activity

Documentation

Overview

Package activity contains functions and types used to implement Cadence activities.

The activity is an implementation of a task to be performed as part of a larger workflow. There is no limitation of what an activity can do. In the context of a workflow, it is in the activities where all operations that affect the desired results must be implemented.

Overview

The client library for Cadence does all the heavy lifting of handling the async communication between the Cadence managed service and the worker running the activity. As such, the implementation of the activity can, for the most part, focus on the business logic. The sample code below shows the implementation of a simple activity that accepts a string parameter, appends a word to it and then returns the result.

import (
	"context"

	"go.uber.org/cadence/activity"
	"go.uber.org/zap"
)

func init() {
	activity.Register(SimpleActivity)
}

func SimpleActivity(ctx context.Context, value string) (string, error) {
	activity.GetLogger(ctx).Info("SimpleActivity called.", zap.String("Value", value))
	return "Processed: ” + value, nil
}

The following sections explore the elements of the above code.

Declaration

In the Cadence programing model, an activity is implemented with a function. The function declaration specifies the parameters the activity accepts as well as any values it might return. An activity function can take zero or many activity specific parameters and can return one or two values. It must always at least return an error value. The activity function can accept as parameters and return as results any serializable type.

func SimpleActivity(ctx context.Context, value string) (string, error)

The first parameter to the function is context.Context. This is an optional parameter and can be omitted. This parameter is the standard Go context.

The second string parameter is a custom activity-specific parameter that can be used to pass in data into the activity on start. An activity can have one or more such parameters. All parameters to an activity function must be serializable, which essentially means that params can’t be channels, functions, variadic, or unsafe pointer.

The activity declares two return values: (string, error). The string return value is used to return the result of the activity. The error return value is used to indicate an error was encountered during execution.

Implementation

There is nothing special about activity code. You can write activity implementation code the same way you would any other Go service code. You can use the usual loggers and metrics collectors. You can use the standard Go concurrency constructs.

Failing the activity

To mark an activity as failed, all that needs to happen is for the activity function to return an error via the error return value.

Activity Heartbeating

For long running activities, Cadence provides an API for the activity code to report both liveness and progress back to the Cadence managed service.

progress := 0
for hasWork {
    // send heartbeat message to the server
    activity.RecordActivityHeartbeat(ctx, progress)
    // do some work
    ...
    progress++
}

When the activity times out due to a missed heartbeat, the last value of the details (progress in the above sample) is returned from the workflow.ExecuteActivity function as the details field of TimeoutError with TimeoutType_HEARTBEAT.

It is also possible to heartbeat an activity from an external source:

// instantiate a Cadence service Client
client.Client client = client.NewClient(...)

// record heartbeat
err := client.RecordActivityHeartbeat(taskToken, details)

The parameters of the RecordActivityHeartbeat function are:

• taskToken: This is the value of the binary “TaskToken” field of the “ActivityInfo” struct retrieved inside the activity
• details: This is the serializable payload containing progress information

Activity Cancellation

When an activity is cancelled (or its workflow execution is completed or failed) the context passed into its function is cancelled which sets its Done channel’s closed state. So an activity can use that to perform any necessary cleanup and abort its execution. Currently cancellation is delivered only to activities that call RecordActivityHeartbeat.

Async/“Manual” Activity Completion

In certain scenarios completing an activity upon completion of its function is not possible or desirable.

One example would be the UberEATS order processing workflow that gets kicked off once an eater pushes the “Place Order” button. Here is how that workflow could be implemented using Cadence and the “async activity completion”:

• Activity 1: send order to restaurant
• Activity 2: wait for restaurant to accept order
• Activity 3: schedule pickup of order
• Activity 4: wait for courier to pick up order
• Activity 5: send driver location updates to eater
• Activity 6: complete order

Activities 2 & 4 in the above flow require someone in the restaurant to push a button in the Uber app to complete the activity. The activities could be implemented with some sort of polling mechanism. However, they can be implemented much simpler and much less resource intensive as a Cadence activity that is completed asynchronously.

There are 2 parts to implementing an asynchronously completed activity. The first part is for the activity to provide the information necessary to be able to be completed from an external system and notify the Cadence service that it is waiting for that outside callback:

// retrieve activity information needed to complete activity asynchronously
activityInfo := activity.GetInfo(ctx)
taskToken := activityInfo.TaskToken

// send the taskToken to external service that will complete the activity
...

// return from activity function indicating the Cadence should wait for an async completion message
return "", activity.ErrResultPending

The second part is then for the external service to call the Cadence service to complete the activity. To complete the activity successfully you would do the following:

// instantiate a Cadence service Client
// the same client can be used complete or fail any number of activities
client.Client client = client.NewClient(...)

// complete the activity
client.CompleteActivity(taskToken, result, nil)

And here is how you would fail the activity:

// fail the activity
client.CompleteActivity(taskToken, nil, err)

The parameters of the CompleteActivity function are:

• taskToken: This is the value of the binary “TaskToken” field of the “ActivityInfo” struct retrieved inside the activity.
• result: This is the return value that should be recorded for the activity. The type of this value needs to match the type of the return value declared by the activity function.
• err: The error code to return if the activity should terminate with an error.

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

For a full example of implementing this pattern see the Expense sample.

Registration

In order to for some workflow execution to be able to invoke an activity type, the worker process needs to be aware of all the implementations it has access to. An activity is registered with the following call:

activity.Register(SimpleActivity)

This call essentially creates an in-memory mapping inside the worker process between the fully qualified function name and the implementation. Unlike in Amazon SWF, workflow and activity types are not registered with the managed service. If the worker receives a request to start an activity execution for an activity type it does not know it will fail that request.

Index

• Variables
• func GetHeartbeatDetails(ctx context.Context, d ...interface{}) error
• func GetLogger(ctx context.Context) *zap.Logger
• func GetMetricsScope(ctx context.Context) tally.Scope
• func GetWorkerStopChannel(ctx context.Context) <-chan struct{}
• func HasHeartbeatDetails(ctx context.Context) bool
• func RecordHeartbeat(ctx context.Context, details ...interface{})
• func Register(activityFunc interface{})
• func RegisterWithOptions(activityFunc interface{}, opts RegisterOptions)
• type Info
  • func GetInfo(ctx context.Context) Info
• type RegisterOptions
• type Type

Variables

var ErrResultPending = internal.ErrActivityResultPending

ErrResultPending is returned from activity's implementation to indicate the activity is not completed when activity method returns. Activity needs to be completed by Client.CompleteActivity() separately. For example, if an activity require human interaction (like approve an expense report), the activity could return ErrResultPending which indicate the activity is not done yet. Then, when the waited human action happened, it needs to trigger something that could report the activity completed event to cadence server via Client.CompleteActivity() API.

Functions

func GetHeartbeatDetails

func GetHeartbeatDetails(ctx context.Context, d ...interface{}) error

GetHeartbeatDetails extract heartbeat details from last failed attempt. This is used in combination with retry policy. An activity could be scheduled with an optional retry policy on ActivityOptions. If the activity failed then server would attempt to dispatch another activity task to retry according to the retry policy. If there was heartbeat details reported by activity from the failed attempt, the details would be delivered along with the activity task for retry attempt. Activity could extract the details by GetHeartbeatDetails() and resume from the progress. See TestActivityEnvironment.SetHeartbeatDetails() for unit test support.

func GetLogger

func GetLogger(ctx context.Context) *zap.Logger

GetLogger returns a logger that can be used in activity

func GetMetricsScope

func GetMetricsScope(ctx context.Context) tally.Scope

GetMetricsScope returns a metrics scope that can be used in activity

func GetWorkerStopChannel

func GetWorkerStopChannel(ctx context.Context) <-chan struct{}

GetWorkerStopChannel returns a read-only channel. The closure of this channel indicates the activity worker is stopping. When the worker is stopping, it will close this channel and wait until the worker stop timeout finishes. After the timeout hit, the worker will cancel the activity context and then exit. The timeout can be defined by worker option: WorkerStopTimeout. Use this channel to handle activity graceful exit when the activity worker stops.

func HasHeartbeatDetails

func HasHeartbeatDetails(ctx context.Context) bool

HasHeartbeatDetails checks if there is heartbeat details from last attempt.

func RecordHeartbeat

func RecordHeartbeat(ctx context.Context, details ...interface{})

RecordHeartbeat sends heartbeat for the currently executing activity If the activity is either cancelled (or) workflow/activity doesn't exist then we would cancel the context with error context.Canceled.

details - the details that you provided here can be seen in the workflow when it receives TimeoutError, you can check error TimeOutType()/Details().

func Register

func Register(activityFunc interface{})

Register - calls RegisterWithOptions with default registration options.

func RegisterWithOptions

func RegisterWithOptions(activityFunc interface{}, opts RegisterOptions)

RegisterWithOptions registers the activity function with options The user can use options to provide an external name for the activity or leave it empty if no external name is required. This can be used as

client.Register(barActivity, RegisterOptions{})
client.Register(barActivity, RegisterOptions{Name: "barExternal"})

An activity takes a context and input and returns a (result, error) or just error. Examples:

func sampleActivity(ctx context.Context, input []byte) (result []byte, err error)
func sampleActivity(ctx context.Context, arg1 int, arg2 string) (result *customerStruct, err error)
func sampleActivity(ctx context.Context) (err error)
func sampleActivity() (result string, err error)
func sampleActivity(arg1 bool) (result int, err error)
func sampleActivity(arg1 bool) (err error)

Serialization of all primitive types, structures is supported ... except channels, functions, unsafe pointer. If function implementation returns activity.ErrResultPending then activity is not completed from the calling workflow point of view. See documentation of activity.ErrResultPending for more info. This method calls panic if activityFunc doesn't comply with the expected format.

Types

type Info

type Info = internal.ActivityInfo

Info contains information about a currently executing activity.

func GetInfo

func GetInfo(ctx context.Context) Info

GetInfo returns information about currently executing activity.

type RegisterOptions

type RegisterOptions = internal.RegisterActivityOptions

RegisterOptions consists of options for registering an activity

type Type

type Type = internal.ActivityType

Type identifies an activity type.