What is a Task?
What is a Task?
A Task is the smallest unit of work, designed to perform a specific operation. Each task represents a distinct operation or process that can be executed, such as processing data, performing calculations, or managing resources. Tasks can operate independently or as components of a more complex set of connected tasks known as a Workflow. Tasks are defined by their code, inputs, and dependencies on other tasks. To create tasks, you need to define the input parameters and specify the action to be performed during execution.
Creating a Task
To create a task in Tilebox, define a class that extends theTask
base class and implements the execute
method. The execute
method is the entry point for the task where its logic is defined. It’s called when the task is executed.
class MyFirstTask(Task)
class MyFirstTask(Task)
MyFirstTask
is a subclass of the Task
class, which serves as the base class for all defined tasks. It provides the essential structure for a task. Inheriting from Task
automatically makes the class a dataclass
, which is useful for specifying inputs. Additionally, by inheriting from Task
, the task is automatically assigned an identifier based on the class name.def execute
def execute
The
execute
method is the entry point for executing the task. This is where the task’s logic is defined. It’s invoked by a task runner when the task runs and performs the task’s operation.context: ExecutionContext
context: ExecutionContext
The
context
argument is an ExecutionContext
instance that provides access to an API for submitting new tasks as part of the same job and features like shared caching.type MyFirstTask struct{}
type MyFirstTask struct{}
MyFirstTask
is a struct that implements the Task
interface. It represents the task to be executed.func (t *MyFirstTask) Execute(ctx context.Context) error
func (t *MyFirstTask) Execute(ctx context.Context) error
The
Execute
method is the entry point for executing the task. This is where the task’s logic is defined. It’s invoked by a task runner when the task runs and performs the task’s operation.The code samples on this page do not illustrate how to execute the task. That will be covered in the
next section on task runners. The reason for that is that executing tasks is a separate concern from implementing tasks.
Input Parameters
Tasks often require input parameters to operate. These inputs can range from simple values to complex data structures. By inheriting from theTask
class, the task is treated as a Python dataclass
, allowing input parameters to be defined as class attributes.
Tasks must be serializable to JSON or to protobuf because they may be distributed across a cluster of task runners.
In Go, task parameters must be exported fields of the task struct (starting with an uppercase letter), otherwise they will not be serialized to JSON.
- Basic types such as
str
,int
,float
,bool
- Lists and dictionaries of basic types
- Nested data classes that are also JSON-serializable or protobuf-serializable
Task Composition and subtasks
Until now, tasks have performed only a single operation. But tasks can be more powerful. Tasks can submit other tasks as subtasks. This allows for a modular workflow design, breaking down complex operations into simpler, manageable parts. Additionally, the execution of subtasks is automatically parallelized whenever possible.ParentTask
submits ChildTask
tasks as subtasks. The number of subtasks to be submitted is based on the num_subtasks
attribute of the ParentTask
. The submit_subtask
method takes an instance of a task as its argument, meaning the task to be submitted must be instantiated with concrete parameters first.
Parent task do not have access to results of subtasks, instead, tasks can use shared caching to share data between tasks.
By submitting a task as a subtask, its execution is scheduled as part of the same job as the parent task. Compared to just directly invoking the subtask’s
execute
method, this allows the subtask’s execution to occur on a different machine or in parallel with other subtasks. To learn more about how tasks are executed, see the section on task runners.Larger subtasks example
A practical workflow example showcasing task composition might help illustrate the capabilities of tasks. Below is an example of a set of tasks forming a workflow capable of downloading a set number of random dog images from the internet. The Dog API can be used to get the image URLs, and then download them. Implementing this using Task Composition could look like this:DownloadRandomDogImages
DownloadRandomDogImages
DownloadRandomDogImages
fetches a specific number of random dog image URLs from an API. It then submits a DownloadImage
task for each received image URL.DownloadImage
DownloadImage
DownloadImage
downloads an image from a specified URL and saves it to a file.DownloadRandomDogImages
submits DownloadImage
tasks as subtasks.
Visualizing the execution of such a workflow is akin to a tree structure where the DownloadRandomDogImages
task is the root, and the DownloadImage
tasks are the leaves. For instance, when downloading five random dog images, the following tasks are executed.
DownloadRandomDogImages
task and five DownloadImage
tasks. The DownloadImage
tasks can execute in parallel, as they are independent. If more than one task runner is available, the Tilebox Workflow Orchestrator automatically parallelizes the execution of these tasks.
Check out job_client.display to learn how this visualization was automatically generated from the task executions.
64
subtasks per task is in place to discourage creating workflows where individual parent tasks submit a large number of subtasks, which can lead to performance issues since those parent tasks are not parallelized. If you need to submit more than 64
subtasks, consider using recursive subtask submission instead.
Recursive subtasks
Tasks can submit other tasks as subtasks, allowing for complex workflows. Sometimes the input to a task is a list, with elements that can be mapped to individual subtasks, whose outputs are then aggregated in a reduce step. This pattern is commonly known as MapReduce. Often times the initial map step—submitting the individual subtasks—might already be an expensive operation. Since this is executed within a single task, it’s not parallelizable, which can bottleneck the entire workflow. Fortunately, Tilebox Workflows offers a solution through recursive subtask submission. A task can submit instances of itself as subtasks, enabling a recursive breakdown into smaller tasks. For example, theRecursiveTask
below is a valid task that submits smaller instances of itself as subtasks.
Recursive subtask example
An example for this is the random dog images workflow mentioned earlier. In the previous implementation, downloading images was already parallelized. But the initial orchestration of the individual download tasks was not parallelized, becauseDownloadRandomDogImages
was responsible for fetching all random dog image URLs and only submitted the individual download tasks once all URLs were retrieved. For a large number of images this setup can bottleneck the entire workflow.
To improve this, recursive subtask submission decomposes a DownloadRandomDogImages
task with a high number of images into two smaller DownloadRandomDogImages
tasks, each fetching half. This process can be repeated until a specified threshold is met, at which point the Dog API can be queried directly for image URLs. That way, image downloads start as soon as the first URLs are retrieved, without initial waiting.
An implementation of this recursive submission may look like this:
Retry Handling
By default, when a task fails to execute, it’s marked as failed. In some cases, it may be useful to retry the task multiple times before marking it as a failure. This is particularly useful for tasks dependent on external services that might be temporarily unavailable. Tilebox Workflows allows you to specify the number of retries for a task using themax_retries
argument of the submit_subtask
method.
Check out the example below to see how this might look like in practice.
A failed task may be picked up by any available runner and not necessarily the same one that it failed on.
Dependencies
Tasks often rely on other tasks. For example, a task that processes data might depend on a task that fetches that data. Tasks can express their dependencies on other tasks by using thedepends_on
argument of the submit_subtask
method. This means that a dependent task will only execute after the task it relies on has successfully completed.
The
depends_on
argument accepts a list of tasks, enabling a task to depend on multiple other tasks.RootTask
submits three PrintTask
tasks as subtasks. These tasks depend on each other, meaning the second task executes only after the first task has successfully completed, and the third only executes after the second completes. The tasks are executed sequentially.
If a task upon which another task depends submits subtasks, those subtasks must also execute before the dependent task begins execution.
Dependencies Example
A practical example is a workflow that fetches news articles from an API and processes them using the News API.Output
Task | Dependencies | Description |
---|---|---|
NewsWorkflow | - | The root task of the workflow. It spawns the other tasks and sets up the dependencies between them. |
FetchNews | - | A task that fetches news articles from the API and writes the results to a file, which is then read by dependent tasks. |
PrintHeadlines | FetchNews | A task that prints the headlines of the news articles to the console. |
MostFrequentAuthors | FetchNews | A task that counts the number of articles each author has written and prints the result to the console. |
PrintHeadlines
and MostFrequentAuthors
tasks. This means they can execute in parallel, which the Tilebox Workflow Orchestrator will do, provided multiple task runners are available.
In this example, the results from
FetchNews
are stored in a file. This is not the recommended method for passing data between tasks. When executing on a distributed cluster, the existence of a file written by a dependent task cannot be guaranteed. Instead, it’s better to use a shared cache.Task Identifiers
A task identifier is a unique string used by the Tilebox Workflow Orchestrator to identify the task. It’s used by task runners to map submitted tasks to a task class and execute them. It also serves as the default name in execution visualizations as a tree of tasks. If unspecified, the identifier of a task defaults to the class name. For instance, the identifier of thePrintHeadlines
task in the previous example is "PrintHeadlines"
. This is good for prototyping, but not recommended for production, as changing the class name also changes the identifier, which can lead to issues during refactoring. It also prevents different tasks from sharing the same class name.
To address this, Tilebox Workflows offers a way to explicitly specify the identifier of a task. This is done by overriding the identifier
method of the Task
class. This method should return a unique string identifying the task. This decouples the task’s identifier from its class name, allowing you to change the identifier without renaming the class. It also allows tasks with the same class name to have different identifiers. The identifier
method can also specify a version number for the task—see the section on semantic versioning below for more details.
In python, the
identifier
method must be defined as either a classmethod
or a staticmethod
, meaning it can be called without instantiating the class.Semantic Versioning
As seen in the previous section, theidentifier
method can return a tuple of two strings, where the first string is the identifier and the second string is the version number. This allows for semantic versioning of tasks.
Versioning is important for managing changes to a task’s execution method. It allows for new features, bug fixes, and changes while ensuring existing workflows operate as expected. Additionally, it enables multiple versions of a task to coexist, enabling gradual rollout of changes without interrupting production deployments.
You assign a version number by overriding the identifier
method of the task class. It must return a tuple of two strings: the first is the identifier and the second is the version number, which must match the pattern vX.Y
(where X
and Y
are non-negative integers). X
is the major version number and Y
is the minor version.
For example, this task has the identifier "tilebox.com/example_workflow/MyTask"
and the version "v1.3"
:
MyTask
is submitted as part of a job. The version is"v1.3"
.- A task runner with version
"v1.3"
ofMyTask
would executes this task. - A task runner with version
"v1.5"
ofMyTask
would also executes this task. - A task runner with version
"v1.2"
ofMyTask
would not execute this task, as its minor version is lower than that of the submitted task. - A task runner with version
"v2.5"
ofMyTask
would not execute this task, as its major version differs from that of the submitted task.
States
A task can be in one of the following states:QUEUED
: the task is queued and waiting to be runRUNNING
: the task is currently running on some task runnerCOMPUTED
: the task has been computed and the output is available. Once in this state, the task will never transition to any other stateFAILED
: the task has failedCANCELLED
: the task has been cancelled due to user request