Dynamic Backpressure for HTTP-based systems
draft-soni-http-dynamic-backpressure-00

Internet Engineering Task Force                                 S. Lasso
Internet-Draft                                             9 August 2024
Intended status: Standards Track                                        
Expires: 10 February 2025

              Dynamic Backpressure for HTTP-based systems
                draft-soni-http-dynamic-backpressure-00

Abstract

   This document describes a mechanism for introducing dynamic
   backpressure into an HTTP-based system, to aid in controlling server
   resources while minimizing tradeoffs.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Background: Why use dynamic backpressure  . . . . . . . . . .   2
   3.  Solution: The Refresh HTTP header . . . . . . . . . . . . . .   3
   4.  Rejected alternatives . . . . . . . . . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   Operating an HTTP service can get expensive.  One of the ways
   services can manage the costs is by providing backpressure.
   Traditionally, HTTP backpressure is implemented variously by refusing
   connections after a limit, holding a connection open and slowly
   trickling data into it, among other techniques.  This document
   describes a simple and widely-deployed mechanism for implementing
   backpressure by telling the client when to make new requests.

1.1.  Requirements Language

   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.

2.  Background: Why use dynamic backpressure

   Consider the scenario: a service that is billed by requests, and an
   Atom feed that gains new entries every 5 minutes.  This service is
   composed of app servers and caching load balancers, with a cache
   policy of 5 minutes.  Yet, the service doesn't want Atom clients to
   send a request every 5 minutes.  This scenario is exactly where
   dynamic backpressure would shine: it doesn't try to slow down or
   throttle the connections (not that it would be able to, due to the
   caches) or anything like that, it simply tells the client "hey, I
   would appreciate it if you waited N seconds before sending another
   request, thanks!".

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3.  Solution: The Refresh HTTP header

   The Refresh header is composed of a non-negative integer number of
   seconds to wait until the next request, and an optional URI to move
   future requests to, according to the following syntax described in
   Augmented BNF [RFC5234], borrowing 'URI-reference' from [RFC3986]:

   refresh-value = wait-seconds [ ";url=" future-URI ]
   wait-seconds = 1*DIGIT
   future-URI = URI-reference

   The value of 'wait-seconds' SHOULD NOT be shorter than the value of
   any caching policy applied to the document.  The client SHOULD wait
   at least 'wait-seconds' before making new requests.

   This syntax is in alignment with Section 7.7 of [WHATWG-HTML].

   Below is a diagram of how the Refresh header can be used in a generic
   control loop, with requests per second as input and wait-seconds as
   output.

                         Control Loop

        req/s  +     +-------------------+   wait-seconds
            -->(+)-->| Control  function |----+------->
                ^-   +-------------------+    |
                |    +-------------------+    |
                +----| Feedback function |<---+
                     +-------------------+

                           Figure 1: Control loop

4.  Rejected alternatives

   The Refresh header is strongly disliked by basically everyone, but it
   may well be the only mechanism that provides the desired properties.
   One may argue the Retry-After header would be a better substitute,
   but the Retry-After header isn't really specified for successful
   content requests such as those used by Atom feeds, nor is it as
   widely deployed as the Refresh header.

5.  IANA Considerations

   This document requests that the IANA update the registration for the
   Refresh header to also point to this document.

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6.  Security Considerations

   This document should not affect the security of the Internet.  After
   all, the Refresh header is already widely deployed.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

7.2.  Informative References

   [WHATWG-HTML]
              van Kesteren, A., "HTML Standard", Living Standard (7
              August 2024), <https://html.spec.whatwg.org/multipage/>.

Acknowledgements

   The authors would like to acknowledge the true power of the Refresh
   header.  It had to be very powerful to survive this long.

Author's Address

   Soni Lasso Terense
   Email: fakedme@gmail.com
   URI:   https://soniex2.autistic.space/

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