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Monikers (Components v2)

A migration from Components v1 to v2 is in progress.

A moniker identifies a specific component instance in the component tree using a topological path.

Types

There are three types of monikers:

  • Child moniker: Denotes a child of a component instance relative to its parent.
  • Relative moniker: Denotes the path from a source component instance to a target component instance. It is expressed as a sequence of child monikers.
  • Absolute moniker: Denotes the path from the root of the component instance tree to a target component instance. It is expressed as a sequence of child monikers. Every component instance has a unique absolute moniker.

Stability

Monikers are stable identifiers. Assuming the component topology does not change, the monikers used to identify component instances in the topology will remain the same.

Uniqueness

Each time a component instance is destroyed and a new component instance with the same name is created in its place in the component topology (as a child of the same parent), the new instance is assigned a unique instance identifier to distinguish it from prior instances in that place.

Monikers include unique instance identifiers to prevent confusion of old component instances with new component instances of the same name as the tree evolves.

Privacy

Monikers may contain privacy-sensitive information about other components that the user is running.

To preserve the encapsulation of the system, components should be unable to determine the identity of other components running outside of their own realm. Accordingly, monikers are only transmitted on a need-to-know basis or in an obfuscated form.

For example, components are not given information about their own absolute moniker because it would also reveal information about their parents and ancestors.

Monikers may be collected in system logs. They are also used to implement the component framework's persistence features.

Notation

This section describes the syntax used for displaying monikers to users.

Instance and Collection Names

Parents assign names to each of their children. Dynamically created children are arranged by their parent into named collections.

Syntax: Each name is a string of 1 to 100 of the following characters: a-z, 0-9, _, ., -.

See component manifest documentation for more details.

Instance Identifiers

Instance identifiers ensure the uniqueness of monikers over time whenever a parent destroys a component instance and creates a new one with the same name.

Syntax: Decimal formatted 32-bit unsigned integer using characters: 0-9.

Child Monikers

Represented by the child's collection name (if any), name, and instance identifier delimited by :.

Syntax: {name}:{id} or {collection}:{name}:{id}

The following diagram shows an example component topology, with the children of alice labeled with their child monikers.


Diagram of Child Monikers

Examples:

  • carol:0: child "carol" (instance id 0)
  • support:dan:1: child "dan" (instance id 1) in collection "support"

Relative Monikers

Represented by the minimal sequence of child monikers encountered when tracing upwards from a source to the common ancestor of the source and target and then downwards to the target.

A relative path begins with . and is followed by path segments. \` denotes an upwards traversal segment./denotes a downwards traversal segment. There is no trailing` or /.

Relative monikers are invertible; a path from source to target can be transformed into a path from target to the source because information about both endpoints are fully encoded by the representation.

In contrast, file system paths are not invertible because they use .. to denote upwards traversal so some inverse traversal information is missing.

To maintain invertibility, the syntax for denoting paths varies slightly for upward and downward traversals. A downward path segment is a child moniker of one of the current component instance's children: ./carol:2. Conversely, an upward path segment is the child moniker of on the current component instance, according to its parent: .\alice:2/bob:0. The reason that the child moniker is explicitly specified in the upward path (instead of a generic "upward traversal" marker like ..) is that otherwise the relative moniker would not be invertible, and would not uniquely identify a component instance. For downward traversals, the paths don't need to include the parent's name to be traceable because a child only has one parent. However, for upward traversals the source path can be one of many children of its parent path.

Syntax: .\{path from source to ancestor}/{path from ancestor to target}

The following diagram shows an example component topology, with all relative monikers that can be derived from the source component alice labeled. Note that support is not a component but rather a collection with two children: dan and jan.


Diagram of Relative Monikers

Examples:

  • .: self - no traversal needed
  • ./carol:2: a child - traverse down carol:2
  • ./carol:2/sandy:1: a grandchild - traverse down carol:2 then down sandy:1
  • .\alice:2/bob:0: a cousin - traverse up alice:2 then down bob:0
  • ./support:dan:1: a child - traverse down into collection child support:dan:1

Absolute Monikers

Represented by the absolute path from the root to the component instance as a sequence of child monikers.

An absolute path begins with / and is followed by downwards traversal path segments delimited by /. There is no trailing /.

Syntax: /{path from root to target}

The following diagram shows an example component topology, all absolute monikers that can be derived from the unnamed root component labeled. The root component is unnamed because it is inherently not the child of any other component and components are named by their parents, not by components themselves. Note that support is not a component but rather a collection with two children: dan and jan.


Diagram of Absolute Monikers

Examples:

  • /: the root itself (it has no name because it has no parent)
  • /alice:2/support:dan:1: from root traverse down alice:2 then down support:dan:1
  • /alice:2/carol:1: from root traverse down alice:2 then down carol:1