Equinox/p2/Concepts

The following is a set of terms and concepts that are prevalent in the Equinox Provisioning work. Some of the terms here are duplicates of those on the general provisioning terminology page.

Agent

The provisioning infrastructure on client machines is generally referred to as the agent. Agents can manage themselves as well as other profiles. An agent may run separate from any other Eclipse system being managed or may be embedded inside of another Eclipse system. Agents can manage many profiles (see below) and indeed, a given system may have many agents running on it. There is no such thing as "the p2 agent" or a bundle that goes by that name. This is because p2 is modular and what you need to run on an embedded device is not the same than what you need on a desktop or an autonomous server.

Artifact

Artifacts are the actual content being installed or managed. Bundle JARs and executable files are examples of artifacts.

Artifact Repository

Artifact repositories hold artifacts

Director

The director is a high level API that combines the work of the planner and the engine. That is, the director invokes the planner to compute the provisioning operations to perform, and then invokes the engine with the planner's output to achieve the desired profile changes.

Engine

The engine is responsible for carrying out the desired provisioning operations as determined by a director. Whereas the subject of the director's work is metadata, the subject of the engine's work is the artifacts and configuration information contained in the IUs selected by the director. Engines cooperate with repositories and transport mechanisms to ensure that the required artifacts are available in the desired locations. The engine runs by invoking a set of engine Phases and working with the various Touchpoints to effect the desired result.

Garbage Collection

Element of repositories (metadata and artifact) can be garbage collected by tracing reachability from a set of known roots. For example, the set of all profiles managed by an agent transitively identifies all IUs that are currently of direct interest to the provisioning agent. Similarly, the IUs identify the artifacts required to run the profiles. Any IUs or artifacts that are not in the transitive list are garbage and can be collected.

Installable Units(IU)

Installable Units are metadata that describe things that can be installed. They are not the things themselves. So an IU for a bundle is not the bundle but a description of the bundle: its name, version, capabilities, requirements, etc.. The bundle JAR is an artifact.

Metadata Repository

A metadata repository holds installable units.

Mirroring

The basic operation of distribution is mirroring. The key here is that metadata and artifacts are not downloaded, they are mirrored. The subtle distinction is that local mirrors are a) simple caches of something that is remote and b) potential sources of further mirroring. This means that locally held information can be deleted and replaced as needed by re-mirroring. Similarly, having local copies act as mirrors opens the path to natural peer-to-peer distribution. Note that metadata and artifacts are quite separate and having an IU mirrored in one repo does not imply that the associated artifacts are in/near/beside/... that repo.

Phase

Provisioning operations generally happen by walking through a set of steps or phases. At each phase a particular kind of activity takes place. For example, during the Fetch phase, the various artifacts required for the operation are Mirrored while during the Configure phase IUs are woven into the underlying runtime system by their associated Touchpoints.

Planner

The planner is responsible for determining what should be done to a given profile to reshape it as requested. That is, given the current state of a profile, a description of the desired end state of that profile and metadata describing the available IUs, a planner produces a list of provisioning operations (e.g., install, update or uninstall) to perform on the related IUs.

Profile

Profiles are the target of install/management operations. They are a list of IUs that go together to make up a system. They are roughly equivalent to the traditional Eclipse configurations. When an IU is installed it is added to a profile. That profile can then be run and the artifacts associated with the installed IUs executed (or whatever). Later the IU can be uninstalled or updated in that profile. The exact same IU can be installed simultaneously in many profiles.

Touchpoint

A part of the engine that is responsible for integrating the provisioning system to a particular runtime or management system. For example, the Eclipse Touchpoint understands how Equinox stores and manages bundles. Different platforms have different Native Touchpoints that integrate with the Windows desktop, RPMs, various registries etc. See also Touchpoint Use Cases.