This document outlines requirements and expectations for dbus interfaces. These are usually specified as requirements due to our dbus architecture or for consistency in implementations.
size and uint64/int64 over explicit sizesDo not over-optimize properties by selecting explicit sizes such as uint8 unless there is a strong basis for it (usually driven by hardware or another protocol's specification).
For countable entities always prefer size, which are an sdbusplus implemented type that maps to the C equivalent of size_t on the architecture. For non-countable values prefer uint64 or int64.
Arbitrary strings should only be utilized for human consumption and never parsed by code. Any arbitrary string is typically expected to have a description such as "... can be shown in user interfaces but this field should not be used for any programmatic interrogation of an object".
The sdbusplus implementation has built-in support for enumerations, which flow across the dbus as uniquely encoded string values but has support in the bindings for automatically converting to a C++ enum type.
In some cases it is useful to have hardware-specific or OEM values for enumerations. In those cases a property may be a string, but should specify that the values contained within are to be sdbusplus-enumerations of a specific pattern. See the software compatiblity and dump interface as two current examples of this.
There are typically two types of associations we use:
Peer associations are rare, but are used to link a single entity that is required to be represented in different dbus path hierarchies due to some overriding aspect of the dbus design.
Directed associations are more common and are used to show a relationship between two different entities. Though it may at times feel contrived, directed associations should be considered to have a "primary" and "secondary" end, which helps establish a pattern for naming consistency. For example, a chassis might be "containing" (primary) any number of other inventory objects which are "contained_by" (secondary) the chassis.
Consider an entity which is contained at /.../foo/entity and /.../bar/entity. The association is what links the foo and bar aspects of the entity in the dbus path hierarchy. Accordingly, the association should be named with end-points "foo" and "bar".
A made-up example of a peer association might be a Inventory.Processor, located under the .../inventory hierarchy, and a Control.Power.Cap for that processor, located under the .../control hierarchy. The peer association allows traversal between the inventory and control namespaces for the single Processor entity.
The end-point names of an association should not codify the types of the interfaces pointed to by the association.
The primary relationship should be a verb with a Present Participle tense (ending in '-ing'). The secondary relationship should be a verb with a Past Participle tense (typically ending in '-ed').
The association end-points should be named in a way that the following sentences are grammatically correct:
{primary element} is {primary association} the {secondary element}.{secondary element} is {secondary association} the {primary element}.These correspond to the mapper D-Bus object paths {primary element}/{primary assocation} with an endpoints property value of {secondary element} and {secondary element}/{secondary association} with an endpoints property value of {primary element}.
In some cases it may be required for grammatical correctness to add a preposition to the secondary association, such as 'by' or 'with'.
Additional information on associations is in the mapper documentation.