lastEventId

LastEventId, often shown as lastEventId in code, is the identifier of the most recently received event in streaming HTTP protocols such as Server-Sent Events (SSE). Each event can include an id field. The lastEventId value is updated whenever a new event with an id is received, and it is commonly used to resume a stream after a disconnection.

During the SSE handshake, when the client reconnects after a break, the client may send an HTTP header named Last-Event-ID containing the value of the lastEventId. This allows the server to identify where the client left off and to optionally resend events that followed that ID. The server's behavior, including whether to replay events, depends on its implementation.

The EventSource API in web browsers exposes a lastEventId property that reflects the last event ID seen by the client. As events arrive with an id: line, lastEventId is updated accordingly.

Considerations: While lastEventId helps with deduplication and resuming streams, it does not guarantee delivery or exact replay semantics across all servers. Applications should design their event streams with id monotonicity, handle duplicates, and implement application-level deduplication if necessary. The Last-Event-ID header is optional and may be ignored by servers that do not support resuming.

See also: Server-Sent Events, EventSource, id field in SSE, Last-Event-ID header.