Distributed Architecture

Introduction to

What is Architecture? A software architecture is an abstract view of a software system distinct from the details of implementation, algorithms, and data representation.

SEI @ Carnegie Mellon

Why Architecture Matters

Accidental Architecture

Every… system has an architecture. While some of these architectures are intentional, most appear to be accidental

Grady Booch

Requirements Analysis

Design

Code and Test

Integration

System Test

Waterfall Process

Requirements Analysis

Design

Code and Test

Integration

System Test

Waterfall Process

Analysis Paralysis

MassiveIntegration

Design divorced from reality

Manifesto for Agile Software Development

We are uncovering better ways of developingsoftware by doing it and helping others do it.Through this work we have come to value:

Individuals and interactions over processes and toolsWorking software over comprehensive documentationCustomer collaboration over contract negotiationResponding to change over following a plan

That is, while there is value in the items onthe right, we value the items on the left more.

Scrum: An agile Process

Architect Code

Techies crave Extremes

Functional requirementsWhat the system should do.• Use cases, User stories, acceptance criteria• Defines passing QA• Day to day you focus on this

Nonfunctional requirementsWhat the system should be.• Extremely expensive or impossible to “fix”

later on• Defined passing Production• Assumed by your customers and users

Nonfunctional Requirements• Availability• Stability• Efficiency• Reliability• Maintainability• Extensibility

• Fault Tolerance• Security• Capacity• Latency• Flexibility• Scalability

Functional: Passed QABeautiful site (in my opinion)Easy to use

Nonfunctional: Failed ProductionAvailability StabilityLatency CapacityThroughput

ArchitectureThere is no best architecture

Pacemakers and Guided Missiles

The Flying Buttress

Patterns

Example: Layered Architecture

Three Tier ArchitectureThe classic 3 tier architecture

N-Tier ArchitectureBecause three tiers wasn’t enough!

N-Tier Architecture

SHAREPOINT

BIZTALK

SQL SERVER

Windows Server 2008

SMS

License managementIIS

Questions about Architecture?

Distributed ArchitectureDesigns appropriate for small brochureware websites fail outrageously when applied to thousand-user, transactional, distributed systems...

Michael T. Nygard

Key Constraints on Distributed Systems

• Stability • Reliability and fault tolerance• Consistency• Capacity and Scalability• Security

Scenario: StabilityYou are the lead developer on a national health care site that will register millions of users a week. You are responsible for the signup process.

Your team must verify the identity of users with a third party API. You call the service and the third party system will return a boolean true if the identity is valid or false otherwise.

private string Register(RegistrationInfo registrationInfo) { try { bool validIdentity = identityService.verifyIdentity(registrationInfo);

if (validIdentity) { database.save(registrationInfo); return “register_success.htm"; }

return “register_failure.htm"; } catch (Exception) { return "errorpage.htm"; }

}

6. RegisteredIdentity

Verification

Database

2. Verify

3. VerifiedSite Server

Stability

4. Save User5. User Saved

1. Register

6. RegisteredIdentity

Verification

Database

2. Verify

3. VerifiedSite Server

Unbalanced Capacities

4. Save User5. User Saved

1. Register

Identity Verification

Blocked Threads

Site ServerVerify sally

Verify bob

joe verified

Verify joe

Verify sally

Verify joe

Verify bobVerify bob

Verify bob

Verify bob

Verify bob

joe verified

Cascading Failures

Site Server

Site Server

Site Server

Site Server

private string Register(RegistrationInfo registrationInfo) { try {

identityService.Timeout = 10000; bool validIdentity = identityService.verifyIdentity(registrationInfo);

if (verified) { db.save(registrationInfo); return “register_success.htm"; }

return “register_failure.htm"; } catch (Exception) { return "errorpage.htm"; } }

Pattern 1: Timeouts

Identity Verification

Pattern 1: Timeouts

Site Server

private void BeginRegister(RegistrationInfo registrationInfo, Function<string> callback) { try {

identityService.EndIdentity += identityService_EndIdentity(callback) identityService.BeginIdentity(registrationInfo);

} catch (Exception) { callback("errorpage"); }

}

private void identityService_EndIdentity(bool success, Function<string> callback) { if (success) { callback("register_success.htm"); } callback("register_failure.htm"); }

Pattern 2: Non-blocking I/O

Pattern 2: Non-blocking I/O

Identity VerificationSite Server

For Users…

==

The site is stable. We’re still failing.

What’s the president recommending?

What might be happeningThank you. We’ll email

you when you are verified.

What might be happening

Identity Verification

“The” Registration ProcessWhen two principles are pushing in opposite directions, some underlying assumption is wrong. Often the word the is the culpritUdi Dahan

Read this againYou are the lead developer on a national health care site that will register millions of users a week. You are responsible for the signup process.

Your team must verify the identity of users with a third party API. You call the service and the third party system will return a boolean true if the identity is valid or false otherwise.

Pattern 3: Decoupling

Site Server

Thank you. We’ll email you when you are

verified

Pending Registration

Database

Pattern 3: Decoupling

Pending Registration

Database

Verification Application

Identity Verification

Scenario 2: ReliabilityYou are the lead developer on a hospital’s prescription filling service. Your RX wholesaler has provided you with an HTTPS endpoint to integrate with.

It is critical a prescription is not accidently prescribed twice and that prescriptions are not lost.

5. Success RX Service

2. Fill RX

3. RX IDSite Server

Reliability

1. Prescribe

Database

4. RX ID and Fill Info5. Record Updated

Invoking the Service private string Prescribe(PrescriptionInfo prescriptionInfo) { try { RxService rxService = new RxService(); int rxID = rxService.Prescribe(prescriptionInfo);

database.Save(rxId, prescriptionInfo);

return "success.htm"; }

catch (Exception) { return "errorpage.htm"; }

}

404 Timeout RX Service

2. Prescribe

Site Server

What if the network goes down?

1. Prescribe

Invoking the Service private string Prescribe(PrescriptionInfo prescriptionInfo) { try { RxService rxService = new RxService(); int rxID = rxService.Prescribe(prescriptionInfo); database.Save(rxId, prescriptionInfo);

return "success.htm"; }

catch (Exception) { return "errorpage.htm"; }

}

RX ServiceSite Server

Can I retry?

Prescribe

RX ServiceSite ServerPrescribe

Got it, but I couldn’t get Back to you.

404 Timeout

RX ServiceSite Server

Pattern 1: Idempotency

Prescribe

RX ServiceSite ServerPrescribe

Got it, but I couldn’t get Back to you.

404 Timeout

Sheesh… I already got it!

5. Success RX Service

2. Fill RX

3. RX IDSite Server

What if the database is down?

1. Prescribe

Database

4. Update Patient Record

Invoking the Service private string Prescribe(PrescriptionInfo prescriptionInfo) { try { RxService rxService = new RxService(); int rxID = rxService.Prescribe(prescriptionInfo);

database.Save(rxId, prescriptionInfo);

return "success.htm"; }

catch (Exception) { return "errorpage.htm"; }

}

Can We Guarantee this code? private string Prescribe(PrescriptionInfo prescriptionInfo) { try { RxService rxService = new RxService(); int rxID = rxService.Prescribe(prescriptionInfo);

database.Save(rxId, prescriptionInfo);

return "success.htm"; }

catch (Exception) { return "errorpage.htm"; }

}

Pattern 2: Transactional Queues

3. PendingSite Server

Transactional Queue

1. Prescribe

Fill RX Message Queue

Inserting into a Queue private string Prescribe(PrescriptionInfo prescriptionInfo) { try { Queue.save(new RXPrescribeMessage(prescriptionInfo)); return "rxPending.htm"; }

catch (Exception) { return "errorpage.htm"; } }

Transactional Queue

Queue

rxPrescribeMessage

RX Service

Fill rx

rx IDHandlerrxDBUpdateMessage

Transactional Queue

QueuerxDBUpdateMessage Save rxHandler Database

Message Handlers public void HandleRXPrescribeMessage(RXPrescribeMessage message) { var prescriptionInfo = message.prescriptionInfo;

RxService rxService = new RxService(); int rxID = rxService.Prescribe(prescriptionInfo); Queue.save(new RXDBUpdateMessage(rxID, prescriptionInfo)); }

public void HandleDBUpdateMessage(RXDBUpdateMessage message) { var rxId = message.rxID; var prescriptionInfo = message.prescriptionInfo

database.Update(message.rxID, message.prescriptionInfo) }

Nygard, Michael T. Cynical software expects bad things to happen and is never surprised when they do. Cynical software doesn’t even trust itself.. It refuses to get too intimate with other systems, because it could get hurt.