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 call 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

There are two types of request ID.

Type	Description
Local request ID	Locally generated unique request ID by each service and different between all services (Nova, Cinder, Glance, Neutron, etc.) involved in that operation. The format is req- + UUID (UUID4).
Global request ID	User specified request ID which is utilized as common identifier by all services (Nova, Cinder, Glance, Neutron, etc.) involved in that operation. This request ID is same among all services involved in that operation. The format is req- + UUID (UUID4).

It is extremely common for clouds to have an ELK (Elastic Search, Logstash, Kibana) infrastructure consuming their logs. The only way to query these flows is if there is a common identifier across all relevant messages. The global request ID immediately makes existing deployed tooling better for managing OpenStack.

Request Header

In each REST API request, you can specify the global request ID in X-Openstack-Request-Id header, starting from microversion 2.46. The format must be req- + UUID (UUID4). If not in accordance with the format, the global request ID is ignored by Nova.

Request header example:

X-Openstack-Request-Id: req-3dccb8c4-08fe-4706-a91d-e843b8fe9ed2

Response Header

In each REST API request, X-Compute-Request-Id is returned in the response header. Starting from microversion 2.46, X-Openstack-Request-Id is also returned in the response header.

X-Compute-Request-Id and X-Openstack-Request-Id are local request IDs. The global request IDs are not returned.

Response header example:

X-Compute-Request-Id: req-d7bc29d0-7b99-4aeb-a356-89975043ab5e
X-Openstack-Request-Id: req-d7bc29d0-7b99-4aeb-a356-89975043ab5e

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: https://docs.openstack.org/api-ref/compute/#servers-actions-servers-os-instance-actions

Logs

All logs on the system, by default, include the global request ID and the local request ID when available. This allows an administrator to track the API request processing as it transitions between all the different nova services or between nova and other component services called by nova during that request.

When nova services receive the local request IDs of other components in the X-Openstack-Request-Id header, the local request IDs are output to logs along with the local request IDs of nova services.

Tip

If a session client is used in client library, set DEBUG level to the keystoneauth log level. If not, set DEBUG level to the client library package. e.g. glanceclient, cinderclient.

Sample log output is provided below. In this example, nova is using local request ID req-034279a7-f2dd-40ff-9c93-75768fda494d, while neutron is using local request ID req-39b315da-e1eb-4ab5-a45b-3f2dbdaba787:

Jun 19 09:16:34 devstack-master nova-compute[27857]: DEBUG keystoneauth.session [None req-034279a7-f2dd-40ff-9c93-75768fda494d admin admin] POST call to network for http://10.0.2.15:9696/v2.0/ports used request id req-39b315da-e1eb-4ab5-a45b-3f2dbdaba787 {{(pid=27857) request /usr/local/lib/python2.7/dist-packages/keystoneauth1/session.py:640}}

Note

The local request IDs are useful to make 'call graphs'.

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. For a server with status ERROR or DELETED, a GET /servers/{server_id} request will include a fault object in the response body for the server resource. For example:

GET https://10.211.2.122/compute/v2.1/servers/c76a7603-95be-4368-87e9-7b9b89fb1d7e
{
   "server": {
      "id": "c76a7603-95be-4368-87e9-7b9b89fb1d7e",
      "fault": {
         "created": "2018-04-10T13:49:40Z",
         "message": "No valid host was found.",
         "code": 500
      },
      "status": "ERROR",
      ...
   }
}

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/response-codes.html

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, it is possible that the operation 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"
            }
        ]
    }
}