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Amazon Simple Workflow Service
Developer Guide (API Version 2012-01-25)

Amazon SWF Workflow History

The progress of every workflow execution is recorded in its workflow history, which Amazon SWF maintains. The workflow history is a detailed, complete, and consistent record of every event that occurred since the workflow execution started. An event represents a discrete change in your workflow execution's state, such as a new activity being scheduled or a running activity being completed. The workflow history contains every event that causes the execution state of the workflow execution to change, such as scheduled and completed activities, task timeouts, and signals.

Operations that do not change the state of the workflow execution do not typically appear in the workflow history. For example, the workflow history does not show poll attempts or the use of visibility operations.

The workflow history has several key benefits:

  • It enables applications to be stateless, because all information about a workflow execution is stored in its workflow history.

  • For each workflow execution, the history provides a record of which activities were scheduled, their current status, and their results. The workflow execution uses this information to determine next steps.

  • The history provides a detailed audit trail that you can use to monitor running workflow executions and verify completed workflow executions.

The following is a conceptual view of the e-commerce workflow history:

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Invoice0001 Start Workflow Execution Schedule Verify Order Start Verify Order Activity Complete Verify Order Activity Schedule Charge Credit Card Start Charge Credit Card Activity Complete Charge Credit Card Activity Schedule Ship Order Start Ship Order Activity

In the preceding example, the order is waiting to ship. In the following example, the order is complete. Because the workflow history is cumulative, the newer events are appended:

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Invoice0001 Start Workflow Execution Schedule Verify Order Start Verify Order Activity Complete Verify Order Activity Schedule Charge Credit Card Start Charge Credit Card Activity Complete Charge Credit Card Activity Schedule Ship Order Start Ship Order Activity Complete Ship Order Activity Schedule Record Order Completion Start Record Order Completion Activity Complete Record Order Completion Activity Close Workflow

Programmatically, the events in the workflow execution history are represented as JavaScript Object Notation (JSON) objects. The history itself is a JSON array of these objects. Each event has the following:

In addition, each type of event has a distinct set of descriptive attributes that are appropriate to that type. For example, the ActivityTaskCompleted event has attributes that contain the IDs for the events that correspond to the time that the activity task was scheduled and when it was started, as well as an attribute that holds result data.

You can obtain a copy of the current state of the workflow execution history by using the GetWorkflowExecutionHistory action. In addition, as part of the interaction between Amazon SWF and the decider for your workflow, the decider periodically receives copies of the history.

Below is a section of an example workflow execution history in JSON format.

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[ { "eventId": 11, "eventTimestamp": 1326671603.102, "eventType": "WorkflowExecutionTimedOut", "workflowExecutionTimedOutEventAttributes": { "childPolicy": "TERMINATE", "timeoutType": "START_TO_CLOSE" } }, { "decisionTaskScheduledEventAttributes": { "startToCloseTimeout": "600", "taskList": { "name": "specialTaskList" } }, "eventId": 10, "eventTimestamp": 1326670566.124, "eventType": "DecisionTaskScheduled" }, { "activityTaskTimedOutEventAttributes": { "details": "Waiting for confirmation", "scheduledEventId": 8, "startedEventId": 0, "timeoutType": "SCHEDULE_TO_START" }, "eventId": 9, "eventTimestamp": 1326670566.124, "eventType": "ActivityTaskTimedOut" }, { "activityTaskScheduledEventAttributes": { "activityId": "verification-27", "activityType": { "name": "activityVerify", "version": "1.0" }, "control": "digital music", "decisionTaskCompletedEventId": 7, "heartbeatTimeout": "120", "input": "5634-0056-4367-0923,12/12,437", "scheduleToCloseTimeout": "900", "scheduleToStartTimeout": "300", "startToCloseTimeout": "600", "taskList": { "name": "specialTaskList" } }, "eventId": 8, "eventTimestamp": 1326670266.115, "eventType": "ActivityTaskScheduled" }, { "decisionTaskCompletedEventAttributes": { "executionContext": "Black Friday", "scheduledEventId": 5, "startedEventId": 6 }, "eventId": 7, "eventTimestamp": 1326670266.103, "eventType": "DecisionTaskCompleted" }, { "decisionTaskStartedEventAttributes": { "identity": "Decider01", "scheduledEventId": 5 }, "eventId": 6, "eventTimestamp": 1326670161.497, "eventType": "DecisionTaskStarted" }, { "decisionTaskScheduledEventAttributes": { "startToCloseTimeout": "600", "taskList": { "name": "specialTaskList" } }, "eventId": 5, "eventTimestamp": 1326668752.66, "eventType": "DecisionTaskScheduled" }, { "decisionTaskTimedOutEventAttributes": { "scheduledEventId": 2, "startedEventId": 3, "timeoutType": "START_TO_CLOSE" }, "eventId": 4, "eventTimestamp": 1326668752.66, "eventType": "DecisionTaskTimedOut" }, { "decisionTaskStartedEventAttributes": { "identity": "Decider01", "scheduledEventId": 2 }, "eventId": 3, "eventTimestamp": 1326668152.648, "eventType": "DecisionTaskStarted" }, { "decisionTaskScheduledEventAttributes": { "startToCloseTimeout": "600", "taskList": { "name": "specialTaskList" } }, "eventId": 2, "eventTimestamp": 1326668003.094, "eventType": "DecisionTaskScheduled" } ]

For a detailed list of the different types of events that can appear in the workflow execution history, see the HistoryEvent data type in the Amazon Simple Workflow Service API Reference.

Amazon SWF stores the complete history of all workflow executions for a configurable number of days after the execution closes. This period, which is known as the workflow history retention period, is specified when you register a Domain for your workflow. Domains are discussed in greater detail later in this section.