Application Initialization and the RequestSink

The only requirement of an Aqueduct application is that it has exactly one RequestSink subclass. This subclass handles the initialization of an application, including setting up routes, authorization and database connections.

A RequestSink subclass is declared in its own file named lib/<application_name>_request_sink.dart, which is exported from the application library file. (For example if the application is named wildfire, the application library file is lib/wildfire.dart.)


Applications are run with the command line tool aqueduct serve. This tool create multiple Isolates (a thread managed by the VM) and creates an instance of the RequestSink for each. Therefore, the channel of RequestControllers created by the RequestSink is replicated for each isolate. Requests are divvied up among each isolate to maximize CPU and resource usage.

Subclassing RequestSink

The responsibility of a RequestSink is to set up routes and initialize services it will use to fulfill requests. There are five initialization methods in RequestSink, each with its own purpose. The methods and the order they are executed in are as follows:

  1. RequestSink.initializeApplication: this static method is called once at the very beginning of an application's startup, before any instances of RequestSink are created.
  2. An instance of RequestSink is created with its default constructor.
  3. The method setupRouter is invoked on the RequestSink instance; initialized properties from the constructor are injected into controllers created here.
  4. The method willOpen is invoked on the RequestSink instance.
  5. The method didOpen is invoked on the RequestSink instance.

Only setupRouter is required, but it is extremely common to provide a constructor and implement RequestSink.initializeApplication. It is rare to use willOpen and rarer still to use didOpen.

Aqueduct applications will create more than one instance of RequestSink and repeat steps 2-5 for each instance. See a later section on multi-threading in Aqueduct applications.

The usage and details for each of these initialization methods is detailed in the following sections.

Use RequestSink.initializeApplication for One-Time Initialization

Since many instances of RequestSink will be created in an Aqueduct application, its instance methods and constructor will be called multiple times. An Aqueduct application may often have to execute one-time startup tasks that must not occur more than once. For this purpose, you may implement a static method with the following name and signature in an application's RequestSink:

static Future initializeApplication(ApplicationConfiguration config) async {
  ... do one time setup ...

A common use of this method is to set up services that will be shared across isolates or unique persistent connections to remote services. This method can modify ApplicationConfiguration prior to RequestSink instances being created. This allows services allocated during this one-time setup method can be accessible by each RequestSink instance.

For example:

static Future initializeApplication(ApplicationConfiguration config) async {        
  config.options["special item"] = "xyz";

RequestSink(ApplicationConfiguration config) {
  var parsedConfigValues = config.options["special item"]; // == xyz

In a more complex example, Aqueduct applications that use scribe will implement initializeApplication to optimize logging. scribe works by spawning a new isolate that writes log statements to disk or the console. Aqueduct isolates send their log messages to the logging isolate so that they can move on to more important things. The logging isolate is spawned in initializeApplication and a reference is stored in ApplicationConfiguration. Each RequestSink grabs the reference so that it can send messages to the logging isolate later.

It is important to note the behavior of isolates as it relates to Aqueduct and the initialization process. Each isolate has its own heap. initializeApplication is executed in the main isolate, whereas each RequestSink is instantiated in its own isolate. This means that any values stored in ApplicationConfiguration must be safe to pass across isolates - i.e., they can't contain references to closures.

Additionally, any static or global variables that are set in the main isolate will not be set in other isolates. Configuration types like HTTPCodecRepository do not share values across isolates. Therefore, they must be set up in the RequestSink constructor.

/// Do not do this!
static Future initializeApplication(ApplicationConfiguration config) async {        
  HTTPCodecRepository.add(new ContentType("application", "xml"), new XMLCodec());

Also, because static methods cannot be overridden in Dart, it is important that you ensure the name and signature of initializeApplication exactly matches what is shown in these code samples. The analyzer can't help you here, unfortunately.

Use RequestSink's Constructor to Initialize Service and Isolate-Specific Configurations

A service, in this context, is something your application will use to fulfill requests. A database connection is an example of a service. Services should be created the constructor of a RequestSink and stored as properties.

The constructor of a RequestSink must be unnamed and take a single argument of type ApplicationConfiguration. This instance of ApplicationConfiguration will have the same values as the instance in the previous initialization step (initializeApplication). The configuration contains a path to the configuration file that the application was started with (this defaults to config.yaml). This file often contains values that set up things like database connections:

The values in ApplicationConfiguration often contain the details for the services that should be created - like database connection information. Here is an example:

class MySink extends RequestSink {
  MySink(ApplicationConfiguration config) : super(config) {
    var appConfigValues = new MyConfig(config.configurationFilePath);
    var databaseConnection = new DatabaseConnection()
      ..connectionInfo = appConfigValues.database;

(See more details about using configuration files here.)

Isolate specific initialization should also be set in this method:

class MySink extends RequestSink {
  MySink(ApplicationConfiguration config) : super(config) {
    HTTPCodecRepository.add(new ContentType("application", "xml"), new XMLCodec());

All of the properties of a RequestSink should be initialized in its constructor. This allows the next phase of initialization - setting up routes - to inject these services into controllers. For example, a typical RequestSink will have some property that holds a database connection; this property should be initialized in the constructor.

A constructor should never call asynchronous functions. Some services require asynchronous initialization - e.g., a database connection has to connect to a database - but those must be fully initialized later. (See a later section on Lazy Services.)

Setting up Routes in setupRouter

Once a RequestSink is instantiated, its setupRouter method is invoked. This method takes a Router that you must configure with all of the routes your application will respond to. (See Routing for more details.)

When setting up routes, you will create many instances of RequestController. Any services these controllers need should be injected in their constructor. For example, Authorizers need an instance of AuthServer to validate a request. The following code is an example of this:

class MySink extends RequestSink {
  MySink(ApplicationConfiguration config) : super(config) {  
    authServer = new AuthServer(...);

  AuthServer authServer;

  void setupRouter(Router router) {
        .pipe(new Authorizer(authServer))
        .listen((req) => new Response.ok("Authorized!"));

This is the only time routes may be set up in an application, as the Router will restructure its registered routes into an optimized, immutable collection after this method is invoked.

You may not call any asynchronous functions in this this method.

After setupRouter has completed, the RequestSink.router property is set to the router this method configured.

Perform Asynchronous Initialization with willOpen

For any initialization that needs to occur asynchronously, you may override RequestSink.willOpen. This method is asynchronous, and the application will wait for this method to complete before sending any HTTP requests to the request sink. In general, you should avoid using this method and read the later section on Lazy Services.

Start Receiving Requests

Once an RequestSink sets up its routes and performs asynchronous initialization, the application will hook up the stream of HTTP requests to the RequestSink and data will start flowing. Just prior to this, one last method is invoked on RequestSink, didOpen. This method is a final callback to the RequestSink that indicates all initialization has completed.

Lazy Services

An Aqueduct application will probably communicate to other servers and databases. A RequestSink will have properties to represent these connections. Services like these must open a persistent network connection, a process that is asynchronous by nature. Following the initialization process of a RequestSink, it may then make sense to create the services in a constructor and then open them in willOpen.

However, an Aqueduct application will run for a long time. It is probable that connections it uses will occasionally be interrupted. If these connections are only opened when the application first starts, the application will not be able to reopen these connections without restarting it. This would be catastrophic.

For that reason, asynchronous services should manage their own opening behavior. For example, a database connection should open it when it is asked to execute a query. If it has a valid connection, it will go ahead and execute the query. Otherwise, it will establish the connection and then execute the query. The caller doesn't care - they get a Future with the desired data.

The pseudo-code looks something like this:

Future execute(String sql) async {
  if (connection == null || !connection.isAvailable) {
    connection = new Connection(...);
    await connection.open();

  return await connection.executeSQL(sql);

From the perspective of an HTTPController, it doesn't care about the underlying connection. It invokes execute, and the connection object figures out if it needs to establish a connection first:

@httpGet getThings() async {
  // May or may not create a new connection, but will either return
  // some things or throw an error.
  var things = await connection.execute("select * from things");


Multi-threaded Aqueduct Applications

Aqueduct applications can - and should - be spread across a number of threads. This allows an application to take advantage of multiple CPUs and serve requests faster. In Dart, threads are called isolates (and have some slight nuances to them that makes the different than a traditional thread). Spreading requests across isolates is an architectural tenet of Aqueduct applications.

When an application is started with aqueduct serve, a option indicates how many isolates the application should run on.

aqueduct serve --isolates 3

The number of isolates defaults to 3. An application will spawn that many isolates and create an instance of RequestSink for each. When an HTTP request is received, one of the isolates - and its RequestSink - will receive the request while the others will never see it. Each isolate works independently of each other, running as their own "web server" within a web server. Because a RequestSink initializes itself in the exact same way on each isolate, each isolate behaves exactly the same way.

An isolate can't share memory with another isolate. Therefore, each RequestSink instance has its own set of services, like database connections. This behavior also makes connection pooling implicit - the connections are effectively pooled by the fact that there is a pool of RequestSinks. If a RequestSink creates a database connection and an application is started with four isolates, there will be four database connections total.

However, there are times where you want your own pool or you want to share a single service across multiple isolates. For example, an API that must register with some other server (like in a system with an event bus) or must maintain a single persistent connection (like the error pipe to Apple's Push Notification Service or a streaming connection to Nest). These types of services should be instantiated in initializeApplication.

The Application Object

Hidden in all of this discussion is the Application<T> object. Because the aqueduct serve command manages creating Application<T> instances, your code rarely concerns itself with this type.

An Application<T> is the top-level object in an Aqueduct application; it setups up HTTP listeners and sends their requests to RequestSink instances. The Application<T> itself is just a generic container for RequestSinks; it doesn't do much other than kick everything off.

The application's start method will initialize at least one instance of the application's RequestSink. If something goes wrong during this initialization process, the application will throw an exception and halt starting the server. For example, setting up an invalid route in a RequestSink subclass would trigger this type of startup exception.

An Application<T> has a number of options that determine how it will listen for HTTP requests, such as which port it is listening on or the SSL certificate it will use. These values are available in the application's configuration property, an instance of ApplicationConfiguration.

Properties of an ApplicationConfiguration and Application<T> are provided through the aqueduct serve command-line options.