How Razorpay scaled their notification system
Delivery of notifications is critical for a FinTech company like Razorpay because it is a way to notify customers about their transactions and connect with external systems through Webhooks.
So, how did they design their notification system? let's find out
Key requirement
maintaining an SLA is very important
guaranteed delivery via SMS, Email, Push, and Webhooks
Existing Setup
Upon every transaction, an event/message was sent to SQS (message broker) which was consumed by a worker that then fanned out the notification through different channels.
Because we want to guarantee delivery, a state was maintained in the database that tells if the notification was successfully sent or not (esp via Webhook).
Hence, there is a component called Scheduler that pulls the unsent notifications from this database and re-queues it in SQS; thus guaranteeing the delivery.
Challenges at scale
huge load on this database
scaling workers were limited by the IPOS on the database
surges, during festive seasons, affected transactional notifications
New architecture
Prioritizing incoming load
In order to ensure that one type of notification does not affect others, every notification type is classified with some priority, and depending on which they are pushed to the corresponding SQS queue.
This ensures that a huge marketing campaign does not affect transactional messages.
Rate Limiting
To ensure mass notification from one customer does not affect others, we add Rate Limiter that would limit the notifications per customer and per type ensuring that critical notifications always meet the SLA.
Reducing DB bottleneck
We could not scale workers because of the high DB load, and hence instead of doing a sync write to the database, the notifications that are unsent and need to be retried are pushed in a sync way to the database.
Because of this async write, we ensure that we write to the database in a staggered way and not put unnecessary load on it.
Observability
To ensure we are maintaining our SLA, we have to exhaustively monitor the entire infra for any anomaly; the metrics like - health of the infra, success rate of delivery, and SLA.
Here's the video of my explaining this in-depth 👇 do check it out
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