Estimation

Users and Activity#

Start with the real world: roughly 10 billion people on Earth, about 1 billion of them are on a platform like Instagram or WhatsApp. Of those 1 billion registered users, assume 50% are Daily Active Users (DAU) — 500 million people using the product on any given day.

Each active user performs about 50 actions per day — likes, comments, DMs, story views. Each action typically generates 1 notification for the receiver (not the sender). So:

500M DAU × 50 actions = 25 billion notifications per day

QPS#

There are approximately 86,400 seconds in a day — round to 100,000 for easy math:

25B / 100,000 = 250,000 notifications/sec  (sustained)

Traffic is never uniform. Peak hours (evening, major events) can spike to 10× sustained:

250K × 10 = 2.5M notification events/sec  (peak)

Multi-Channel Multiplier#

Here's what candidates often miss: one notification event can trigger sends on multiple channels. A user might have push + email both enabled. The system supports 3 channels — push, SMS, email. On average, assume 2 channel sends per event:

2.5M events/sec × 2 channels = 5M actual sends/sec at peak

This is the harder number. The fan-out infrastructure (Kafka, channel workers) must handle 5M dispatches per second at peak, not 2.5M.

Storage#

Each notification record contains: - ID: 8 bytes - User ID, sender ID: 16 bytes
- Title + body: ~200 bytes - Deep link + metadata: ~100 bytes - Status, timestamps: ~50 bytes

Total: ~300–400 bytes per notification record. Call it 400 bytes.

2.5M notifications/sec × 400 bytes = 1GB/sec
1GB/sec × 86,400 = ~86TB/day

Wait — that seems large. The key insight is notifications are not permanent. Nobody queries "did I get that Instagram like in January?" After 30–90 days, notifications are useless. Cap retention at 90 days:

86TB/day × 90 days = ~7.7PB

That's still large. But most of this is cold storage — only recent notifications (last 7 days) are hot. And you can compress heavily since notification payloads are repetitive text. With 10× compression, cold storage drops to ~770TB for 90 days. Manageable.

Bandwidth#

At peak, 5M sends/sec, each notification payload ~300 bytes:

5M × 300 bytes = 1.5GB/sec outbound
1.5GB/sec × 8 = 12Gbps

A standard NIC handles 10Gbps. At peak you need slightly more than one NIC's worth of capacity — so you need either: - Multiple dispatch nodes (each handling a shard of the traffic), or - 25Gbps NICs on dispatcher machines

This is not a problem in practice since you'll have many dispatcher nodes anyway, but it's worth flagging that a single-machine dispatcher falls over at peak.

Summary Table#