senlin/doc/source/developer/receiver.rst

Receiver

Concept

A Receiver is an abstract resource created in Senlin engine to handle operation automation. You can create a receiver to trigger a specific action on a cluster on behalf of a user when some external alarms or events are fired.

A receiver can be of different types. The type of a receiver is specified when being created. For example, the most common type of receiver is webhook, where a Webhook is a URI that can be accessed from any user or program.

A receiver encapsulates the information needed for triggering an action. These information include:

• actor: the credential of a user on whose behalf the action will be triggered. This is usually the user who created the receiver, but it can be any other valid user explicitly specified when the receiver is created.
• cluster_id: the ID of the targeted cluster.
• action: the name of an action that is applicable on a cluster.
• params: a dictionary feeding argument values (if any) to the action.

In the long term, senlin may support user-defined actions where action will be interpreted as the UUID or name of a user-defined action.

Creating a Receiver

When a user requests the creation of a receiver by invoking senlin command line tool, the request comes with at least two parameters: the targeted cluster and the intended action to invoke when the receiver is triggered. Optionally, the user can provide some additional parameters to use and/or the credentials of a different user. By default, a receiver of type "webhook" is created. In future, we may support more receiver types.

When the Senlin API service receives the request, it does three things:

• Validates the request and rejects it if any of the following conditions is met:

  • the targeted cluster could not be found;
  • the receiver type specified is not supported;
  • the targeted cluster is not owned by the requester and the requester does not have an "admin" role in the project;
  • the provided action is not applicable on a cluster.

• Creates a receiver object that contains all necessary information that will be used to trigger the specified action on the specified cluster.

• Creates a "channel" which is specific to the receiver type. For example, a receiver of type "webhook" will contain a key named "alarm_url" in its channel which looks like:

  http://{host:port}/v1/webhooks/{webhook_id}/trigger?V=1

  NOTE: The V=1 above is used to encode the current webhook triggering protocol. When the protocol changes in future, the value will be changed accordingly.

Finally, Senlin engine returns a dictionary containing the properties of the receiver object.

Triggering a Receiver

Different types of receivers are triggered in different ways. For example, a webhook receiver is triggered via the alarm_url channel; a message queue receiver can be triggered via messages delivered in a shared queue.

Triggering a Webhook

When triggering a webhook, a user or a software sends a POST request to the receiver's alarm_url channel, which is a specially encoded URL. This request is first processed by the webhook middleware before arriving at the Senlin API service.

The webhook middleware checks this request and parses the format of the request URL. The middleware attempts to find the receiver record from Senlin database and see if the named receiver does exist. If the receiver is found, it then tries to load the saved credentials. An error code 404 will be returned if the receiver is not found.

After having retrieved the credentials, the middleware will proceed to get a Keystone token using credentials combined with Senlin service account info. Using this token, the triggering request can proceed along the pipeline of middlewares. An exception will be thrown if the authentication operation fails.

When the senlin engine service receives the webhook triggering request it creates an action based on the information stored in the receiver object. The newly created action is then dispatched and scheduled by a scheduler to perform the expected operation.

Credentials

When requesting the creation of a receiver, the requester can choose to provide some credentials by specifying the actor property of the receiver. This information will be used for invoking the webhook in the future. There are several options to provide these credentials.

If the credentials to use is explicitly specified, Senlin will save it in the receiver DB record. When the webhook is invoked later, the saved credentials will be used for authentication with Keystone. Senlin engine won't check if the provided credentials actually works when creating the receiver. The check is postponed to the moment when the receiver is triggered.

If the credentials to use is not explicitly provided, Senlin will assume that the receiver will be triggered in the future using the the requester's credential. To make sure the future authentication succeeds, Senlin engine will extract the user ID from the invoking context and create a trust between the user and the senlin service account, just like the way how Senlin deals with other operations.

The requester must be either the owner of the targeted cluster or he/she has the admin role in the project. This is enforced by the policy middleware. If the requester is the admin of the project, Senlin engine will use the cluster owner's credentials (i.e. a trust with the Senlin user in this case).