Slides from the talk I gave at dev weekend at Southern Methodist University.

1. Distributed
Architecture
Introduction to

2. 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

3. Why Architecture Matters

4. Accidental Architecture
Every… system has an
architecture. While
some of these
architectures are
intentional, most
appear to be accidentalGrady Booch

5. Requirements
Analysis
Design
Code and Test
Integration
System Test
Waterfall Process

6. Requirements
Analysis
Design
Code and Test
Integration
System Test
Waterfall Process
Analysis
Paralysis
Massive
Integration
Design divorced
from reality

7. Manifesto for Agile Software Development
We are uncovering better ways of developing
software by doing it and helping others do it.
Through this work we have come to value:
Individuals and interactions over processes and tools
Working software over comprehensive documentation
Customer collaboration over contract negotiation
Responding to change over following a plan
That is, while there is value in the items on
the right, we value the items on the left more.

8. Scrum: An agile Process

9. Architect Code
Techies crave Extremes

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

11. Nonfunctional requirements
What the system should be.
• Extremely expensive or impossible to “fix”
later on
• Defined passing Production
• Assumed by your customers and users

12. Nonfunctional Requirements
• Availability
• Stability
• Efficiency
• Reliability
• Maintainability
• Extensibility
• Fault Tolerance
• Security
• Capacity
• Latency
• Flexibility
• Scalability

13. Functional:
Passed QA
Beautiful site (in my opinion)
Easy to use
Nonfunctional:
Failed Production
Availability
Stability
Latency
Capacity
Throughput

15. ArchitectureThere is no best architecture

16. Pacemakers and Guided Missiles

17. The Flying Buttress

18. Patterns

19. Example: Layered Architecture

20. Three Tier Architecture
The classic 3 tier architecture

21. N-Tier Architecture
Because three tiers wasn’t enough!

22. N-Tier Architecture
SHAREPOINT
BIZTALK
SQL SERVER
Windows Server 2008
SMS
License
managementIIS

23. Questions about Architecture?

24. Distributed Architecture
Designs appropriate for
small brochureware
websites fail outrageously
when applied to thousand-
user, transactional,
distributed systems...
Michael T. Nygard

25. Key Constraints on Distributed Systems
• Stability
• Reliability and fault tolerance
• Consistency
• Capacity and Scalability
• Security

26. Scenario: Stability
You 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.

27. 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";
}
}

28. 6. Registered
Identity
Verification
Database
2. Verify
3. Verified
Site Server
Stability
4. Save User5. User Saved
1. Register

29. 6. Registered
Identity
Verification
Database
2. Verify
3. Verified
Site Server
Unbalanced Capacities
4. Save User5. User Saved
1. Register

30. Identity
Verification
Blocked Threads
Site Server
Verify sally
Verify bob
joe verified
Verify joe
Verify sally
Verify joe
Verify bob
Verify bob
Verify bob
Verify bob
Verify bob
joe verified

31. Cascading Failures
Site Server
Site
Server
Site
Server
Site
Server

32. 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

33. Identity
Verification
Pattern 1: Timeouts
Site Server

34. 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

35. Pattern 2: Non-blocking I/O
Identity
Verification
Site Server

36. For Users…
==

37. The site is stable. We’re still failing.

38. What’s the president recommending?

39. What might be happening
Thank you. We’ll email
you when you are
verified.

40. What might be happening
Identity
Verification

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

42. Read this again
You 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.

43. Pattern 3: Decoupling
Site Server
Thank you. We’ll email
you when you are
verified
Pending
Registration
Database

44. Pattern 3: Decoupling
Pending
Registration
Database
Verification
Application
Identity
Verification

45. Scenario 2: Reliability
You 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.

46. 5. Success
RX Service
2. Fill RX
3. RX ID
Site Server
Reliability
1. Prescribe
Database
4. RX ID and Fill Info5. Record Updated

47. Invoking the Serviceprivate 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";
}
}

48. 404 Timeout
RX Service
2. Prescribe
Site Server
What if the network goes down?
1. Prescribe

49. Invoking the Serviceprivate 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";
}
}

50. RX ServiceSite Server
Can I retry?
Prescribe
RX Service
Site Server
Prescribe
Got it, but I couldn’t get Back to you.
404 Timeout

51. RX ServiceSite Server
Pattern 1: Idempotency
Prescribe
RX Service
Site Server
Prescribe
Got it, but I couldn’t get Back to you.
404 Timeout
Sheesh… I already got it!

52. 5. Success
RX Service
2. Fill RX
3. RX ID
Site Server
What if the database is down?
1. Prescribe
Database
4. Update Patient Record

53. Invoking the Serviceprivate 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";
}
}

54. 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";
}
}

55. Pattern 2: Transactional Queues

56. 3. Pending
Site Server
Transactional Queue
1. Prescribe
Fill RX Message
Queue

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

58. Transactional Queue
Queue
rxPrescribeMessage
RX Service
Fill rx
rx ID
Handler
rxDBUpdateMessage

59. Transactional Queue
Queue
rxDBUpdateMessage Save rx
Handler Database

60. 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)
}

61. 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.