1. .NET Mobile Application Development Distributed Application Design

2. Introduction  In previous lectures we have examined >Characteristics of mobile and distributed applications >Contemporary mobile and distributed computing technologies  In this session we give an overview of >Distributed application design >Common distributed application software architectures

3. Technology Limitations  Design must take account of practicalities of existing distributed technologies >failure to do so may produce elegant but unworkable solutions  Current distributed technologies offer a compromise between networking technology and object-oriented design  Distributed components are not full partners in an OO design and should >not be perceived as abstractions of real-world entities >be considered as service providers which aggregate operations

4. Remote Objects Are Not Objects  Local and remote code are used (transparently) in the same way but should be designed differently  Local objects provide >an abstraction of a real-world entity >methods/properties that abstract real-world behaviour and may be used in conjunction with each other  Remote objects >involve network communication and delays ­orders of magnitude slower than local interaction >should be designed as collection of utility functions ­reduces network traffic and latencies and increases overall system performance

5. Example: Transferring Funds // Server code public class Account { private int _accountID; private int _accountID; private decimal _balance; private decimal _balance; public int AccountID { public int AccountID { get {return _accountID;} set {_accountID = value;} get {return _accountID;} set {_accountID = value;} } public decimal Balance { public decimal Balance { get {return _balance;} set {_balance = value;} get {return _balance;} set {_balance = value;} } public void Update() { public void Update() { // Update account record in database } // Update account record in database } public void Insert() { public void Insert() { // Inserts new record into database } // Inserts new record into database } public void Delete() { public void Delete() { // Deletes record corresponding to this class } // Deletes record corresponding to this class } public void Fill() { public void Fill() { // Gets database record which matches this class } // Gets database record which matches this class }} // Client code void TransferFunds(decimal amount) { Account accA = new Account(); Account accA = new Account(); Account accB = new Account;(); Account accB = new Account;(); // Retreive account balances // Retreive account balances accA.AccountID = 100; accA.AccountID = 100; accA.Fill(); accA.Fill(); accB.AccountID = 201; accB.AccountID = 201; accB.Fill(); accB.Fill(); accA.Balance -= amount; accA.Balance -= amount; accB.Balance += amount; accB.Balance += amount; accA.Update(); accA.Update(); accB.Update(); accB.Update();}

6. A Service-Provider Solution // Server code public class AccountUtility { public void UpdateAccount(int accountID, decimal balance) { // Update account record in database } public void InsertAccount( int accountID, decimal balance) { // Inserts new account record into database } public void Delete(int accountID) { // Deletes corresponding database record } public void TransferFunds( int accountFromID, int accountToID, decimal balance) { // Uses database transaction to modify the two accounts }

7. Remote Objects and State  Remote objects may be >Stateless – retain no information between calls >Stateful - store state information in memory between calls  Stateful objects >use memory in storing state; this works well with small number of clients but memory load increases drastically with large number of clients >are effectively bound to a single machine, making it hard to use load balancing for good distributed performance  Stateless objects are best design choice  XML Web Services are completely stateless >State can be maintained using session tracking but this is inefficient and not recommended

8. Design Guidelines for Distributed and Mobile Applications  Design components with distributed architecture in mind >deploy them to separate computers only when required  Distributed architecture is needed >to support unmodifiable legacy apps or multi-platform 3 rd party apps >if scalability issues are more important than small-scale performance >to support interaction from a mobile device ­mobile devices have resource limitations (e.g. memory capacity, processing power) and it may be necessary to shift computation / data storage to another device in the network

9. Design Principles  The following principles are useful in designing distributed applications  Aim to reduce communication frequency >a few ‘chunky’ interactions are better than many ‘chatty’ interactions  Follow the principle of locality >interacting objects should be close together  Remote objects should be >be stateless wherever possible and designed as service providers  Successful distributed applications >guide clients towards proper usage >restrict performance limiting options

10. Layered Design  Layered design is sound software engineering >can be difficult to avoid!  3-tier designs have >Presentation layer ­user interface logic ­typically the only role of a mobile client >Business logic layer ­business-specific rules >Data logic layer ­management of back-end data store Presentation Layer User Interface Object Business Logic Layer Business Rules Data Logic Layer DatabaseBinary FilesXML Data Store

11. Business Rules  Strict 3-tier design requires business rules to be placed in the middle layer >This is not always practical; early designs used stateful objects to achieve this and performance was dreadful  Some business rules belong in the user interface >e.g. validating user input should not require interaction with remote objects  Other business rules have to be in the data logic layer >e.g. verifying referential integrity requires access to other data already in the database  Performance gains can result from implementing business rules as stored database procedures >For maximum performance, aim for one stored procedure which performs all required operations on a single business object

12.  In this session we have discussed >Design principles for distributed applications >Performance and scalability >Layered designs  In the next session we conclude our study of mobile devices in distributed applications by considering device and application security Summary

13. Reading and Resources Reading  Matthew MacDonald, Microsoft.NET Distributed Applications: Integrating XML Web services and.NET Remoting, Microsoft Press, 2003 Chapter 10, pp 329 - 358 Resources  Microsoft Patterns and Practices >Enterprise Solution Patterns using Microsoft.NET, http://msdn.microsoft.com/architecture/patterns/default.aspx?pull=/library/en-us/dnpatterns/html/Esp.asp http://msdn.microsoft.com/architecture/patterns/default.aspx?pull=/library/en-us/dnpatterns/html/Esp.asp >Application Architecture for.NET: Designing Applications and Services, http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnbda/html/distapp.asp http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnbda/html/distapp.asp  Yukon Basics, http://msdn.microsoft.com/msdnmag/issues/04/02/YukonBasics/default.aspx http://msdn.microsoft.com/msdnmag/issues/04/02/YukonBasics/default.aspx