Architecting for the Cloud

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Architecting for the CloudRichard Blewett

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Agenda

The Problem Space

Managing State

Architect for Change

Dealing with Failure

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ScalingMainframe Approach

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ScalingCloud Approach

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Effects of scaling

Applications run on many machines concurrently

Every request may hit a different machine

Hardware fails

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Statelessness is King

Statelessness: Local state of any kind is unreliable

Store critical state in Azure Storage or SQL Azure

See Eric Nelson’s talk for more details

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.NET Services Service Bus

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Don’t be Scared of the Service Bus

Messaging backbone for the cloud

User Service Bus to bridge in-flight cloud data to full data on premise

Enables rich message exchange patterns

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Prefer Asynchrony

Connect components using queuesAzure queue storage

Synchrony is a form of coupling

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Asynchrony = Load Levelling

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Asynchrony = Scalability

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Its OK to Degrade

Some functionality is mission critical

Other functionality can wait

Degrade service to maintain mission critical functionality

You can do everything else later

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Not Everyone Needs Perfect Consistency

Many services move though “inconsistent” states

EmailIMSMS

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Update is Hard

Difficult to update and remain available

Design for rolling updateAzure supports two concurrent deployed versionsAzure will support update groups

Update Group 1 Update Group 2 Update Group 3

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Update Code *or* Data

Do not update code and data at the same time

Design data to handle multiple code versionsDesign code to handle multiple data versions

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Maintain Compatibility

Make sure that you can roll back change without breaking everything

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Timing of Update can be Hard

Depends on location of users

Depends on work pattern of users

Don’t forget the Pacific Ocean is big with a low population

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Change is Inevitable

Expect things to changeDon’t hard-code valuesUse azure config

string val = RoleManager.GetConfigurationSetting("LoggingLevel");

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Azure configuration

<ServiceConfiguration serviceName="PhotoGallery"> <Role name="WebRole"> <Instances count="1"/> <ConfigurationSettings> <Setting name="LoggingLevel" value="Error"/> </ConfigurationSettings> </Role></ServiceConfiguration>

<ServiceDefinition name="PhotoGallery"> <WebRole name="WebRole"> ... <ConfigurationSettings> <Setting name="LoggingLevel" /> </ConfigurationSettings> </WebRole></ServiceDefinition>

.csdef

.cscfg

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What Just Happened?

Debugging and Diagnostics non-trivial

Use local fabric for testingTest against local storageTest against cloud storage

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Tracing is Key

Add trace statements in codeCan be filtered on log level from configCritical errors raised as alerts

if( RoleManager.GetConfigurationSetting("LoggingLevel") == "Verbose"){ RoleManager.WriteToLog("Information", "Product Purchased");}

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Failures Often Transient

Build retry logic into your code

Remember to stop retrying eventually

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Failures Can be “Catastrophic”

Don’t assume your “shutdown” logic will be executed

Try to keep state consistent enough at all times

Think about sanity checking on start

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Fault Domains

Fault domains allow you divide application for fault tolerance

Not available yet

Fault Domain 1 Fault Domain 2 Fault Domain 3

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The Challenges are Not Just Technical

CostEstimatingActual

Lock-inVendors have vested interest

LegalWhere is my data?

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Q & AThanks for coming

richard@rocksolidknowledge.comhttp://rocksolidknowledge.com/blogs