Follow-up
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iOS:
APN: A remote notification service built by Apple to push notification to iOS devices.
Android:
Firebase Cloud Messaging: Instead of APNs, FCM is commonly used to send notifications to mobile devices.
SMS:
Third party SMS providers such as Twillo, Nexmo, etc.
Email:
Set up their own email servers
Or commercial email service such as Sendgrid, Mailchimp, etc.
Provider builds notification with device token and a notification payload.
Notification servers could be the bottleneck
A single notification server means a single point of failure
Performance bottleneck: Processing and sending notifications can be resource intensive. For example, constructing HTML pages and waiting for responses from third party services could take time. Handling everything in one system can result in the system overload, especially during peak hours.
Add message queue to decouple:
Move the database and cache out of the notification server.
Introduce message queues to decouple the system components.
Asynchronously executed
Follow a user: Merge users' timeline into news feed asynchronously
Unfollow a user: Pick out tweets from news feed asynchronously
Benefits:
Fast response to users.
Disadvantages:
Consistency. After unfollow and refreshing newsfeed, users' info still there.
Tweet table
id
integer
userId
foreign key
content
text
createdAt
timestamp
likeNums
integer
commentNums
integer
retweetNums
integer
Like table
id
integer
userId
foreignKey
tweetId
foreignKey
createdAt
timestamp
Select Count in Like Table where tweet id == 1
Denormalize:
Store like_numbers within Tweet Table
Need distributed transactions.
Might resulting in inconsistency, but not a big problem.
Could keep consistency with a background process.