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Web ServerArch i tec tu res

CS 4244: Internet Programming

Dr. Eli TilevichBased on Flash: An Efficient and Portable Web Server, Vivek S. Pai, PeterDruschel, and Willy Zwaenepoel, 1999 Annual Usenix Technical Conference,Monterey, CA, June 1999.

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Design Goa ls

Performance & Quality of Service (Systems) Good respons iveness; low la t enc y Good c oncur rency

Can support multiple clients simultaneously High t hroughput

Grac efu l degradat ion

Low mem ory c onsum pt ion

Ease of development (Software Engineering) Sim ple t o underst and, f ine-t une, add new features,

debug, e t c .

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What Web Servers Do

In response to a Web client request(e.g., http://google.com/index.html) a Web server: netw ork c onnec t ion

t he request (index .h tm l ) f i le from d isk o r runs a dynam ic c on ten t

generator

c ont ent (headers and body) bac k

http://google.com/index.htmlhttp://google.com/index.html

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Single-Threaded Web Server

One process sequentially handles all client connections

Simple requires no synchronization Does not scale (one client at a time)

AcceptConnection

Parse HTTPRequest

Read File or

generate contentSend Data

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Opt imizat ions?

Caching Pat hnam e t rans la t ion

Som e dynam ic c on ten t

Fi le operat ions

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Addi t ional Feat ures of Web Servers

Logging Security (e.g., access control)

Traffic analysis

Require centralized data structures to implement

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Main Server Arc h i t ec t ures

Multi-process (Apache on Unix) Multi-threaded (Apache on NT/XP)

Single process event driven (Zeus, thttpd)

Asymmetric multi-process event-driven (Flash)

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Mul t i -Proc ess Arc h i t ec t ure

Utilizes multiple processors

Easy to debug

Can pre-fork a pool of processes

Inter Process Communication is difficult and expensive

High memory cost, context switches

AcceptConnection

Parse HTTPRequest

Read File or

generate contentSend Data

Accept

Connection

Parse HTTP

Request

Read File or

generate content

Send Data

Process 1

Process N

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Mul t i -Threaded Arc h i t ec t ure

Utilizes multiple threads, good performance

Easy to change threading policies

Need to synchronize, to avoid data races

Resources utilization (kernel and user-level): m emory consumpt ion, c on tex t sw i tc hes, s ta r tup

Blocking I/O can cause deadlocks

AcceptConnection

Parse HTTPRequest

Read File or

generate contentSend Data

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Sing le Proc ess Even t -Dr iven

Use a selector to check for ready file descriptors

Uses a finite state machine to determine how to move to the next

processing stage

No context switching, no synchronization, single address space

Modern OS do not provide adequate support for asynchronous disk

operations

AcceptConnection

Parse HTTPRequest

Read File or

generate contentSend Data

Event Dispatcher

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Asym m et r ic Mul t i -Proc ess Event -Dr iven

Similar to Single Process Event-Driven but with helpers

for blocking I/O (e.g., disk requests)

AcceptConnection

Parse HTTPRequest

Read File or

generate contentSend Data

Event Dispatcher

Helper 1 Helper N

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Real Work load (On Solar is )

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Perform anc e in WAN(Adding Cl ient s)

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Per form anc e Com par isonMP MT SPED AMPED

Disk

Blocking

+(only one

proc.)

+ - (wholeserver proc.

blocks)

+

Memory

Cons.

-(separate

mem. spaceper proc.)

+ + +

Disk

Usage

+ + - (one diskrequest at a

time)

+

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Chal lenges of Using Threads

Need for synchronization Deadlocks, starvation

Race conditions

Scheduling can be tricky

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Chal lenges of Using Event s

Only one process/thread is used High latency for long running handlers

Control flow is obscure

Difficult to write, understand, and modify

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Hybrid Approac h

SEDA: Staged Event-Driven Architecture

, Mat t Welsh , David Cul le r, and EricBrew er . In Proc eedings o f t he Eight eent h Sym posium onOpera t ing Syst em s Prin c ipl es (SOSP-18), Banff , Canada,

Oc t ober , 2001.

Uses thread pools and events

Events organized into stages

Each stage has a thread pool

Load conditioning; graceful degradation