliseconds), and operations against Redis should typically be a lot less than that. But we don’t work in an ideal world, and since Redis is single-threaded, expensive application commands against Redis or even internal Redis operations can cause commands to at times exceed this threshold and result in a timeout. In such cases, you don’t want an operation to fail because of a one-off spike, so just give it a little extra (3 seconds should be reasonable). However, if you get lots of timeouts, you should review your code and look for bottlenecks or blocking operations.

Once you have the means to create a connection (as above), just get the lazy value, and from it get a handle on one of the 16 Redis databases (by default it’s database 0 if not specified):

var db = conn.Value.GetDatabase();

I’ve seen a lot of code in the past that just calls GetDatabase() all the time, for each operation. That’s fine, because the Basic Usage documentation states that:

“The object returned from GetDatabase is a cheap pass-thru object, and does not need to be stored.”

Despite this, I see no point in having an unnecessary extra level of indirection in my code, so I like to store this and work directly with it. Your mileage may vary.

Once you’ve got hold of your Redis database, you can perform your regular Redis operations on it.

            db.StringSet("x", 1);
            var x = db.StringGet("x");

Update 25th October 2016: Just looking to quickly set up a Redis connection? Check out the followup article. Read on for a more detailed article on the topic.

StackExchange.Redis is a pretty good .NET client for Redis. Unfortunately, it can be a little bit tricky to use, and the existing documentation is far from comprehensive.

After installing StackExchange.Redis via NuGet, a Redis connection can be obtained via a special ConnectionMultiplexer object. Working with this is already tricky in itself, and many get this wrong. For instance, check out the implementation in this answer:

public static ConnectionMultiplexer RedisConnection;
public static IDatabase RedisCacheDb;

protected void Session_Start(object sender, EventArgs e)
    {
        if (ConfigurationManager.ConnectionStrings["RedisCache"] != null)
        {
            if (RedisConnection == null || !RedisConnection.IsConnected)
            {
                RedisConnection = ConnectionMultiplexer.Connect(ConfigurationManager.ConnectionStrings["RedisCache"].ConnectionString);
            }
            RedisCacheDb = RedisConnection.GetDatabase();
        }
    }

As I pointed out in my question, this is a bad form of lazy initialization because it lacks thread safety: multiple threads may get through the checks and initialize multiple connections, resulting in connection leaks.

It is not hard to prove that this code is leaky in multithreaded environments. First, let’s set up the ConfigurationOptions with a client name so that we can identify connections coming from our program:

        private static Lazy<ConfigurationOptions> configOptions
            = new Lazy<ConfigurationOptions>(() => 
            {
                var configOptions = new ConfigurationOptions();
                configOptions.EndPoints.Add("localhost:6379");
                configOptions.ClientName = "LeakyRedisConnection";
                configOptions.ConnectTimeout = 100000;
                configOptions.SyncTimeout = 100000;
                return configOptions;
            });

Then, we provide a property with the faulty lazy initialization:

        private static ConnectionMultiplexer conn;

        private static ConnectionMultiplexer LeakyConn
        {
            get
            {
                if (conn == null || !conn.IsConnected)
                    conn = ConnectionMultiplexer.Connect(configOptions.Value);
                return conn;
            }
        }

Finally, we write some code that runs some Redis stuff in parallel:

        static void Main(string[] args)
        {
            for (int i = 0; i < 3; i++)
            {
                Task.Run(() =>
                    {
                        var db = LeakyConn.GetDatabase();
                        Console.WriteLine(i);

                        string key = "key" + i;

                        db.StringSet(key, i);
                        Thread.Sleep(10);
                        string value = db.StringGet(key);
                    }
                );
            }

            Console.WriteLine("Done");
            Console.ReadLine();
        }

When the program does its work, even with just 3 iterations, we get a total of six connections (when normally a single ConnectionMultiplexer should have at most 2 physical connections):

Another approach from this answer is to use an exclusive lock:

private static ConnectionMultiplexer _redis;
private static readonly Object _multiplexerLock = new Object();

private void ConnectRedis()
{
    try
    {
        _redis = ConnectionMultiplexer.Connect("...<connection string here>...");
    }
    catch (Exception ex)
    {
        //exception handling goes here
    }
}


private ConnectionMultiplexer RedisMultiplexer
{
    get
    {
        lock (_multiplexerLock)
        {
            if (_redis == null || !_redis.IsConnected)
            {
                ConnectRedis();
            }
            return _redis;
        }
    }
}

However, since Redis is often used as a cache in highly concurrent web applications, this approach essentially forces code to degrade into something sequential, and has obvious performance implications.

The correct approach to using ConnectionMultiplexer is described by this answer. It involves use of Lazy<T> for thread-safe lazy initialization (see Jon Skeet’s article on Singletons). Additionally:

• It sets “abortConnect=false”, which means if the initial connect attempt fails, the ConnectionMultiplexer will silently retry in the background rather than throw an exception.
• It does not check the IsConnected property, since ConnectionMultiplexer will automatically retry in the background if the connection is dropped.

With this info, we can now fix our code:

        private static Lazy<ConfigurationOptions> configOptions
            = new Lazy<ConfigurationOptions>(() => 
            {
                var configOptions = new ConfigurationOptions();
                configOptions.EndPoints.Add("localhost:6379");
                configOptions.ClientName = "SafeRedisConnection";
                configOptions.ConnectTimeout = 100000;
                configOptions.SyncTimeout = 100000;
                configOptions.AbortOnConnectFail = false;
                return configOptions;
            });

        private static Lazy<ConnectionMultiplexer> conn
            = new Lazy<ConnectionMultiplexer>(
                () => ConnectionMultiplexer.Connect(configOptions.Value));

        private static ConnectionMultiplexer SafeConn
        {
            get
            {
                return conn.Value;
            }
        }

        static void Main(string[] args)
        {
            for (int i = 0; i < 3; i++)
            {
                Task.Run(() =>
                    {
                        var db = SafeConn.GetDatabase();
                        Console.WriteLine(i);

                        string key = "key" + i;

                        db.StringSet(key, i);
                        Thread.Sleep(10);
                        string value = db.StringGet(key);
                    }
                );
            }

            Console.WriteLine("Done");
            Console.ReadLine();
        }

If you run this, you’ll find that there are now only two physical connections generated by the application, which is normal.