This course immerses the learner into the design of systems and makes them ready for any new design problems.

sys-design.tar.gz

basic

This course provides an alternate perspective to system design where we learn about building blocs first and then use them to build bigger, more capable systems.

Copy-System Design-The Missing Pieces

A task is a piece of computational work that requires resources (CPU time, memory, storage, network bandwidth, etc.) for some specified time. For example, uploading a photo or a video on Facebook or Instagram consists of the following background tasks:

1. Encode the photo/video, in multiple resolutions
2. Validate the photo/video, to check for #key# content monetization: Content monetization is a way of leveraging content so that a service can profit from it as users consume it. #key# terms or copyrights, etc. 

Successful execution of all the above tasks makes the photo/video visible. Though photo/video uploader does not need to block on the above tasks to complete.

Another example is posting a comment on Facebook. We don't hold the comment poster until that comment is delivered to all the followers. That delivery can be delegated to an asynchronous task scheduler to do offline.

In a system, there are much more tasks present contending for limited computational resources. A system that mediates between tasks and resources by intelligently allocating resources to tasks such that task-level and system-level goals are met is called a **task scheduler**.
# Motivation
Task scheduler is a critical component of a system for getting the work done efficiently. It allows us to complete a large number of tasks using limited resources. It also aids in fully utilizing the system's resources, providing users with an #key# uninterrupted execution experience: Running periodic tasks without user intervention to initiate the task execution again and again. #key#, and so on. Following are some of the use cases of task scheduling.
* In a **single-OS-based node**, there are many processes/tasks contending for the node's limited computational resources. So, there is a local OS task scheduler that efficiently allocates resources to the tasks. It uses multi-feedback queues to pick some of them and to run them on some processor.
* With the advent of **cloud computing services**, where there are many distributed resources and various tasks from multiple tenants, there is a strong need for a task scheduler to utilize cloud computing resources efficiently and meet tenants' demands. A local OS task scheduler isn't sufficient for this purpose because now the tasks are in billions, the source of the tasks is not single, and the resources to manage are not in a single machine. We have to go for a distributed solution.
* In **large distributed systems**, many tasks run in the background against a single request by a user, and there are millions to billions of users of a popular system like Facebook, WhatsApp, or Instagram. These systems require a task scheduler for handling billions of tasks. Facebook schedules the tasks against billions of parallel #key# asynchronous requests: Asynchronous requests are those that are not on a client's critical path and often have some delay tolerance. Users request such tasks, and the system completes the tasks while the requester does not need to wait for the completion of the work. Notifications about the final state of the task are communicated to the requester sometime in the future. #key# by its users using **Async**.

> Async is Facebook's own distributed task scheduler that schedules all its tasks. Some tasks are more time-sensitive, like the tasks that should run to notify the users that the Live Streaming of an event is started. If the users get the notification of the Live Streaming when it is going to be finished, then that notification would be useless for the users. Some tasks can be delayed, like tasks behind friend suggestions. Async schedules tasks based on the priorities.

Learn about distributed task scheduling and our requirements.

Introduction to Distributed Task Scheduler

What is a task scheduler?