Presence for online status in Supabase - Time & Space Complexity

Time Complexity: Presence for online status

O(n)

Understanding Time Complexity

We want to understand how the time it takes to track online users grows as more users connect or disconnect.

Specifically, how does the system handle many users updating their online status?

Scenario Under Consideration

Analyze the time complexity of the following operation sequence.

// User connects
const { data, error } = await supabase
  .from('presence')
  .upsert({ user_id: userId, status: 'online' })

// User disconnects
await supabase
  .from('presence')
  .delete()
  .eq('user_id', userId)

// Listen for presence changes
const subscription = supabase
  .channel('presence')
  .on('postgres_changes', { event: '*', schema: 'public', table: 'presence' }, payload => {
    // update UI with online users
  })
  .subscribe()

This sequence updates a user's online status and listens for changes to show who is online.

Identify Repeating Operations

• Primary operation: Upsert or delete calls to the 'presence' table for each user status change.
• How many times: Once per user connection or disconnection event.
• Secondary operation: Real-time subscription listens for all presence changes.
• Dominant operation: The upsert/delete calls scale with the number of users changing status.

How Execution Grows With Input

Each user connecting or disconnecting triggers one database operation.

Input Size (n users)	Approx. API Calls/Operations
10	About 10 upsert or delete calls
100	About 100 upsert or delete calls
1000	About 1000 upsert or delete calls

Pattern observation: The number of operations grows directly with the number of users changing status.

Final Time Complexity

Time Complexity: O(n)

This means the time to update presence grows linearly with the number of users changing their online status.

Common Mistake

[X] Wrong: "Updating one user's status updates all users at once, so time stays constant."

[OK] Correct: Each user's status update is a separate operation, so more users mean more updates and more time.

Interview Connect

Understanding how presence updates scale helps you design systems that handle many users smoothly and keep interfaces responsive.

Self-Check

"What if we batch multiple user status updates into a single database call? How would the time complexity change?"

Practice

1. What is the main purpose of using presence in Supabase realtime channels?

easy

A. To show which users are currently online in real-time

B. To store user passwords securely

C. To send emails to users

D. To create database backups automatically

Solution

1. Step 1: Understand presence feature

   Presence in Supabase is designed to track and show who is online at any moment.

2. Step 2: Identify main use case

   It helps applications display real-time online user lists, not for security or backups.

3. Final Answer:

   To show which users are currently online in real-time -> Option A

4. Quick Check:

   Presence = Online user tracking [OK]

Hint: Presence means tracking who is online live [OK]

Common Mistakes:

• Confusing presence with security features
• Thinking presence handles backups
• Assuming presence sends emails

2. Which of the following is the correct way to listen for presence updates in Supabase realtime channels?

easy

A. channel.on('presence', callback)

B. channel.on('update', callback)

C. channel.on('message', callback)

D. channel.on('sync', callback)

Solution

1. Step 1: Recall Supabase presence event names

   Supabase presence uses the 'sync' event to notify when presence data updates.

2. Step 2: Match event to correct listener

   The 'sync' event triggers the callback with the updated online user list.

3. Final Answer:

   channel.on('sync', callback) -> Option D

4. Quick Check:

   Presence updates use 'sync' event [OK]

Hint: Presence updates come from 'sync' event, not 'presence' [OK]

Common Mistakes:

• Using 'presence' event which does not exist
• Confusing 'message' event with presence updates
• Using 'update' event which is unrelated

3. Given this code snippet, what will be logged when a user joins the presence channel?

const channel = supabase.channel('online-users').on('presence', () => {
  console.log(channel.presenceState())
}).subscribe()

medium

A. An object showing all currently online users

B. An error because presenceState() is not a function

C. An empty array always

D. A string message 'User joined'

Solution

1. Step 1: Understand presenceState() method

   presenceState() returns the current presence data, showing online users grouped by user ID.

2. Step 2: Analyze code behavior on 'presence' event

   When a user joins, 'presence' triggers and logs the updated presence state object.

3. Final Answer:

   An object showing all currently online users -> Option A

4. Quick Check:

   presenceState() = online users object [OK]

Hint: presenceState() returns online users object on presence event [OK]

Common Mistakes:

• Thinking presenceState() is undefined
• Expecting a string message instead of object
• Assuming it returns an empty array always

4. You wrote this code to track online users but no updates appear:

const channel = supabase.channel('online-users')
channel.on('presence', () => {
  console.log('Users online:', channel.presenceState())
})

What is the likely problem?

medium

A. The event name 'presence' is incorrect

B. You forgot to call subscribe() on the channel

C. presenceState() does not exist on channel

D. You need to use 'message' event instead

Solution

1. Step 1: Check channel subscription

   Without calling subscribe(), the channel does not connect and receive events.

2. Step 2: Confirm event and method correctness

   'presence' event and presenceState() are correct, so missing subscribe() is the issue.

3. Final Answer:

   You forgot to call subscribe() on the channel -> Option B

4. Quick Check:

   Must call subscribe() to receive events [OK]

Hint: Always call subscribe() after setting up channel listeners [OK]

Common Mistakes:

• Skipping subscribe() call
• Using wrong event name
• Expecting presenceState() without subscription

5. You want to show a live list of online users with their usernames using Supabase presence. Which approach correctly combines presence tracking and user data fetching?

hard

A. Subscribe to 'update' events and update UI without querying user data

B. Query user profiles once on page load, then listen to 'message' events for updates

C. Listen to 'presence' event, get user IDs from presenceState(), then query user profiles from database

D. Use presenceState() to get usernames directly without extra queries

Solution

1. Step 1: Understand presenceState() data

   presenceState() gives user IDs and metadata, but not full usernames.

2. Step 2: Combine presence with database queries

   After getting IDs from presenceState(), query the database to fetch usernames for display.

3. Final Answer:

   Listen to 'presence' event, get user IDs from presenceState(), then query user profiles from database -> Option C

4. Quick Check:

   Presence + DB query = live usernames [OK]

Hint: Use presenceState() IDs then query DB for usernames [OK]

Common Mistakes:

• Expecting presenceState() to contain usernames
• Not querying user profiles after presence update
• Listening to wrong events like 'message' or 'update'