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 is a feature which could be useful for executing particular tasks on specific Mistral executors. In fact, there are 2 cases:
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 ...
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.
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.
Defines whether Mistral Engine should put the workflow on pause or not before starting a task.
Defines a delay in seconds that Mistral Engine should wait before starting a task.
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’.
Defines a period of time in seconds after which a task will be failed automatically by engine if hasn’t completed.
Defines a pattern how task should be repeated.
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 flow control allows to synchronize multiple parallel workflow branches and aggregate their data.
Full Join (join: all).
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)
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”.
---
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.
For Mistral used in production, it is often hardly to control the number of workflow executions. The number of workflow executions is significantly growing for the long time of Mistral running. The purpose of this feature to delete old workflow executions which has been already completed. The criteria is the time when a workflow execution was updated last time.
By default this feature is disabled.
In order to configure this feature, please open and edit configuration file and specify time in minutes:
[execution_expiration_policy]
older_than = 10080 # Workflow executions older than 1 week will be deleted automatically.