Health Policy V1.0

Health Policy V1.0

The health policy is designed to automate the failure detection and recovery process for a cluster.

Applicable Profile Types

The policy is designed to handle both os.nova.server and os.heat.stack profile types.

Actions Handled

The policy is capable of handling the following actions:

  • CLUSTER_RECOVER: an action that carries some optional parameters as its inputs. The parameters are specific to the profile type of the target cluster.
  • CLUSTER_DEL_NODES: an action that carries a list value named candidates in its inputs value.
  • CLUSTER_SCALE_IN: an action that carries an optional integer value named count in its inputs value.
  • CLUSTER_RESIZE: an action that carries various key-value pairs as arguments to the action in its inputs value.
  • NODE_DELETE: an action that has a node associated with it. This action has to be originated from a RPC request directly so that it will be processed by the health policy.

The policy will be checked BEFORE a CLUSTER_RECOVER action is executed. It will derive the appropriate inputs to the action based on the policy’s properties.

The policy will be checked BEFORE and AFTER any one of the CLUSTER_DEL_NODES, CLUSTER_SCALE_IN, CLUSTER_RESIZE and the NODE_DELETE action is executed. Under the condition that any of these actions are originated from RPC requests, Senlin is aware of the fact that a cluster is losing node member(s) because of a normal cluster membership management operation initiated by users rather than unexpected node failures. The health policy will temporarily disable the health manager function on the cluster in question and re-enable the health management after the action has completed.

The health policy can be treated as an interface for the health manager engine running inside the senlin-engine process. Its specification contains two main “sections”, detection and recovery, each of which specifies how to detect node failures and how to recover a node to a healthy status respectively.

Failure Detection

The health policy is designed to be flexible regarding node failure detection. The current vision is that the health policy will support following types of failure detection:

  • NODE_STATUS_POLLING: the health manager periodically polls a cluster and check if there are nodes inactive.
  • LIFECYCLE_EVENTS: the health manager listens to event notifications sent by the backend service (e.g. nova-compute).
  • LB_STATUS_POLLING: the health manager periodically polls the load balancer (if any) and see if any node has gone offline.

The third option above (LB_STATUS_POLLING) is not usable yet due to an outstanding issue in the LBaaS service. But we are still tracking its progress considering that metrics from the load-balancer is more trust-worthy and more useful because they originate from the data plane rather than the control plane.

Yet another option regarding load-balancer based health detection is to have the load-balancer emit event notifications when node status changes. This is also an ongoing work which may take some time to land.

Proactive Node Status Polling

The most straight-forward way of node failure detection is by checking the backend service about the status of the physical resource represented by a node. If the type of detection is set to “NODE_STATUS_POLLING” (optionally, with an interval value specified), the health manager will periodically check the resource status by querying the backend service and see if the resource is active. Below is a sample configuration:

type: senlin.policy.health
version: 1.0
properties:
  detection:
    type: NODE_STATUS_POLLING
    options:
      internal: 120

  ...

NOTE: The current polling logic is only about checking with the backend service whether a resource is in “ACTIVE” status. However, in future, this may get extended to having the health manager ping the IP address of a nova server or posting a “GET” request to a specific URL. We believe such extensions can better reveal whether a specific node is operating.

Once such a policy object is attached to a cluster, Senlin registers the cluster to the health manager engine for failure detection, i.e., node health checking. A thread is created to issue a CLUSTER_CHECK RPC request to the senlin-engine periodically at the specified interval. The CLUSTER_CHECK action only refreshes the status of each and every node in the cluster.

When one of the senlin-engine services is restarted, a new health manager engine will be launched. This new engine will check the database and see if there are clusters which have health policies attached and thus having its health status maintained by a health manager that is no longer alive. The new health manager will pick up these clusters for health management.

Listening to Event Notifications

For some profile types (currently os.nova.server), the backend service may emit an event notification on the control plane message bus. These events are more economic ways for node failure detection, assuming that all kinds of status changes will be captured and reported by the backend service. Actually, we have verified that most lifecycle events related to a VM server are already captured and reported by Nova. For other profile types such as os.heat.stack, there also exists such a possibility although based on our knowledge Heat cannot detect all stack failures.

Event listening is a cheaper way for node failure detection when compared to the status polling approach described above. To instruct the health manager to listen to event notifications, users can attach their cluster(s) a health policy which looks like the following example:

type: senlin.policy.health
version: 1.0
properties:
  detection:
    type: LIFECYCLE_EVENTS

  ...

When such a policy is attached to a cluster, Senlin registers the cluster to the health manager engine for failure detection, i.e., node health checking. A listener thread is created to listen to events that indicate certain node has failed. For nova server nodes, the current implementation treats all of the following event types as indication of node failures:

  • compute.instance.pause.end: A server has been accidentally paused.
  • compute.instance.power_off.end: A server has been stopped accidentally.
  • compute.instance.rebuild.error: A server rebuild has failed.
  • compute.instance.shutdown.end: A server has been shut down for unknown reasons.
  • compute.instance.soft_delete.end: A server has been soft deleted.

When any one of such an event is heard by the listener thread, it will issue a NODE_RECOVER RPC request to the senlin-engine service. For the health policy to make a smarter decision on the proper recover operation, the RPC request is augmented with some parameters as hints to the recovery operation as exemplified below:

{
  "event": "SHUTDOWN",
  "state": "shutdown",
  "instance_id": "449ad837-3db2-4aa9-b324-ecd28e14ab14",
  "timestamp": "2016-11-27T12:10:58Z",
  "publisher": "nova-compute:node1",
}

Ideally, a health management solution can react differently based on the different types of failures detected. For example, a server stopped by accident can be simply recovered by start it again; a paused server can be unpaused quickly instead of being recreated.

When one of the senlin-engine services is restarted, a new health manager engine will be launched. This new engine will check the database and see if there are clusters which have health policies attached and thus having its health status maintained by a health manager that is no longer alive. The new health manager will pick up these clusters for health management.

Recovery Actions

The value of the recovery actions key for recovery is modeled as a list, each of which specifies an action to try. The list of actions are to be adjusted by the policy before passing on to a base Profile for actual execution. An example (imaginary) list of actions is shown below:

type: senlin.policy.health
version: 1.0
properties:
  ...
  recovery:
    actions:
      - name: REBOOT
        params:
          type: soft
      - name: REBUILD
      - name: my_evacuation_workflow
        type: MISTRAL_WORKFLOW
        params:
          node_id: {{ node.physicalid }}

The above specification basically tells Senlin engine to try a list of recovery actions one by one. The first thing to try is to “reboot” (an operation only applicable on a Nova server) the failed node in question. If that didn’t solve the problem, the engine is expected to “rebuild” (also a Nova server specific verb) the failed node. If this cannot bring the node back to healthy status, the engine should execute a Mistral workflow named “my_evacuation_workflow” and pass in the physical ID of the node.

The health policy is triggered when a CLUSTER_RECOVER action is to be executed. Using the above example, the policy object will fill in the data field of the action object with the following content:

{
  "health": {
    "recover_action": [
      {
        "name": "REBOOT",
        "params": {
          "type": "soft"
        }
      },
      {
        "name": "REBUILD"
      },
      {
        "name": "my_evacuation_workflow",
        "type": "MISTRAL_WORKFLOW",
        "params": {
          "node_id": "7a753f4b-417d-4c10-8065-681f60db0c9a"
        }
      }
    ]
    ...
  }
}

This action customization is eventually passed on to the Profile base class where the actual actions are performed.

NOTE: Currently, we only support a single action in the list. The support to Mistral workflow is also an ongoing work.

Default Recovery Action

Since Senlin is designed to manage different types of resources, each resource type, i.e. profile type, may support different sets of operations that can be used for failure recovery.

A more practical and more general operation to recover a failed resource is to delete the old one followed by creating a new one, thus a RECREATE operation. Note that the RECREATE action is although generic enough, it may and may not be what users want. For example, there is not guarantee that a recreated Nova server will preserve its physical ID or its IP address. The temporary status of the original server will be lost for sure.

Profile-specific Recovery Actions

Each profile type supports a unique set of operations, some of which are relevant to failure recovery. For example, a Nova server may support many operations that can be used for failure recovery, a Heat stack may support only the STACK_UPDATE operation for recovery. This set of actions that can be specified for recovery is profile specific, thus an important part for the policy to check and validate.

External Recovery Actions

In real-life deployments, there are use cases where a simple recovery of a node itself is not sufficient to bring back the business services or applications that were running on those nodes. There are other use cases where appropriate actions must be taken on the storage and/or network used for a full failure recovery. These are the triggers for the Senlin team to bring in support to Mistral workflows as special actions.

The current design is to allow for a mixture of built-in recovery actions and user provided workflows. In the foreseeable future, Senlin does not manage the workflows to be executed and the team has no plan to support the debugging of workflow executions. Users have to make sure their workflows are doing things they want.

Fencing Support

The term “fencing” is used to describe the operations that make sure a seemingly failed resource is dead for sure. This is a very important aspect in all high-availability solutions. Take a Nova server failure as an example, there are many causes which can lead the server into an inactive status. A physical host crash, a network connection breakage etc. can all result in a node unreachable. From Nova controller’s perspective, it may appear that the host has gone offline, however, what really happened could be just the management network is experiencing some problems. The host is actually still there, all the VM instances on it are still active, which means they are still processing requests and they are still using the IP addresses allocated to them by a DHCP server.

There are many such cases where a seemingly inactive node is still working and these nodes will bring the whole cluster into unpredictable status if we only attempt an immature recovery action without considering the possibility that the nodes are still alive.

Considering this, we are working on modeling and implementing support to fencing in the health policy.

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