Design Specifications for Masakari
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Implement RESERVED_HOST recovery action for host failure workflow

This spec talks about adding RESERVED_HOST recovery action for host failure workflow. In masakari each failover segment has recovery_method defined for it, so that if any of the host within that failover segment goes down then recovery action will be executed to evacuate "HA_Enabled" or all instances depending on "evacuate_all_instances" configuration option from that host based on recovery_method.


In each failover segment operators will keep some hosts as reserved by disabling the compute service on those hosts and setting "reserved" property of that host as "True". As a result, these hosts are not selected by the nova scheduler while provisioning new instances or for performing any other instance actions such as resize, migration etc. These hosts can be used by masakari engine for evacuating the instances from the failed host. Once the reserved host is used for evacuating the instances it is no longer treated as reserved and nova scheduler can use that host for scheduling the instances.

Problem description

Masakari provides a driver interface for implementing the workflows synchronously or asynchronously. Whoever wants to implement the workflow can inherit the masakari driver and implement the workflows.

For implementing the RESERVED_HOST recovery action masakari engine should provide list of reserved hosts associated with its failover segment to the driver. Its then job of the driver to execute the workflow and use this list for evacuating the instances from failed host. One of the task of the workflow is to enable the compute service on reserved host so that instances can be evacuated on that host. At the same time "reserved" property of that host needs to be set to False. There is a possibility for multiple host failures under one failover segment may take the same reserved host and start the recovery workflows at the same time. To avoid this situation, lock with current reserved host name will be acquired on each of the task and that reserved host will be skipped if the lock acquired and evacuation will be done on the next reserved host from the list.

Use Cases

Operator may want to execute host_failure workflow using 'RESERVED_HOST' recovery method.

Proposed change

Masakari engine should execute the workflows synchronously only. Masakari engine will load all the drivers. Whoever is going to implement the new driver, it should be the responsibility of that driver to get the result of workflow and send it back to the masakari engine. If someone wants to add a driver which will execute the workflow on a different host and not on the same host where masakari engine is running then they will need to design that driver in such a way that workflow will execute on any host in asynchronous way and send back the result to the masakari engine, so that masakari engine will set the notification status to "ERROR" or "FINISHED" based on the results.

To implement "reserved_host" recovery method, we need to implement lock mechanism over reserved host so that masakari-engine don't use same reserved host for multiple failure notifications. There are two ways to implement the lock mechanism:

  1. Use oslo_concurrency.lockutils file based lock: Easy to implement, but cannot manage lock among multiple nodes.
  2. Implement lock mechanism using Tooz: Operator would want to deploy multiple masakari-engine services on different nodes. For this purpose we recommend to use distributed locking mechanism provided by Tooz library. By default Tooz would be configured to use file locks, so everything will work as oslo_concurrency lock mechanism. If operator would want to run multiple masakari-engine services he/she would need to configure Tooz backend service and set it in masakari.conf. Currently most reliable Tooz backends are ZooKeeper and Redis.

As of now, in masakari file based lock (oslo_concurrency.lockutils) is already used. Same mechanism will be used to acquire lock on reserved host. Tooz support will be added later and all the existing locks will be migrated to use Tooz locking mechanism.


  1. No need to change current masakari engine implementation.
  2. It's easy to implement other recovery actions such as "AUTO_PRIORITY" and "RH_PRIORITY" with this design.


Execute the workflows asynchronously i.e. either workflow can be executed on same host where masakari engine is running or on a different host altogether. In this case masakari engine might not get the results from the workflow execution and will not able to update notification status and set reserved hosts to False.

To achieve this masakari engine will generate a callback URL based on the notification_id and pass it to the driver. Sample callback URL will be like,


Driver will further pass this URL and the required information such as reserved host list, host name etc. to the workflow. Workflow will be responsible to call the masakari using callback URL with notification status and reserved hosts used by workflow as a body of the request.

Once this request is received by masakari, then based on the notification_id it will map it to the notification from the database table and update the status of the notification accordingly. Also masakari will get the list of used reserved hosts in request body, so it will loop through it and set those host's "reserved" property as False.

Rest API can be:

method: PUT
URL: URL that is passed to the workflow(contains notification_id)
result {
    notification_status: status of the notification, either 'error' or
    'finished', used_reserved_hosts: list of actually used reserved_hosts
    by workflow


  1. Better approach to implement new workflow drivers.


  1. The other service which is going to request masakari using REST api should have required admin credentials to call the API.
  2. Need to change current driver (taskflow) implementation to adopt this design.
  3. Need to modify PUT api to incorporate this change.

Data model impact


REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact


Other deployer impact


Developer impact




Primary assignee:

Dinesh Bhor <> Abhishek Kekane <>

Work Items

  • Implement RESERVED_HOST recovery_method for host_failure recovery in synchronous way for taskflow driver
  • Add unit tests for the coverage





Documentation Impact