Udi Dahan – The Software Simplist Session Code: ARC 304

What will you see here? Distributed Systems Architecture UI Design Domain Model What won’t you see here? Code

Distributed Systems Architecture

Common Distributed Architecture DB

Let’s deal with the obvious…

Collaboration is good – isn’t it ? Get data Change data User is looking at stale data

So why go through all the layers? DB Cache

Why transform between tiers? Use EF to map from tables to domain objects Map from DTOs and WS to domain object Map from DTOs & WS to view model DB WSWS UIUI Cache

What was the point of all this again?

Queries – showing data to the user

Why not be upfront about it? Data correct as of 10 minutes ago List of customers

Only data, no behavior: not an object

Keep it simple - 2-Tier Persistent View Model UIUI Query only For each view in the UI, have a view in the DB

Incrementally add from there

Commands – accepting user input

Can we fully trust users decisions?

Validation and Business Rules Validation: Is the input potentially good? Structured correctly? Ranges, lengths, etc Rules:Should we do this? Based on the current system state What the user saw is irrelevant

Command Processing Layers DB

TransactionTransaction Command Processing Tiers DB WSWS InputInput ValidationValidation ValidationValidation Command Get current state RulesRules Persist

Should we do what the user asks?

User Interface Design

Traditional User Interfaces IDTotalDateShippedAccountetc 317$37.871/9/09YesA17T5 318$99.993/7/09YesA17T5 319$ /8/09YesP313Z 320$69.479/9/09NoP599Z Orders CancelCancelSaveSave

Capturing user intent Why is it important? Differentiating between: Correcting a mistyped shipping address The user moved – reroute shipments underway Sometimes users accidentally modify fields when tabbing between columns

Reservation systems

Not capturing user intent In a traditional UI – what we have is the checkbox Why do users select multiple seats? Because they’re reserving for a family / friends But then, concurrency happens Somebody else got in first on one of the seats Try to find a block of seats somewhere else

Capturing user intent Group reservation: Small group – sitting together Large group – several small groups Enter number of people Enter preferred seat type – indicates cost System s back when reservation can be filled Include waiting list functionality

Usability benefits More users get what they want with less clicks

Scalability benefits Thousands of seats, hundreds of thousands of requests No need to show actual status

Revisiting the command

What’s a good command? The kind you can reply with: “Thank you. Your confirmation will arrive shortly” Inherently asynchronous Not really related to an entity

Commands versus Entities It’s easier to validate the command Less data More specific Is this potentially good Validating large entities is complex

Domain Models

What aren’t they for? Validation Commands are validated before the domain model is called Queries Entity relationships for reading are unnecessary

What are they for? Answering the question: Should I do what this valid command is asking? If the answer is yes, change its state

Putting it all together

QueriesQueries CommandsCommands DB WSWS InputInput ValidationValidation ValidationValidation RulesRules View Model QueriesQueries

QueriesQueries CommandsCommands DB WSWS InputInput ValidationValidation ValidationValidation RulesRules QueriesQueries For better scalability, add pub/sub View Model CacheUpdaterCacheUpdater Publish

Summary

Keep queries simple – 2-Tier if possible No data transfer objects Have commands reflect user intent Adjust UI design to capture intent Get the benefits of asynchronous programming Simple validation, focused business rules

Thank you Udi Dahan – The Software Simplist Enterprise Development Expert & SOA Specialist

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