dos

Dos - Step 3

This step will follow the same general path as Uno, but we'll also iterate on what we made there to bring some of the code into a more functional style.

`project.json` Changes

We only need to add the RethinkDB package to what we had at the end of step 2 (under dependencies):

1:	`"RethinkDb.Driver": "2.3.15"`

Run dotnet restore to pull in that dependency. Also, we'll bring across the entire Data directory we created in Uno during this step. We'll be able to use Table.cs and EnvironmentExtensions.cs as is (except for changing the namespace to Dos.Data).

Configurable Connection

Since appsettings.json is a .NET Core thing, we will not use it here. We can still use JSON to configure our connection, though; here's the data-config.json file:

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{
  "Hostname": "my-rethinkdb-server",
  "Database": "O2F2"
}

To support this, we'll need to change a couple of other things. First, in our DataConfig file, we'll add the following using statement and static method:

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using Newtonsoft.Json;
...
public static DataConfig FromJson(string json) => JsonConvert.DeserializeObject<DataConfig>(json);

Now we can create our configuration from this JSON once we read it in. We're also going to modify the CreateConnection() method; we'll also add some more usings and a support property.

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using System;
using System.Collections.Generic;
using System.Linq;
using static RethinkDb.Driver.Net.Connection;
...
private IEnumerable<Func<Builder, Builder>> Blocks
{
    get
    {
        yield return builder => null == Hostname ? builder : builder.Hostname(Hostname);
        yield return builder => 0 == Port ? builder : builder.Port(Port);
        yield return builder => null == AuthKey ? builder : builder.AuthKey(AuthKey);
        yield return builder => null == Database ? builder : builder.Db(Database);
        yield return builder => 0 == Timeout ? builder : builder.Timeout(Timeout);
    }
}

public IConnection CreateConnection() =>
    Blocks.Aggregate(RethinkDB.R.Connection(), (builder, block) => block(builder)).Connect();

If this is the first time you've seen code like this, you may be thinking "Why would we do something like that?" We're moving from an imperative style, like we used in Uno, to a more declarative style. Blocks is an enumeration (or sequence) where each item yielded takes a connection builder, and returns either the original connection builder it received or one that has been modified with a configuration parameter. If you didn't understand that last sentence, look at the code, then read it again; understanding this structure will pay dividends once we're knee-deep in F#.

Once you understand the Blocks property, take a look at the CreateConnection() method. This uses the LINQ method Aggregate, which takes two parameters: an initial state; and a function that will be given the current state and each item, and will return the "current" state after that item has been processed. If that makes no sense, imagine you had a sequence exSeq with the letters "A", "B", and "C". If you were to run var str = exSeq.Aggregate("", (state, letter) => String.Format("{0},{1}", state, letter));, str would hold the string ",A,B,C". The "initial state" is simply the starting value; but, every iteration must return a value of that same type.

Hopefully Aggregate is making sense at this point. Taking a forward-looking side trip - we're going to see it, with a different parameter order, as the fold function in our F# code. You've likely heard the term "map/reduce" - this describes a process where, given a data collection, you can transform it into a shape you need (map) and distill that data into the answer you need (reduce). (Yes, purists, this is a bit of a simplification.) F# provides map and reduce implementations for several collection types; however, reduce cannot produce a type different from that of the underlying collection - fold is what does that.

Back from our side trip, what this code does is:

Seeds Aggregate with RethinkDB.R.Connection(), which is an instance of Connection.Builder (Builder is a nested type within RethinkDb.Driver.Net.Connection; the using static makes it visible the way we've used it here.)
Loops through each item of our enumeration. Since each item is a Func<Builder, Builder>, we pass the item the current builder; it returns a builder that may have been further configured ("aggregating" our configuration).
Once the Aggregate has completed, we're ready to call Connect() on our connection builder, and return that from our method.

Seeing a more functional style with C# should help when we start seeing F# collections.

Dependency Injection

With Nancy, if you want to add forks to the SDHP, you have to provide a bootstrapper that will handle the startup code. For most purposes, the best way is to simply override DefaultNancyBootstrapper; that way, any code you don't provide will use the default, and you can call base methods from your overridden ones, so all the SDHP magic continues to work.

Here's the custom bootstrapper we'll use:

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namespace Dos
{
    using Dos.Data;
    using Nancy;
    using Nancy.TinyIoc;
    using System.IO;
    
    public class DosBootstrapper : DefaultNancyBootstrapper
    {
        public DosBootstrapper() : base() { }
        
        protected override void ConfigureApplicationContainer(TinyIoCContainer container)
        {
            base.ConfigureApplicationContainer(container);
            
            var cfg = DataConfig.FromJson(File.ReadAllText("data-config.json"));
            var conn = cfg.CreateConnection();
            conn.EstablishEnvironment(cfg.Database).GetAwaiter().GetResult();
            container.Register(conn);
        }
    }
}

This looks very similar to the code from the ASP.NET Core implementation, with the exception of how we're getting the configuration. We're all done except for two minor fixes. First, we need to tell Nancy to use this bootstrapper instead of the default. This is Startup.cs:

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public void Configure(IApplicationBuilder app) =>
    app.UseOwin(x => x.UseNancy(options => options.Bootstrapper = new DosBootstrapper()));

Finally, we need to specify that our data-config.json file should be copied to the output directory; otherwise, it will just sit on the hard drive while you scratch your head trying to figure out why your application can't connect. (Ask me how I know...) This change is in project.json, just under the emitEntryPoint declaration (included here for context):

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"emitEntryPoint": true,
"copyToOutput": {
  "include": "data-config.json"
}

At this point, you should be able to dotnet run and, once the console says that it's listening, you should be able to see the database, tables, and indexes in the O2F2 database.

Back to Step 3

objects () |> functions

Dos - Step 3

project.json Changes

Configurable Connection

Dependency Injection

`project.json` Changes