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7.3. Resources
This section examines common features for resources.
7.3.1. Retrieving a Resource
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The API retrieves the state of a resource with a
GET request on a URI obtained from a collection listing.
7.3.2. Updating a Resource
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The API modifies resource properties with a
PUT request containing an updated description from a previous GET request for the resource URI. Details on modifiable properties are found in the individual resource type documentation.
A
PUT request requires a Content-Type: application/xml header. This informs the API of the XML representation in the body content as part of the request.
This does not include immutable resource properties that an API user has attempted to modify. If an attempt is made to modify a strictly immutable resource property, the API reports a
409 Conflict error with a fault representation in the response body.
Properties omitted from the representation are ignored and not changed.
7.3.3. Deleting a Resource
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The API deletes a resource with a
DELETE request sent to its URI.
DELETE /api/collection/resource_id HTTP/1.1 Accept: application/xml HTTP/1.1 204 No Content
DELETE /api/collection/resource_id HTTP/1.1
Accept: application/xml
HTTP/1.1 204 No Content
Some cases require optional body content in the
DELETE request to specify additional properties. A DELETE request with optional body content requires a Content-Type: application/xml header to inform the API of the XML representation in the body content. If a DELETE request contains no body content, omit the Content-Type: application/xml header.
7.3.4. Sub-Collection Relationships
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A sub-collection relationship defines a hierarchical link between a resource and a sub-collection. The sub-collection exists or has some meaning in the context of a parent resource. For example, a volume contains bricks, which means the API maps the relationship between the volume resource and the bricks sub-collection.
Sub-collections are used to model the following relationships types:
- 1:N mappings, where mapped resources are dependent on a parent resources. Without the parent resource, the dependent resource cannot exist. For example, the link between a volume and its bricks.
- 1:N mappings, where mapped resources exist independently from parent resources but data is still associated with the relationship. For example, the link between a network and a cluster.
The API defines a relationship between a resource and a sub-collection using the
link rel= attribute:
The API user now queries the sub-collection.
7.3.5. XML Element Relationships
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XML element links act as an alternative to sub-collections to express relationships between resources. XML element links are simply elements with a "href" attribute that points to the linked element.
XML element links are used to model simple 1:N mappings between resources without a dependency and without data associated with the relationship. For example, the relationship between a host and a cluster.
Examples of such relationships include:
- Backlinks from a resource in a sub-collection to a parent resource; or
- Links between resources with an arbitrary relationship.
Example 7.6. Backlinking from a sub-collection resource to a resource using an XML element
7.3.6. Actions
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Most resources include a list of action links to provide functions not achieved through the standard HTTP methods.
The API invokes an action with a
POST request to the supplied URI. The body of the POST requires an action representation encapsulating common and task-specific parameters.
| Element | Description |
|---|---|
async | If true, the server responds immediately with 202 Accepted and an action representation contains a href link to be polled for completion. |
grace_period | a grace period in milliseconds, which must expire before the action is initiated. |
Individual actions and their parameters are documented in the individual resource type's documentation. Some parameters are mandatory for specific actions and their absence is indicated with a
fault response.
An action also requires a
Content-Type: application/xml header since the POST request requires an XML representation in the body content.
When the action is initiated asynchronously, the immediate
202 Accepted response provides a link to monitor the status of the task:
A subsequent
GET on the action URI provides an indication of the status of the asynchronous task.
| Status | Description |
|---|---|
pending | Task has not yet started. |
in_progress | Task is in operation. |
complete | Task completed successfully. |
failed | Task failed. The returned action representation would contain a fault describing the failure. |
Once the task has completed, the action is retained for an indeterminate period. Once this has expired, subsequent
GETs are 301 Moved Permanently redirected back to the target resource.
An action representation also includes some links that are identified by the
rel attribute:
| Type | Description |
|---|---|
parent | A link back to the resource of this action. |
replay | A link back to the original action URI. POSTing to this URI causes the action to be re-initiated. |
7.3.7. Permissions
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Each resource contains a
permissions sub-collection. Each permission contains a user, an assigned role and the specified resource. For example:
A resource acquires a new permission when an API user sends a
POST request with a permission representation and a Content-Type: application/xml header to the resource's permissions sub-collection. Each new permission requires a role and a user:
7.3.8. Handling Errors
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Some errors require further explanation beyond a standard HTTP status code. For example, the API reports an unsuccessful resource state update or action with a
fault representation in the response entity body. The fault contains a reason and detail strings. Clients must accommodate failed requests via extracting the fault or the expected resource representation depending on the response status code. Such cases are clearly indicated in the individual resource documentation.
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