Building a Real-Time Notification System for Marketplaces

Most classified marketplace projects get notifications wrong for the same reason. They build the listing page, the search filters, the payment flow, and then bolt notifications on at the end with three endpoints, a Firebase call, and a technical debt comment in the codebase. The fundamental issue is that notification delivery is not merely a feature; it is core infrastructure. Infrastructure decisions made late in a project are the ones you pay for the longest. A buyer who misses a price-drop alert moves on, and a seller who fails to see a message reply may migrate to a competitor, leaving the listing closed without a transaction while the platform gains no insights.

This post is intended for developers building classified marketplace platforms who want to address the notification layer before it becomes a bottleneck. The architecture itself is not overly complex, but the design decisions matter, and following a specific sequence helps avoid common failure modes. According to research published in the IJARCST journal, event-driven architectures—where individual services emit events into a shared event bus rather than calling delivery endpoints directly—serve as the foundational model that allows notification systems to scale without suffering from severe fragmentation.

Stock photo for illustration only, not from the actual event

Separating the publishing mechanism from the delivery layer represents the primary architectural milestone. Every backend service publishes events, while a single dedicated notification service consumes them. Before selecting a transport mechanism, mapping out event urgency is critical because not all events share the same time sensitivity. High-urgency events, such as a new message from a buyer, an incoming bid, or flagged suspicious activity, require delivery within seconds. Medium-urgency events, including listing approvals or expiry warnings, tolerate a delay of a few minutes. Low-urgency events, like weekly performance summaries, function effectively as batch jobs rather than real-time transmissions.

When selecting a transport protocol, teams must evaluate whether the client requires real-time bidirectional communication and how many concurrent connections the infrastructure must sustain. Polling offers simplicity for low-traffic environments but drains server resources at scale. Server-Sent Events (SSE) maintain a long-lived HTTP connection allowing unidirectional push from server to client, making it the ideal default for notification feeds according to developer guides. Meanwhile, WebSockets establish persistent bidirectional channels, making them necessary specifically for live interactive buyer-seller chat interfaces where both parties transmit data simultaneously.

Source: Dev.to