Internet DRAFT - draft-frindell-httpbis-partial-post-replay
draft-frindell-httpbis-partial-post-replay
httpbis Working Group A. Frindell
Internet-Draft Facebook
Intended status: Informational June 28, 2019
Expires: December 30, 2019
HTTP Partial POST Replay
draft-frindell-httpbis-partial-post-replay-00
Abstract
This memo introduces a method of exchanging HTTP [RFC7230] messages
between a web server and a cooperating intermediary - such as a
reverse proxy load balancer - that enables faster restarts for the
web server with minimal disruption for users.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 30, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Frindell Expires December 30, 2019 [Page 1]
Internet-Draft HTTP-PPR June 2019
1. Introduction
Web servers need to drain traffic periodically for configuration
changes, software updates and maintenance. As continuous deployment
becomes more common, the frequency of such events increases. When a
server shuts down, it chooses whether to let all existing requests
run to completion, or abort some or all in-progress requests.
Aborted requests lead to poor user experiences including error
messages or additional latency while the request is resent. Partial
POST Replay makes it possible to eliminate this class of errors by
handing off in-process requests to another server within a
deployment.
1.1. Conventions and Definitions
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. Partial POST Replay
This section describes the Partial POST Replay mechanism for handing
off a request with a partially transferred entity body to another
server instance.
2.1. Response Message
When the server begins restarting, it responds to any unprocessed
requests with incomplete entity bodies with a new 3xx status code
(TBD). The HTTP/1.1 status message is Partial POST Replay. Once
this status is sent the server MUST NOT process this request other
than is specified in this document.
The server MUST have prior knowledge that the intermediary supports
Partial POST Replay before sending the 3xx response. If a server
sends this response to an intermediary that does not understand it,
the response will likely be forwarded back to the client.
2.1.1. Response Headers
Each request header is echoed in the response message with the prefix
"Echo-". For example, the "User-Agent: Foo" request header would be
included in the response as "Echo-User-Agent: Foo". HTTP/2 [RFC7540]
and HTTP/3 {{?HTTP3} request pseudo-headers (beginning with ':') are
echoed in the response message with the prefix "Pseudo-Echo-", and
with the ':' removed. For example, ":path: /" is echoed as "pseudo-
Frindell Expires December 30, 2019 [Page 2]
Internet-Draft HTTP-PPR June 2019
echo-path: /". The server MUST NOT insert any Echo- or Pseudo-Echo
headers in the response if the corresponding header was not present
in the request.
Because there might be request body bytes in flight to the server
when the 3xx response is generated, the length of the response body
is unknown. The response SHOULD NOT include a "Content-Length"
header (but will include a "Echo-Content-Length" header, if the
request contained "Content-Length"). If the request protocol is
HTTP/1.1, the server SHOULD use chunked transfer encoding for the
response.
HTTP/1.1 server SHOULD include a "Connection: close" header in the
response to prevent the intermediary from reusing the connection for
a new request. HTTP/2 and HTTP/3 servers SHOULD emit a GOAWAY frame
on each open connection when shutdown is initiated.
2.2. Intermediary Processing
Intermediaries MUST track the number of body bytes forwarded to the
server for any request that could be replayed by the server. When an
intermediary receives a 3xx status code from the server, it stops
forwarding any new HTTP data from the client to this server. The
intermediary does not forward the 3xx response to the client, but
instead reconstructs the original HTTP request message from headers
in the response beginning with the "Echo-" or "Pseudo-Echo" prefixes.
Alternatively, if the intermediary retained a copy of the request it
MAY use that and discard the response headers.
The intermediary can choose to buffer the response before selecting a
new server, or can immediately select a new server and begin
forwarding the request there. When the entire replayed request body
has been sent to the new server, the intermediary can begin
forwarding new HTTP data from the client to the new server.
If the intermediary receives more body bytes from the server than it
forwarded, or if the response is terminated before receiving all
forwarded bytes, the intermediary MUST fail the request with a 5xx
status.
2.3. Original Request Termination
When the intermediary has received in the response body all of the
request bytes forwarded to the original server, it completes the
request message to the original server, according to the semantics of
the transport protocol:
o For HTTP/1.0, the intermediary half-closes the connection
Frindell Expires December 30, 2019 [Page 3]
Internet-Draft HTTP-PPR June 2019
o For HTTP/1.1, the intermediary sends the final chunk terminator,
or half-closes the connection if the request did not use chunked
transfer encoding.
o For HTTP/2, the intermediary sends a DATA frame with the
END_STREAM flag set on the request stream
o For HTTP/3, the intermediary sends a FIN on the request stream
When the server processes the end of the request, it completes the
response message according to the semantics of the transport
protocol.
Note that some HTTP server implementations treat the termination of
the request with fewer bytes than specified in the Content-Length
header as an error. Because all required information has been
transferred to the intermediary before this error occurs, the server
can abort the response and ignore the error without impacting the
final status of the request.
It is possible that the entire entity body was sent by the
intermediary before it received the Partial POST Replay status
message. In this case the intermediary will receive the entire
entity body in the response.
2.4. Preventing Loops
To prevent the intermediary from becoming stuck in an infinite
redirect loop, it SHOULD add a 'Partial-Post-Replay: 1' header
whenever forwarding to a new server. An intermediary that receives a
redirect response with more "Echo-Partial-Post-Replay" headers than
it supports SHOULD fail original request with a 5xx response.
3. Existing Solutions
There are several existing solutions to handling requests while
draining traffic from a web server, but each has drawbacks that
Partial POST Replay does not.
3.1. Drain Timeout
When servers stop accepting new connections, they often set a timeout
during which existing requests can continue processing. At the end
of the timeout, the server will abort any unfinished requests.
During this phase, the server is not operating at full capacity, and
requests that exceed the timeout are still terminated with error.
Frindell Expires December 30, 2019 [Page 4]
Internet-Draft HTTP-PPR June 2019
3.2. GOAWAY
HTTP/2 introduced the GOAWAY frame which a server can use to indicate
which requests will not be processed, and which can be safely retried
by the client. There are two problems with this mechanism.
First, the server cannot use this mechanism to refuse requests with
stream IDs lower than the highest stream ID it has already processed.
For example, if the server has received a partial request on stream
ID=3, but has already begun processing a request on stream ID=5, it
cannot send a GOAWAY with a Last-Stream-ID lower than 5. HTTP/2 does
not have a status code that indicates an individual request is
retryable
Second, an intermediary cannot seamlessly retry a POST request unless
it has buffered the entire request body. Buffering all request
bodies presents an enormous scalability challenge for intermediaries.
3.3. State Handover
Another possible technique is to pass state from a draining web
server to a new instance. Such deployments start a new instance to
handle new work in parallel with the instance that is shutting down.
This requires that the system have enough resources to run two
instances of the server simultaneously, for a potentially very long
time.
4. Security Considerations
An intermediary must trust the server to echo back the headers and
body of the original request. A malicious server could replay a
different request to the intermediary, who would then send it to
another server. The response to this forged request would be
interpreted as a response to the original request.
5. IANA Considerations
This document has no IANA actions.
6. References
6.1. 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>.
Frindell Expires December 30, 2019 [Page 5]
Internet-Draft HTTP-PPR June 2019
[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>.
6.2. Informative References
[HTTP3] Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-latest (work in progress).
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
Acknowledgments
This draft evolved from a feature developed at Facebook. Thanks to
Mohammad Husain, Woo Xie and David Langevin who worked on the initial
implementation and deployment of this feature.
Author's Address
Alan Frindell
Facebook
Email: afrind@fb.com
Frindell Expires December 30, 2019 [Page 6]
