Worker Sessions - Temporal Go SDK feature guide

This page shows how to do the following:

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.

Enable Worker Sessions

How to enable Worker Sessions using the Go SDK.

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())
// ...
}

Change the maximum concurrent Sessions of a Worker.

How to change the maximum concurrent Sessions of a Worker using the Go SDK.

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,
// ...
}
// ...

Create a Worker Session

How to create a Worker Session using the Go SDK.

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.

Enable Worker Sessions​

Change the maximum concurrent Sessions of a Worker.​

Create a Worker Session​

Additional Session usage information​

Recreate Session​

Enable Worker Sessions

Change the maximum concurrent Sessions of a Worker.

Create a Worker Session

Additional Session usage information

Recreate Session