Unicomplex & Cube Bootstrapping

squbs comes with a default bootstrap class org.squbs.unicomplex.Bootstrap. This can be started from IDEs, command line, sbt, or even Maven. Bootstrap scans the class loader and finds META-INF/squbs-meta.<ext> in each loaded jar resource. If squbs metadata is available, the jar resource is treated as squbs cube or extension and initialized according to the metadata declarations. The bootstrap then first initializes extensions, cubes, then service handlers last regardless of their sequence in the classpath.

Given normal circumstances, bootstrapping detail are of not much significance. However, one may need to programmatically bootstrap squbs in different ways. This is especially common in test cases needing custom configuration and needing to run in parallel. Please see Testing for more information. The syntax for bootstrapping squbs is as follows:

Option 1) Start with user-defined configuration

  .createUsing {(name, config) => ActorSystem(name, config)}

Option 2) Start with default configuration

UnicomplexBoot {(name, config) => ActorSystem(name, config)}

Lets take a look at each component.

  1. Creating the UnicomplexBoot (boot) object. This can be done by passing a custom config or an actor system creator closure to UnicomplexBoot.apply().

  2. The configuration object shown as customConfig in the example above. This is a configuration object obtained from the Typesafe Config library's parse functions. This config object is not yet merged with reference.conf. It is optional and substitutes other application.conf configurations defined.

  3. The ActorSystem creator passes a function or closure to create the ActorSystem. The actual creation happens in the start phase (item 7, below). The default function is {(name, config) => ActorSystem(name, config)}. The input name is the intended ActorSystem name read from the configuration. The config is the loaded configuration object after merging with any provided config. Most use cases would want to create the ActorSystem this way and thus the function need not be provided. createUsing can be avoided altogether.

  4. Scanning components looking for cubes, services, or extensions using the scanResources() function. This is mandatory as there would otherwise be no components to start. If no arguments are passed, the squbs bootstrap will scan its class loader. Test cases may want to target certain components to scan only. This can be done by passing the location of additional squbs-meta.conf file locations (as a variable argument to scanResources), such as .scanResources("component1/META-INF/squbs-meta.conf", "component2/META-INF/squbs-meta.conf"). This will scan your classpath and the given resources in addition. If you do not want classpath to be scanned, pass withClassPath = false or just false as the first argument before the resource list: .scanResources(withClassPath = false, "component1/META-INF/squbs-meta.conf", "component2/META-INF/squbs-meta.conf")

  5. Initialize the extensions using the initExtension function. This will initialize the extensions scanned. Extension initialization is done before the ActorSystem is created. For multiple Unicomplex cases (multiple ActorSystems), the same extension must not be initialized more than once. An extension can only be used by one test case. In some test cases we do not want to initialize the extensions at all and would not call initExtension altogether.

  6. Stopping the JVM on exit. This is enabled by calling the stopJVMOnExit function. This option should generally not be used for test cases. It is used by squbs' bootstrap to make sure squbs shuts down and exits properly.

  7. Starting the Unicomplex by calling start(). This is a mandatory step. Without it no ActorSystem starts and no Actor would be able to run. The start call blocks until the system is fully up and running, or a timeout occurs. When start times out, some components may still be initializing, leaving the system in Initializing state. However, any single component failure will flip the system state to Failed at timeout. This would allow for system components like system diagnostics to run and complete. The default start timeout is set to 60 seconds. For tests that expect timeouts, it can be set lower by passing the desired timeout as an argument to start() such as start(Timeout(5 seconds)) or in short start(5 seconds) using implicit conversions from duration to a timeout.

Configuration Resolution

squbs chooses one application configuration and merges it with the aggregated application.conf and reference.conf in the classpath. The application configuration being merged are chosen from the following order.

  1. If a configuration is provided when creating the boot object, this configuration is chosen. This is the customConfig field from the example above.

  2. If an application.conf file is provided in the external config directory, this application.conf is chosen. The external config dir is configured by setting the config property squbs.external-config-dir and defaults to squbsconfig. Not that this directory cannot be changed or overridden by supplied configuration or an external configuration (as the directory itself is determined using the config property.)

  3. Otherwise, the application.conf provided with the application, if any, will be used. This then falls back to the reference.conf.

Drop-in Modular System

squbs divides applications into modules called cubes. Modules in squbs are intended to be run in modular isolation as well as on a flat classpath. Modular isolation is intended for true loose coupling of the modules, without incurring any classpath conflicts due to the dependencies.

The current implementation bootstraps from a flat classpath. On bootstrapping, squbs will automatically detect the modules by classpath scanning. Scanned cubes are detected and started automatically.

Cube Jars

All cubes are represented by a top-level jar file with the cube logic itself. All cubes must have the cube metadata residing in META-INF/squbs-meta.<ext>. Supported extensions are .conf, .json, and .properties. The format follows the Typesafe config format.

At the minimum, the cube metadata uniquely identifies the cube and version and declares and configures one or more of the followings:

Actor: Identifies the well known actors automatically started by squbs.

Service: Identifies a squbs service.

Extension: Identifies a squbs framework extension. The extension entry point has to extend from org.squbs.lifecycle.ExtensionLifecycle trait.

Configuration Resolution

Providing application.conf for a cube may cause issues when multiple cubes try to provide their internal application.conf. The precedence rules for merging such configuration is undefined. It is recommended cubes only provide a reference.conf and can be overridden by an external application.conf for deployment.

Well Known Actors

Well known actors are just Akka actors as defined by the Akka documentation. They are started by a supervisor actor that is created for each cube. The supervisor carries the name of the cube. Therefore any well known actor has a path of /<CubeName>/<ActorName> and can be looked up using the ActorSelection call under /user/<CubeName>/<ActorName>.

A well known actor can be started as a singleton actor or with a router. To declare a well known actor as a router, add: with-router = true in the actor declaration. Router, dispatcher, and mailbox configuration for well known actors are done in reference.conf or application.conf following the Akka documentation.

Following is a sample cube declaration META-INF/squbs-meta.conf declaring a well known actor:

cube-name = org.squbs.bottlecube
cube-version = "0.0.2"
squbs-actors = [
    class-name = org.squbs.bottlecube.LyricsDispatcher
    name = lyrics
    with-router = false  # Optional, defaults to false
    init-required = false # Optional

The init-required parameter is used for actors that need to signal back their fully initialized status for the system to be considered initialized. Please refer to the Startup Hooks section of the Runtime Lifecycles & API documentation for a full discussion of startup/initialization hooks.

If an actor is configured with-router (with-router = true) and a non-default dispatcher, the intention is usually to schedule the actor (routee) on the non-default dispatcher. The router will assume the well known actor name, not the routee (your actor implementation). A dispatcher set on the router will only affect the router, not the routee. To affect the routee, you need to create a separate configuration for the routees and append "/*" to the name. Next you want to configure the dispatcher in the routee section as the following example. {

  # Router configuration
  /bottlecube/lyrics {
    router = round-robin-pool
    resizer {
      lower-bound = 1
      upper-bound = 10

  # Routee configuration. Since it has a '*', the name has to be quoted.
  "/bottlecube/lyrics/*" {
    # Configure the dispatcher on the routee.
    dispatcher = blocking-dispatcher

Router concepts, examples, and configuration, are documented in the Akka documentation.


Services are described in full detail in Implementing HTTP(S) Services. Service metadata is declared in META-INF/squbs-meta.conf as shown in the following example:

cube-name = org.squbs.bottlesvc
cube-version = "0.0.2"
squbs-services = [
    class-name = org.squbs.bottlesvc.BottleSvc
    web-context = bottles # You can also specify bottles/v1, for instance.

    # The listeners entry is optional, and defaults to 'default-listener'.
    listeners = [ default-listener, my-listener ]

    # Optional, defaults to a default pipeline.
    pipeline = some-pipeline

    # Optional, disables the default pipeline if set to off.  If missing, it is set to on.
    defaultPipeline = on

    # Optional, only applies to actors.
    init-required = false

Please see Service Registration for a full description.


Extensions for squbs are low level facilities that need to be started for the environment. The extension initializer has to extend from the org.squbs.lifecycle.ExtensionLifecycle trait and override the proper callbacks. An extension has great capabilities to introspect the system and provide additional functionality squbs by itself does not provide. An extension must not be combined with an actor or a service in the same cube.

Extensions are started serially, one after another. Providers of extensions can provide a sequence number for the extension startup by specifying: sequence = [number] in the extension declaration. If the sequence is not specified, it defaults to Int.maxValue. This means it will start after all extensions that provide a sequence number. If there is more than one extension not specifying the sequence or specifying the same sequence number, the order between starting these is indeterministic. The shutdown order is the reverse of the startup order.

Shutting Down squbs

The squbs runtime can be properly shutdown by sending the Unicomplex() a GracefulStop message.

The default startup main method, org.squbs.unicomplex.Bootstrap, registers a JVM shutdown hook that sends a GracefulStop message to Unicomplex. Accodingly, if a squbs app is started by using the default main method, the system will be shutdown gracefully when JVM receives a SIGTERM.

If some other monitor process is responsible for shutting down the app, e.g. JSW, org.squbs.unicomplex.Shutdown can be set as the main method to gracefully shutdown the system. This Shutdown main method sends a GracefulStop message to Unicomplex as well.

In some use cases, it is desirable to add a delay to the shutdown. For instance, if a load balancer checks the health of the application every 5 seconds and the app is shutdown 1 second after a health check, the app will keep getting requests for the next 4 seconds until the next health check; however, it won't be able to serve these requests. If you are using one of the methods above, org.squbs.unicomplex.Bootstrap or org.squbs.unicomplex.Shutdown, you can add a delay to shutdown by adding the following in configuration:

squbs.shutdown-delay = 5 seconds

With the above configuration, the GracefulStop message to Unicomplex will be scheduled to be sent with a 5 second delay.

After receiving the GracefulStop message, the Unicomplex actor will stop the service and propagate the GracefulStop message to all cube supervisors. Each supervisor will be responsible for stopping the actors in its cube (by propagating the GracefulStop message to its children who wants to perform a graceful stop), ensure they stopped successfully or re-send a PoisonPill after timeout, and then stop itself. Once all cube supervisors and services are stopped, the squbs system shuts down. Then, a shutdown hook will be invoked to stop all the extensions and finally exits the JVM.

There is currently no standard console to a web container allowing users of squbs to build their own. The web console could provide proper user shutdown by sending a stop message to Unicomplex as follows:

  Unicomplex() ! GracefulStop