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Features - Go SDK feature guide

The Features section of the Temporal Developer's guide provides basic implementation guidance on how to use many of the development features available to Workflows and Activities in the Temporal Platform.

In this section you can find the following:

How to develop with Signals​

A Signal is a message sent to a running Workflow Execution.

Signals are defined in your code and handled in your Workflow Definition. Signals can be sent to Workflow Executions from a Temporal Client or from another Workflow Execution.

How to define a Signal​

A Signal has a name and can have arguments.

  • The name, also called a Signal type, is a string.
  • The arguments must be serializable.

Structs should be used to define Signals and carry data, as long as the struct is serializable via the Data Converter. The Receive() method on the Data Converter decodes the data into the Struct within the Workflow. Only public fields are serializable.

MySignal struct {
	Message string // serializable
	message string // not serializable
}

How to handle a Signal​

Workflows listen for Signals by the Signal's name.

Use the GetSignalChannel() API from the go.temporal.io/sdk/workflow package to get the Signal Channel.

A common use-case is to block a Workflow while waiting for a Signal, like in the following snippet:

func YourWorkflowDefinition(ctx workflow.Context, param YourWorkflowParam) error {
  // ...
  var signal MySignal
  signalChan := workflow.GetSignalChannel(ctx, "your-signal-name")
  signalChan.Receive(ctx, &signal)
  if len(signal.Message) > 0 && signal.Message != "SOME_VALUE" {
      return errors.New("signal")
  }
  // ...
}

Alternatively, you might want the Workflow to proceed and still be capable of handling external Signals.

func YourWorkflowDefinition(ctx workflow.Context, param YourWorkflowParam) error {
   var signal MySignal
   signalChan := workflow.GetSignalChannel(ctx, "your-signal-name")
 	workflow.Go(ctx, func(ctx workflow.Context) {
 		for {
 			selector := workflow.NewSelector(ctx)
 			selector.AddReceive(signalChan, func(c workflow.ReceiveChannel, more bool) {
 				c.Receive(ctx, &mySignal)
 			})
 			selector.Select(ctx)
 		}
 	})
   // submit activity one
   // signal can be received while activity one is pending

 }

In the example above, the Workflow code uses workflow.GetSignalChannel to open a workflow.Channel for the Signal type (identified by the Signal name).

Before completing the Workflow or using Continue-As-New, make sure to do an asynchronous drain on the Signal channel. Otherwise, the Signals will be lost.

How to send a Signal from a Temporal Client​

When a Signal is sent successfully from the Temporal Client, the WorkflowExecutionSignaled Event appears in the Event History of the Workflow that receives the Signal.

Use the SignalWorkflow() method on an instance of the Go SDK Temporal Client to send a Signal to a Workflow Execution.

Pass in both the Workflow Id and Run Id to uniquely identify the Workflow Execution. If only the Workflow Id is supplied (provide an empty string as the Run Id param), the Workflow Execution that is Running receives the Signal.

// ...
signal := MySignal {
  Message: "Some important data",
}
err = temporalClient.SignalWorkflow(context.Background(), "your-workflow-id", runID, "your-signal-name", signal)
if err != nil {
	log.Fatalln("Error sending the Signal", err)
	return
}
// ...

Possible errors:

  • serviceerror.NotFound
  • serviceerror.Internal
  • serviceerror.Unavailable

How to send a Signal from a Workflow​

A Workflow can send a Signal to another Workflow, in which case it's called an External Signal.

When an External Signal is sent:

A Signal can be sent from within a Workflow to a different Workflow Execution using the SignalExternalWorkflow API from the go.temporal.io/sdk/workflow package.

// ...
func YourWorkflowDefinition(ctx workflow.Context, param YourWorkflowParam) error {
  //...
  signal := MySignal {
    Message: "Some important data",
  }
  err :=  workflow.SignalExternalWorkflow(ctx, "some-workflow-id", "", "your-signal-name", signal).Get(ctx, nil)
  if err != nil {
    // ...
  }
// ...
}

How to Signal-With-Start​

Signal-With-Start is used from the Client. It takes a Workflow Id, Workflow arguments, a Signal name, and Signal arguments.

If there's a Workflow running with the given Workflow Id, it will be signaled. If there isn't, a new Workflow will be started and immediately signaled.

Use the SignalWithStartWorkflow() API on the Go SDK Temporal Client to start a Workflow Execution (if not already running) and pass it the Signal at the same time.

Because the Workflow Execution might not exist, this API does not take a Run ID as a parameter

// ...
signal := MySignal {
  Message: "Some important data",
}
err = temporalClient.SignalWithStartWorkflow(context.Background(), "your-workflow-id", "your-signal-name", signal)
if err != nil {
	log.Fatalln("Error sending the Signal", err)
	return
}

How to develop with Queries​

A Query is a synchronous operation that is used to get the state of a Workflow Execution.

How to define a Query​

A Query has a name and can have arguments.

  • The name, also called a Query type, is a string.
  • The arguments must be serializable.

In Go, a Query type, also called a Query name, is a string value.

queryType := "your_query_name"

How to handle a Query​

Queries are handled by your Workflow.

Don’t include any logic that causes Command generation within a Query handler (such as executing Activities). Including such logic causes unexpected behavior.

Use the SetQueryHandler API from the go.temporal.io/sdk/workflow package to set a Query Handler that listens for a Query by name.

The handler must be a function that returns two values:

  1. A serializable result
  2. An error

The handler function can receive any number of input parameters, but all input parameters must be serializable. The following sample code sets up a Query Handler that handles the current_state Query type:

func YourWorkflow(ctx workflow.Context, input string) error {
  currentState := "started" // This could be any serializable struct.
  queryType := "current_state"
  err := workflow.SetQueryHandler(ctx, queryType, func() (string, error) {
    return currentState, nil
  })
  if err != nil {
    currentState = "failed to register query handler"
    return err
  }
  // Your normal Workflow code begins here, and you update the currentState as the code makes progress.
  currentState = "waiting timer"
  err = NewTimer(ctx, time.Hour).Get(ctx, nil)
  if err != nil {
    currentState = "timer failed"
    return err
  }
  currentState = "waiting activity"
  ctx = WithActivityOptions(ctx, yourActivityOptions)
  err = ExecuteActivity(ctx, YourActivity, "your_input").Get(ctx, nil)
  if err != nil {
    currentState = "activity failed"
    return err
  }
  currentState = "done"
  return nil
}

For example, suppose your query handler function takes two parameters:

err := workflow.SetQueryHandler(ctx, "current_state", func(prefix string, suffix string) (string, error) {
    return prefix + currentState + suffix, nil
})

How to send a Query​

Queries are sent from a Temporal Client.

Use the QueryWorkflow() API or the QueryWorkflowWithOptions API on the Temporal Client to send a Query to a Workflow Execution.

// ...
response, err := temporalClient.QueryWorkflow(context.Background(), workflowID, runID, queryType)
if err != nil {
  // ...
}
// ...

You can pass an arbitrary number of arguments to the QueryWorkflow() function.

// ...
response, err := temporalClient.QueryWorkflow(context.Background(), workflowID, runID, queryType, "foo", "baz")
if err != nil {
  // ...
}
// ...

The value of response returned by the Query needs to be decoded into result. Because this is a future, use Get() on response to get the result, such as a string in this example.

var result string
if err != response.Get(&result); err != nil {
  // ...
}
log.Println("Received Query result. Result: " + result)

How to develop with Updates​

An Update is an operation that can mutate the state of a Workflow Execution and return a response.

How to define an Update in Go​

In Go, you define an Update type, also known as an Update name, as a string value. You must ensure the arguments and result are serializable. When sending and receiving the Update, use the Update name as an identifier. The name does not link to the data type(s) sent with the Update. Ensure that every Workflow listening to the same Update name can handle the same Update arguments.

View the source code

in the context of the rest of the application code.

// YourUpdateName holds a string value used to correlate Updates.
const YourUpdateName = "your_update_name"
// ...
func YourUpdatableWorkflow(ctx workflow.Context, param WFParam) (WFResult, error) {
// ...
	err := workflow.SetUpdateHandler(ctx, YourUpdateName, func(ctx workflow.Context, arg YourUpdateArg) (YourUpdateResult, error) {
// ...
	}
// ...
}

How to handle an Update in Go​

Register an Update handler for a given name using the SetUpdateHandler API from the go.temporal.io/sdk/workflow package. The handler function can accept multiple serializable input parameters, but we recommend using only a single parameter. This practice enables you to add fields in future versions while maintaining backward compatibility. You can optionally include a workflow.Context parameter in the first position of the function. The function can return either a serializable value with an error or just an error. The Workflow's WorkflowPanicPolicy configuration determines how panics are handled inside the Handler function. WorkflowPanicPolicy is set in the Worker Options.

Update handlers, unlike Query handlers, can change Workflow state.

View the source code

in the context of the rest of the application code.

// ...
func YourUpdatableWorkflow(ctx workflow.Context, param WFParam) (WFResult, error) {
	counter := param.StartCount
	err := workflow.SetUpdateHandler(ctx, YourUpdateName, func(ctx workflow.Context, arg YourUpdateArg) (YourUpdateResult, error) {
		counter += arg.Add
		result := YourUpdateResult{
			Total: counter,
		}
		return result, nil
	})
// ...
}

How to set an Update validator function in go​

Validate certain aspects of the data sent to the Workflow using an Update validator function. For instance, a counter Workflow might never want to accept a non-positive number. Invoke the SetUpdateHandlerWithOptions API and define a validator function as one of the options.

When you use a Validator function, the Worker receives the Update first, before any Events are written to the Event History. If the Update is rejected, it's not recorded in the Event History. If it's accepted, the WorkflowExecutionUpdateAccepted Event occurs. Afterwards, the Worker executes the accepted Update and, upon completion, a WorkflowExecutionUpdateCompleted Event gets written into the Event History. The Validator function, unlike the Update Handler, can not change the state of the Workflow.

The platform treats a panic in the Validator function as a rejection of the Update."

View the source code

in the context of the rest of the application code.

// UpdatableWorkflowWithValidator is a Workflow Definition.
// This Workflow Definition has an Update handler that uses the isPositive() validator function.
// After setting the Update hanlder it sleeps for 1 minutue.
// Updates can be sent to the Workflow during this time.
func UpdatableWorkflowWithValidator(ctx workflow.Context, param WFParam) (WFResult, error) {
	counter := param.StartCount
	err := workflow.SetUpdateHandlerWithOptions(
		ctx, YourValidatedUpdateName,
		func(ctx workflow.Context, arg YourUpdateArg) (YourUpdateResult, error) {
// ...
		},
		// Set the isPositive validator.
		workflow.UpdateHandlerOptions{Validator: isPositive},
	)
	if err != nil {
		return WFResult{}, err
	}
// ...
}

// isPositive is a validator function.
// It returns an error if the int value is below 1.
// This function can not change the state of the Workflow.
// workflow.Context can be used to log
func isPositive(ctx workflow.Context, u YourUpdateArg) error {
	log := workflow.GetLogger(ctx)
	if u.Add < 1 {
		log.Debug("Rejecting non-positive number, positive integers only", "UpdateValue", u.Add)
		return fmt.Errorf("addend must be a positive integer (%v)", u.Add)
	}
	log.Debug("Accepting Update", "UpdateValue", u.Add)
	return nil
}

How to send an Update from a Temporal Client in Go​

Invoke the UpdateWorkflow() method on an instance of the Go SDK Temporal Client to dispatch an Update to a Workflow Execution.

You must provide the Workflow Id, but specifying a Run Id is optional. If you supply only the Workflow Id (and provide an empty string as the Run Id param), the currently running Workflow Execution receives the Update.

View the source code

in the context of the rest of the application code.

func main() {
// ...
	// Set the Update argument values.
	updateArg := updates.YourUpdateArg{
		Add: n,
	}
	// Call the UpdateWorkflow API.
	// A blank RunID means that the Update is routed to the most recent Workflow Run of the specified Workflow ID.
	updateHandle, err := temporalClient.UpdateWorkflow(context.Background(), updates.YourUpdateWFID, "", updates.YourUpdateName, updateArg)
	if err != nil {
		log.Fatalln("Error issuing Update request", err)
	}
	// Get the result of the Update.
	var updateResult updates.YourUpdateResult
	err = updateHandle.Get(context.Background(), &updateResult)
	if err != nil {
		log.Fatalln("Update encountered an error", err)
	}
	log.Println("Update succeeded, new total: ", updateResult.Total)
}

Workflow timeouts​

Each Workflow timeout controls the maximum duration of a different aspect of a Workflow Execution.

Workflow timeouts are set when starting the Workflow Execution.

Create an instance of StartWorkflowOptions from the go.temporal.io/sdk/client package, set a timeout, and pass the instance to the ExecuteWorkflow call.

Available timeouts are:

  • WorkflowExecutionTimeout
  • WorkflowRunTimeout
  • WorkflowTaskTimeout
workflowOptions := client.StartWorkflowOptions{
  // ...
  // Set Workflow Timeout duration
  WorkflowExecutionTimeout: time.Hours * 24 * 365 * 10,
  // WorkflowRunTimeout: time.Hours * 24 * 365 * 10,
  // WorkflowTaskTimeout: time.Second * 10,
  // ...
}
workflowRun, err := c.ExecuteWorkflow(context.Background(), workflowOptions, YourWorkflowDefinition)
if err != nil {
  // ...
}

Workflow retries​

A Retry Policy can work in cooperation with the timeouts to provide fine controls to optimize the execution experience.

Use a Retry Policy to retry a Workflow Execution in the event of a failure.

Workflow Executions do not retry by default, and Retry Policies should be used with Workflow Executions only in certain situations.

Create an instance of a RetryPolicy from the go.temporal.io/sdk/temporal package and provide it as the value to the RetryPolicy field of the instance of StartWorkflowOptions.

retrypolicy := &temporal.RetryPolicy{
  InitialInterval:    time.Second,
  BackoffCoefficient: 2.0,
  MaximumInterval:    time.Second * 100,
}
workflowOptions := client.StartWorkflowOptions{
  RetryPolicy: retrypolicy,
  // ...
}
workflowRun, err := temporalClient.ExecuteWorkflow(context.Background(), workflowOptions, YourWorkflowDefinition)
if err != nil {
  // ...
}

How to set Activity timeouts​

Each Activity timeout controls the maximum duration of a different aspect of an Activity Execution.

The following timeouts are available in the Activity Options.

An Activity Execution must have either the Start-To-Close or the Schedule-To-Close Timeout set.

To set an Activity Timeout in Go, create an instance of ActivityOptions from the go.temporal.io/sdk/workflow package, set the Activity Timeout field, and then use the WithActivityOptions() API to apply the options to the instance of workflow.Context.

Available timeouts are:

  • StartToCloseTimeout
  • ScheduleToClose
  • ScheduleToStartTimeout
activityoptions := workflow.ActivityOptions{
  // Set Activity Timeout duration
  ScheduleToCloseTimeout: 10 * time.Second,
  // StartToCloseTimeout: 10 * time.Second,
  // ScheduleToStartTimeout: 10 * time.Second,
}
ctx = workflow.WithActivityOptions(ctx, activityoptions)
var yourActivityResult YourActivityResult
err = workflow.ExecuteActivity(ctx, YourActivityDefinition, yourActivityParam).Get(ctx, &yourActivityResult)
if err != nil {
  // ...
}

How to set an Activity Retry Policy​

A Retry Policy works in cooperation with the timeouts to provide fine controls to optimize the execution experience.

Activity Executions are automatically associated with a default Retry Policy if a custom one is not provided.

To set a RetryPolicy, create an instance of ActivityOptions from the go.temporal.io/sdk/workflow package, set the RetryPolicy field, and then use the WithActivityOptions() API to apply the options to the instance of workflow.Context.

retrypolicy := &temporal.RetryPolicy{
  InitialInterval:    time.Second,
  BackoffCoefficient: 2.0,
  MaximumInterval:    time.Second * 100, // 100 * InitialInterval
  MaximumAttempts: 0, // Unlimited
  NonRetryableErrorTypes: []string, // empty
}

Providing a Retry Policy here is a customization, and overwrites individual Field defaults.

retrypolicy := &temporal.RetryPolicy{
  InitialInterval:    time.Second,
  BackoffCoefficient: 2.0,
  MaximumInterval:    time.Second * 100,
}

activityoptions := workflow.ActivityOptions{
  RetryPolicy: retrypolicy,
}
ctx = workflow.WithActivityOptions(ctx, activityoptions)
var yourActivityResult YourActivityResult
err = workflow.ExecuteActivity(ctx, YourActivityDefinition, yourActivityParam).Get(ctx, &yourActivityResult)
if err != nil {
  // ...
}

How to Heartbeat an Activity​

An Activity Heartbeat is a ping from the Worker Process that is executing the Activity to the Temporal Service. Each Heartbeat informs the Temporal Service that the Activity Execution is making progress and the Worker has not crashed. If the Temporal Service does not receive a Heartbeat within a Heartbeat Timeout time period, the Activity will be considered failed and another Activity Task Execution may be scheduled according to the Retry Policy.

Heartbeats may not always be sent to the Temporal Service—they may be throttled by the Worker.

Activity Cancellations are delivered to Activities from the Temporal Service when they Heartbeat. Activities that don't Heartbeat can't receive a Cancellation. Heartbeat throttling may lead to Cancellation getting delivered later than expected.

Heartbeats can contain a details field describing the Activity's current progress. If an Activity gets retried, the Activity can access the details from the last Heartbeat that was sent to the Temporal Service.

To Heartbeat in an Activity in Go, use the RecordHeartbeat API.

import (
    // ...
    "go.temporal.io/sdk/workflow"
    // ...
)

func YourActivityDefinition(ctx, YourActivityDefinitionParam) (YourActivityDefinitionResult, error) {
    // ...
    activity.RecordHeartbeat(ctx, details)
    // ...
}

When an Activity Task Execution times out due to a missed Heartbeat, the last value of the details variable above is returned to the calling Workflow in the details field of TimeoutError with TimeoutType set to Heartbeat.

You can also Heartbeat an Activity from an external source:

// The client is a heavyweight object that should be created once per process.
temporalClient, err := client.Dial(client.Options{})
// Record heartbeat.
err := temporalClient.RecordActivityHeartbeat(ctx, taskToken, details)

The parameters of the RecordActivityHeartbeat function are:

  • taskToken: The value of the binary TaskToken field of the ActivityInfo struct retrieved inside the Activity.
  • details: The serializable payload containing progress information.

If an Activity Execution Heartbeats its progress before it failed, the retry attempt will have access to the progress information, so that the Activity Execution can resume from the failed state. Here's an example of how this can be implemented:

func SampleActivity(ctx context.Context, inputArg InputParams) error {
    startIdx := inputArg.StartIndex
    if activity.HasHeartbeatDetails(ctx) {
        // Recover from finished progress.
        var finishedIndex int
        if err := activity.GetHeartbeatDetails(ctx, &finishedIndex); err == nil {
            startIdx = finishedIndex + 1 // Start from next one.
        }
    }

    // Normal Activity logic...
    for i:=startIdx; i<inputArg.EndIdx; i++ {
        // Code for processing item i goes here...
        activity.RecordHeartbeat(ctx, i) // Report progress.
    }
}

How to set a Heartbeat Timeout​

A Heartbeat Timeout works in conjunction with Activity Heartbeats.

To set a Heartbeat Timeout, Create an instance of ActivityOptions from the go.temporal.io/sdk/workflow package, set the RetryPolicy field, and then use the WithActivityOptions() API to apply the options to the instance of workflow.Context.

activityoptions := workflow.ActivityOptions{
  HeartbeatTimeout: 10 * time.Second,
}
ctx = workflow.WithActivityOptions(ctx, activityoptions)
var yourActivityResult YourActivityResult
err = workflow.ExecuteActivity(ctx, YourActivityDefinition, yourActivityParam).Get(ctx, &yourActivityResult)
if err != nil {
  // ...
}

How to asynchronously complete an Activity​

Asynchronous Activity Completion enables the Activity Function to return without the Activity Execution completing.

There are three steps to follow:

  1. The Activity provides the external system with identifying information needed to complete the Activity Execution. Identifying information can be a Task Token, or a combination of Namespace, Workflow Id, and Activity Id.

  2. The Activity Function completes in a way that identifies it as waiting to be completed by an external system.

  3. The Temporal Client is used to Heartbeat and complete the Activity.

  4. Provide the external system with a Task Token to complete the Activity Execution. To do this, use the GetInfo() API from the go.temporal.io/sdk/activity package.

// Retrieve the Activity information needed to asynchronously complete the Activity.
activityInfo := activity.GetInfo(ctx)
taskToken := activityInfo.TaskToken
// Send the taskToken to the external service that will complete the Activity.
  1. Return an activity.ErrResultPending error to indicate that the Activity is completing asynchronously.
return "", activity.ErrResultPending
  1. Use the Temporal Client to complete the Activity using the Task Token.
// Instantiate a Temporal service client.
// The same client can be used to complete or fail any number of Activities.
// The client is a heavyweight object that should be created once per process.
temporalClient, err := client.Dial(client.Options{})

// Complete the Activity.
temporalClient.CompleteActivity(context.Background(), taskToken, result, nil)

The following are the parameters of the CompleteActivity function:

  • taskToken: The value of the binary TaskToken field of the ActivityInfo struct retrieved inside the Activity.
  • result: The return value to record for the Activity. The type of this value must match the type of the return value declared by the Activity function.
  • err: The error code to return if the Activity terminates with an error.

If error is not null, the value of the result field is ignored.

To fail the Activity, you would do the following:

// Fail the Activity.
client.CompleteActivity(context.Background(), taskToken, nil, err)

How to start a Child Workflow Execution​

A Child Workflow Execution is a Workflow Execution that is scheduled from within another Workflow using a Child Workflow API.

When using a Child Workflow API, Child Workflow related Events (StartChildWorkflowExecutionInitiated, ChildWorkflowExecutionStarted, ChildWorkflowExecutionCompleted, etc...) are logged in the Workflow Execution Event History.

Always block progress until the ChildWorkflowExecutionStarted Event is logged to the Event History to ensure the Child Workflow Execution has started. After that, Child Workflow Executions may be abandoned using the default Abandon Parent Close Policy set in the Child Workflow Options.

To be sure that the Child Workflow Execution has started, first call the Child Workflow Execution method on the instance of Child Workflow future, which returns a different future.

Then get the value of an object that acts as a proxy for a result that is initially unknown, which is what waits until the Child Workflow Execution has spawned.

To spawn a Child Workflow Execution in Go, use the ExecuteChildWorkflow API, which is available from the go.temporal.io/sdk/workflow package.

The ExecuteChildWorkflow call requires an instance of workflow.Context, with an instance of workflow.ChildWorkflowOptions applied to it, the Workflow Type, and any parameters that should be passed to the Child Workflow Execution.

workflow.ChildWorkflowOptions contain the same fields as client.StartWorkflowOptions. Workflow Option fields automatically inherit their values from the Parent Workflow Options if they are not explicitly set. If a custom WorkflowID is not set, one is generated when the Child Workflow Execution is spawned. Use the WithChildOptions API to apply Child Workflow Options to the instance of workflow.Context.

The ExecuteChildWorkflow call returns an instance of a ChildWorkflowFuture.

Call the .Get() method on the instance of ChildWorkflowFuture to wait for the result.

func YourWorkflowDefinition(ctx workflow.Context, params ParentParams) (ParentResp, error) {

  childWorkflowOptions := workflow.ChildWorkflowOptions{}
  ctx = workflow.WithChildOptions(ctx, childWorkflowOptions)

  var result ChildResp
  err := workflow.ExecuteChildWorkflow(ctx, YourOtherWorkflowDefinition, ChildParams{}).Get(ctx, &result)
  if err != nil {
    // ...
  }
  // ...
  return resp, nil
}

func YourOtherWorkflowDefinition(ctx workflow.Context, params ChildParams) (ChildResp, error) {
  // ...
  return resp, nil
}

To asynchronously spawn a Child Workflow Execution, the Child Workflow must have an "Abandon" Parent Close Policy set in the Child Workflow Options. Additionally, the Parent Workflow Execution must wait for the ChildWorkflowExecutionStarted Event to appear in its Event History before it completes.

If the Parent makes the ExecuteChildWorkflow call and then immediately completes, the Child Workflow Execution does not spawn.

To be sure that the Child Workflow Execution has started, first call the GetChildWorkflowExecution method on the instance of the ChildWorkflowFuture, which will return a different Future. Then call the Get() method on that Future, which is what will wait until the Child Workflow Execution has spawned.

import (
  // ...
  "go.temporal.io/api/enums/v1"
)

func YourWorkflowDefinition(ctx workflow.Context, params ParentParams) (ParentResp, error) {

  childWorkflowOptions := workflow.ChildWorkflowOptions{
    ParentClosePolicy: enums.PARENT_CLOSE_POLICY_ABANDON,
  }
  ctx = workflow.WithChildOptions(ctx, childWorkflowOptions)

  childWorkflowFuture := workflow.ExecuteChildWorkflow(ctx, YourOtherWorkflowDefinition, ChildParams{})
  // Wait for the Child Workflow Execution to spawn
  var childWE workflow.Execution
  if err := childWorkflowFuture.GetChildWorkflowExecution().Get(ctx, &childWE); err != nil {
     return err
  }
  // ...
  return resp, nil
}

func YourOtherWorkflowDefinition(ctx workflow.Context, params ChildParams) (ChildResp, error) {
  // ...
  return resp, nil
}

How to set a Parent Close Policy​

A Parent Close Policy determines what happens to a Child Workflow Execution if its Parent changes to a Closed status (Completed, Failed, or Timed Out).

The default Parent Close Policy option is set to terminate the Child Workflow Execution.

In Go, a Parent Close Policy is set on the ParentClosePolicy field of an instance of workflow.ChildWorkflowOptions. The possible values can be obtained from the go.temporal.io/api/enums/v1 package.

  • PARENT_CLOSE_POLICY_ABANDON
  • PARENT_CLOSE_POLICY_TERMINATE
  • PARENT_CLOSE_POLICY_REQUEST_CANCEL

The Child Workflow Options are then applied to the instance of workflow.Context by using the WithChildOptions API, which is then passed to the ExecuteChildWorkflow() call.

import (
  // ...
  "go.temporal.io/api/enums/v1"
)

func YourWorkflowDefinition(ctx workflow.Context, params ParentParams) (ParentResp, error) {
  // ...
  childWorkflowOptions := workflow.ChildWorkflowOptions{
    // ...
    ParentClosePolicy: enums.PARENT_CLOSE_POLICY_ABANDON,
  }
  ctx = workflow.WithChildOptions(ctx, childWorkflowOptions)
  childWorkflowFuture := workflow.ExecuteChildWorkflow(ctx, YourOtherWorkflowDefinition, ChildParams{})
  // ...
}

func YourOtherWorkflowDefinition(ctx workflow.Context, params ChildParams) (ChildResp, error) {
  // ...
  return resp, nil
}

How to Continue-As-New​

Continue-As-New enables a Workflow Execution to close successfully and create a new Workflow Execution in a single atomic operation if the number of Events in the Event History is becoming too large. The Workflow Execution spawned from the use of Continue-As-New has the same Workflow Id, a new Run Id, and a fresh Event History and is passed all the appropriate parameters.

To cause a Workflow Execution to Continue-As-New, the Workflow API should return the result of the NewContinueAsNewError() function available from the go.temporal.io/sdk/workflow package.

func SimpleWorkflow(ctx workflow.Context, value string) error {
    ...
    return workflow.NewContinueAsNewError(ctx, SimpleWorkflow, value)
}

To check whether a Workflow Execution was spawned as a result of Continue-As-New, you can check if workflow.GetInfo(ctx).ContinuedExecutionRunID is not empty (i.e. "").

Notes

  • To prevent Signal loss, be sure to perform an asynchronous drain on the Signal channel. Failure to do so can result in buffered Signals being ignored and lost.
  • Make sure that the previous Workflow and the Continue-As-New Workflow are referenced by the same alias. Failure to do so can cause the Workflow to Continue-As-New on an entirely different Workflow.

What is a Timer?​

A Workflow can set a durable timer for a fixed time period. In some SDKs, the function is called sleep(), and in others, it's called timer().

A Workflow can sleep for months. Timers are persisted, so even if your Worker or Temporal Service is down when the time period completes, as soon as your Worker and Temporal Service are back up, the sleep() call will resolve and your code will continue executing.

Sleeping is a resource-light operation: it does not tie up the process, and you can run millions of Timers off a single Worker.

To set a Timer in Go, use the NewTimer() function and pass the duration you want to wait before continuing.

timer := workflow.NewTimer(timerCtx, duration)

To set a sleep duration in Go, use the sleep() function and pass the duration you want to wait before continuing. A zero or negative sleep duration causes the function to return immediately.

sleep = workflow.Sleep(ctx, 10*time.Second)

For more information, see the Timer example in the Go Samples repository.

How to Schedule a Workflow​

Scheduling Workflows is a crucial aspect of any automation process, especially when dealing with time-sensitive tasks. By scheduling a Workflow, you can automate repetitive tasks, reduce the need for manual intervention, and ensure timely execution of your business processes

Use any of the following action to help Schedule a Workflow Execution and take control over your automation process.

How to create a Schedule in Go​

Schedules are initiated with the create call. The user generates a unique Schedule ID for each new Schedule.

To create a Schedule in Go, use Create() on the Client. Schedules must be initialized with a Schedule ID, Spec, and Action in client.ScheduleOptions{}.

View the source code

in the context of the rest of the application code.

func main() {
// ...
	scheduleID := "schedule_id"
	workflowID := "schedule_workflow_id"
	// Create the schedule.
	scheduleHandle, err := temporalClient.ScheduleClient().Create(ctx, client.ScheduleOptions{
		ID:   scheduleID,
		Spec: client.ScheduleSpec{},
		Action: &client.ScheduleWorkflowAction{
			ID:        workflowID,
			Workflow:  schedule.ScheduleWorkflow,
			TaskQueue: "schedule",
		},
	})
// ...
}
// ...

How to backfill a Schedule in Go​

Backfilling a Schedule executes Workflow Tasks ahead of the Schedule's specified time range. This is useful for executing a missed or delayed Action, or for testing the Workflow ahead of time.

To backfill a Schedule in Go, use Backfill() on ScheduleHandle. Specify the start and end times to execute the Workflow, along with the overlap policy.

View the source code

in the context of the rest of the application code.

func main() {
// ...
	err = scheduleHandle.Backfill(ctx, client.ScheduleBackfillOptions{
		Backfill: []client.ScheduleBackfill{
			{
				Start:   now.Add(-4 * time.Minute),
				End:     now.Add(-2 * time.Minute),
				Overlap: enums.SCHEDULE_OVERLAP_POLICY_ALLOW_ALL,
			},
			{
				Start:   now.Add(-2 * time.Minute),
				End:     now,
				Overlap: enums.SCHEDULE_OVERLAP_POLICY_ALLOW_ALL,
			},
		},
	})
	if err != nil {
		log.Fatalln("Unable to Backfill Schedule", err)
	}
// ...
}
// ...

How to delete a Schedule in Go​

Deleting a Schedule erases a Schedule. Deletion does not affect any Workflows started by the Schedule.

To delete a Schedule, use Delete() on the ScheduleHandle.

View the source code

in the context of the rest of the application code.

func main() {
// ...
	defer func() {
		log.Println("Deleting schedule", "ScheduleID", scheduleHandle.GetID())
		err = scheduleHandle.Delete(ctx)
		if err != nil {
			log.Fatalln("Unable to delete schedule", err)
		}
	}()
// ...

How to describe a Schedule in Go​

Describe retrieves information about the current Schedule configuration. This can include details about the Schedule Spec (such as Intervals), CronExpressions, and Schedule State.

To describe a Schedule, use Describe() on the ScheduleHandle.

View the source code

in the context of the rest of the application code.

func main() {
// ...
	scheduleHandle.Describe(ctx)
// ...

How to list a Schedule in Go​

The List action returns all available Schedules and their respective Schedule IDs.

To return information on all Schedules, use ScheduleClient.List().

View the source code

in the context of the rest of the application code.

func main() {
// ...
	listView, _ := temporalClient.ScheduleClient().List(ctx, client.ScheduleListOptions{
		PageSize: 1,
	})

	for listView.HasNext() {
		log.Println(listView.Next())
	}
// ...

How to pause a Schedule in Go​

Pause and Unpause enable the start or stop of all future Workflow Runs on a given Schedule.

Pausing a Schedule halts all future Workflow Runs. Pausing can be enabled by setting State.Paused to true, or by using Pause() on the ScheduleHandle.

Unpausing a Schedule allows the Workflow to execute as planned. To unpause a Schedule, use Unpause() on ScheduleHandle.

View the source code

in the context of the rest of the application code.

func main() {
// ...
	err = scheduleHandle.Pause(ctx, client.SchedulePauseOptions{
		Note: "The Schedule has been paused.",
	})
// ...
	err = scheduleHandle.Unpause(ctx, client.ScheduleUnpauseOptions{
		Note: "The Schedule has been unpaused.",
	})

How to trigger a Schedule in Go​

Triggering a Schedule immediately executes an Action defined in that Schedule. By default, trigger is subject to the Overlap Policy.

To trigger a Scheduled Workflow Execution, use trigger() on ScheduleHandle.

View the source code

in the context of the rest of the application code.

func main() {
// ...
	for i := 0; i < 5; i++ {
		scheduleHandle.Trigger(ctx, client.ScheduleTriggerOptions{
			Overlap: enums.SCHEDULE_OVERLAP_POLICY_ALLOW_ALL,
		})
		time.Sleep(2 * time.Second)
	}
// ...

How to update a Schedule in Go​

Updating a Schedule changes the configuration of an existing Schedule. These changes can be made to Workflow Actions, Action parameters, Memos, and the Workflow's Cancellation Policy.

Use Update() on the ScheduleHandle to modify a Schedule.

View the source code

in the context of the rest of the application code.

func main() {
// ...
	updateSchedule := func(input client.ScheduleUpdateInput) (*client.ScheduleUpdate, error) {
		return &client.ScheduleUpdate{
			Schedule: &input.Description.Schedule,
		}, nil
	}

	_ = scheduleHandle.Update(ctx, client.ScheduleUpdateOptions{
		DoUpdate: updateSchedule,
	})
}
// ...

How to use Temporal Cron Jobs​

A Temporal Cron Job is the series of Workflow Executions that occur when a Cron Schedule is provided in the call to spawn a Workflow Execution.

A Cron Schedule is provided as an option when the call to spawn a Workflow Execution is made.

Create an instance of StartWorkflowOptions from the go.temporal.io/sdk/client package, set the CronSchedule field, and pass the instance to the ExecuteWorkflow call.

  • Type: string
  • Default: None
workflowOptions := client.StartWorkflowOptions{
  CronSchedule: "15 8 * * *",
  // ...
}
workflowRun, err := c.ExecuteWorkflow(context.Background(), workflowOptions, YourWorkflowDefinition)
if err != nil {
  // ...
}

Side Effects​

Side Effects are used to execute non-deterministic code, such as generating a UUID or a random number, without compromising deterministic in the Workflow. This is done by storing the non-deterministic results of the Side Effect into the Workflow Event History.

A Side Effect does not re-execute during a Replay. Instead, it returns the recorded result from the Workflow Execution Event History.

Side Effects should not fail. An exception that is thrown from the Side Effect causes failure and retry of the current Workflow Task.

An Activity or a Local Activity may also be used instead of a Side effect, as its result is also persisted in Workflow Execution History.

note

You shouldn’t modify the Workflow state inside a Side Effect function, because it is not reexecuted during Replay. Side Effect function should be used to return a value.

Use the SideEffect function from the go.temporal.io/sdk/workflow package to execute a Side Effect directly in your Workflow.

Pass it an instance of context.Context and the function to execute.

The SideEffect API returns a Future, an instance of converter.EncodedValue.

Use the Get method on the Future to retrieve the result of the Side Effect.

Correct implementation

The following example demonstrates the correct way to use SideEffect:

encodedRandom := workflow.SideEffect(ctx, func(ctx workflow.Context) interface{} {
 return rand.Intn(100)
})

var random int
encodedRandom.Get(&random)
// ...
}

Incorrect implementation

The following example demonstrates how NOT to use SideEffect:

// Warning: This is an incorrect example.
// This code is non-deterministic.
var random int
workflow.SideEffect(func(ctx workflow.Context) interface{} {
      random = rand.Intn(100)
      return nil
})
// random will always be 0 in replay, so this code is non-deterministic.

On replay the provided function is not executed, the random number will always be 0, and the Workflow Execution could take a different path, breaking determinism.

Mutable Side Effects​

Mutable Side Effects execute the provided function once, and then it looks up the History of the value with the given Workflow ID.

  • If there is no existing value, then it records the function result as a value with the given Workflow Id on the History.
  • If there is an existing value, then it compares whether the existing value from the History has changed from the new function results, by calling the equals function.
    • If the values are equal, then it returns the value without recording a new Marker Event
    • If the values aren't equal, then it records the new value with the same ID on the History.
note

During a Workflow Execution, every new Side Effect call results in a new Marker recorded on the Workflow History; whereas Mutable Side Effects only records a new Marker on the Workflow History if the value for the Side Effect ID changes or is set the first time.

During a Replay, Mutable Side Effects will not execute the function again. Instead, it returns the exact same value that was returned during the Workflow Execution.

To use MutableSideEffect() in Go, provide a unique name within the scope of the workflow.

if err := workflow.MutableSideEffect(ctx, "configureNumber", get, eq).Get(&number); err != nil {
    panic("can't decode number:" + err.Error())
  }

How to create and manage Namespaces​

You can create, update, deprecate, or delete your Namespaces using either the Temporal CLI or SDK APIs.

Use Namespaces to isolate your Workflow Executions according to your needs. For example, you can use Namespaces to match the development lifecycle by having separate dev and prod Namespaces. You could also use them to ensure Workflow Executions between different teams never communicate - such as ensuring that the teamA Namespace never impacts the teamB Namespace.

On Temporal Cloud, use the Temporal Cloud UI to create and manage a Namespace from the UI, or tcld commands to manage Namespaces from the command-line interface.

On self-hosted Temporal Service, you can register and manage your Namespaces using the Temporal CLI (recommended) or programmatically using APIs. Note that these APIs and Temporal CLI commands will not work with Temporal Cloud.

Use a custom Authorizer on your Frontend Service in the Temporal Service to set restrictions on who can create, update, or deprecate Namespaces.

You must register a Namespace with the Temporal Service before setting it in the Temporal Client.

How to register Namespaces​

Registering a Namespace creates a Namespace on the Temporal Service or Temporal Cloud.

On Temporal Cloud, use the Temporal Cloud UI or tcld commands to create Namespaces.

On self-hosted Temporal Service, you can register your Namespaces using the Temporal CLI (recommended) or programmatically using APIs. Note that these APIs and Temporal CLI commands will not work with Temporal Cloud.

Use a custom Authorizer on your Frontend Service in the Temporal Service to set restrictions on who can create, update, or deprecate Namespaces.

Use Register API with the NamespaceClient interface to register a Namespace and set the Retention Period for the Workflow Execution Event History for the Namespace.

You can also register Namespaces using the Temporal CLI command-line tool.

client, err := client.NewNamespaceClient(client.Options{HostPort: ts.config.ServiceAddr})
        //...
    err = client.Register(ctx, &workflowservice.RegisterNamespaceRequest{
        Namespace: your-namespace-name,
        WorkflowExecutionRetentionPeriod: &retention,
    })

The Retention Period setting using WorkflowExecutionRetentionPeriod is mandatory. The minimum value you can set for this period is 1 day.

Once registered, set Namespace using Dial in a Workflow Client to run your Workflow Executions within that Namespace. See how to set Namespace in a Client in Go for details.

Note that Namespace registration using this API takes up to 10 seconds to complete. Ensure that you wait for this registration to complete before starting the Workflow Execution against the Namespace.

To update your Namespace, use the Update API with the NamespaceClient.

To update your Namespace using the Temporal CLI, use the temporal operator namespace update command.

How to manage Namespaces​

You can get details for your Namespaces, update Namespace configuration, and deprecate or delete your Namespaces.

On Temporal Cloud, use the Temporal Cloud UI or tcld commands to manage Namespaces.

On self-hosted Temporal Service, you can manage your registered Namespaces using the Temporal CLI (recommended) or programmatically using APIs. Note that these APIs and Temporal CLI commands will not work with Temporal Cloud.

Use a custom Authorizer on your Frontend Service in the Temporal Service to set restrictions on who can create, update, or deprecate Namespaces.

You must register a Namespace with the Temporal Service before setting it in the Temporal Client.

On Temporal Cloud, use the Temporal Cloud UI or tcld commands to manage Namespaces.

On self-hosted Temporal Service, you can manage your registered Namespaces using the Temporal CLI (recommended) or programmatically using APIs. Note that these APIs and Temporal CLI commands will not work with Temporal Cloud.

  • Update information and configuration for a registered Namespace on your Temporal Service:

    • With the Temporal CLI: temporal operator namespace update Example

    • Use the UpdateNamespace API to update configuration on a Namespace. Example

      //...
        err = client.Update(context.Background(), &workflowservice.UpdateNamespaceRequest{
        Namespace:         "your-namespace-name",
        UpdateInfo:        &namespace.UpdateNamespaceInfo{ //updates info for the namespace "your-namespace-name"
            Description:   "updated namespace description",
            OwnerEmail:    "newowner@mail.com",
            //Data:        nil,
            //State:       0,
        },
        /*other details that you can update:
        Config:            &namespace.NamespaceConfig{ //updates the configuration of the namespace with the following options
            //WorkflowExecutionRetentionTtl: nil,
            //BadBinaries:                   nil,
            //HistoryArchivalState:          0,
            //HistoryArchivalUri:            "",
            //VisibilityArchivalState:       0,
            //VisibilityArchivalUri:         "",
        },
        ReplicationConfig: &replication.NamespaceReplicationConfig{ //updates the replication configuration for the namespace
            //ActiveClusterName: "",
            //Clusters:          nil,
            //State:             0,
        },
        SecurityToken:     "",
        DeleteBadBinary:   "",
        PromoteNamespace:  false,
        })*/
      //...

  • Get details for a registered Namespace on your Temporal Service:

    • With the Temporal CLI: temporal operator namespace describe

    • Use the DescribeNamespace API to return information and configuration details for a registered Namespace. Example

      //...
        client, err := client.NewNamespaceClient(client.Options{})
        //...
        client.Describe(context.Background(), "default")
      //...

  • Get details for all registered Namespaces on your Temporal Service:

    //...
        namespace.Handler.ListNamespaces(context.Context(), &workflowservice.ListNamespacesRequest{ //lists 1 page (1-100) of namespaces on the active Temporal Service. You can set a large PageSize or loop until NextPageToken is nil
            //PageSize:        0,
            //NextPageToken:   nil,
            //NamespaceFilter: nil,
            })
    //...

  • Delete a Namespace: The DeleteNamespace API deletes a Namespace. Deleting a Namespace deletes all running and completed Workflow Executions on the Namespace, and removes them from the persistence store and the visibility store. Example:

    //...
    client.OperatorService().DeleteNamespace(ctx, &operatorservice.DeleteNamespaceRequest{...
    //...

How to use Worker Session APIs​

Support, stability, and dependency info
  • This feature is currently available only in the Go SDK.

A Worker Session is a feature that provides a straightforward API for Task Routing to ensure that Activity Tasks are executed with the same Worker without requiring you to manually specify Task Queue names.

How to enable Worker Sessions​

Enable the Worker to use Sessions.

Set EnableSessionWorker to true in the Worker options.

View the source code

in the context of the rest of the application code.

// ...
func main() {
// ...
	// Enable Sessions for this Worker.
	workerOptions := worker.Options{
		EnableSessionWorker: true,
// ...
	}
	w := worker.New(temporalClient, "fileprocessing", workerOptions)
	w.RegisterWorkflow(sessions.SomeFileProcessingWorkflow)
	w.RegisterActivity(&sessions.FileActivities{})
	err = w.Run(worker.InterruptCh())
// ...
}

How to change the maximum concurrent Sessions of a Worker.​

You can adjust the maximum concurrent Sessions of a Worker.

To limit the number of concurrent Sessions running on a Worker, set the MaxConcurrentSessionExecutionSize field of worker.Options to the desired value. By default, this field is set to a very large value, so there's no need to manually set it if no limitation is needed.

If a Worker hits this limitation, it won't accept any new CreateSession() requests until one of the existing sessions is completed. If the session can't be created within CreationTimeout, CreateSession() returns an error .

View the source code

in the context of the rest of the application code.

func main() {
// ...
	workerOptions := worker.Options{
// ...
		// This configures the maximum allowed concurrent sessions.
		// Customize this value only if you need to.
		MaxConcurrentSessionExecutionSize: 1000,
// ...
}
// ...

How to create a Worker Session​

Within the Workflow code use the Workflow APIs to create a Session with whichever Worker picks up the first Activity Task.

Use the CreateSession API to create a Context object that can be passed to calls to spawn Activity Executions.

Pass an instance of workflow.Context and SessionOptions to the CreateSession API call and get a Session Context that contains metadata information of the Session.

Use the Session Context to spawn all Activity Executions that should belong to the Session. All associated Activity Tasks are then processed by the same Worker Entity. When the CreateSession API is called, the Task Queue name that is specified in ActivityOptions (or in StartWorkflowOptions if the Task Queue name is not specified in ActivityOptions) is used, and a Session is created with one of the Workers polling that Task Queue.

The Session Context is cancelled if the Worker executing this Session dies or CompleteSession() is called. When using the returned Session Context to spawn Activity Executions, a workflow.ErrSessionFailed error is returned if the Session framework detects that the Worker executing this Session has died. The failure of Activity Executions won't affect the state of the Session, so you still need to handle the errors returned from your Activities and call CompleteSession() if necessary.

If the context passed in already contains an open Session, CreateSession() returns an error. If all the Workers are currently busy and unable to handle a new Session, the framework keeps retrying until the CreationTimeout period you specified in SessionOptions has passed before returning an error. (For more details, check the "Concurrent Session Limitation" section.)

CompleteSession() releases the resources reserved on the Worker, so it's important to call it as soon as you no longer need the Session. It cancels the session context and therefore all the Activity Executions using that Session Context. It is safe to call CompleteSession() on a failed Session, meaning that you can call it from a defer function after the Session is successfully created.

If the Worker goes down between Activities, any scheduled Activities meant for the Session Worker are canceled. If not, you get a workflow.ErrSessionFailed error when the next call of workflow.ExecuteActivity() is made from that Workflow.

View the source code

in the context of the rest of the application code.

package sessions

import (
	"time"

	"go.temporal.io/sdk/workflow"
)

// ...
// SomeFileProcessingWorkflow is a Workflow Definition.
func SomeFileProcessingWorkflow(ctx workflow.Context, param FileProcessingWFParam) error {
	activityOptions := workflow.ActivityOptions{
		StartToCloseTimeout: time.Minute,
	}
	ctx = workflow.WithActivityOptions(ctx, activityOptions)
// ...
	sessionOptions := &workflow.SessionOptions{
		CreationTimeout:  time.Minute,
		ExecutionTimeout: time.Minute,
	}
	// Create a Session with the Worker so that all Activities execute with the same Worker.
	sessionCtx, err := workflow.CreateSession(ctx, sessionOptions)
	if err != nil {
		return err
	}
	defer workflow.CompleteSession(sessionCtx)
// ...
	err = workflow.ExecuteActivity(sessionCtx, a.DownloadFile, param).Get(sessionCtx, &downloadResult)
// ...
	err = workflow.ExecuteActivity(sessionCtx, a.ProcessFile, processParam).Get(sessionCtx, &processResult)
// ...
	err = workflow.ExecuteActivity(sessionCtx, a.UploadFile, uploadParam).Get(sessionCtx, nil)
// ...
}

Additional Session usage information​

type SessionInfo struct {
 // A unique Id for the session
 SessionID         string
 // The hostname of the worker that is executing the session
 HostName          string
 // ... other unexported fields
}

func GetSessionInfo(ctx Context) *SessionInfo

The Session Context also stores some Session metadata, which can be retrieved by the GetSessionInfo() API. If the Context passed in doesn't contain any Session metadata, this API will return a nil pointer.

Recreate Session​

For long-running Sessions, you may want to use the ContinueAsNew feature to split the Workflow into multiple runs when all Activities need to be executed by the same Worker. The RecreateSession() API is designed for such a use case.

func RecreateSession(ctx Context, recreateToken []byte, sessionOptions *SessionOptions) (Context, error)

Its usage is the same as CreateSession() except that it also takes in a recreateToken, which is needed to create a new Session on the same Worker as the previous one. You can get the token by calling the GetRecreateToken() method of the SessionInfo object.

token := workflow.GetSessionInfo(sessionCtx).GetRecreateToken()

Is there a complete example?

Yes, the file processing example in the temporalio/samples-go repo has been updated to use the session framework.

What happens to my Activity if the Worker dies?

If your Activity has already been scheduled, it will be canceled. If not, you will get a workflow.ErrSessionFailed error when you call workflow.ExecuteActivity().

Is the concurrent session limitation per process or per host?

It's per Worker Process, so make sure there's only one Worker Process running on the host if you plan to use this feature.

Future Work

  • Right now, a Session is considered failed if the Worker Process dies. However, for some use cases, you may only care whether the Worker host is alive or not. For these use cases, the Session should be automatically re-established if the Worker Process is restarted.

  • The current implementation assumes that all Sessions are consuming the same type of resource and there's only one global limitation. Our plan is to allow you to specify what type of resource your Session will consume and enforce different limitations on different types of resources.

Error Handling in Go​

An Activity, or a Child Workflow, might fail, and you could handle errors differently based on the different error cases.

If the Activity returns an error as errors.New() or fmt.Errorf(), that error is converted into *temporal.ApplicationError.

If the Activity returns an error as temporal.NewNonRetryableApplicationError("error message", details), that error is returned as *temporal.ApplicationError.

There are other types of errors such as *temporal.TimeoutError, *temporal.CanceledError and *temporal.PanicError. Following is an example of what your error code might look like:

Here's an example of handling Activity errors within Workflow code that differentiates between different error types.

err := workflow.ExecuteActivity(ctx, YourActivity, ...).Get(ctx, nil)
if err != nil {
	var applicationErr *ApplicationError
	if errors.As(err, &applicationErr) {
		// retrieve error message
		fmt.Println(applicationError.Error())

		// handle Activity errors (created via NewApplicationError() API)
		var detailMsg string // assuming Activity return error by NewApplicationError("message", true, "string details")
		applicationErr.Details(&detailMsg) // extract strong typed details

		// handle Activity errors (errors created other than using NewApplicationError() API)
		switch applicationErr.Type() {
		case "CustomErrTypeA":
			// handle CustomErrTypeA
		case CustomErrTypeB:
			// handle CustomErrTypeB
		default:
			// newer version of Activity could return new errors that Workflow was not aware of.
		}
	}

	var canceledErr *CanceledError
	if errors.As(err, &canceledErr) {
		// handle cancellation
	}

	var timeoutErr *TimeoutError
	if errors.As(err, &timeoutErr) {
		// handle timeout, could check timeout type by timeoutErr.TimeoutType()
        switch err.TimeoutType() {
        case commonpb.ScheduleToStart:
                // Handle ScheduleToStart timeout.
        case commonpb.StartToClose:
                // Handle StartToClose timeout.
        case commonpb.Heartbeat:
                // Handle heartbeat timeout.
        default:
        }
	}

	var panicErr *PanicError
	if errors.As(err, &panicErr) {
		// handle panic, message and call stack are available by panicErr.Error() and panicErr.StackTrace()
	}
}

Go SDK Selectors​

Overview​

In Go, the select statement lets a goroutine wait on multiple communication operations. A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.

However, a normal Go select statement can not be used inside of Workflows directly because of the random nature. Temporal's Go SDK Selectors are similar and act as a replacement. They can block on sending and receiving from Channels but as a bonus can listen on Future deferred work. Usage of Selectors to defer and process work (in place of Go's select) are necessary in order to ensure deterministic Workflow code execution (though using select in Activity code is fine).

Full API Example​

The API is sufficiently different from select that it bears documenting:

func SampleWorkflow(ctx workflow.Context) error {
	// standard Workflow setup code omitted...

	// API Example: declare a new selector
	selector := workflow.NewSelector(ctx)

	// API Example: defer code execution until the Future that represents Activity result is ready
	work := workflow.ExecuteActivity(ctx, ExampleActivity)
	selector.AddFuture(work, func(f workflow.Future) {
		// deferred code omitted...
	})

	// more parallel timers and activities initiated...

	// API Example: receive information from a Channel
	var signalVal string
	channel := workflow.GetSignalChannel(ctx, channelName)
	selector.AddReceive(channel, func(c workflow.ReceiveChannel, more bool) {
		// matching on the channel doesn't consume the message.
	 	// So it has to be explicitly consumed here
		c.Receive(ctx, &signalVal)
		// do something with received information
	})

	// API Example: block until the next Future is ready to run
	// important! none of the deferred code runs until you call selector.Select
	selector.Select(ctx)

	// Todo: document selector.HasPending
}

Using Selectors with Futures​

You usually add Futures after Activities:

// API Example: defer code execution until after an activity is done
work := workflow.ExecuteActivity(ctx, ExampleActivity)
selector.AddFuture(work, func(f workflow.Future) {
	// deferred code omitted...
})

selector.Select(ctx) is the primary mechanism which blocks on and executes Future work. It is intentionally flexible; you may call it conditionally or multiple times:

	// API Example: blocking conditionally
  if somecondition != nil {
		selector.Select(ctx)
  }

	// API Example: popping off all remaining Futures
  for i := 0; i < len(someArray); i++ {
		selector.Select(ctx) // this will wait for one branch
		// you can interrupt execution here
	}

A Future matches only once per Selector instance even if Select is called multiple times. If multiple items are available, the order of matching is not defined.

Using Selectors with Timers​

An important use case of futures is setting up a race between a timer and a pending activity, effectively adding a "soft" timeout that doesn't result in any errors or retries of that activity.

For example, the Timer sample shows how you can write a long running order processing operation where:

  • if processing takes too long, we send out a notification email to user about the delay, but we won't cancel the operation
  • if the operation finishes before the timer fires, then we want to cancel the timer.
var processingDone bool
f := workflow.ExecuteActivity(ctx, OrderProcessingActivity)
selector.AddFuture(f, func(f workflow.Future) {
	processingDone = true
	// cancel timerFuture
	cancelHandler()
})

// use timer future to send notification email if processing takes too long
timerFuture := workflow.NewTimer(childCtx, processingTimeThreshold)
selector.AddFuture(timerFuture, func(f workflow.Future) {
	if !processingDone {
		// processing is not done yet when timer fires, send notification email
		_ = workflow.ExecuteActivity(ctx, SendEmailActivity).Get(ctx, nil)
	}
})

// wait the timer or the order processing to finish
selector.Select(ctx)

We create timers with the workflow.NewTimer API.

Using Selectors with Channels​

selector.AddReceive(channel, func(c workflow.ReceiveChannel, more bool) {}) is the primary mechanism which receives messages from Channels.

// API Example: receive information from a Channel
var signalVal string
channel := workflow.GetSignalChannel(ctx, channelName)
selector.AddReceive(channel, func(c workflow.ReceiveChannel, more bool) {
	c.Receive(ctx, &signalVal)
	// do something with received information
})

Merely matching on the channel doesn't consume the message; it has to be explicitly consumed with a c.Receive(ctx, &signalVal) call.

Querying Selector State​

You can use the selector.HasPending API to ensure that signals are not lost when a Workflow is closed (e.g. by ContinueAsNew).

Learn More​

Usage of Selectors is best learned by example:

  • Setting up a race condition between an Activity and a Timer, and conditionally execute (Timer example)
  • Receiving information in a Channel (Mutex example)
  • Looping through a list of work and scheduling them all in parallel (DSL example)
  • Executing activities in parallel, pick the first result, cancel remainder (Pick First example)

Start Delay​

How to delay the start of a Workflow Execution using Start Delay with the Temporal Go SDK.

Use StartDelay to schedule a Workflow Execution at a specific one-time future point rather than on a recurring schedule.

Create an instance of StartWorkflowOptions from the go.temporal.io/sdk/client package, set the StartDelay field, and pass the instance to the ExecuteWorkflow call.

workflowOptions := client.StartWorkflowOptions{
  // ...
  // Start the workflow in 12 hours
  StartDelay: time.Hours * 12,
  // ...
}
workflowRun, err := c.ExecuteWorkflow(context.Background(), workflowOptions, YourWorkflowDefinition)
if err != nil {
  // ...
}