What are Structured and Unstructured Resource Addresses?
Addressing resources in oneM2M can be done in different ways. oneM2M defines three formats that can be used for addressing resources and in resource IDs
Structured
Identifiers that use the structured format look like a path in a file system. The path elements are constructed from the resource names of the resources in a CSE's resource tree. Using this format it is also possible to address resources where the resource identifier is unknown.
Example: CseName/MyAeName/MyContainerName
Unstructured
The unstructured format is used to directly address a resource where the resource identifier is known. Sometimes it is simpler to use this format. Also, the identifier is a bit shorter which may be important in when communicating over constrained communication channels. In general, a CSE is using unstructured references to address resources.
Example: MyContainerID
Hybrid
The hybrid format is is a combination of the unstructured and structured formats and used only to address virtual resources while using also the unstructured format. This is necessary because virtual resources only have a resource name, but no resource identifier. While virtual resources can be easily addressed using the path-like structured format, the unstructured format is therefore not available here.
An address in hybrid format has two elements: The unstructured resource ID of the virtual resource's parent, and the resource name of the virtual resource.
Example: MyContainerID/la - this addresses the <latest> resource of a parent <container> resource with the resource identifier "MyContainerID".
Example Resource Tree
Figure 1: Example Resource Tree1
In the example tree in Figure 1, the resources can be addressed in the following ways:
| Resource | Structured | Unstructured (Resource ID) |
Hybrid |
|---|---|---|---|
| CSE | CseName | 1001 | N/A |
| AE | CseName/MyAeName | 1002 | N/A |
| Container | CseName/MyAeName/MyContainerName | 1003 | N/A |
| latest | CseName/MyAeName/MyContainerName/la | N/A | 1003/la |
Summary
An Application Entity (AE) might choose to use any of these formats to address resources in a oneM2M system. Sometimes is is more convenient to use the structured format, sometimes the unstructured format. The hybrid format is only used to address virtual resources.
Read more about addressing resources in a distributed CSE deployment and see more examples in Resource Identifier Scopes and Formats.
by Andreas Kraft, 2023-08-08
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Author: Andreas Kraft ↩