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3-Tier Reduces the Number of Sessions • Partition the set of presentation devices (e.g., 10
3 PSs per WFC) (^3) • 100 WFC × ( 2 PS/WFC + 10TS/WFC) = 110,000 sessions
WorkflowController TxnServer • Typo on page 50 in textbook, paragraph 4: 10 routers would have 10,000 sessionsto their presentation servers => 105,000 sessions, not 15,000 sessions.
TxnServer
Pres’n^ Pres’n^.. .ServerServer^ WorkflowController^ TxnServer
Pres’n^ Pres’n^.. .ServerServer
Txn.. .Server
2-Tier vs. 3 Tier — Other Issues • In early 90’s people argued whether 2-Tier was enough – Scalability was the decisive factor, but there were other issues • Database Servers – Nonstandard stored procedure language, usually less expressivewith weaker development tools and it’s another language tolearn – Limited interoperability of cross-server calls – Limited interoperability of distributed transactions – Poor fit with OO design, which are inherently 3-tier(client, business rules, business objects) • Application Servers – more system complexity
11.3 Web Servers • Presentation independence - application isindependent of the display device used – Today, this is via http and html – In the past, it was via a display controller or middle-tierminicomputer whose presentation functions insulatedthe rest of the back-end system from different devicetypes • Web server performs presentation functions:– Gathering input– Validating input
- DB caching– Authentication -^ They also do some basic request routing^ – Constructing requests - Invoking applications -^ Examples - IIS (MS), Apache, Netscape Server
Gathering Input
-^ Gathering input - Select transaction type (menu item,etc.), and fill in a form (request’s parameters)^ –^ Today, Web forms, moving to XML (XForms, XSLT, …) •^ 30 year^ evolution of presentation devices^ –^ Teletype, character-at-a-time terminal (async), block-modeterminal (IBM 3270)^ –^ Specialized devices - ATMs, bar code readers, gas pumps,robots, credit card authorization, cash registers, ticketprinters, etc.^ –^ 4GL on a PC - ActiveX controls accessed from Visual Basic(VB), PowerBuilder, Delphi, etc.
Input Validation
-^ Validate input against locally cached tables^ –^ E.g., product types, department numbers •^ Avoids wasting communications and server resourcesfor obvious input errors^ –^ Fewer round-trips to the DBMS^ –^ And faster feedback to the end user •^ “Cache” is part of the web page^ –^ List boxes, script^ –^ Cache size is a factor (it affects page access time)
Authentication
-^ Authentication - determining the identity of a user and/or^ display device^ –^ Client system (e.g., PC) may do authentication, but the serverusually does it too (doesn’t trust clients)^ –^ Encrypt the wire to avoid wiretapping and spoofing(on the Web, use https = http over secure socket layer) •^ Geographical entitlement - check that a particular
device^ is
allowed access (e.g., security trading room) • Need system mgmt functions to create accounts, initializepasswords, bracket hours of access (simplify it using a role abstraction)
Application Invocation • Request arrives as an http message. – Need to call a program (i.e. a WFC), to perform the request • Common Gateway Interface – Write a script, store it as a file in cgi-bin – Web server creates a process to execute the request (Slow!!) • ISAPI (Microsoft) and NSAPI (Netscape) – Web server calls an in-proc .dll instead of creating a process – Web server can cache the .dll – More complex programming model, but much faster • Active Server Pages and Java Server Pages – Offers the performance of ISAPI with programmability of CGI
