What is Server-Sent Events?

Server-Sent Events (SSE) is a web standard that allows a browser (or scraper) to receive automatic updates from a server via an HTTP connection. The client initiates the connection with an Accept: text/event-stream header, and the server responds with a 200 OK but does not close the connection.

The server then pushes data in plain text blocks separated by double newlines (\n\n). Each block can contain an event type, an id, and the data payload itself. Because it operates over standard HTTP, it seamlessly traverses firewalls and proxies that might otherwise block non-HTTP traffic.

When scraping an SSE endpoint, you cannot use a standard synchronous HTTP GET request, as the response body never technically finishes downloading. Instead, you must open a streaming HTTP request and read the response line-by-line in an asynchronous loop.

As lines arrive, your parser must buffer them until it encounters an empty line, which signals the end of an event block. The buffered lines are then parsed to extract the data: payload, which is typically a JSON string containing the real-time update (e.g., a price change or a new chat message).

A common pitfall when scraping SSE is assuming the stream contains all the data you need. SSE is almost exclusively used to send deltas (changes), not the full state. If you connect to a live sports scoreboard via SSE, you will receive "Player X scored," but you won't receive the current total score.

To build a complete dataset, your pipeline must first make a standard REST API call to fetch the "base state" (the current score), and then immediately open the SSE connection to apply the incoming deltas to that base state in memory.

We treat SSE endpoints as continuous ingestion streams rather than discrete scrape jobs. Our async worker fleet establishes persistent connections using static ISP proxies, bypassing the aggressive connection-termination policies of standard residential proxy pools.

Each worker maintains a local state machine tracking the Last-Event-ID. If a network partition occurs, the worker automatically reconnects, passing the ID to ensure no events are dropped. The raw events are immediately published to an internal Kafka topic, allowing our aggregation layer to merge the deltas and deliver clean, up-to-the-second datasets to clients.

In a standard web browser, the native EventSource API handles SSE automatically, including silent reconnections and managing the Last-Event-ID header. However, when building a scraper in Python or Go, you are responsible for implementing this entire recovery logic yourself. Failing to implement the ID tracking means every connection drop results in permanent data loss.