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An extension to WildFly will likely want to make use of services provided by the WildFly kernel, may want to make use of services provided by other subsystems, and may wish to make functionality available to other extensions. Each of these cases involves integration between different parts of the system. In releases prior to WildFly 10, this kind of integration was done on an ad-hoc basis, resulting in overly tight coupling between different parts of the system and overly weak integration contracts. For example, a service installed by subsystem A might depend on a service installed by subsystem B, and to record that dependency A's authors copy a ServiceName from B's code, or even refer to a constant or static method from B's code. The result is B's code cannot evolve without risking breaking A. And the authors of B may not even intend for other subsystems to use its services. There is no proper integration contract between the two subsystems.

Beginning with WildFly Core 2 and WildFly 10 the WildFly kernel's management layer provides a mechanism for allowing different parts of the system to integrate with each other in a loosely coupled manner. This is done via WildFly Capabilities. Use of capabilities provides the following benefits:

  1. A standard way for system components to define integration contracts for their use by other system components.
  2. A standard way for system components to access integration contracts provided by other system components.
  3. A mechanism for configuration model referential integrity checking, such that if one component's configuration has an attribute that refers to an other component (e.g. a socket-binding attribute in a subsystem that opens a socket referring to that socket's configuration), the validity of that reference can be checked when validating the configuration model.

Capabilities

A capability is a piece of functionality used in a WildFly Core based process that is exposed via the WildFly Core management layer. Capabilities may depend on other capabilities, and this interaction between capabilities is mediated by the WildFly Core management layer.

Some capabilities are automatically part of a WildFly Core based process, but in most cases the configuration provided by the end user (i.e. in standalone.xml, domain.xml and host.xml) determines what capabilities are present at runtime. It is the responsibility of the handlers for management operations to register capabilities and to register any requirements those capabilities may have for the presence of other capabilities. This registration is done during the MODEL stage of operation execution

A capability has the following basic characteristics:

  1. It has a name.
  2. It may install an MSC service that can be depended upon by services installed by other capabilities. If it does, it provides a mechanism for discovering the name of that service.
  3. It may expose some other API not based on service dependencies allowing other capabilities to integrate with it at runtime.
  4. It may depend on, or require other capabilities.

During boot of the process, and thereafter whenever a management operation makes a change to the process' configuration, at the end of the MODEL stage of operation execution the kernel management layer will validate that all capabilities required by other capabilities are present, and will fail any management operation step that introduced an unresolvable requirement. This will be done before execution of the management operation proceeds to the RUNTIME stage, where interaction with the process' MSC Service Container is done. As a result, in the RUNTIME stage the handler for an operation can safely assume that the runtime services provided by a capability for which it has registered a requirement are available.

Comparison to other concepts

Capabilities vs modules

A JBoss Modules module is the means of making resources available to the classloading system of a WildFly Core based process. To make a capability available, you must package its resources in one or more modules and make them available to the classloading system. But a module is not a capability in and of itself, and simply copying a module to a WildFly installation does not mean a capability is available. Modules can include resources completely unrelated to management capabilities.

Capabilities vs Extensions

An extension is the means by which the WildFly Core management layer is made aware of manageable functionality that is not part of the WildFly Core kernel. The extension registers with the kernel new management resource types and handlers for operations on those resources. One of the things a handler can do is register or unregister a capability and its requirements. An extension may register a single capability, multiple capabilities, or possibly none at all. Further, not all capabilities are registered by extensions; the WildFly Core kernel itself may register a number of different capabilities.

Capability Names

Capability names are simple strings, with the dot character serving as a separator to allow namespacing.

The 'org.wildfly' namespace is reserved for projects associated with the WildFly organization on github (https://github.com/wildfly).

Statically vs Dynamically Named Capabilities

The full name of a capability is either statically known, or it may include a statically known base element and then a dynamic element. The dynamic part of the name is determined at runtime based on the address of the management resource that registers the capability. For example, the management resource at the address '/socket-binding-group=standard-sockets/socket-binding=web' will register a dynamically named capability named 'org.wildlfy.network.socket-binding.web'. The 'org.wildlfy.network.socket-binding' portion is the static part of the name.

All dynamically named capabilities that have the same static portion of their name should provide a consistent feature set and set of requirements.

Service provided by a capability

Typically a capability functions by registering a service with the WildFly process' MSC ServiceContainer, and then dependent capabilities depend on that service. The WildFly Core management layer orchestrates registration of those services and service dependencies by providing a means to discover service names.

Custom integration APIs provided by a capability

Instead of or in addition to providing MSC services, a capability may expose some other API to dependent capabilities. This API must be encapsulated in a single class (although that class can use other non-JRE classes as method parameters or return types).

Capability Requirements

A capability may rely on other capabilities in order to provide its functionality at runtime. The management operation handlers that register capabilities are also required to register their requirements.

There are three basic types of requirements a capability may have:

  • Hard requirements. The required capability must always be present for the dependent capability to function.
  • Optional requirements. Some aspect of the configuration of the dependent capability controls whether the depended on capability is actually necessary. So the requirement cannot be known until the running configuration is analyzed.
  • Runtime-only requirements. The dependent capability will check for the presence of the depended upon capability at runtime, and if present it will utilize it, but if it is not present it will function properly without the capability. There is nothing in the dependent capability's configuration that controls whether the depended on capability must be present. Only capabilities that declare themselves as being suitable for use as a runtime-only requirement should be depended upon in this manner.

Hard and optional requirements may be for either statically named or dynamically named capabilities. Runtime-only requirements can only be for statically named capabilities, as such a requirement cannot be specified via configuration, and without configuration the dynamic part of the required capability name is unknown.

Supporting runtime-only requirements

Not all capabilities are usable as a runtime-only requirement.

Any dynamically named capability is not usable as a runtime-only requirement.

For a capability to support use as a runtime-only requirement, it must guarantee that a configuration change to a running process that removes the capability will not impact currently running capabilities that have a runtime-only requirement for it. This means:

  • A capability that supports runtime-only usage must ensure that it never removes its runtime service except via a full process reload.
  • A capability that exposes a custom integration API generally is not usable as a runtime-only requirement. If such a capability does support use as a runtime-only requirement, it must ensure that any functionality provided via its integration API remains available as long as a full process reload has not occurred.

Capability Contract

A capability provides a stable contract to users of the capability. The contract includes the following:

  • The name of the capability (including whether it is dynamically named).
  • Whether it installs an MSC Service, and if it does, the value type of the service. That value type then becomes a stable API users of the capability can rely upon.
  • Whether it provides a custom integration API, and if it does, the type that represents that API. That type then becomes a stable API users of the capability can rely upon.
  • Whether the capability supports use as a runtime-only requirement.

Developers can learn about available capabilities and the contracts they provide by reading the WildFly capabilty registry.

Capability Registry

The WildFly organization on github maintains a git repo where information about available capabilities is published.

https://github.com/wildfly/wildfly-capabilities

Developers can learn about available capabilities and the contracts they provide by reading the WildFly capabilty registry.

The README.md file at the root of that repo explains the how to find out information about the registry.

Developers of new capabilities are strongly encouraged to document and register their capability by submitting a pull request to the wildfly-capabilities github repo. This both allows others to learn about your capability and helps prevent capability name collisions. Capabilities that are used in the WildFly or WildFly Core code base itself must have a registry entry before the code referencing them will be merged.

External organizations that create capabilities should include an organization-specific namespace as part their capability names to avoid name collisions.

Using Capabilities

Now that all the background information is presented, here are some specifics about how to use WildFly capabilities in your code.

Basics of Using Your Own Capability

Creating your capability

A capability is an instance of the immutable org.jboss.as.controller.capability.RuntimeCapability class. A capability is usually registered by a resource, so the usual way to use one is to store it in constant in the resource's ResourceDefinition. Use a RuntimeCapability.Builder to create one.

That creates a statically named capability named com.example.foo.

If the capability is dynamically named, add the dynamic parameter to state this:

Most capabilities install a service that requiring capabilities can depend on. If your capability does this, you need to declare the service's value type (the type of the object returned by org.jboss.msc.Service.getValue()). For example, if FOO_CAPABILITY provides a Service<javax.sql.DataSource>:

For a dynamic capability:

If the capability provides a custom integration API, you need to instantiate an instance of that API:

and provide it to the builder:

For a dynamic capability:

A capability can provide both a custom integration API and install a service:

Registering and unregistering your capability

Once you have your capability, you need to ensure it gets registered with the WildFly Core kernel when your resource is added. This is easily done simply by providing a reference to the capability to the resource's ResourceDefinition. This assumes your resource definition is a subclass of the standard org.jboss.as.controller.SimpleResourceDefinition. SimpleResourceDefinition provides a Parameters class that provides a builder-style API for setting up all the data needed by your definition. This includes a setCapabilities method that can be used to declare the capabilities provided by resources of this type.

Your add handler needs to extend the standard org.jboss.as.controller.AbstractAddStepHandler class or one of its subclasses:

AbstractAddStepHandler's logic will register the capability when it executes.

Your remove handler must also extend of the standard org.jboss.as.controller.AbstractRemoveStepHandler or one of its subclasses.

AbstractRemoveStepHandler's logic will deregister the capability when it executes.

If for some reason you cannot base your ResourceDefinition on SimpleResourceDefinition or your handlers on AbstractAddStepHandler and AbstractRemoveStepHandler then you will need to take responsibility for registering the capability yourself. This is not expected to be a common situation. See the implementation of those classes to see how to do it.

Installing, accessing and removing the service provided by your capability

If your capability installs a service, you should use the RuntimeCapability when you need to determine the service's name. For example in the Stage.RUNTIME handling of your "add" step handler. Here's an example for a statically named capability:

If the capability is dynamically named, get the dynamic part of the name from the OperationContext and use that when getting the service name:

The same patterns should be used when accessing or removing the service in handlers for remove, write-attribute and custom operations.

If you use ServiceRemoveStepHandler for the remove operation, simply provide your RuntimeCapability to the ServiceRemoveStepHandler constructor and it will automatically remove your capability's service when it executes.

Basics of Using Other Capabilities

When a capability needs another capability, it only refers to it by its string name. A capability should not reference the RuntimeCapability object of another capability.

Before a capability can look up the service name for a required capability's service, or access its custom integration API, it must first register a requirement for the capability. This must be done in Stage.MODEL, while service name lookups and accessing the custom integration API is done in Stage.RUNTIME.

Registering a requirement for a capability is simple.

Registering a hard requirement for a static capability

If your capability has a hard requirement for a statically named capability, simply declare that to the builder for your RuntimeCapability. For example, WildFly's JTS capability requires both a basic transaction support capability and IIOP capabilities:

When your capability is registered with the system, the WildFly Core kernel will automatically register any static hard requirements declared this way.

Registering a requirement for a dynamically named capability

If the capability you require is dynamically named, usually your capability's resource will include an attribute whose value is the dynamic part of the required capability's name. You should declare this fact in the AttributeDefinition for the attribute using the SimpleAttributeDefinitionBuilder.setCapabilityReference method.

For example, the WildFly "remoting" subsystem's "org.wildfly.remoting.connector" capability has a requirement for a dynamically named socket-binding capability:

If the "add" operation handler for your resource extends AbstractAddStepHandler and the handler for write-attribute extends AbstractWriteAttributeHandler, the declaration above is sufficient to ensure that the appropriate capability requirement will be registered when the attribute is modified.

Depending upon a service provided by another capability

Once the requirement for the capability is registered, your OperationStepHandler can use the OperationContext to discover the name of the service provided by the required capability.

For example, the "add" handler for a remoting connector uses the OperationContext to find the name of the needed {{SocketBinding} service:

That service name is then used to add a dependency on the SocketBinding service to the remoting connector service.

If the required capability isn't dynamically named, OperationContext exposes an overloaded getCapabilityServiceName variant. For example, if a capability requires a remoting Endpoint:

Using a custom integration API provided by another capability

In your Stage.RUNTIME handler, use OperationContext.getCapabilityRuntimeAPI to get a reference to the required capability's custom integration API. Then use it as necessary.

Runtime-only requirements

If your capability has a runtime-only requirement for another capability, that means that if that capability is present in Stage.RUNTIME you'll use it, and if not you won't. There is nothing about the configuration of your capability that triggers the need for the other capability; you'll just use it if it's there.

In this case, use OperationContext.hasOptionalCapability in your Stage.RUNTIME handler to check if the capability is present:

The WildFly Core kernel will not register a requirement for the "com.example.bar" capability, so if a configuration change occurs that means that capability will no longer be present, that change will not be rolled back. Because of this, runtime-only requirements can only be used with capabilities that declare in their contract that they support such use.

Using a capability in a DeploymentUnitProcessor

A DeploymentUnitProcessor is likely to have a need to interact with capabilities, in order to create service dependencies from a deployment service to a capability provided service or to access some aspect of a capability's custom integration API that relates to deployments.

If a DeploymentUnitProcessor associated with a capability implementation needs to utilize its own capability object, the DeploymentUnitProcessor authors should simply provide it with a reference to the RuntimeCapability instance. Service name lookups or access to the capabilities custom integration API can then be performed by invoking the methods on the RuntimeCapability.

If you need to access service names or a custom integration API associated with a different capability, you will need to use the org.jboss.as.controller.capability.CapabilityServiceSupport object associated with the deployment unit. This can be found as an attachment to the DeploymentPhaseContext:

Once you have the CapabilityServiceSupport you can use it to look up service names:

It's important to note that when you request a service name associated with a capability, the CapabilityServiceSupport will give you one regardless of whether the capability is actually registered with the kernel. If the capability isn't present, any service dependency your DUP creates using that service name will eventually result in a service start failure, due to the missing dependency. This behavior of not failing immediately when the capability service name is requested is deliberate. It allows deployment operations that use the rollback-on-runtime-failure=false header to successfully install (but not start) all of the services related to a deployment. If a subsequent operation adds the missing capability, the missing service dependency problem will then be resolved and the MSC service container will automatically start the deployment services.

You can also use the CapabilityServiceSupport to obtain a reference to the capability's custom integration API:

Note that here, unlike the case with service name lookups, the CapabilityServiceSupport will throw a checked exception if the desired capability is not installed. This is because the kernel has no way to satisfy the request for a custom integration API if the capability is not installed. The DeploymentUnitProcessor will need to catch and handle the exception.

Detailed API

The WildFly Core kernel's API for using capabilities is covered in detail in the javadoc for the RuntimeCapability and RuntimeCapability.Builder classes and the OperationContext and CapabilityServiceSupport interfaces.

Many of the methods in OperationContext related to capabilities have to do with registering capabilities or registering requirements for capabilities. Typically non-kernel developers won't need to worry about these, as the abstract OperationStepHandler implementations provided by the kernel take care of this for you, as described in the preceding sections. If you do find yourself in a situation where you need to use these in an extension, please read the javadoc thoroughly.

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