# API Reference: **futures_orchestra**
This document provides a detailed API reference for the `futures_orchestra` library.
## 1. Introduction / Core Concepts
`futures_orchestra` is a library designed for managing and executing asynchronous tasks (futures) in a controlled, concurrent environment within the Tokio runtime. It provides mechanisms for limiting concurrency, queuing tasks, labeling tasks for group operations, and cooperative cancellation.
**Core Concepts & Primary Structs/Classes:**
* **`FuturePoolManager<R>`**:
* **Description**: The central component and main entry point for the library. It manages a pool of workers, a task queue, and a semaphore to control concurrency. It's responsible for accepting task submissions, scheduling their execution, and handling the pool's lifecycle, including shutdown.
* **Interaction**: Users create an instance of `FuturePoolManager` (typically an `Arc<FuturePoolManager<R>>`) and use it to submit tasks and manage the pool. `R` is the success type returned by the futures executed by the pool.
* **`TaskHandle<R>`**:
* **Description**: A handle returned when a task is successfully submitted to the `FuturePoolManager`. It allows users to interact with an individual task, such as awaiting its result or requesting its cancellation. `R` matches the result type of the submitted future.
* **Task (`TaskToExecute<R>`)**:
* **Description**: Represents the actual asynchronous work to be done. It's a type alias for `Pin<Box<dyn Future<Output = R> + Send + 'static>>`. This means any future that is `Send` (can be sent between threads) and `'static` (does not hold non-static references) and produces a value of type `R` can be submitted.
* **`TaskLabel`**:
* **Description**: A type alias for `String`. Labels are used to categorize tasks. A task can have multiple labels. These are primarily used for performing bulk operations, such as canceling all tasks that share a particular label.
* **Concurrency Control**:
* **Description**: Achieved via an internal `tokio::sync::Semaphore`. The `FuturePoolManager` is configured with a concurrency limit, and the semaphore ensures that no more than this number of tasks are actively running simultaneously.
* **Task Queuing**:
* **Description**: Tasks submitted when the concurrency limit is reached are placed into an internal, bounded, asynchronous MPMC (Multi-Producer Multi-Consumer) queue (implemented using `kanal`). Tasks are dequeued and processed as semaphore permits become available.
* **Cooperative Cancellation**:
* **Description**: Cancellation is signaled through `tokio_util::sync::CancellationToken`. The `FuturePoolManager`'s worker loop uses `tokio::select!` to race the execution of a task's future against its associated cancellation token. If the token is cancelled, the future's execution is aborted by the pool.
* **Shutdown Mechanism**:
* **Description**: The pool can be shut down gracefully (allowing active tasks to complete) or forcefully (attempting to cancel active tasks). The `FuturePoolManager` also implements `Drop` for implicit signaling of shutdown if all `Arc` references are dropped.
**Pervasive Types/Patterns:**
* **`PoolError`**: The primary error enum used throughout the library for all fallible operations.
* **`Result<T, PoolError>`**: Most public methods that can fail return this standard Rust `Result` type, specialized with `PoolError`.
* **`Arc<FuturePoolManager<R>>`**: The `FuturePoolManager::new` constructor returns an `Arc`-wrapped manager, facilitating shared ownership and usage across different parts of an application (e.g., different async tasks or threads).
## 2. Configuration
Configuration primarily happens during the instantiation of the `FuturePoolManager`.
* **`FuturePoolManager::new(...)` parameters**:
* `concurrency_limit: usize`: The maximum number of tasks that can be actively running. Must be at least 1.
* `queue_capacity: usize`: The maximum capacity of the internal task queue. Must be at least 1.
* `tokio_handle: tokio::runtime::Handle`: A handle to an existing Tokio runtime. Tasks managed by the pool will be spawned onto this runtime.
* `pool_name: &str`: A descriptive name for the pool, primarily used for logging and tracing.
* **`ShutdownMode` enum**:
* **Description**: Specifies the behavior of the pool during an explicit shutdown requested via `FuturePoolManager::shutdown()`.
* **Variants**:
* `Graceful`: The pool stops accepting new tasks. It waits for currently executing tasks to complete. Queued tasks that have not yet started will not be executed.
* `ForcefulCancel`: The pool stops accepting new tasks. It attempts to cancel all currently executing tasks. Queued tasks that have not yet started will not be executed.
## 3. Main Types and Their Public Methods
### `struct FuturePoolManager<R: Send + Sync + 'static>`
The main struct for managing and interacting with the task pool.
**Constructors:**
* `pub fn new(concurrency_limit: usize, queue_capacity: usize, tokio_handle: tokio::runtime::Handle, pool_name: &str) -> std::sync::Arc<Self>`
* Creates a new `FuturePoolManager` instance, wrapped in an `Arc`.
* `R` is the success type of the futures this pool will manage.
**Methods:**
* `pub fn name(&self) -> &str`
* Returns the configured name of the pool.
* `pub fn active_task_count(&self) -> usize`
* Returns the current number of tasks that are actively running (i.e., have acquired a semaphore permit and have been spawned).
* `pub fn queued_task_count(&self) -> usize`
* Returns the current number of tasks waiting in the internal queue for a semaphore permit.
* `pub async fn submit(&self, labels: std::collections::HashSet<TaskLabel>, task_future: TaskToExecute<R>) -> Result<TaskHandle<R>, PoolError>`
* Submits a new task to the pool.
* `labels`: A set of `TaskLabel` (strings) to associate with this task.
* `task_future`: The future to be executed, of type `TaskToExecute<R>`.
* Returns a `TaskHandle<R>` on successful submission, or a `PoolError` if submission fails (e.g., pool is shutting down, queue send error).
* `pub fn cancel_tasks_by_label(&self, label_to_cancel: &TaskLabel)`
* Requests cancellation for all currently active tasks that have the specified `label_to_cancel`.
* `pub fn cancel_tasks_by_labels(&self, labels_to_cancel: &std::collections::HashSet<TaskLabel>)`
* Requests cancellation for all currently active tasks that have one or more of the labels in `labels_to_cancel`.
* `pub async fn shutdown(self: std::sync::Arc<Self>, mode: ShutdownMode) -> Result<(), PoolError>`
* Initiates an explicit shutdown of the pool. This method consumes an `Arc<Self>`.
* `mode`: The `ShutdownMode` (Graceful or ForcefulCancel) to use.
* Awaits the termination of the internal worker loop. Returns `Ok(())` on successful shutdown, or a `PoolError` if an issue occurs during shutdown (though primarily errors relate to internal state issues, not task execution errors).
**`Drop` Implementation:**
`FuturePoolManager<R>` implements `std::ops::Drop`. When the last `Arc<FuturePoolManager<R>>` is dropped, if an explicit shutdown has not already been initiated, it will:
1. Cancel the internal global shutdown token.
2. Close the task submission queue (`task_queue_tx`).
This effectively signals the worker loop to terminate but does *not* block or await its completion. For guaranteed cleanup and worker joining, an explicit `shutdown()` call is recommended.
### `struct TaskHandle<R: Send + 'static>`
A handle to a task that has been submitted to the pool. `R` is the success type of the task's future.
**Methods:**
* `pub fn id(&self) -> u64`
* Returns the unique ID assigned to this task by the pool.
* `pub fn labels(&self) -> std::collections::HashSet<TaskLabel>`
* Returns a clone of the set of `TaskLabel`s associated with this task at the time of submission.
* `pub fn is_cancellation_requested(&self) -> bool`
* Checks if cancellation has been requested for this specific task (either via `TaskHandle::cancel()` or by a label-based cancellation affecting this task).
* `pub fn cancel(&self)`
* Requests cancellation of this specific task by triggering its internal `CancellationToken`.
* `pub async fn await_result(mut self) -> Result<R, PoolError>`
* Awaits the completion of the task and returns its outcome.
* This method consumes the `TaskHandle` (takes `self` by value).
* Returns `Ok(R)` if the task completes successfully with a value of type `R`.
* Returns `Err(PoolError)` if the task panics (`PoolError::TaskPanicked`), is cancelled (`PoolError::TaskCancelled`), the result channel is broken (`PoolError::ResultChannelError`), or if this method has already been called (`PoolError::ResultUnavailable`).
## 4. Public Traits and Their Methods
There are no public traits defined in this library for external implementation by users.
## 5. Public Enums (Non-Config)
* **`PoolError`** (See [9. Error Handling](#9-error-handling) for variants)
* **Description**: Represents all possible errors that can occur within the `futures_orchestra` pool operations.
## 6. Public Functions (Free-standing)
There are no public free-standing functions exported at the crate root. All primary interactions are through methods on `FuturePoolManager` and `TaskHandle`.
## 7. Public Type Aliases
* `pub type TaskLabel = String;`
* Alias for `String`, used for labeling tasks.
* `pub type TaskToExecute<R> = std::pin::Pin<Box<dyn std::future::Future<Output = R> + Send + 'static>>;`
* Alias for the type of future that can be submitted to the pool.
* `R`: The output type of the future.
* The future must be `Send` (sendable between threads) and `'static` (has no non-static lifetimes).
## 8. Public Constants
There are no public constants exported at the crate root.
## 9. Error Handling
### `enum PoolError`
The primary error type used throughout the library. It implements `std::error::Error` (via `thiserror::Error`) and `std::fmt::Debug`, `PartialEq`.
**Variants:**
* `QueueSendError(String)`: Failed to submit a task to the pool's internal queue. The `String` contains details about the send error.
* `ResultChannelError(String)`: An error occurred with the `tokio::sync::oneshot` channel used to communicate the task's result back to its `TaskHandle`. This can happen if the task panicked before sending, was cancelled abruptly, or the `TaskHandle` (receiver) was dropped. The `String` provides more context.
* `ResultUnavailable`: `TaskHandle::await_result()` was called when the result had already been taken or the channel was otherwise not available (e.g., called multiple times).
* `SemaphoreClosed`: The pool's internal `tokio::sync::Semaphore` was unexpectedly closed, preventing new tasks from acquiring permits.
* `QueueSendChannelClosed`: The sender side of the pool's internal task queue (`kanal::AsyncSender`) was unexpectedly closed.
* `QueueReceiveChannelClosed`: The receiver side of the pool's internal task queue (`kanal::AsyncReceiver`) was unexpectedly closed.
* `TaskPanicked`: The submitted task future panicked during its execution.
* `TaskCancelled`: The task was cancelled before it could complete.
* `PoolShuttingDown`: An operation was attempted (e.g., submitting a new task) while the pool is in the process of shutting down or has already shut down.
**Standard Result Type:**
The library does not define a custom `Result` type alias (like `LibResult<T>`). It uses the standard `std::result::Result<T, PoolError>` for operations that can fail with a `PoolError`.
## 10. Modules
The public API is primarily exposed from the crate root. Key types are:
* `futures_orchestra::FuturePoolManager`
* `futures_orchestra::TaskHandle`
* `futures_orchestra::PoolError`
* `futures_orchestra::ShutdownMode`
* `futures_orchestra::TaskLabel` (type alias)
* `futures_orchestra::TaskToExecute<R>` (type alias)