deb-mistral/doc/source/main_features.rst

Mistral Main Features

Task result / Data Flow

Mistral supports transferring data from one task to another. In other words, if taskA produces a value then taskB which follows taskA can use it. In order to use this data Mistral relies on query language called YAQL. YAQL is powerful yet simple tool that allows to filter needed information, transform data and call functions. Find more information about it in YAQL official documentation . This mechanism allowing to transfer data plays one of the central roles in workflow concept and is referred to as Data Flow.

Below is a simple example of how Mistral Data Flow looks like from DSL (workflow language) perspective:

version: '2.0'

my_workflow:
  input:
    - host
    - username
    - password

  tasks:
    task1:
      action: std.ssh host=<% $.host %> username=<% $.username %> password=<% $.password %>
      input:
        cmd: "cd ~ && ls"
      on-complete: task2

    task2:
      action: do_something data=<% task(task1).result %>

Task called "task1" produces a result that contains a list of files in a user home folder of a host (both username and host are provided as workflow input) and task "task2" uses this data using YAQL expression "task(task1).result". "task()" here is a function registered in YAQL by Mistral to get information about a task by its name.

Task affinity

Task affinity is a feature which could be useful for executing particular tasks on specific Mistral executors. In fact, there are 2 cases:

1. Need to execute the task on single executor.
2. Need to execute the task on one of executor in executors group which has one name.

For enabling task affinity feature, edit section "executor" host property in configuration file:

[executor]
host = my_favorite_executor

Then start (restart) executor. Use target task property to specify this executor in Workflow DSL:

... Workflow YAML ...
task1:
  ...
  target: my_favorite_executor
... Workflow YAML ...

Task policies

Any Mistral task regardless of its workflow type can optionally have configured policies. Policies itself control the flow of the task - for example, policy can delay task execution before task starts or after task completes.

YAML example

my_task:
  action: my_action
  pause-before: true
  wait-before: 2
  wait-after: 4
  timeout: 30
  retry:
    count: 10
    delay: 20
    break-on: <% $.my_var = true %>

There are different types of policies in Mistral.

1. pause-before

Defines whether Mistral Engine should put the workflow on pause or not before starting a task.

2. wait-before

Defines a delay in seconds that Mistral Engine should wait before starting a task.

3. wait-after

Defines a delay in seconds that Mistral Engine should wait after a task has completed before starting next tasks defined in 'on-success', 'on-error' or 'on-complete'.

4. timeout

Defines a period of time in seconds after which a task will be failed automatically by engine if hasn't completed.

5. retry

Defines a pattern how task should be repeated.

• count - Defines a maximum number of times that a task can be repeated.
• delay - Defines a delay in seconds between subsequent task iterations.
• break-on - Defines a YAQL expression that will break iteration loop if it evaluates to 'true'. If it fires then the task is considered error.
• continue-on - Defines a YAQL expression that will continue iteration loop if it evaluates to 'true'. If it fires then the task is considered successful.

Retry policy can also be configured on a single line as:

task1:
  action: my_action
  retry: count=10 delay=5 break-on=<% $.foo = 'bar' %>

All parameter values for any policy can be defined as YAQL expressions.

Join

Join flow control allows to synchronize multiple parallel workflow branches and aggregate their data.

Full Join (join: all).

YAML example

register_vm_in_load_balancer:
  ...
  on-success:
    - wait_for_all_registrations

register_vm_in_dns:
  ...
  on-success:
    - wait_for_all_registrations

try_to_do_something_without_registration:
  ...
  on-error:
    - wait_for_all_registrations

wait_for_all_registrations:
  join: all
  action: send_email

When a task has property "join" assigned with value "all" the task will run only if all upstream tasks (ones that lead to this task) are completed and corresponding conditions have triggered. Task A is considered an upstream task of Task B if Task A has Task B mentioned in any of its "on-success", "on-error" and "on-complete" clauses regardless of YAQL guard expressions.

Partial Join (join: 2)

YAML example

register_vm_in_load_balancer:
  ...
  on-success:
    - wait_for_all_registrations

register_vm_in_dns:
  ...
  on-success:
    - wait_for_all_registrations

register_vm_in_zabbix:
  ...
  on-success:
    - wait_for_all_registrations

wait_for_two_registrations:
  join: 2
  action: send_email

When a task has property "join" assigned with a numeric value then the task will run once at least this number of upstream tasks are completed and corresponding conditions have triggered. In the example about task "wait_for_two_registrations" will run if two any of "register_vm_xxx" tasks complete.

Discriminator (join: one)

Discriminator is a special case of Partial Join when "join" property has value 1. In this case instead of 1 it is possible to specify special string value "one" which is introduced for symmetry with "all". However, it's up to the user whether to use "1" or "one".

Processing Collections (with-items)

YAML example

---
version: '2.0'

create_vms:
  description: Creating multiple virtual servers using "with-items".
  input:
    - vm_names
    - image_ref
    - flavor_ref
  output:
    vm_ids: <% $.vm_ids %>

  tasks:
    create_servers:
      with-items: vm_name in <% $.vm_names %>
      action: nova.servers_create name=<% $.vm_name %> image=<% $.image_ref %> flavor=<% $.flavor_ref %>
      publish:
        vm_ids: <% $.create_servers.id %>
      on-success:
        - wait_for_servers

    wait_for_servers:
      with-items: vm_id in <% $.vm_ids %>
      action: nova.servers_find id=<% $.vm_id %> status='ACTIVE'
      retry:
        delay: 5
        count: <% $.vm_names.len() * 10 %>

Workflow "create_vms" in this example creates as many virtual servers as we provide in "vm_names" input parameter. E.g., if it is specified vm_names=["vm1", "vm2"] then it'll create servers with these names based on same image and flavor. It is possible because of using "with-items" keyword that makes an action or a workflow associated with a task run multiple times. Value of "with-items" task property contains an expression in the form: <variable_name> in <% YAQL_expression %>.

The most common form is:

with-items:
  - var1 in <% YAQL_expression_1 %>
  - var2 in <% YAQL_expression_2 %>
  ...
  - varN in <% YAQL_expression_N %>

where collections expressed as YAQL_expression_1, YAQL_expression_2, YAQL_expression_N must have equal sizes. When a task gets started Mistral will iterate over all collections in parallel, i.e. number of iterations will be equal to length of any collections.

Note that in case of using "with-items" task result accessible in workflow context as <% $.task_name %> will be a list containing results of corresponding action/workflow calls. If at least one action/workflow call has failed then the whole task will get into ERROR state. It's also possible to apply retry policy for tasks with "with-items" property. In this case retry policy will be relaunching all action/workflow calls according to "with-items" configuration. Other policies can also be used the same way as with regular non "with-items" tasks.

Execution expiration policy

When Mistral is used in production it can be difficult to control the number of completed workflow executions. By default Mistral will store all executions indefinitely and over time the number stored will accumulate. This can be resolved by setting an expiration policy.

By default this feature is disabled.

When enabled, the policy will define a maximum age of an execution in minutes since the last updated time. To enable and set a policy, edit the Mistral configuration file and specify older_than and evaluation_interval in minutes.

[execution_expiration_policy]
older_than = 10080  # 1 week
evaluation_interval = 120  # 2 hours

For the expiration policy to be enabled, both of these configuration options must be set.

• older_than

This defines the maximum age of an execution in minutes since it was last updated. It must be greater or equal to 1.

• evaluation_interval

The evaluation interval defines how frequently Mistral will check and expire old executions. In the above example it is set to two hours, then every two hours Mistral will clean up and look for expired executions.