The HTTP SUBSCRIBE Method

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Table of Contents

1. Introduction

Modern web applications require efficient, low-latency, and real-time communication channels from the server to the client. Examples include chat applications, stock market tickers, collaborative document editing, and live dashboards.¶

Historically, real-time push has been achieved using three primary workarounds:¶

1. Long Polling: Recreating HTTP requests continuously. This is highly inefficient and creates substantial connection overhead.¶

2. WebSockets: Upgrades the connection from HTTP to a separate bidirectional TCP-based protocol. While efficient, it bypasses HTTP intermediaries (like load balancers, reverse proxies, and Web Application Firewalls), breaks semantic caching, complicates authentication, and often struggles with strict enterprise firewalls.¶

3. Server-Sent Events (SSE): Utilizes standard HTTP GET requests with text/event-stream responses. However, because GET requests do not support a request body, clients must pass subscription parameters (such as filter expressions, selected fields, or authentication tokens) within the URL query string. This leads to problems with URL length limits, logging of sensitive data, and overall architectural rigidity.¶

This specification introduces the SUBSCRIBE method to address these issues. SUBSCRIBE is a safe HTTP method that allows the client to send subscription parameters in the request body while establishing a long-lived, server-push event stream.¶

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. The SUBSCRIBE Method

The SUBSCRIBE method is used to request a persistent, real-time event stream from a target resource.¶

• Safe: Yes. A SUBSCRIBE request is read-only; it MUST NOT modify the state of the target resource on the server.¶

• Idempotent: Yes. Multiple identical SUBSCRIBE requests will yield identical event stream configurations.¶

• Request Body: Allowed. The request body contains subscription parameters (e.g., query filters, requested fields, backfill start time, or sub-topics).¶

• Response Body: Allowed. The response body is a long-lived stream of events or data updates.¶

SUBSCRIBE /events/trades HTTP/1.1
Host: api.example.com
Content-Type: application/json
Accept: text/event-stream

{
  "ticker": "GOOG",
  "min_volume": 100,
  "fields": ["price", "volume", "timestamp"]
}

HTTP/1.1 200 OK
Content-Type: text/event-stream
Cache-Control: no-cache, no-store
Connection: keep-alive

data: {"price": 175.20, "volume": 150, "timestamp": 1719999999}

data: {"price": 175.25, "volume": 500, "timestamp": 1720000005}

4. Client and Server Behavior

5. Caching Considerations

Because SUBSCRIBE responses represent dynamic, real-time streams of events, they MUST NOT be cached by shared caches or HTTP intermediaries.¶

Servers MUST include a Cache-Control: no-cache, no-store header in the response. Intermediaries MUST immediately forward both the request and response stream without buffering.¶

6. Security Considerations

7. IANA Considerations

IANA is requested to register the SUBSCRIBE method in the "HTTP Method Registry" under the Hypertext Transfer Protocol (HTTP) Parameters registry:¶

• Method Name: SUBSCRIBE¶

• Safe: Yes¶

• Idempotent: Yes¶

• Reference: This document¶

8. References

Author's Address