System requirements

Functional:

• Task creation - users should be able to create task specifying the task name, execution time, and recurrence interval is needed. There could be some kind of user interface such as web interface.
• Task scheduling - the system should efficiently schedule tasks for execution based on the specified time and recurrence.
• Task execution - tasks should be executed accurately at the scheduled time.
• Task monitoring - users should be able to monitor the status of tasks, whether pending, completed, or failed.
• Task rescheduling - users should have the option to reschedule or cancel tasks that are already scheduled.
• Concurrency handling - the system should handle multiple tasks running concurrently without conflicts.
• Error handling - the system should have robust error handling mechanisms to deal with failures during task execution.
• Task persistence - task data should be stored persistently to ensure that scheduled tasks are not lost in case of system failure.
• Scalability - the system should be able to handle a large number of tasks efficiently without significant delay

Non-Functional:

• Performance - The system should have low latency and be able to handle large number of scheduled tasks efficiently. Let's say the system needs to support scheduling 10,000 tasks per minute with a maximum delay of 1 second.
• Reliability - The system should be highly reliable, ensuring that scheduled tasks are executed as expected without failures.
• Scalability - The system should be designed to scale horizontally to accommodate an increasing number of tasks over time. It should be able to scale to support up to 100,000 tasks per minute.
• Security - The system should have robust security measures in place to protect task data and prevent unauthorized access.
• Monitoring - The system should provide monitoring capabilities to track task execution, system performance, and resource utilization.
• Audit-ability - The system should have logging and auditing mechanisms to track task scheduling, execution and any system events.

Capacity estimation

Let's consider the following estimates for capacity and bandwidth:

• Task Creation Frequency: Let's assume an average of 100 tasks are created per second.
• Task Execution Frequency: Assuming an average of 80 tasks are executed per second.
• Task Data Size: Let's estimate each task data to be around 1 KB in size.
• Bandwidth: Assuming an average bandwidth consumption of 1 MB/s for task creation and execution.

Based on these estimates, we can calculate the required capacity and bandwidth for the Task Scheduler system:

1. Capacity Estimation:
2. Task Creation Capacity: 100 tasks/second * 60 seconds = 6000 tasks/minute
3. Task Execution Capacity: 80 tasks/second * 60 seconds = 4800 tasks/minute
4. Bandwidth Estimation:
5. Task Creation Bandwidth: 1 KB/task * 100 tasks/second = 100 KB/s = 0.1 MB/s
6. Task Execution Bandwidth: 1 KB/task * 80 tasks/second = 80 KB/s = 0.08 MB/s

Considering each task data size as 1 KB and the creation of 6000 tasks per minute, the system will need a database capable of storing and managing this data efficiently. Therefore, the database should be able to handle a large volume of data insertion and retrieval operations.

API design

CreateTask - creates a task

input: task name, code function to be executed (this could be literal code passed in or reference to some file with entry point)

output: creation success or failure, 201 https code response.

ScheduleTask - schedule the task for a specify time to run

input: date time to execute the task, name of the task to be executed

output: schedule success or failure, 201 https code response.

ExecuteTask - immediately executes the task by queuing it up.

input: task name

output: job id

RescheduleTask - reschedules the task for a different time to run.

input: job id

output: job id

ListTasks - list tasks

input: none

output: list of tasks

UpdateTask - updates a task

input: task name, code function or reference to code file

output: update success or failure

DeleteTask - deletes a task

input: task id

output: success or failure

Database design

We will design the database schema in InfluxDB for a Task Scheduler system, we can follow a structured approach incorporating the key components of InfluxDB's time-series data model.

Measurement: task_schedule

• Tags:
• task_id (string)
• Fields:
• task_name (string)
• execution_time (timestamp)
• task_status (string)
• recurrence_interval (string)
• start_date (timestamp)
• end_date (timestamp)

Measurement: task_metrics

• Tags:
• task_id (string)
• Fields:
• cpu_utilization (float)
• memory_utilization (float)
• disk_usage (float)
• timestamp (timestamp)

Measurement: task_logs

• Tags:
• task_id (string)
• Fields:
• log_message (string)
• log_level (string)
• log_timestamp (timestamp)

task_schedule: Stores scheduled task information including task name, execution time, task status (pending, in progress, completed), recurrence interval, start date, and end date.

task_metrics: Contains performance metrics data related to task execution such as CPU utilization, memory utilization, disk usage, and timestamp.

task_logs: Records log messages generated during task execution with details like log message, log level, and log timestamp.

Why InfluxDB:

InfluxDB is purpose-built for handling time-series data, making it highly efficient for storing and querying timestamped data points. This aligns well with the nature of scheduling tasks with execution times.

Secondly, InfluxDB provides excellent write performance for ingesting time-series data rapidly. This is crucial for a Task Scheduler system where tasks may be created, updated, and executed frequently, requiring efficient data write operations.

High-level design

User Interface:

• Represents the interface through which users interact with the Task Scheduler system.
• Accepts user requests for task scheduling, monitoring, and management, facilitating user interaction with the system.

Task Scheduler:

• Core component responsible for managing task scheduling and execution.
• Orchestrates the scheduling of tasks, handles task execution requests, and coordinates interactions between different system components.

Database - InfluxDB :

• Stores task data, performance metrics, and monitoring information in a time-series database.
• Manages the persistent storage of task-related data, allowing for efficient retrieval and analysis of time-stamped information.

Monitoring Service:

• Monitors the performance and health of the Task Scheduler system.
• Collects performance data, generates insights, and alerts system administrators (Admin) about potential issues or abnormalities within the system.

Notification Service:

• Handles the generation and delivery of notifications to users.
• Sends notifications to users (represented by Users) based on specific events or triggers within the system, such as task completion or errors.

Admin:

• System administrator responsible for managing and overseeing the Task Scheduler system.
• Manages system configurations, resolves issues, sets up monitoring parameters, and ensures the smooth operation of the system.

Task Executor:

• Executes the scheduled tasks within the system.
• Receives task execution requests from the Task Scheduler or Task Queue, processes and executes tasks, and updates the task status in the database.

Task Queue:

• Manages the queuing and prioritization of task execution requests.
• Acts as a buffer for incoming task execution requests, ensures orderly task processing, and forwards tasks to the Task Executor for execution.

Task Processor:

• Processing component that updates task status and performs tasks related to task execution.
• Receives updates on task execution progress from the Task Executor, updates task status in the database, and manages the execution flow for efficient task processing.

ZooKeeper:

• ZooKeeper would serve as a coordination layer for tasks, ensuring that all nodes in the system are in sync and facilitating leader election.
• ZooKeeper can help in electing a leader node among the distributed nodes. The leader node can then be responsible for task scheduling and coordination.

Request flows

1. User Request Handling:
2. An external user interacts with the User Interface to perform actions such as creating, updating, or monitoring tasks within the Task Scheduler system.
3. The User Interface receives the user's request and forwards it to the Task Scheduler for processing.
4. Task Scheduling and Execution:
5. When a user request is received, the Task Scheduler processes the request and decides on the scheduling and execution logic for the task based on the input provided.
6. If the request involves creating a new task, the Task Scheduler stores the task data in the Database - InfluxDB under the appropriate measurement for task scheduling.
7. The Task Scheduler may also generate associated performance data and store it in InfluxDB for monitoring purposes.
8. Task Execution Request:
9. The Task Scheduler holds the responsibility of managing task execution requests. When it determines that a task needs to be executed, it places the task in the Task Queue.
10. The Task Queue manages the sequence of task execution, ensures proper task prioritization, and forwards tasks to the Task Executor for processing.
11. The Task Scheduler interacts with ZooKeeper to check for the leader node, ensuring that tasks are coordinated among distributed nodes effectively.
12. ZooKeeper helps in maintaining shared task status information, ensuring that all nodes have a consistent view of the system state.
13. When a new task is created, the Task Scheduler updates the shared configuration data in ZooKeeper to reflect the task status.
14. ZooKeeper assists in leader election and consensus, helping in establishing a reliable and coordinated task execution workflow among the distributed nodes.
15. Task Execution:
16. The Task Executor, upon receiving a task from the Task Queue, proceeds with executing the task logic as per the scheduled parameters.
17. As part of task execution, the Task Executor interacts with the Database (InfluxDB) to fetch relevant task information and update task status after completion.
18. Monitoring and Notifications:
19. During and after task execution, the Task Executor may generate performance data and logs, which are stored in InfluxDB for monitoring.
20. The Monitoring Service continuously monitors system performance and task execution metrics to ensure system reliability.
21. If specific events or thresholds are met, the Notification Service sends notifications to users or system administrators to provide updates or alerts about task status or system health.
22. Task Status Update:
23. The Task Executor communicates the task execution status to the Task Processor, which updates the task status in the Database (InfluxDB) to reflect the current state of the task.
24. This update ensures that users and administrators can access real-time task status information and monitor task progress within the system.

Detailed component design

Scheduling algorithm

Priority-Based Task Execution:

• We can implement a priority queue to manage tasks based on their priority levels.
• Assign priority values to tasks and reorder the task queue accordingly.
• Ensure that high-priority tasks are executed before lower-priority tasks.

we can utilize Dijkstra's algorithm to assign priorities to tasks based on their dependencies, execution times, or any other criteria. Here's how Dijkstra's algorithm can be applied for task prioritization:

1. Define the Graph: Represent tasks as nodes in a graph where edges denote dependencies or relationships between tasks.
2. Assign Weights: Assign weights to edges based on task attributes such as execution time, importance, or resource requirements.
3. Run Dijkstra's Algorithm: Execute Dijkstra's algorithm on the graph to compute the shortest path from a selected source node (starting task) to all other nodes (tasks).
4. Extract Priorities: Based on the shortest distances calculated by Dijkstra's algorithm, assign priorities to tasks. Tasks with shorter paths from the source node receive higher priorities.

Handling Overlapping Task Schedules:

Some tasks with dependencies need to be executed in a specific order. Overlapping schedules can disrupt this order, causing tasks to run out of sequence and leading to incorrect results. In addition, some tasks may have different priorities based on their importance. With overlapping schedules, lower priority tasks might delay higher priority tasks if resources are allocated based on the order of arrival rather than priority.

To address overlapping task schedules we can take the following approaches:

When a new task is scheduled to run at the same time as an existing task, we can implement conflict resolution mechanisms to prioritize or reschedule tasks based on predefined rules (e.g., priority levels or task dependencies).

Higher priority task will be executed first. If there's a dependency issue then we will order the tasks using algorithm like topological sort and execute the task in that order.

The task scheduler can maintain a graph data structure where nodes represent tasks and edges represent dependencies between tasks. When a task is scheduled, the scheduler can traverse the graph to ensure all dependencies are met before executing the task.

Flow for how the task scheduler handles this scenario

• The Task Scheduler receives a new task to be scheduled.
• It first checks for any dependencies required by the task. If dependencies are not met, the scheduler waits until they are resolved before proceeding.
• If no dependencies are found or once dependencies are resolved, the scheduler checks for concurrency constraints.
• If the task can be executed concurrently with other tasks, it proceeds to execute the task.
• If there is a concurrency conflict with existing tasks, the scheduler waits until the conflicting tasks are completed before executing the task.

Concurrency can be managed using techniques like locking mechanisms or thread pools to control access to shared resources and ensure tasks are executed safely without conflicts.

Resiliency Against Sudden Spikes:

Load Balancing Strategies:

• Distribute incoming task requests evenly across multiple task processing nodes to prevent overloading a single node during spikes.
• Monitor the workload of each task executor and dynamically adjust task assignment.
• Consider factors like task complexity, execution time, and resource utilization for load balancing.

Fault-Tolerant Mechanisms:

Task Checkpointing:

Task checkpointing involves saving the state of a task at specific points in its execution to enable recovery in case of failure.

Checkpointing allows the system to resume task execution from the last saved state rather than restarting the task from scratch, minimizing the impact of failures. Checkpointing can be implemented by periodically saving the task state to persistent storage (e.g., database or disk) or through in-memory snapshots.

Below is a diagram illustrating checkpointing:

Replication:

In addition to checkpointing we should also replicate task scheduler components and data across multiple servers to ensure high availability.

Finally, we can use a cloud service such as AWS to host our Task Scheduler service. AWS will cover non functional requirements such as scalability and reliability. This way we don't need to worry about consensus algorithms like Paxos or Raft. AWS abstracts these details away.

Trade offs/Tech choices

Consistency vs. Availability:

In the context of task scheduling, the system may lean towards favoring availability over strong consistency. For example, in scenarios where it is critical for tasks to be scheduled and executed without delays, ensuring that the system remains available and operational becomes a higher priority than strict consistency across all nodes.

Failure scenarios/bottlenecks

Task Execution Failure:

A task scheduled for execution may fail due to various reasons such as exceptions, errors in the task logic, or external dependencies not being available.

System Crashes:

The Task Scheduler system itself may crash or become unresponsive due to hardware failures, software bugs, or resource exhaustion.

Network Issues:

Network failures or interruptions can disrupt communication between system components, leading to task scheduling or execution failures.

Future improvements

Retry Mechanism

It would be interesting to discuss more on the retry mechanism for failed tasks to automatically retry execution a certain number of times before marking the task as failed.

Logging

More discussion on logging and monitoring would contribute more to the solution.

Fallback Logic

More discussion on how to incorporate fallback logic or alternative paths for critical tasks to handle failures gracefully and ensure continuity.

Algorithms

We can also consider more advanced scheduling algorithms beyond Dijkstra's algorithm by exploring real-time scheduling heuristics and adaptive strategies to cope with varying task complexities and priorities.