Server Sent Events Streaming

Overview

Server-Sent Events (SSE) is a lightweight way to receive a continuous stream of real-time data from the Timebase Historian over a standard HTTP connection. SSE is simpler than the WebSocket (SignalR) API: the client opens one HTTP request and the Historian pushes events as data arrives. There is no bidirectional messaging — SSE is one-way, from the Historian to your client.

SSE is well suited for web dashboards, monitoring displays, and third-party integrations that only need to receive data, not send commands.

Two SSE endpoints are available:

• Data stream — streams live TVQ (Timestamp-Value-Quality) data points as tags are updated
• Tag stream — streams tag metadata changes (tag created, updated, or deleted)

How to use SSE

Endpoints

Replace {dataset} with the name of the dataset you want to subscribe to.

Subscribing with the pattern parameter

Use the pattern query parameter to specify which tags to subscribe to. You can provide multiple patterns in a single request:

GET /api/datasets/MyDataset/data/stream?pattern=Area1.TT.001.PV&pattern=Area2.#

Each pattern can be an exact tag name or a wildcard expression. Multiple patterns in one request are combined — if a tag matches any pattern, its data is included.

Wildcard patterns

Delimiters supported in tag names: . and /.

Initial values on connect

When you open a data stream connection, the Historian immediately sends the last known TVQ for every tag that matches your pattern(s), before streaming any new data. This ensures your display shows current values instantly rather than waiting for the next update.

Message format

SSE messages use standard event: and data: fields.

Data event (new TVQ value)

event: data
data: {"n":"Area1.TT.001.PV","t":"2026-04-29T10:30:00.000Z","v":56.78,"q":192}

Tag metadata event (tag created/updated/deleted)

event: tag
data: {"n":"Area1.TT.001.PV","tp":"Updated","t":{...},"dt":"2026-04-29T10:30:00.000Z"}

Connecting from a browser

const source = new EventSource(
  '/api/datasets/MyDataset/data/stream?pattern=Area1.TT.#'
);

source.addEventListener('data', (event) => {
  const tvq = JSON.parse(event.data);
  console.log(tvq.n, tvq.v, tvq.t);
});

// To disconnect:
source.close();

Connecting from the command line (curl)

curl -N "http://localhost:4511/api/datasets/MyDataset/data/stream?pattern=Area1.TT.#"

Authentication

SSE endpoints require authentication when the Historian has Pulse authentication enabled. Pass a Bearer token in the Authorization header:

Authorization: Bearer <your-access-token>

Note: The browser EventSource API does not support custom headers. For authenticated browser SSE, use the Fetch API with an AbortController instead, or pass the token as a query parameter if your deployment supports it.

Disconnecting

Close the HTTP connection to unsubscribe. The Historian cleans up all resources automatically when the connection closes. There is no separate unsubscribe call.

SSE vs WebSocket (SignalR)

Troubleshooting