Common class structure

Here is a common template for class declarations. Note, that it is in the YAML format.

 1Name: class name
 2Namespaces: namespaces specification
 3Extends: [list of parent classes]
 4Properties: properties declaration
 6    methodName:
 7        Arguments:
 8            - list
 9            - of
10            - arguments
11        Body:
12            - list
13            - of
14            - instructions

Thus MuranoPL class is a YAML dictionary with predefined key names, all keys except for Name are optional and can be omitted (but must be valid if specified).

Class name

Class names are alphanumeric names of the classes. Traditionally, all class names begin with an upper-case letter symbol and are written in PascalCasing.

In MuranoPL all class names are unique. At the same time, MuranoPL supports namespaces. So, in different namespaces you can have classes with the same name. You can specify a namespace explicitly, like ns:MyName. If you omit the namespace specification, MyName is expanded using the default namespace =:. Therefore, MyName equals =:MyName if = is a valid namespace.


Namespaces declaration specifies prefixes that can be used in the class body to make long class names shorter.

    std: io.murano

In the example above, the srv: Something class name is automatically translated to

= means the current namespace, so that MyClass means

If the class name contains the period (.) in its name, then it is assumed to be already fully namespace qualified and is not expanded. Thus ns.Myclass remains as is.


To make class names globally unique, we recommend specifying a developer’s domain name as a part of the namespace.


MuranoPL supports multiple inheritance. If present, the Extends section shows base classes that are extended. If the list consists of a single entry, then you can write it as a scalar string instead of an array. If you do not specify any parents or omit the key, then the class extends io.murano.Object. Thus, io.murano.Object is the root class for all class hierarchies.


Properties are class attributes that together with methods create public class interface. Usually, but not always, properties are the values, and reference other objects that have to be entered in an environment designer prior to a workflow invocation.

Properties have the following declaration format:

    Contract: property contract
    Usage: property usage
    Default: property default


Contract is a YAQL expression that says what type of the value is expected for the property as well as additional constraints imposed on a property. Using contracts you can define what value can be assigned to a property or argument. In case of invalid input data it may be automatically transformed to confirm to the contract. For example, if bool value is expected and user passes any not null value it will be converted to True. If converting is impossible exception ContractViolationException will be raised.

The following contracts are available:



an integer value (may be null). String values consisting of digits are converted to integers
a mandatory integer
a string. If the value is not a string, it is converted to a string
bools are true and false. 0 is converted to false, other integers to true
value must be a reference to an instance of specified class name
value must be a dictionary with object-model representation of specified class name
$.class(ns:ClassName, ns:DefaultClassName)
create instance of the ns:DefaultClassName class if no instance provided
$.class(ns:Name).check($.p = 12)
the value must be of the ns:Name type and have the p property equal to 12
a current object must be direct or indirect owner of the value
the value must be owned by any object except current one
an array of integers. Similar to other types.
[$.int().check($ > 0)]
an array of the positive integers (thus not null)
[$.int(), $.string()]
an array that has at least two elements, first is int and others are strings
[$.int(), 2]
[$.int(), 2, 5]
an array of ints with at least 2 items
an array of ints with at least 2 items, and maximum of 5 items
{ A: $.int(), B: [$.string()] }
the dictionary with the A key of the int type and B - an array of strings
any scalar or data structure as is
any array
any dictionary
{ $.string().notNull(): $.int().notNull() }
dictionary string -> int
A: StringMap
$.string().notNull(): $
the dictionary with the A key that must be equal to StringMap, and other keys be
any scalar or data structure
$.check($ in $this.myStaticMethod())
the value must be equal to one of a member of a list returned by static method of the class
the static method of the class must return true for the value

In the example below property port must be int value greater than 0 and less than 65536; scope must be a string value and one of ‘public’, ‘cloud’, ‘host’ or ‘internal’, and protocol must be a string value and either ‘TCP’ or ‘UDP’. When user passes some values to these properties it will be checked that values confirm to the contracts.

  =: io.murano.apps.docker
  std: io.murano

Name: ApplicationPort

    Contract: $.int().notNull().check($ > 0 and $ < 65536)

    Contract: $.string().notNull().check($ in list(public, cloud, host, internal))
    Default: private

    Contract: $.string().notNull().check($ in list(TCP, UDP))
    Default: TCP

        port: $.port
        scope: $.scope
        protocol: $.protocol

The template contract does the same validation as the class contract, but does not require the actual object to be passed as a property or argument. Instead it allows to create an object from the given template later. Also you can exclude some of the properties from validation and provide them later in the body of the method.

Consider the following example:

  =: io.murano.applications
  res: io.murano.resources
  std: io.murano

Name: TemplateServerProvider

    Contract: $.template(res:Instance, excludeProperties => [name]).notNull()
    Contract: $.string().notNull()
    Contract: $.int().check($ > 0)

      - index:
          Contract: $.int().notNull()
      - owner:
          Contract: $.class(std:Object)
      - If: $index < $this.threshold
          - $template: $this.template
          - $ $this.serverNamePattern.format($index)
          - $template['?'].name: format('Server {0}', $index)
          - Return: new($template, $owner)
          - Return: null

In the example above the class has the template property that is validated by the template contract. It holds the template of the object of the Instance class or its inheritor. In the createReplica method template is used to dynamically create instances in runtime considering some conditions and customizing the name property of an instance, as it was excluded from validation.

You still can pass an actual object to the property or argument with the template contract, but it will be automatically converted to its object model representation.

Property usage

Usage states the purpose of the property. This implies who and how can access it. The following usages are available:

Input property. Values of such properties are obtained from a user and cannot be modified in MuranoPL workflows. This is the default value for the Usage key.
A value is obtained from executing MuranoPL workflow and cannot be modified by a user.
A value can be modified both by user and by workflow.
The same as In but once workflow is executed a property cannot be changed neither by a user nor by a workflow.
A property is visible only from within workflows. It is neither read from input nor serialized to a workflow output.
Property is defined on a class rather than on an instance. See Static methods and properties for details.
A property allows to have per-class configuration. A value is obtained from the config file rather than from the object model. These config files are stored in a special folder that is configured in the [engine] section of the Murano config file under the class_configs key.

The usage attribute is optional and can be omitted (which implies In).

If the workflow tries to write to a property that is not declared with one of the types above, it is considered to be private and accessible only to that class (and not serialized to output and thus would be lost upon the next deployment). An attempt to read the property that was not initialized results in an exception.


Default is a value that is used if the property value is not mentioned in the input object model, but not when it is set to null. Default, if specified, must conform to a declared property contract. If Default is not specified, then null is the default.

For properties that are references to other classes, Default can modify a default value of the referenced objects. For example:

 Contract: $.class(MyClass)
 Default: {a: 12}

This overrides default for the a property of MyClass for instance of MyClass that is created for this property.


Workflows are the methods that describe how the entities that are represented by MuranoPL classes are deployed.

In a typical scenario, the root object in an input data model is of the io.murano.Environment type, and has the deploy method. This method invocation causes a series of infrastructure activities (typically, a Heat stack modification) and the deployment scripts execution initiated by VM agents commands. The role of the workflow is to map data from the input object model, or a result of previously executed actions, to the parameters of these activities and to initiate these activities in a correct order.


Methods have input parameters, and can return a value to a caller. Methods are defined in the Workflow section of the class using the following template:

    Scope: Public
       - list
       - of
       - arguments
       - list
       - of
       - instructions

Public is an optional parameter that specifies methods to be executed by direct triggering after deployment.

Method arguments

Arguments are optional too, and are declared using the same syntax as class properties. Same as properties, arguments also have contracts and optional defaults.

Unlike class properties Arguments may have a different set of Usages:

Regular method argument. Holds a single value based on its contract. This is the default value for the Usage key.
A variable length argument. Method body sees it as a list of values, each matching a contract of the argument.
A keywrod-based argument, Method body sees it as a dict of values, with keys being valid keyword strings and values matching a contract of the argument.

Arguments example:

    - rcName:
        Contract: $.string().notNull()
    - newSize:
        Contract: $.int().notNull()
    - rest:
        Contract: $.int()
        Usage: VarArgs
    - others:
        Contract: $.int()
        Usage: KwArgs

Method body

The Method body is an array of instructions that get executed sequentially. There are 3 types of instructions that can be found in a workflow body:

  • Expressions,

  • Assignments,

  • Block constructs.

Method usage

Usage states the purpose of the method. This implies who and how can access it. The following usages are available:

Normal instance method.
Static method that does not require class instance. See Static methods and properties for details.
Extension static method that extends some other type. See Extension methods for details.
Method can be invoked from outside (using Murano API). This option is deprecated for the package format versions > 1.3 in favor of Scope: Public and occasionally will be no longer supported. See Murano actions for details.

The Usage attribute is optional and can be omitted (which implies Runtime).

Method scope

The Scope attribute declares method visibility. It can have two possible values:

  • Session - regular method that is accessible from anywhere in the current execution session. This is the default if the attribute is omitted;

  • Public - accessible anywhere, both within the session and from outside through the API call.

The Scope attribute is optional and can be omitted (which implies Session).


Expressions are YAQL expressions that are executed for their side effect. All accessible object methods can be called in the expression using the $obj.methodName(arguments) syntax.



invoke method ‘methodName’ on this (self) object
invocation of method on object that is in property
$.method(1, 2, 3)
methods can have arguments
$.method(1, 2, thirdParameter => 3)
named parameters also supported
list($.foo().bar($, $p)
complex expressions can be constructed


Assignments are single key dictionaries with a YAQL expression as a key and arbitrary structure as a value. Such a construct is evaluated as an assignment.



$x: value
assigns value to the local variable $x
$.x: value
$this.x: value
assign value to the object’s property
$.x: $.y
copies the value of the property y to the property x
$x: [$a, $b]
sets $x to the array of two values: $a and $b
NestedKey: $variable
structures of any level of complexity can be evaluated
$.x[0]: value
assigns value to the first array entry of the x property
$.x: $.x.append(value)
appends value to the array in the x property
$.x: $.x.insert(1, value)
inserts value into position 1 of the array in the x property
$x: list($a, $b).delete(0)
sets $x to the list without the item at index 0
$.x.key.subKey: value
$.x[key][subKey]: value
deep dictionary modification

Block constructs

Block constructs control a program flow. They are dictionaries that have strings as all their keys.

The following block constructs are available:



Return: value
Returns value from a method
If: predicate()
- code
- block
- code
- block
predicate() is a YAQL expression that must be evaluated to True or False
The Else section is optional
One-line code blocks can be written as scalars rather than an array.
While: predicate()
- code
- block
predicate() must be evaluated to True or False
For: variableName
In: collection
- code
- block
collection must be a YAQL expression returning iterable collection or evaluatable array as in assignment instructions, for example, [1, 2, $x]
Inside a code block loop, a variable is accessible as $variableName
- code
- block
Repeats the code block specified number of times
Breaks from loop
- code
- block
- code
- block
Value: $valExpression()
- code
- block
Matches the result of $valExpression() against a set of possible values (cases). The code block of first matched case is executed.
If no case matched and the default key is present than the Default code block get executed.
The case values are constant values (not expressions).
- code
- block
- code
- block
- code
- block
All code blocks that have their predicate evaluated to True are executed, but the order of predicate evaluation is not fixed.
The Default key is optional.
If no predicate evaluated to True, the Default code block get executed.
- code
- block
Limit: 5
Executes all instructions in code block in a separate green threads in parallel.
The limit is optional and means the maximum number of concurrent green threads.
- code
- block
With: keyError
As: e
- code
- block
- code
- block
- code
- block
Try and Catch are keywords that represent the handling of exceptions due to data or coding errors during program execution. A Try block is the block of code in which exceptions occur. A Catch block is the block of code, that is executed if an exception occurred.
Exceptions are not declared in Murano PL. It means that exceptions of any types can be handled and generated. Generating of exception can be done with construct: Throw: keyError.
The Else is optional block. Else block is executed if no exception occurred.
The Finally also is optional. It’s a place to put any code that will be executed, whether the try-block raised an exception or not.

Notice, that if you have more than one block construct in your workflow, you need to insert dashes before each construct. For example:

  - If: predicate1()
      - code
      - block
  - While: predicate2()
      - code
      - block

Object model

Object model is a JSON serialized representation of objects and their properties. Everything you do in the OpenStack dashboard is reflected in an object model. The object model is sent to the Application catalog engine when the user decides to deploy the built environment. On the engine side, MuranoPL objects are constructed and initialized from the received Object model, and a predefined method is executed on the root object.

Objects are serialized to JSON using the following template:

 2    "?": {
 3        "id": "globally unique object ID (UUID)",
 4        "type": "fully namespace-qualified class name",
 6        "optional designer-related entries can be placed here": {
 7            "key": "value"
 8        }
 9    },
11    "classProperty1": "propertyValue",
12    "classProperty2": 123,
13    "classProperty3": ["value1", "value2"],
15    "reference1": {
16        "?": {
17            "id": "object id",
18            "type": "object type"
19        },
21        "property": "value"
22    },
24    "reference2": "referenced object id"

Objects can be identified as dictionaries that contain the ? entry. All system fields are hidden in that entry.

There are two ways to specify references:

  1. reference1 as in the example above. This method allows inline definition of an object. When the instance of the referenced object is created, an outer object becomes its parent/owner that is responsible for the object. The object itself may require that its parent (direct or indirect) be of a specified type, like all applications require to have Environment somewhere in a parent chain.

  2. Referring to an object by specifying other object ID. That object must be defined elsewhere in an object tree. Object references distinguished from strings having the same value by evaluating property contracts. The former case would have $.class(Name) while the later - the $.string() contract.