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

Let's discuss the high-level and detailed design of a newsfeed system based on the requirements discussed in the previous lesson.  
 
# High-level design

Primarily, the newsfeed system is responsible for the following two tasks. 

1. **Feed generation:** The newsfeed is generated by aggregating friends' and followers' posts (or feed items) based on some ranking mechanism. 

2. **Feed publishing:** When a feed is published, the relevant data is written into the cache and database. This data could be textual or any media content. A post containing the data from friends and followers is populated to a user's newsfeed. 



Let's move to the high-level design of our system, consisting of the above two essential parts, shown in the following figure.







Let's discuss the main components shown in the high-level design:

1.  **User(s):** Users can make a post with some content or request their newsfeed. 

2. **Load balancer:** It redirects traffic to one of the web servers.  

3. **Web servers:** The web servers encapsulate the back-end services and work as an intermediate layer between users and various services. Apart from enforcing authentication and rate-limiting, web servers are responsible for redirecting traffic to other back-end services.

4. **Notification service:** It informs the newsfeed generation service whenever a new post is available from one's friends or followers and sends out a push notification.

5. **Newsfeed generation service:** This service generates newsfeeds from the posts of followers/friends of a user and keeps them in the newsfeed cache.

6. **Newsfeed publishing service:** This service is responsible for publishing newsfeeds to a users' timeline from the newsfeed cache. It also appends a thumbnail of the media content from the blob storage and its link to the newsfeed intended for a user. 

7. **Post service:** Whenever a user requests to create a post, the post-service is called, and the created post is stored on the post database and corresponding cache. The media content in the post is stored on the blob storage.  





## API design
APIs are the primary ways for clients to communicate with servers. Usually, newsfeed APIs are HTTP-based that allow clients to perform actions, including posting a status, retrieving newsfeeds, adding friends, etc. We aim to generate and get a user's newsfeed; therefore, the following APIs are essential. 

### Generating user newsfeed
The following could be the API for generating a user's newsfeed.

```
generateNewsfeed(user_id)
```
This API takes users' IDs via the parameter **user_id**, and determines their friends and followers. Via this API newsfeeds are generated consisting of several posts. Since internal system components will use this API; therefore, it can be called offline to pre-generate newsfeeds for users. The pre-generated newsfeeds are stored on persistent storage and associated cache.  

### Getting user newsfeed

The following could be the definition of API for getting a user's newsfeed. 

```
getNewsfeed(user_id, count)
```
Let's explain each parameter of the given API. 

* **user_id:** The ID of the user for whom the system will fetch the newsfeed. 
  

* **count:** This parameter specifies the number of feed items (posts) to be retrieved per request. 


The `getNewsfeed(.)` API call returns a JSON object consisting of a list of posts.

Understand the high-level and detailed design of a newsfeed system. 

Design of Newsfeed System

Understand the high-level and detailed design of a newsfeed system.