Cadence

# Versioning

The definition code of a Cadence workflow must be deterministic because Cadence uses event sourcing to reconstruct the workflow state by replaying the saved history event data on the workflow definition code. This means that any incompatible update to the workflow definition code could cause a non-deterministic issue if not handled correctly.

# workflow.GetVersion()

Consider the following workflow definition:

func MyWorkflow(ctx workflow.Context, data string) (string, error) {
    ao := workflow.ActivityOptions{
        ScheduleToStartTimeout: time.Minute,
        StartToCloseTimeout:    time.Minute,
    }
    ctx = workflow.WithActivityOptions(ctx, ao)
    var result1 string
    err := workflow.ExecuteActivity(ctx, ActivityA, data).Get(ctx, &result1)
    if err != nil {
        return "", err
    }
    var result2 string
    err = workflow.ExecuteActivity(ctx, ActivityB, result1).Get(ctx, &result2)
    return result2, err
}

Now let's say we have replaced ActivityA with ActivityC, and deployed the updated code. If there is an existing workflow execution that was started by the original version of the workflow code, where ActivityA had already completed and the result was recorded to history, the new version of the workflow code will pick up that workflow execution and try to resume from there. However, the workflow will fail because the new code expects a result for ActivityC from the history data, but instead it gets the result for ActivityA. This causes the workflow to fail on the non-deterministic error.

Thus we use workflow.GetVersion().

var err error
v := workflow.GetVersion(ctx, "Step1", workflow.DefaultVersion, 1)
if v == workflow.DefaultVersion {
    err = workflow.ExecuteActivity(ctx, ActivityA, data).Get(ctx, &result1)
} else {
    err = workflow.ExecuteActivity(ctx, ActivityC, data).Get(ctx, &result1)
}
if err != nil {
    return "", err
}
var result2 string
err = workflow.ExecuteActivity(ctx, ActivityB, result1).Get(ctx, &result2)
return result2, err

When workflow.GetVersion() is run for the new workflow execution, it records a marker in the workflow history so that all future calls to GetVersion for this change ID--Step 1 in the example--on this workflow execution will always return the given version number, which is 1 in the example.

If you make an additional change, such as replacing ActivityC with ActivityD, you need to add some additional code:

v := workflow.GetVersion(ctx, "Step1", workflow.DefaultVersion, 2)
if v == workflow.DefaultVersion {
    err = workflow.ExecuteActivity(ctx, ActivityA, data).Get(ctx, &result1)
} else if v == 1 {
    err = workflow.ExecuteActivity(ctx, ActivityC, data).Get(ctx, &result1)
} else {
    err = workflow.ExecuteActivity(ctx, ActivityD, data).Get(ctx, &result1)
}

Note that we have changed maxSupported from 1 to 2. A workflow that had already passed this GetVersion() call before it was introduced will return DefaultVersion. A workflow that was run with maxSupported set to 1, will return 1. New workflows will return 2.

After you are sure that all of the workflow executions prior to version 1 have completed, you can remove the code for that version. It should now look like the following:

v := workflow.GetVersion(ctx, "Step1", 1, 2)
if v == 1 {
    err = workflow.ExecuteActivity(ctx, ActivityC, data).Get(ctx, &result1)
} else {
    err = workflow.ExecuteActivity(ctx, ActivityD, data).Get(ctx, &result1)
}

You'll note that minSupported has changed from DefaultVersion to 1. If an older version of the workflow execution history is replayed on this code, it will fail because the minimum expected version is 1. After you are sure that all of the workflow executions for version 1 have completed, then you can remove 1 so that your code would look like the following:

_ := workflow.GetVersion(ctx, "Step1", 2, 2)
err = workflow.ExecuteActivity(ctx, ActivityD, data).Get(ctx, &result1)

Note that we have preserved the call to GetVersion(). There are two reasons to preserve this call:

  1. This ensures that if there is a workflow execution still running for an older version, it will fail here and not proceed.
  2. If you need to make additional changes for Step1, such as changing ActivityD to ActivityE, you only need to update maxVersion from 2 to 3 and branch from there.

You only need to preserve the first call to GetVersion() for each changeID. All subsequent calls to GetVersion() with the same change ID are safe to remove. If necessary, you can remove the first GetVersion() call, but you need to ensure the following:

  • All executions with an older version are completed.
  • You can no longer use Step1 for the changeID. If you need to make changes to that same part in the future, such as change from ActivityD to ActivityE, you would need to use a different changeID like Step1-fix2, and start minVersion from DefaultVersion again. The code would look like the following:
v := workflow.GetVersion(ctx, "Step1-fix2", workflow.DefaultVersion, 1)
if v == workflow.DefaultVersion {
    err = workflow.ExecuteActivity(ctx, ActivityD, data).Get(ctx, &result1)
} else {
    err = workflow.ExecuteActivity(ctx, ActivityE, data).Get(ctx, &result1)
}

Upgrading a workflow is straightforward if you don't need to preserve your currently running workflow executions. You can simply terminate all of the currently running workflow executions and suspend new ones from being created while you deploy the new version of your workflow code, which does not use GetVersion(), and then resume workflow creation. However, that is often not the case, and you need to take care of the currently running workflow executions, so using GetVersion() to update your code is the method to use.

However, if you want your currently running workflows to proceed based on the current workflow logic, but you want to ensure new workflows are running on new logic, you can define your workflow as a new WorkflowType, and change your start path (calls to StartWorkflow()) to start the new workflow type.

# Sanity checking

The Cadence client SDK performs a sanity check to help prevent obvious incompatible changes. The sanity check verifies whether a decision made in replay matches the event recorded in history, in the same order. The decision is generated by calling any of the following methods:

  • workflow.ExecuteActivity()
  • workflow.ExecuteChildWorkflow()
  • workflow.NewTimer()
  • workflow.Sleep()
  • workflow.SideEffect()
  • workflow.RequestCancelWorkflow()
  • workflow.SignalExternalWorkflow()
  • workflow.UpsertSearchAttributes()

Adding, removing, or reordering any of the above methods triggers the sanity check and results in a non-deterministic error.

The sanity check does not perform a thorough check. For example, it does not check on the activity's input arguments or the timer duration. If the check is enforced on every property, then it becomes too restricted and harder to maintain the workflow code. For example, if you move your activity code from one package to another package, that changes the ActivityType, which technically becomes a different activity. But, we don't want to fail on that change, so we only check the function name part of the ActivityType.

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