System requirements

Functional:

List functional requirements for the system (Ask the chat bot for hints if stuck.)...

• The service can support the following statuses: online, idle, and offline.
• The service can be configured by having the extra statuses and timeouts between online, idle, and offline.
• Users can get their current status
• Users can get their status history within a time range.
• Users can change their status.

Non-Functional:

List non-functional requirements for the system..

• The service should be highly available and highly reliable
• The service should persist the received statuses.
• The service should be stateless and fault-tolerant.

Capacity estimation

Estimate the scale of the system you are going to design...

Let's have 1 million DAU.

Every status has userId, a status value, and a timestamp.

UserId takes 8 bytes, status value takes 1 byte, and a timestamp takes 8 bytes. One status event takes 20 bytes.

Every user's status is changed 5 times daily.

It would give 100 million bytes every day of new statuses.

Let's keep this information for one year, and it takes 100 Gigabytes (two replications)

Let's use a key-value storage like AWS DynamoDB.

API design

Define what APIs are expected from the system...

getUserStatus(userId,fromDate,toDate) returns the user status list for the given time range

updateUserStatus(userId, status, currentTime) update the user status

Database design

Defining the system data model early on will clarify how data will flow among different components of the system. Also you could draw an ER diagram using the diagramming tool to enhance your design...

We would use a NOSQL solution like AWS DynamoDB.

It has concurrent write and read speeds.

The key is userId

The value is the timestamp and status.

UserId would be the partition key, and timestamp and status are created as the secondary index.

High-level design

You should identify enough components that are needed to solve the actual problem from end to end. Also remember to draw a block diagram using the diagramming tool to augment your design. If you are unfamiliar with the tool, you can simply describe your design to the chat bot and ask it to generate a starter diagram for you to modify...

Request flows

Explain how the request flows from end to end in your high level design. Also you could draw a sequence diagram using the diagramming tool to enhance your explanation...

Detailed component design

Dig deeper into 2-3 components and explain in detail how they work. For example, how well does each component scale? Any relevant algorithm or data structure you like to use for a component? Also you could draw a diagram using the diagramming tool to enhance your design...

Trade offs/Tech choices

Explain any trade offs you have made and why you made certain tech choices...

Failure scenarios/bottlenecks

Try to discuss as many failure scenarios/bottlenecks as possible.

Future improvements

What are some future improvements you would make? How would you mitigate the failure scenario(s) you described above?