Dolittle Documentation

Getting started

WebAssembly support is to be considered experimental. Seeing that we have less experience with it than our server side and the fact that the underlying Mono support for WebAssembly being used is still in test stages, you’re at your own risk. However, that being said - we have so far had good experience with it on projects we’ve used, we just need a bit more time with it.

This section describes how you can get started with the Dolittle WebAssembly support. If you’re looking for a finished sample with everything set up, you can go here.

If you’re going to use all of Dolittle and not just the fundamentals part, we recommend you get familiar with Dolittle, its runtime, building blocks and SDK.

You can use bits and pieces and you do not need to take in all of Dolittle to be able to make use of our WebAssembly support.


The Dolittle WebAssembly support can be used from do different perspectives:

  • Want to have a client / server version and be able to go to client and be offline, sharing code between them
  • Client only - no server, or the code on the server is different from the client

Today, the WebAssembly support is oriented around .NET and leveraging the work of the Mono Project. On top of this you’ll need a Web based project based on the framework of your liking, be it a Single Page Framework or server side rendering. The mechanics for the Web part is standard and totally agnostic. However, since we at Dolittle internally uses Aurelia, we’ve added extra love for Aurelia to make it even simpler for hooking up things.

It is recommended that you have a look at our Quickstart and our walktrough, as this guide builds upon its shoulders.

For the time being, we don’t have boilerplates for WebAssembly, and that means we need to do a bit more manual work to get started.

If you want to get a video walkthrough, have a look below:

.NET Core

At the root of it sits .NET Core. All our support is built around this with both the code for it to run and the tooling around it. So to get started you’ll need to do create a classlib representing the starting point. Create a folder for your client e.g. Client.

$ dotnet new classlib

The Mono SDK for WebAssembly has been packaged into a single NuGet package, called Dolittle.Interaction.WebAssembly.Core. This is a heavy package with everything needed for Monos WebAssembly support inside it.

Dolittle has a specific build pipeline, and we do need that as well. This walkthrough also assumes the Dolittle.SDK and also assumes Autofac as the IoC container, MongoDB as database and our own development Event Store.

After creating the classlib, Add the following dependencies to your .csproj file:

    <PackageReference Include="Dolittle.SDK" Version="3.*" />

    <PackageReference Include="Dolittle.DependencyInversion.Autofac" Version="3.*" />

    <PackageReference Include="Dolittle.Interaction.WebAssembly" Version="3.*" />
    <PackageReference Include="Dolittle.Interaction.WebAssembly.Core" Version="3.*" />

    <PackageReference Include="Dolittle.Events.WebAssembly.Dev" Version="3.*" />
    <PackageReference Include="Dolittle.ReadModels.MongoDB.WebAssembly" Version="3.*" />

    <!-- Build pipeline -->
    <PackageReference Include="Dolittle.Build.MSBuild" Version="3.*"/>
    <PackageReference Include="Dolittle.SDK.Build" Version="3.*" />
    <PackageReference Include="Dolittle.Interaction.WebAssembly.Build" Version="3.*" />

Dolittle has a specific build pipeline that does a few things, for instance it generates metadata about your application in the form of a topology and all artifacts within that topology. The topology is a map of the application in Dolittle terminology of Application, Bounded Context, Modules and Features. Within these are artifacts. The SDK build tool is responsible for creating this information and then an additional build tool for our WebAssembly stack sits on top of this and embeds the information into the application being built.

The SDK part can be configured, according to the documentation found here. For the most part we can probably leave it with the default values. All depending on what your starting point is and what you’re trying to achieve.

Below is an example of setting up the properties in your .csproj

    <!-- The relative path from this .csproj file to the bounded-context.json configuration file -->

    <!-- Whether or not to use modules or not when generating bounded context topology structure -->

    <!--  A | separated Key/Value pair map of namespace segments to strip -->

    <!-- Whether or not the build tool should generate proxies -->

    <!-- The relative path to put proxies if generated-->

    <!-- The relative path where the .dolittle folder is located for generating artifacts, 
    this can be taken out if you're not having a side-by-side server scenario where you're sharing
    artifacts/topology -->
The Dolittle tooling relies on a configuration file called bounded-context.json, this governs the types of resources being used for things like Read Models and Event Store. This configuration file does not have the dimension of types of runtime environments such as WebAssembly as an interaction layer. You therefor need a copy of this file if you want to support both a server/client scenario and a WebAssembly scenario side-by-side for the same codebase.


The next thing we need is a class holding the entrypoint of the application. Add a C# file called Program.cs.

We then need the following using statement:

using Dolittle.Booting;

Add a static Main() method:

static void Main(string[] args)

Inside this we can then configure Dolittle and the Bootloader.

var bootResult = Bootloader.Configure(_ => _

You should then have a file looking like something like this.

using Dolittle.Booting;

namespace Client

    class Program
        static void Main(string[] args)
            var bootResult = Bootloader.Configure(_ => _

Being early days, not all moving parts are in place. As you can notice, the .NoLogging() option will give no logging. If you want to have logging on with the Dolittle logging infrastructure, you will have to add your own ILogAppender implementation.

Logging has a noticeable impact on performance as well, so in production you really don’t want to have logging on.

To enable logging using the Dolittle logging infrastructure, add a CustomLogAppender file to your project and add the following code:

using Dolittle.Logging;

public class CustomLogAppender : ILogAppender
    public void Append(string filePath, int lineNumber, string member, LogLevel level, string message, Exception exception = null)
        if (exception != null)
            System.Console.WriteLine($"{System.DateTime.UtcNow} - ({filePath} - {member}[{lineNumber}]) - {level} - {message} - {exception.Message} - {exception.StackTrace}");
            System.Console.WriteLine($"{System.DateTime.UtcNow} - ({filePath} - {member}[{lineNumber}]) - {level} - {message}");

Instead of the .NoLogging() you can then do .UseLogAppender(new CustomLogAppender())

You can now build using dotnet build from your shell and you should be seeing at the end of the build something similar:

Perform post build tasks
    Copying files for output (Post Task: 'Dolittle.Interaction.WebAssembly.Build.CopyFiles, Dolittle.Interaction.WebAssembly.Build, Version=, Culture=neutral, PublicKeyToken=null')
      Copied 191 to output folder './publish/managed'
      Copied 104 to output folder './publish/managed'
      Copied 1 to output folder './publish'
      Copied 2 to output folder './publish'

  Time Elapsed 0:00:00:02.2163990 (Dolittle)

Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:07.31

This means that all the artifacts from the WebAssembly side is now available in the ./publish folder. This is what we want to make use of in the Web solution.



In the publish folder you’ll find a file called mono-config.js. This is an optional file, if you just want to add a <script/> tag and get started without module loading or WebPack. You simply just include it in your HTML file.


The assemblies.json file in the publish folder holds a list of all the assemblies to include in loading, and if you built the .NET solution with the default debug configuration, it will also include the PDB - debugging files. This file can be used directly in your JavaScript code by importing it and use the array directly when configuring Mono to start.

Service Worker

If you want to support an offline scenario, you want to make all the files needed to run available for offline use. We use a service worker for this purpose. Luckily, we have a WebPack plugin that takes information coming from the .NET build and generates the necessary file needed for this purpose.

All you need to do is add the package to your NPM based Web project:

$ npm install --save-dev @dolittle/webassembly.webpack

…or with yarn

$ yarn install -D @dolittle/webassembly.webpack

All you then need to do is import the plugin and add it to the plugins.

const { ServiceWorkerGenerator } = require('@dolittle/webassembly.webpack');

// Assuming you have an instance of your WebPack config object
config.plugins.push(new ServiceWorkerGenerator());


In WebPack you can also leverage the copy-webpack-plugin to copy the necessary files from the Client .NET projects output:

const CopyWebpackPlugin = require('copy-webpack-plugin');

// Assuming you have an instance of your WebPack config object
config.plugins.push(new CopyWebpackPlugin([
    { from: 'publish/managed/**/*', to: 'managed', flatten: true },
    { from: 'publish/mono.js', to: 'mono.js', flatten: true },
    { from: 'publish/mono.wasm', to: 'mono.wasm', flatten: true },
    { from: 'manifest.json', to: 'manifest.json', flatten: true },
    { from: 'dolittle.png', to: 'dolittle.png', flatten: true }


As mentioned, we have a specific Aurelia plugin to make things easier for Aurelia developers. You simply add a reference to it in your project and configure it as below:

import * as assemblies from '../publish/assemblies.json';

export function configure(aurelia) {
        .plugin(PLATFORM.moduleName('@dolittle/webassembly.aurelia'), {
            entryPoint: "[Client] Client.Program:Main", // Method to call to start your application
                                                        // Format:
                                                        // [Assembly] Namespace.Class:Method
            assemblies: assemblies.default,
            offline: false

Non Aurelia

The plugin for Aurelia simplifies things for developers using Aurelia - what it does can be seen here.

Basically, it gets the Mono WASM runtime initialized and also sets up the Dolittle EventStore and gets the MongoDB/Minimongo initialized and ready to be used.

Configuring Mono in the browser

import { EventProcessorOffsetRepository } from '@dolittle/';
import { storage } from '@dolittle/';
import * as assemblies from '../../Client/publish/assemblies.json';

        .then(_ => EventProcessorOffsetRepository.preload())
        .then(_ => EventStore.preload()).catch(error => {
    console.error('Error preloading', error);
}).then(_ => {
    // Now the rest of the system should be ready for booting
    window.Module = {};

    window.Module.onRuntimeInitialized = () => {
            'managed',          // Virtual filesystem prefix
            'managed',          // folder to look for managed DLLs
            0,                  // Wether or not to enable debugging (0: false, 1: true)
            assemblies,         // 
            () => {
                BINDING.call_static_method("[Client] Client.Program:Main", []); // Method to call to start your application
                                                                                // Format:
                                                                                // [Assembly] Namespace.Class:Method

    let monoScript = document.createElement('script');
    monoScript.async = true;
    monoScript.src = config.monoScript;

    if (config.offline === true) {

    window._mongoDB = new MongoDB();
    window._eventStore = {
        eventProcessorOffsetRepository: new EventProcessorOffsetRepository(),
        eventStore: new EventStore()
Right now, our interop uses global variables. This is something we’re looking into improving. As you can see in code above, it adds _mongoDB and _eventStore as global variables. These are needed for the C# implementations to be able to work with the underlying data storage mechanism in the browser; IndexedDB.