MC714: Sistemas DistribuídosWeb proxy caching Basic idea (cnt’d) Cooperative caching, by which you ﬁrst check your neighbors on a cache miss Web proxy Web server Web proxy Web

MC714: Sistemas Distribuıdos

Prof. Lucas Wanner

Instituto de Computacao, Unicamp

Aula 27: Sistemas Web Distribuıdos

Distributed Web-based systems

EssenceThe WWW is a huge client-server system with millions of servers; each server hosting thousandsof hyperlinked documents.

Documents are often represented in text (plain text, HTML, XML)Alternative types: images, audio, video, applications (PDF, PS)Documents may contain scripts, executed by client-side software

Client machine

Browser

OS

Server machine

Web server

1. Get document request (HTTP)

3. Response

2. Server fetchesdocument fromlocal file

Source: Maarten van Steen, Distributed Systems: Principles and Paradigms 2 / 13

Multi-tiered architectures

ObservationAlready very soon, Web sites were organized into three tiers.

Web server Database serverCGI process

CGI program

1. Get request

3. Start process to fetch document

5. HTML document created

HTTP request handler6. Return result

4. Database interaction


Web services

ObservationAt a certain point, people started recognizing that it is was more than just user↔ site interaction:sites could offer services to other sites⇒ standardization is then badly needed.

Service description (WSDL)

Client machine

Client application

Stub

Server application

Stub

Communication subsystem

Communication subsystem

SOAP

Service description (WSDL)Service description (WSDL)

Directory service (UDDI)

Publish serviceLook up

a service

Generate stub from WSDL description

Server machine

Generate stub from WSDL description


Clients: Web browsers

ObservationBrowsers form the Web’s most important client-side sofware. They used to be simple, butthat was long ago.

User interface

Browser engine

Rendering engine

Network comm.

HTML/XML parser

Display back end

Client-side script

interpreter


Apache Web server

Observation:More than 46% of all ∼170 million active Web sites are Apache.

Hook Hook Hook Hook

Function

... ... ...

Module Module Module

Apache coreFunctions called per hook

Link between function and hook

Request Response


Server clusters

EssenceTo improve performance and availability, WWW servers are often clustered in a way that istransparent to clients.

Frontend

Webserver

Webserver

Webserver

Webserver

Request Response

Front end handlesall incoming requestsand outgoing responses

LAN


Server clusters

ProblemThe front end may easily get overloaded, so that special measures need to be taken.

Transport-layer switching: Front end simply passes the TCP request to one of theservers, taking some performance metric into account.Content-aware distribution: Front end reads the content of the HTTP request andthen selects the best server.


Server Clusters

SwitchClient

Webserver

Webserver

Distributor

Distributor

Dis-patcher

1. Pass setup requestto a distributor

2. Dispatcher selectsserver

3. Hand offTCP connection

4. InformswitchSetup request

Other messages

5. Forwardothermessages

6. Server responses


Web proxy caching

Basic idea (cnt’d)Cooperative caching, by which you first check your neighbors on a cache miss

Webproxy

Webserver

Webproxy

WebproxyCache

Cache

Cache

Client

Client

ClientClient

Client

ClientClient

Client

Client

2. Ask neighboring proxy caches

1. Look inlocal cache

HTTP Get request

3. Forward requestto Web server


Replication in Web hosting systems

ObservationBy-and-large, Web hosting systems are adopting replication to increase performance. Muchresearch is done to improve their organization. Follows the lines of self-managing systems.

Web hosting system

Metric estimation

Analysis

+/-+/-+/-

Reference input

Initial configuration

Uncontrollable parameters (disturbance / noise)

Observed output

Measured outputAdjustment triggers

Corrections

Replica placement

Consistency enforcement

Request routing


Handling flash crowds

ObservationWe need dynamic adjustment to balance resource usage. Flash crowds introduce aserious problem.

(a) (b)

(c) (d)

2 days 2 days

6 days 2.5 days


Server replication

Content Delivery NetworkCDNs act as Web hosting services to replicate documents across the Internet providingtheir customers guarantees on high availability and performance (example: Akamai).

Origin server

Client

CDN server

CDN DNS server

Regular DNS system

Cache

1. Get base document

2. Document with refs to embedded documents

6. Get embedded documents (if not already cached)

5. Get embedded documents

7. Embedded documentsReturn IP address client-best server

DNS lookups 3

4
