State machine structure in Amazon States Language for Step Functions workflows
State machines are defined using JSON text that represents a structure containing the following fields.
-
Comment
(Optional) -
A human-readable description of the state machine.
-
StartAt
(Required) -
A string that must exactly match (is case sensitive) the name of one of the state objects.
-
TimeoutSeconds
(Optional) -
The maximum number of seconds an execution of the state machine can run. If it runs longer than the specified time, the execution fails with a
States.Timeout
Error Name. -
Version
(Optional) -
The version of the Amazon States Language used in the state machine (default is "1.0").
-
States
(Required) -
An object containing a comma-delimited set of states.
The States
field contains States.
{
"State1" : {
},
"State2" : {
},
...
}
A state machine is defined by the states it contains and the relationships between them.
The following is an example.
{
"Comment": "A Hello World example of the Amazon States Language using a Pass state",
"StartAt": "HelloWorld",
"States": {
"HelloWorld": {
"Type": "Pass",
"Result": "Hello World!",
"End": true
}
}
}
When an execution of this state machine is launched, the system begins with the state
referenced in the StartAt
field ("HelloWorld"
). If this state has
an "End": true
field, the execution stops and returns a result. Otherwise,
the system looks for a "Next":
field and continues with that state
next. This process repeats until the system reaches a terminal state (a state with
"Type": "Succeed"
, "Type": "Fail"
, or "End": true
), or a runtime
error occurs.
The following rules apply to states within a state machine:
-
States can occur in any order within the enclosing block, but the order in which they're listed doesn't affect the order in which they're run. The contents of the states determines this order.
-
Within a state machine, there can be only one state that's designated as the
start
state, designated by the value of theStartAt
field in the top-level structure. This state is the one that is executed first when the execution starts. -
Any state for which the
End
field istrue
is considered anend
(orterminal
) state. Depending on your state machine logic—for example, if your state machine has multiple branches of execution—you might have more than oneend
state. -
If your state machine consists of only one state, it can be both the
start
state and theend
state.
Common state fields in workflows
The following fields are common to all state elements.
-
Type
(Required) -
The state's type.
-
Next
-
The name of the next state that is run when the current state finishes. Some state types, such as
Choice
, allow multiple transition states.If the current state is the last state in your workflow, or a terminal state, such as Succeed workflow state or Fail workflow state, you don't need to specify the
Next
field. -
End
-
Designates this state as a terminal state (ends the execution) if set to
true
. There can be any number of terminal states per state machine. Only one ofNext
orEnd
can be used in a state. Some state types, such asChoice
, or terminal states, such as Succeed workflow state and Fail workflow state, don't support or use theEnd
field. -
Comment
(Optional) -
Holds a human-readable description of the state.
-
InputPath
(Optional) -
A path that selects a portion of the state's input to be passed to the state's task for processing. If omitted, it has the value
$
which designates the entire input. For more information, see Input and Output Processing. -
OutputPath
(Optional) -
A path that selects a portion of the state's output to be passed to the next state. If omitted, it has the value
$
which designates the entire output. For more information, see Input and Output Processing.