Requirements

Imagine you are building a robust download manager for audio and video files from online sources. Users should be able to add multiple files to a download queue, specify download locations, control download speed, and handle potential errors gracefully. The system should support resuming interrupted downloads and be easily extensible to support new download protocols and file types. This requires a design that is modular, extensible, and handles concurrent downloads efficiently. We need to consider scenarios such as network disruptions, server errors, and file corruption. Furthermore, the design should allow for different download strategies such as sequential downloads or concurrent downloads with limits on the number of concurrent downloads. # Requirements ## Functional Requirements 1. **Add Downloads:** Allow users to add audio and video files to a download queue, specifying the URL and the local file path where the downloaded content should be saved. 2. **Start/Pause/Resume/Cancel Downloads:** Provide functionality to start, pause, resume, and cancel individual downloads or all downloads in the queue. Canceling should ensure any partially downloaded file is properly cleaned up. 3. **Download Speed Control:** Implement a mechanism to limit the download speed for each download or globally for all active downloads. This might involve throttling the read operations from the input stream. 4. **Download Progress Tracking:** Display the download progress (percentage completed, bytes downloaded, estimated time remaining) for each download. 5. **Error Handling:** Implement robust error handling for common download failures such as network errors, server errors (e.g., 404, 500), insufficient disk space, and file corruption. Provide meaningful error messages to the user. 6. **Resume Interrupted Downloads:** Support resuming downloads that were interrupted (e.g., due to network disconnection or program termination). The system should maintain metadata about the download state to facilitate resuming. 7. **Concurrent Downloads:** Support downloading multiple files concurrently, with a configurable limit on the number of simultaneous downloads. 8. **Different Download Protocols**: Support HTTP and HTTPS download protocols. 9. **Authentication**: Support basic HTTP authentication where the URL requires a username and password. 10. **File Metadata**: Extract file metadata such as file size, content type, and last modified date before starting the download (if available via HTTP headers). 11. **Download Prioritization**: Allow users to prioritize downloads in the queue. ## Non-Functional Requirements 1. **Thread Safety:** Ensure that all operations related to the download queue and individual downloads are thread-safe. Multiple threads might access and modify the download queue and individual download states concurrently. Use appropriate synchronization mechanisms (e.g., locks, concurrent collections) to prevent race conditions and data corruption. 2. **Extensibility:** The design should be easily extensible to support new download protocols (e.g., FTP, SFTP), file types, and download strategies without requiring significant code changes. Use design patterns like Strategy or Template Method to achieve this. 3. **Modularity:** The system should be divided into well-defined modules with clear responsibilities. This will improve maintainability and testability. 4. **Testability:** The code should be designed to be easily testable, with clear interfaces and minimal dependencies. Use dependency injection to facilitate unit testing. 5. **Resource Management:** Ensure efficient resource management, especially for network connections and file handles. Properly close resources when downloads are completed or canceled. Consider using a thread pool to manage download threads. 6. **Resilience**: The system must be resilient to transient network errors. Implement retry mechanisms with exponential backoff. 7. **Configuration**: The number of concurrent downloads, retry attempts, and download speed limits should be configurable. 8. **Observability**: Provide logging and metrics to monitor the download process and identify potential issues. ## Core Entities 1. **`DownloadTask`:** Represents a single download task. Holds information about the URL, local file path, download status, progress, and error details. It encapsulates the logic for downloading a single file. 2. **`DownloadQueue`:** Manages the queue of `DownloadTask` objects. Provides methods for adding, removing, starting, pausing, resuming, and canceling downloads. Responsible for scheduling and coordinating downloads. This will probably use a thread-safe data structure. 3. **`DownloadManager`:** The main entry point for the download manager. It holds the `DownloadQueue` and provides methods for users to interact with the download system. It can configure global settings like max concurrent downloads. 4. **`DownloadStrategy`:** An interface that defines the strategy for downloading a file. Different implementations could support different download protocols (e.g., HTTP, FTP). This allows to introduce new protocols without modifying the core `DownloadTask`. 5. **`DownloadProgressListener`:** An interface for receiving download progress updates. Implementations can update the UI or log progress information. 6. **`RetryStrategy`**: Defines how retries are handled (number of retries, backoff strategy). 7. **`AuthenticationStrategy`**: Defines how authentication is handled (e.g., Basic Auth). 8. **`SpeedLimiter`**: Controls the download speed to prevent overwhelming the network or disk.

Design online audio/video file downloader.

Requirements

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