Mod_spdy Architectural Overview

5/25/2018 Mod_spdy Architectural Overview

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mod_spdy:

SPDY in ApacheMatthew Steele

April 2012


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Why make mod_spdy?

SPDY makes the web faster

Apache is the most popular webserver bymarket share.

To help increase SPDY adoption, we want a

dead-easy-to-use, drop-in SPDY module for

Apache.


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History of mod_spdy

Work began January 2010. We

implemented framing, and serial

multiplexing.

The project was placed on indefinite hold a

few months later, pending better SPDY

experimental results.

Resumed the project November 2011. We

now have a complete SPDY implementation.


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Critical SPDY features

Next Protocol Negotiation (NPN)

Framing and header compression

Stream multiplexing

Server push


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How Apache works

The MPMaccepts connections and assigns

each one to a child process/thread.

For each connection, most of the work is

done by handlersand filters.

A handlertakes control of completely processing a

connection or an HTTP request.

A filtertransforms the input or output data passingbetween the handler and the socket.


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How Apache works

Apache's behavior can be extended by

modules, which register hooks.

Hook functions are called at various

interesting events, and give the module a

chance to do something in response.

For example, a module could register a hook

to insert a filter for each request that arrives.


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Stream multiplexing

Server push


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server client


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server client

Hello!

SSL handshake


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client

I support: - spdy/3 - spdy/2

- foo/1.1

SSL handshake

server


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server client


- foo/1.1

I choose:

spdy/2

SSL handshake


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Our patched version of mod_ssl provides

two NPN-related hooks.


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The first hook lets us add protocols to be

advertised.

The second hook informs us which protocol

was chosen by the client.


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The first hook lets us add protocols to be

advertised.

The second hook informs us which protocol

was chosen by the client.

Other modules could use these hooks too.


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Apache server


clientmod_ssl

mod_spdy

mod_foo


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Apache server


clientmod_ssl

mod_spdy

mod_foo

Hello!

SSL handshake


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Apache server


clientmod_ssl

mod_spdy

mod_foo

What do we

support?

NPN advertise hook

Hello!

SSL handshake


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Apache server


clientmod_ssl

mod_spdy

mod_foo

spdy/3

spdy/2

NPN advertise hook

foo/1.1

Hello!

SSL handshake


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Apache server


clientSSL handshakemod_ssl

mod_spdy

mod_foo


- foo/1.1


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Apache server


client


- foo/1.1

I choose:

spdy/2

SSL handshakemod_ssl

mod_spdy

mod_foo


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Apache server


clientmod_ssl

mod_spdy

mod_foo

The client

chose spdy/2

NPN done hook


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Apache server


clientmod_ssl

mod_spdy

mod_foo

The client

chose spdy/2

Hooray!

Aww...


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Stream multiplexing

Server push


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We insert filters to translate from SPDY

frames to equivalent HTTP data, and back.

Do this at the connection (not request) level,

to maintain header compression state.

This approach lets the site owner use the

same, HTTP-specific Apache modules to

process requests as without mod_spdy.


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Stream multiplexing

Server push


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How Apache works


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How Apache works

MPM

server

socket


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How Apache works

MPM

server

socket

client

socket

client

socket

client

socket


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How Apache works

MPM

server

socket

connectionobject client

socket

connection

object clientsocket


socket


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thread/process 1

How Apache works

MPM

server

socket


socket

thread/process 2

connection

object clientsocket

thread/process 3


socket


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thread/process 1

connectionobject

How Apache works

MPM

server

socket

client

socket


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thread/process 1

connectionobject

How Apache works

MPM

server

socket

client

socket

networkfilters


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thread/process 1

connectionobject

How Apache works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

networkfilters


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thread/process 1

connectionobject

How Apache works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

connectionhandler

(e.g. HTTP)

networkfilters


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thread/process 1

connectionobject

How Apache works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

networkfilters

request

object

connectionhandler

(e.g. HTTP)


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thread/process 1

connectionobject

How Apache works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

connectionhandler

(e.g. HTTP)

requestfilters

(e.g. gzip)

requesthandler

(e.g. CGI)

networkfilters

request

object


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thread/process 1

connectionobject

How Apache works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

connectionhandler

(e.g. HTTP)

networkfilters


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Stream multiplexing

Once a request handler takes over, it does

not return control until the request is

finished.

Apache (and modules) assume onethread

per connection; the core data structures are

notthread-safe.

So we can't be event-driven, and we can't be

multithreaded. What to do?


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

networkfilters


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY

stream

SPDYstream

SPDY

stream


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SPDY thread 3

SPDY thread 2

SPDY thread 1

thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY

stream

SPDYstream

SPDY

stream


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY thread 1SPDY

stream


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY thread 1

connection

object

SPDY

stream


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY thread 1

connection

objectfake

socket

SPDY

stream


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY thread 1

connection

objectfake

socket

SPDY

stream

SSL filters


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY thread 1

connection

objectfake

socket

SPDY

stream


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY thread 1

connection

objectfake

socket

SPDY

stream

network

filters


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY thread 1

connection

objectfake

socket

SPDY

stream


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY thread 1

connection

objectfake

socket

SPDY

stream

connectionhandler

(e.g. HTTP)


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY thread 1

connection

objectfake

socket

SPDY/HTTPconversion

filters

connectionhandler

(e.g. HTTP)

SPDY

stream


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY thread 1

connection

objectfake

socket

SPDY

stream

requestobject

SPDY/HTTPconversion

filters

connectionhandler

(e.g. HTTP)


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thread/process 1

connectionobject

How mod_spdy works

MPM

server

socket

client

socket

connectionfilters

(e.g. SSL)

SPDYconnection

handler

networkfilters

SPDY thread 1

connection

objectfake

socket

request

filters(e.g. gzip)

request

handler(e.g. CGI)

requestobject

SPDY

stream

SPDY/HTTPconversion

filters

connectionhandler

(e.g. HTTP)


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Stream multiplexing

Server push


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Server push

mod_spdy provides a server-push function

to other modules.

When called (e.g. from a stream request

handler), the main SPDY connection thread

creates a new server-push stream.

We synthesize a request to imaginethat the

client sent us, and allow Apache to respond

accordingly.


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Server push

One wrinkle: by the spec, we must send the

pushed SYN_STREAM beforethe

associated stream's FLAG_FIN.


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Server push




But we don't have response headers yet,

and don't want to impose a synchronization

between stream threads.


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Server push




But we don't have response headers yet,

and don't want to impose a synchronization

between stream threads.

Solution: send SYN_STREAM sans

headers; follow up with HEADERS frame


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Stream multiplexing

Server push


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Remaining tasks/challenges

Get SSL changes into Apache trunk.https://issues.apache.org/bugzilla/show_bug.cgi?id=52210

Support SPDY v3 in addition to v2.

Experiment further with server push.
https://issues.apache.org/bugzilla/show_bug.cgi?id=52210


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Open source project:

http://code.google.com/p/mod-spdy/

Download DEB/RPM packages:

https://developers.google.com/speed/spdy/mod_spdy/
http://code.google.com/p/mod-spdy/https://developers.google.com/speed/spdy/mod_spdy/http://code.google.com/p/mod-spdy/

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Mod_spdy Architectural Overview

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