Choice states and examples - CloudFlow - Alibaba Cloud Documentation Center

This topic describes Choice states and provides related examples. This topic also describes how to use conditional expressions in Choice states.

Overview

A Choice state allows the flow to execute different states based on conditions, and is similar to the switch-case statement in programming languages. A Choice state contains multiple choice branches and a default branch. Each choice branch contains the Condition field that specifies a conditional expression and the Next field that specifies the state to execute if the condition is met.

When a flow proceeds to a Choice state, the system evaluates whether the conditional expressions of the choice branches return True in the defined sequence.

If the conditional expression of a choice branch returns True, the states (if any) and the Next instruction (if any) defined in the choice branch are executed.
If no conditional expressions of the choice branches return True, the states and the Next instruction defined in the default branch are executed.

The following table describes the attributes that a Choice state contains.

Attribute	Type	Required	Description	Example
Name	string	Yes	The name of the state.	my state
Description	string	No	The description of the state.	describe it here
Type	string	Yes	The type of the state.	Choice
InputConstructor	map[string]any	No	The input constructor.	See InputConstructor.
Default	string	Yes	The default branch.	my next task
Branches	ChoiceBranch	Yes	The choice branches.	See ChoiceBranch.

ChoiceBranch

Attribute	Type	Required	Description	Example
Condition	string	Yes	The value of this attribute is a conditional expression.	$Input.status=="ready"
Next	string	Yes	The name of the next state that the workflow transits to if the condition is met.	my next task

Important

Choice states do not support the End and Next attributes. The Default attribute of Choice states specifies the state that the workflow transits to by default when no condition in Choice branches is met. The feature of the Default attribute is the same as the feature of the Next attribute. ChoiceBranch can contain the Next attribute.

Example

The following sample flow contains a Choice state.

If the value of status in the input is ready, Pass2 is executed, as specified in the first choice branch.
If the value of status in the input is not ready, Pass1 is executed, as specified in the default branch.

Type: StateMachine
Name: my-wkfl
SpecVersion: v1
StartAt: Choice1
States:
  - Type: Choice
    Name: Choice1
    Branches:
      - Condition:$Input.status=="ready"
        Next: Pass2
    Default: Pass1
  - Type: Pass
    Name: Pass1
    End: true
  - Type: Pass
    Name: Pass2
    End: true

Conditional expressions

The conditional expressions supported by the system consist of the following operations and variables. You can determine whether the conditions specified by a conditional expression are met based on the Boolean value returned by the expression.

Comparison operations: >>, >=, <, <=, ==, and ! =. The comparison operations are suitable for strings and numbers.
Logic operations: || and &&.
String constants: A string constant is enclosed in double quotation marks (") or grave accents (`). Examples: "foobar" or `foobar`.
Numeric constants: Examples: 1 and 12.5.
Boolean constants: true and false.
Prefixes: ! and -.
Contain: in, which is used to determine whether an array contains a value or whether an object contains a key value.

Sample expression

The following example shows the results of different conditional expressions based on the specified input. You can use the $Context and $Input variables to reference data in the context and input of a state in conditional expressions. For more information, see Data passing.

{
	"a": 1,
	"b": {
		"b1": true,
		"b2": "ready"
	},
	"c": [1, 2, 3],
	"d": 1,
	"e": 1,
	"f": {
		"f1": false,
		"f2": "inprogress"
	}
}

Conditional expression	Execution results
`$Input.a==1`	true
`$Input.a==2`	false
`$Input.a>0`	true
`0<$Input.a`	true
`$Input.a>=1`	true
`$Input.a!=2`	true
`$Input.b.b1`	true
`$Input.b.b1==true`	true
`$Input.b.b1==false`	false
`$Input.b.b2=="ready"`	true
`$Input.b.b2=="inprogress"`	false
`$Input.a==1 && $Input.b.b1`	true
`$Input.a==2 && $Input.b.b1`	false
`$Input.c[0]==1`	true
`$Input.c[0]==$Input.a`	true