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Designing a real-time chat

Requirements

Functional Requirements:

  • users can chat with another user that is in his contact list
  • users can chat in a group with multiple users
  • users can see the status of another user (online, offline, away)

Non-functional requirements:

  • system should store the history of the chat
  • system should be highly available

Extended requirements:

  • when the user is offline, the user should receive a notification/email when there are new messages
  • users should be able to send files/photos
  • users should be able to send emoji/stickers

Capacity Estimations and Constraints

Traffic: Let's start with a rough estimate. Let's say that we are expecting 500 messages per second, and the read to write ratio is 100:1 (there are more reads on the system than write), then our system's QPS (query per second) is:

Write QPS
Rate: 500 msg/s
1 day traffic: 43.2M/s
1 month traffic: 1.3B/s
1 year traffic: 15.6B/s

Read QPS:
Rate: 50k/s
1 day traffic: 4.32B/s
1 month traffic: 130B/s
1 year traffic: 1.56T/s

Storage: If we are planning to store the messages for 5 years:

1 year storage = 500 msg/s * 86400 * 30 * 365 * 5 * 140 bytes / 1024 ^ 4 = 301 TB
5 year storage = 1.5 PB

Bandwidth: Assuming that the messages are short, and does not exceed 140 characters:

1 char = 1 byte
Write per second: 68KB/s
Write per month: 1.3B/s * 140 bytes = 169.5 GB/s

Read per second: 6.8MB/s
Read per month: 1.7TB/s

Memory: Based on pareto principles, 80% of the read comes from 20% of the messages. We want to cache this data in the storage:

With 50000 msg/s, in one day:
50000 * 86400 * 500 bytes = ~2TB
20% * 2TB = 400 GB

We need 400 GB to cache 20% of the requests.

APIs

What apis do we need?

sendMessage(from, to User, msg string)
broadcastMessage(from User, group Group, msg string)
joinGroup(id User, group Group)
leaveGroup(id User, group Group)
status(id User) bool
getHistory(id User, since Time, limit int)
notify(id User, msg string)

Extension

  • how to notify user's presence? (when user is online or offline)
  • how to decide which user to show (show the ones last communicated with, and the rest based on the last added date, and optional for user to search for friends)
  • how to create group chat
  • how to attach different media types (links, image)
  • how to moderate content
  • how to create a pluggable chat system