nova/api-guide/source/faults.rst

Faults

This doc explains how to understand what has happened to your API request.

Every HTTP request has a status code. 2xx codes signify the API was a success. However, that is often not the end of the story. That generally only means the request to start the operation has been accepted. It does not mean the action you requested has successfully completed.

Tracking Errors by Request ID

Every request made has a unique Request ID. This is returned in a response header. Here is an example response header:

X-Compute-Request-ID: req-4b9e5c04-c40f-4b4f-960e-6ac0858dca6c

Server Actions

There is an API for end users to list the outcome of Server Actions, referencing the requested action by request id.

For more details, please see: http://developer.openstack.org/api-ref-compute-v2.1.html#os-instance-actions-v2.1

Logs

All logs on the system, by default, include the request-id when available. This allows an administrator to track the API request processing as it transitions between all the different nova services.

Instance Faults

Nova often adds an instance fault DB entry for an exception that happens while processing an API request. This often includes more administrator focused information, such as a stack trace. However, there is currently no API to retrieve this information.

Notifications

In many cases there are also notifications emitted that describe the error. This is an administrator focused API, that works best when treated as structured logging.

Synchronous Faults

If an error occurs while processing our API request, you get a non 2xx API status code. The system also returns additional information about the fault in the body of the response.

Example: Fault: JSON response

{
   "itemNotFound":{
      "code": 404,
      "message":"Aggregate agg_h1 could not be found."
   }
}

The error code is returned in the body of the response for convenience. The message section returns a human-readable message that is appropriate for display to the end user. The details section is optional and may contain information—for example, a stack trace—to assist in tracking down an error. The details section might or might not be appropriate for display to an end user.

The root element of the fault (such as, computeFault) might change depending on the type of error. The following link contains a list of possible elements along with their associated error codes.

For more information on possible error code, please see: http://specs.openstack.org/openstack/api-wg/guidelines/http.html#http-response-codes

Asynchronous faults

An error may occur in the background while a server is being built or while a server is executing an action.

In these cases, the server is usually placed in an ERROR state. For some operations, like resize, its possible that the operations fails but the instance gracefully returned to its original state before attempting the operation. In both of these cases, you should be able to find out more from the Server Actions API described above.

When a server is placed into an ERROR state, a fault is embedded in the offending server. Note that these asynchronous faults follow the same format as the synchronous ones. The fault contains an error code, a human readable message, and optional details about the error. Additionally, asynchronous faults may also contain a created timestamp that specifies when the fault occurred.

Example: Server in error state: JSON response

{
    "server": {
        "id": "52415800-8b69-11e0-9b19-734f0000ffff",
        "tenant_id": "1234",
        "user_id": "5678",
        "name": "sample-server",
        "created": "2010-08-10T12:00:00Z",
        "hostId": "e4d909c290d0fb1ca068ffafff22cbd0",
        "status": "ERROR",
        "progress": 66,
        "image" : {
            "id": "52415800-8b69-11e0-9b19-734f6f007777"
        },
        "flavor" : {
            "id": "52415800-8b69-11e0-9b19-734f216543fd"
        },
        "fault" : {
            "code" : 500,
            "created": "2010-08-10T11:59:59Z",
            "message": "No valid host was found. There are not enough hosts available.",
            "details": [snip]
        },
        "links": [
            {
                "rel": "self",
                "href": "http://servers.api.openstack.org/v2/1234/servers/52415800-8b69-11e0-9b19-734f000004d2"
            },
            {
                "rel": "bookmark",
                "href": "http://servers.api.openstack.org/1234/servers/52415800-8b69-11e0-9b19-734f000004d2"
            }
        ]
    }
}