Martin: Created page with "= Multi Tier Architecture = Client Server architecture with presentation, processing, data management being seperated Typically: * Layer - Logical * Tier - Physical Presenta..."

2021-09-14T21:19:18Z

Created page with "= Multi Tier Architecture = Client Server architecture with presentation, processing, data management being seperated Typically: * Layer - Logical * Tier - Physical Presenta..."

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= Multi Tier Architecture =
Client Server architecture with presentation, processing, data management being seperated

Typically:
* Layer - Logical
* Tier - Physical

Presentation Tier <-> Business processing <-> Data management

= Two Tier =
Disadvantages:
* Fails to separate business logic from presentation
** less reuse
* Easily compromised integrity
** Each client may have different business rules
* Difficult to administer
* Exposure to security issues
** Distributed DB access in each client
* Limited scalability
** Network resources required for each client
** Limited resources such as DB connections required for each client
** Slow network - more DB strain
** can not add more machines to business tier
* Restricted clients
* Limited presentations - PC/Phone/...
* Difficult to debug/fault find/monitor
* Hard to maintain - difficult to update clients/fix/ensure everyone has right version

= Three Tier =
Split Presentation tier from Business tier

Used in most ERP Enterprise Resource Planning solutions
* systems specialising in high volume transactions CICS, Tuxedo

Problems:
* complexity
** distribution, multi threading, security
* system level complexities of distribution
** use of application frameworks such as TP Transaction Processing
** application server
* Lack of portability - each framework has own API
* Vendor incompatibility - no standard interoperable protocol
* Limited adoption - limited tools programmers typically only support one framework
* Historically not web compatible - proprietary client/appserver protocol - most now support web

= J2EE Application Architecure =

Supports 2 and 3 tier architecture

[[Image:2tier.png]] [[Image:3tier.png]]

== 4 Programming Environments ==
# Web container - presentation logic
# EJB container - business logic
# Application client container - J2EE app client - see [[J2EE_API and Services#JEE_5_API_and_Services]]
# Applet container

== Advantages of EJB Development environment ==
* Simplicity provides frameworks for: security, transactions, muti threading, distribution, resource pooling, secure protocols
* Standards - Application portability - eases porting to other vendors platforms
* Component reuse - Beans - reusable business block
** can customise per deployment
** multiple applications share same bean and business logic
* clearly defined roles allow larger teams
* deployment in different environments - separates business logic from deployment configuration
** encourages tooling from vendors to support and reduce errors
* distributed environment independent from topology
* application interoperability
* integrates with systems other than java
** JCA and JMS allows integration with ERP, DBMS in a standard way
* Tooling and resources - standard lead to tools for development and maintenance

Customer Benefits
* choice of server - price - features
* application management - sophisticated tools for management/monitoring/analysing
* integration with existing systems and data - reuse
* security - separation of roles - both people and software

More tiers
* increase extensibility, manageability, security
* decrease performance - increases latency

2 tier vs 3 tier
* Advantages
** gain performance network latency by bringing closer application server and DB
** better reliability - have control over server environment
** easier to extend
** easier to manage/optimise

* Disadvantages
** loose performance if CPU bound

Key terms:

=== Availability ===
* often coupled with SLA
availability = (available time)/(time possible)

=== Capacity ===
number of jobs/time unit

=== Extensibility ===
Economically add/modify functionality

=== Flexibility ===
Ability to support architecture/hardware changes - location independent code helps

=== Manageability ===
Mange health of system - upgrades

=== Reliability ===
Integrity and consistency of transactions and application
* MTBF 10^5 hours - for 1 box => MTBF 10^15 hours for 3 boxes

=== Performance ===
Response time
* SLA
* ability to control expensive requests
* process & network

=== Scalability ===
Increase quality of service as load increases
* Vertically scale - add more memory/CPUs
* Horizontally scale - add more servers - requires load balancer - '''also increases availability/reliability'''

=== Security ===
Ensure information modifications and disclosures meet security policy

=== Testability ===
Confirm expected results
* monitoring

== Two tier ==
Good for:
* homogeneous environment
* static business rules
* small business <100 users
* often decision support services
* low interaction
Moving business logic to DB limits growth - difficult to scale
* stored procedures lack rich development tools - debugging/test/VCS

== Three tier ==
Good for:
* 100s of users
* batch jobs (tend to tie up 2 tier clients)
* high transaction load
* changeable business rules

== Load balancing ==
Distributes load across machines

== Replication ==
http://en.wikipedia.org/wiki/Replication_(computer_science)
=== Active replication ===
* Clustering
* Each request is applied on every system
* An interceptor is used to
* Equivalent to a hot backup
* Multi-primary - (multi-master in DB terms)
* Requires distributed concurrency - e.g. distributed lock manager
* Cluster is horizontally scalable by adding more machines

=== Passive replication ===
* High availability
* A request is applied on one system then replicated to the others
* Equivalent to a warm backup

'''Backup - copy of data unchanged for long periods - replicas - frequently updated quickly loose historical state'''

=== Applet Client Tier ===

http://java.sun.com/blueprints/qanda/client_tier/index.html

Applet client should talk to web container not EJB container

# JRE may not support direct EJB communication
# Firewalls may prevent direct EJB communication
# Exposing the EJB container to the internet so client can communication is a security risk
# Servlet layer decouples the applet from the business layer - [[Facade Pattern]]

== J2EE Notes ==
Annotations: replace deployment descriptors
* put resources in code next to where they are used

Dependency injection: Hides lookup and creation of resources
* EJB, Web container and clients
* container inserts references to components/resources using annotations

Solves scalability, accessibility, manageability
* Splits business and presentation tiers

[[Image:Tier.png]]

Considered 3 tier due to number of machines
* Makes business tier multi-threaded - and web tier

Security: can be defined at deployment time
* provides a standard login mechanism - authentication

J2EE component: self contained software unit
* application - classes & files
* communicates with other components
* well formed
* compliant with specification
* deployed and managed by J2EE server

# Application clients and applets - run on client
# Servlets/JFS/JPS - web components -run on web server
# EJB - business components - run on application server

* Web tier - interface, session, input, dialogue, display management
* Appserver tier - process managment, development, enactment, monitoring, resourcing
* EIS tier - DBMS - data and file services - ensures consistency, locking, replication

Resources named rather than location based - aids flexibility (moving)/scalability (adding)

J2EE Tiering - Revision history

Martin: Created page with "= Multi Tier Architecture = Client Server architecture with presentation, processing, data management being seperated Typically: * Layer - Logical * Tier - Physical Presenta..."