Difference between revisions of "Contexts"

Revision as of 14:38, 26 January 2009

Within service implementations, some functionality needs to be accessed by different components and for different purposes.

The most obvious case of shared functionality is that required to expose the configuration of the service, or the configuration of its RI, or the configuration of the gHN on which the RI is deployed. There is also information related to scope and security that is dynamically acquired and needed by different components. Beyond information access, there are utilities and behaviours - general-purpose or service-specific - which may be called upon from multiple and functionally unrelated components.

The gCF approach to shared functionality is to centralise it in distinguished components, called contexts, which are directly accessible from any other implementation component. gCF contexts are singleton classes, in that they are designed to be instantiated only once and to expose the single instance to all their clients.

There are different type of contexts in gCF, each of which centralises functionality that is conceptually associated with the key entities of service implementations: the service and its RI, the gHN on which the RI is deployed, the port-types of the service, and even the port-types of the target services of the RI. Some are fully defined within gCF (e.g. the context of the gHN). Others are only partially implemented and need to be specialised within service implementations. Collectively, the context form a inheritance hierarchy contained in the contexts package:

As shown in the diagram, the entire framework has dependencies on the contexts packages, in line with the definition of contexts as repositories of shared functionality. The high-level roles of the components in the package is summarised below:

• GCUBEContext: the base implementation for all contexts in gCF.

• GCUBEGHNContext: a concrete specialisation of GCUBEContext for a gCube Hosting Node.

• GCUBEServiceContext: a concrete specialisation of GCUBEContext for a gCube Service and its RIs.

• GCUBEPortTypeContext: a concrete specialisation of GCUBEContext for a port-type of gCube Service.

• GCUBEStatefulPortTypeContext: a concrete specialisation of GCUBEPortTypeContext for a stateful port-type of a gCube Service.

• GCUBERemotePortTypeContext: a concrete specialisation of GCUBEContext for a port-type of a target gCube Service.

Before discussing each context class in detail, we refine our the service model to introduce key context classes from the perspective of the service developer:

The figure shows that the GHNContext and GCUBERemotePortTypeContext are fully implemented by the framework and live outside the boundary of specific service implementations. In contrast, service and the port-type contexts fall in the scope of service implementations as specialisations of gCF context classes.

Common Facilities

All gCF contexts offer basic facilities for configuration and local file management.

The primary form of configuration supported by gC contexts is based on named access to arbitrary objects. The underlying configuration mechanism is based on a dedicated implementation of the JNDI interface; in particular, gCF contexts act as a wrappers of standard JNDI contexts and offer a much simplified interface to named access.

The underlying JNDI implementation remains opaque to both clients and implementers of context classes. Developers are only exposed to the configuration files that are then used to populate the JNDI implementation wrapped by the contexts. As we discuss below, the direct subclasses of GCUBEContext are responsible for establishing conventions on the naming and location of such files, for the definition and documentation of pre-defined configuration properties, and for loading all the configuration properties from the files. As most service developers specialise these subclasses, they need only to author the configuration files. The syntax for authoring JNDI configuration is based on XML, and its definition is lifted from technologies that underlies gCF. Conventions on context-specific configuration authoring are discussed below in relation to each type of context.

As to file management, gCF contexts offers facilities to access:

• files stored in the local file system for reading or writing purposes. Here the main goal is to abstract over the location in which the files are stored and thus to avoid gHN-specific dependencies. While actual location in which files are stored is specific to the type of context, as we discuss below, all contexts offer basic backup facilities: transparently to their clients, they create copies of files that are about to be modified and used backups whenever the original files are corrupted. In the second case, access is read-only

• files packaged along with the context, or more generally on the classpath, for read-only purposes.

In both cases, the gCF contexts are agnostic to the content of the files. This may be service-specific configuration that lives outside the standard JNDI framework, or any form of state that is deemed suitable for file storage.

The basic operations for configuration and file management are following (see the code documentation for signature details):

• getProperty(name,mandatory?): resolves a configuration property from its name. An optional flag indicates whether the presence property is deemed mandatory or else is optional (default). The flag is used for error and log management purposes.
• getResource(path): returns a classpath file resource given its path. A relative path is resolved with respect to the context implementation, while an absolute path is resolved directly against the classpath.
• getFile(path,mode): returns a java.io.File for read or write access. A write access mode induces backups and a read access (default) relies on backups to recover from failures. In write mode, directories that are specified in the path but do not exist are automatically created, while paths that identify directories are disallowed.

The following refinement of the service model emphasises the configuration and file management role of contexts:

The gHN Context

The GHNContext specialises GCUBEContext to serve as the gCF context for gCube Hosting Nodes. In particular, the GHNContext is the gCF component that is responsible for:

• managing the lifetime of the gHN, from its initialisation to its shutdown and failure.

This functionality supports the internal operation of the gHN and typically remains transparent to most service developers.

• exposing the configuration of the gHN and its hosting environment.

This functionality allows service developers to inspect at runtime the environment in which their code is deployed.

Clients can obtain the single instance of the GHNContext by invoking its static method getContext(). The first invocation of the method triggers the initialisation of the instance.

Lifetime Management

For lifetime management purposes, the GHNContext models the gHN context as the finite state machine shown below.

GHNStates.jpg

The different states of a gHN are represented by the values of the inner enumeration GHNContext.GHNSatus:

• Status.DOWN: this is the state of a gHN that is not running within the current JVM (or not running at all).

Detected during the initialisation of the GHNContext, this is at once an initial and terminal state for the gHN. When the gHN is in this state, the GHNContext can only be used to access the gHN configuration and hardware-related information, as described below.

• Status.RUNNING: this is the state of a gHN that is running within the current JVM.

When the gHN is in this state, the GHNContext engages in the following activities:

• at regular intervals, it publishes the gHN profile in the infrastructure.

During its initialisation, the GHNContext synthesises a GCUBEHostingNode resource that models the local gHN (either from scratch or from local traces of previous gHN startups). It then periodically derives the gHN profile from a serialisation of the state of the resource.

• it maintains and exposes a record of the RIs of the deployed services, along with their current state.

As discussed later, RIs register with the GHNContext as part of their initialisation process and the GHNContext subscribes with them as a consumer of lifetime events. Clients can then access the service context of some or of all registered RIs (cf. getServiceContext(), getServiceContexts()). One distinguished client in gCF - the handlers.GCUBEHandler - relies on this information to block calls made to RIs that are no longer operative or else are not ready to process them yet.

• Status.UPDATED: this is the transitional state of a gHN that has changed some of its state.

This is a short-lived state that is used internally to trigger publication of the gHN profile within the infrastructure. After publication, the gHN reverts immediately to the GHNStatus.RUNNING.

Note: Only privileged clients and the gHN itself can trigger state transitions (cf. setStatus()). All clients, however, can inspect the current state of the gHN (cf. getStatus()) and be notified of state transitions, as discussed next.

Configuration Management

The gHN configuration exposed by the GHNContext includes:

• configuration related to the infrastructure in which the gHN operates, as described in detail in the Administrator's Guide.

Some configuration properties - most noticeably those properties that benefit from some form of post-processing - are exposed by dedicated methods (cf. getInfrastructure(), getStartScopes(), getMode(), getType(), getHostame(), getPort(), etc.). Others are available via the GCUBEHostingNode resource maintained by the GHNContext (cf. getGHN()). Yet other configuration properties can be accessed with the generic facilities inherited from GCUBEContext (cf. getProperty()).

• configuration related to the implementations of core gCF interfaces, e.g. discovery, publishing, and remote notification interfaces (cf. getImplementation()).
• statistics and other information related to the underlying hardware (getFreeSpace), getUptime(), getMemoryUsage(), getCPUInfo(), getLoadStatistics(), etc.).

Note: The GHNContext can expose gHN configuration and hardware-related information even when the status of the gHN is Status.DOWN.

Note: The facilities for file management that GHNContextinherits from GCUBEContext (cf. getFile()) are for internal use and service developers should not use them. Instead, service developers access the file system using dedicated facilities in their service context.

Event Management

The GHNContext allows clients to subscribe to be notified of GHN lifetime events, such as state transition events and RI registration events (cf. subscribeGHNEvents(), GHNContext.GHNTopic). Subscription requires the registration of a dedicated event consumer (cf. GHNContext.GHNConsumer). Clients can derive GHNContext.GHNConsumer and selectively implement only the callbacks that correspond to the events of interest.

Service Contexts

[coming soon]

PortType Contexts

[coming soon]