LAB OF SOFTWARE ARCHITECTURES AND INFORMATION SYSTEMS FACULTY OF INFORMATICS MASARYK UNIVERSITY, BRNO AND CONFLICTS BETWEEN THEM Barbora Bühnová buhnova@fi.muni.cz FOCUS ON QUALITY ATTRIBUTES Wheredo we stand? We already know many techniques for code-level quality: • Clean code principles • SOLID (Single responsibility,Open/closed, Liskov substitution, etc.) • GRASP (High cohesion, Low coupling, Polymorphism, etc.) • Bad code smells • Abstraction levels, dependencies, cohesion, naming conventions, etc. • Refactoring • When, where and how Is this enough to ensure code-level quality? © B. Bühnová, PV260 Software Quality What"quality"meanstoyou? …andyourcustomer? …andyourmanager? © B. Bühnová, PV260 Software Quality Outlineofthelecture • Bad code smells for • Performance • Scalability • Reliability • Testability • Maintainability • Tactics for • Discussed quality attributes • Conflicts between them © B. Bühnová, PV260 Software Quality Our big five Outlineofthelecture • Bad code smells for • Performance • Scalability • Reliability • Testability • Maintainability • Tactics for • Discussed quality attributes • Conflicts between them © B. Bühnová, PV260 Software Quality BadcodesmellsforPerformance • Let’s assume our code is perfectly CLEAN • What about performance? Are there any performance code smells we could check for? Let’s discuss four performance smells: • Smell #1: RedundantWork • Smell #2: One by One Processing • Smell #3: Long Critical Section • Smell #4: BusyWaiting © B. Bühnová, PV260 Software Quality Motivatingexample#1: FibonacciSequence • 1, 1, 2, 3, 5, 8, 13, 21, … • Fib(0) = Fib(1) = 1 Fib(n+2) = Fib(n+1) + Fib(n) where n≥0 In Java: © PatrycjaWegrzynowicz [2] public int fibonacci(int n) { if(n <= 1) return 1; return fibonacci(n-1) + fibonacci(n-2); } Smell#1:RedundantWork • Description • A time-consuming method computes the same many times in a single execution path • Consequences • A slower execution time since the time-consuming operation is performed multiple times • Solution • Call the heavy method only once and store the result for further reuse Note: Applies also in more complex scenarios, such as caching of database results in distributed systems. © PatrycjaWegrzynowicz [2] Example#1:Fibonaccirefactored © PatrycjaWegrzynowicz [2] Map cache1 = new HashMap(); long fibonacci(int n) { if (cache1.containsKey(n)) return cache1.get(n); if (n==0 || n==1) { int var1 = 1; cache1.put(n, var1); return var1; } int var2 = fibonacci(n-1) + fibonacci(n-2); cache1.put(n, var2); return var2; } Motivatingexample#2:Search © PatrycjaWegrzynowicz [2] private ArrayList list = new ArrayList(); List findGreaterThan(int value) { List ret = new ArrayList(); for (Item item : list) { if (item.isGreaterThan(value)) { ret.add(item); } } return ret; } Smell#2:One byOneProcessing • Description • Overused linear search/processing • Consequences • Slower performance • Solution • Use smarter algorithms and/or data structures (binary search, sorted collections, map with precomputed search predicates) Note: Become familiar with the performance of operations you execute on different types of data structures. And think about the complexity of your algorithms. © PatrycjaWegrzynowicz [2] Example#2:Searchrefactored © PatrycjaWegrzynowicz [2] private List list = new ArrayList(); private List var1 = new SortedList( ... ); ... List findGreaterThan(int value) { return subList(var1, value); } Motivatingexample#3: PasswordCracking © PatrycjaWegrzynowicz [2] static List passwordsToCheck; // launch 100 threads and FOR each thread void run() { while (!passwordsToCheck.isEmpty()) { synchronized(passwordsToCheck) { if (!passwordsToCheck.isEmpty()) { String pwd = passwordsToCheck.remove(0); checkPassword(pwd); } } } } void checkPassword() { ... } Smell#3: LongCriticalSection • Description • Unnecessary code performed in a critical section • Consequences • More like single-threaded model • Solution • Move the code outside the critical section Note: Sometimes it is favorable to use multiple locks within a class to enable partial locking of an object. See an example below. © PatrycjaWegrzynowicz [2] Example#3:PasswordCrackingrefactored © PatrycjaWegrzynowicz [2] static List passwordsToCheck; // launch 100 threads and FOR each thread void run() { while (!passwordsToCheck.isEmpty()) { synchronized(passwordsToCheck) { if (!passwordsToCheck.isEmpty()) { String pwd = passwordsToCheck.remove(0); } } checkPassword(pwd); } } void checkPassword() { ... } Example#3.b:Multiplelockswithinaclass © B. Bühnová, PV260 Software Quality public class MyUpdater { private long var1 = 0; private long var2 = 0; public void updateVar1() { synchronized(this) { // update var1 } } public void updateVar2() { synchronized(this) { // update var2 } } } private Object lock1 = new Object(); private Object lock2 = new Object(); public void updateVar1() { synchronized(lock1) { // update var1 } } public void updateVar2() { synchronized(lock2) { // update var2 } } Smell#4:BusyWaiting • Description • Repeatedly checking if something interesting happened (e.g. value changed, user input arrived). • Consequences • A lot of work with mostly no value, slowing down the system • Solution • Hollywood principle: “Don't call us, we'll call you.” • Observer pattern (Gang of Four book) © B. Bühnová, PV260 Software Quality Outlineofthelecture • Bad code smells for • Performance • Scalability • Reliability • Testability • Maintainability • Tactics for • Discussed quality attributes • Conflicts between them © B. Bühnová, PV260 Software Quality Badsmells(beliefs)forScalability • Smell #1: Distribution improves performance • Not always. Distributed systems must use network I/O, more CPU to maintain coherence, partitioning and replication. • Smell #2: Just performance • If you want to get distributed, there are many lessons to learn in reliability, maintainability, security, testability, and many other domains. • Smell #3: My framework takes care of it • Distributed applications must address many new concerns: • State sharing • Data consistency • Caching Fowler’s First Law of Distributed Object Design: Don't distribute your objects. Advice: Better clean up your application and stay local, if you can. © B. Bühnová, PV260 Software Quality • Load balancing • Failure management Outlineofthelecture • Bad code smells for • Performance • Scalability • Reliability • Testability • Maintainability • Tactics for • Discussed quality attributes • Conflicts between them © B. Bühnová, PV260 Software Quality BadcodesmellsforReliability • Smell #1: Input Kludge • Check all inputs for validity!On all user interfaces and service interfaces. • Smell #2: Blind Faith • Do not trust others (limit access to your code, check bug fixes), nor yourself (check the correctness of your results). • Smell #3: Poorly Handled Exceptions • Smell #4: Unguarded Sequential Coupling • Assumptions on the right ordering of method calls without control. • Smell #5: Fashionable Coding • Usage of all the new cool technologies and constructs you do not really understand. © B. Bühnová, PV260 Software Quality Outlineofthelecture • Bad code smells for • Performance • Scalability • Reliability • Testability • Maintainability • Tactics for • Discussed quality attributes • Conflicts between them © B. Bühnová, PV260 Software Quality BadcodesmellsforTestability • Smell #1: Global State • Do not allow your objects to communicate secretly. • Smell #2: Lack of Dependency Injection • Make your dependencies explicit. • Smell #3: Law of Demeter violation • Only talk to your immediate friends. • Smell #4: Misplaced and Hard Coded new Operator • Do not mix factory and service code. Note: In over 90% of cases, Global State is the problem. General advice: If your code is difficult to test, do not ask how to hack it, but what is wrong with that code! © B. Bühnová, PV260 Software Quality Motivatingexample#1:SecretCommunication © Miško Hevery [4] class X { ... X() { ... } public int doSomething() { ... } } int a = new X().doSomething(); int b = new X().doSomething(); Does a==b ?? Motivatingexample#1:SecretCommunication © Miško Hevery [4] a = new X() → b = new X() → X Y Q Z X Y Q Z Motivatingexample#1:SecretCommunication © Miško Hevery [4] a = new X() → a.doSomething() b = new X() → b.doSomething() a==b ✔ X Y Q Z X Y Q Z Motivatingexample#1:SecretCommunication © Miško Hevery [4] a = new X() → a.doSomething() b = new X() → b.doSomething() a==b ✖ X Y Q X Y Q GS Z Z Smell#1:GlobalState • Multiple executions can produce different results • Test flakiness • Order of tests matters • Cannot run tests in parallel • Unbounded location of state • Transitive dependencies • Hidden global states • System.currentTime() • Database © Miško Hevery [4] What about Singletons? Motivatingexample#2: DeceptiveAPI testCharge() { CreditCard cc; cc = new CreditCard(“1234567890121234”); cc.charge(100); } © Miško Hevery [4] Motivatingexample#2: DeceptiveAPI testCharge() { CreditCard cc; cc = new CreditCard(“1234567890121234”); cc.charge(100); } java.lang.NullPointerException at talk3.CreditCard.charge(CredicCard.java:48) © Miško Hevery [4] Motivatingexample#2: DeceptiveAPI testCharge() { CreditCardProcessor.init(); CreditCard cc; cc = new CreditCard(“1234567890121234”); cc.charge(100); } © Miško Hevery [4] Motivatingexample#2: DeceptiveAPI testCharge() { CreditCardProcessor.init(); CreditCard cc; cc = new CreditCard(“1234567890121234”); cc.charge(100); } java.lang.NullPointerException at talk3.CreditCardProcessor.init(CredicCardProcessor.java:146) © Miško Hevery [4] Motivatingexample#2: DeceptiveAPI testCharge() { OfflineQueue.start(); CreditCardProcessor.init(); CreditCard cc; cc = new CreditCard(“1234567890121234”); cc.charge(100); } © Miško Hevery [4] Motivatingexample#2: DeceptiveAPI testCharge() { OfflineQueue.start(); CreditCardProcessor.init(); CreditCard cc; cc = new CreditCard(“1234567890121234”); cc.charge(100); } java.lang.NullPointerException at talk3.OfflineQueue.start(OfflineQueue.java:16) © Miško Hevery [4] Motivatingexample#2: DeceptiveAPI testCharge() { Database.connect(...); OfflineQueue.start(); CreditCardProcessor.init(); CreditCard cc; cc = new CreditCard(“1234567890121234”); cc.charge(100); } © Miško Hevery [4] Motivatingexample#2: DeceptiveAPI testCharge() { Database.connect(...); OfflineQueue.start(); CreditCardProcessor.init(); CreditCard cc; cc = new CreditCard(“1234567890121234”); cc.charge(100); } © Miško Hevery [4] • CreditCard API lies • It pretends to not need the CreditCardProcessor even though in reality it does. Motivatingexample#2: BetterAPI testCharge() { ?? CreditCard cc; cc = new CreditCard(“1234567890121234”, ccProc); cc.charge(100); } © Miško Hevery [4] Motivatingexample#2: BetterAPI testCharge() { ?? ccProc = new CreditCardProcessor(queue); CreditCard cc; cc = new CreditCard(“1234567890121234”, ccProc); cc.charge(100); } © Miško Hevery [4] Motivatingexample#2: BetterAPI testCharge() { ?? queue = new OfflineQueue(db); ccProc = new CreditCardProcessor(queue); CreditCard cc; cc = new CreditCard(“1234567890121234”, ccProc); cc.charge(100); } © Miško Hevery [4] Motivatingexample#2: BetterAPI testCharge() { db = new Database(...); queue = new OfflineQueue(db); ccProc = new CreditCardProcessor(queue); CreditCard cc; cc = new CreditCard(“1234567890121234”, ccProc); cc.charge(100); } © Miško Hevery [4] Motivatingexample#2: BetterAPI testCharge() { db = new Database(...); queue = new OfflineQueue(db); ccProc = new CreditCardProcessor(queue); CreditCard cc; cc = new CreditCard(“1234567890121234”, ccProc); cc.charge(100); } © Miško Hevery [4] Dependency Injection Smell#2:Lack of DependencyInjection • Dependency injection makes your dependencies explicit • It does not make the dependencies in your code better or worse • It only makes them visible • If there are too many dependencies, do not blame DI! • The dependencies have always been there, DI only showed them to you • Dependency injection enforces the order of initialization at compile time • Compiler helps to prevent illegal test setup © Miško Hevery [4] Won’t my system get flooded with arguments passed around? Smell#2:Lack of DependencyInjection • Dependency injection makes your dependencies explicit • It does not make the dependencies in your code better or worse • It only makes them visible • If there are too many dependencies, do not blame DI! • The dependencies have always been there, DI only showed them to you • Dependency injection enforces the order of initialization at compile time • Compiler helps to prevent illegal test setup © Miško Hevery [4] testCharge() { db = new Database(...); queue = new OfflineQueue(db); ccProc = new CreditCardProcessor(queue); CreditCard cc; cc = new CreditCard(“1234567890121234”, ccProc); cc.charge(100); } Won’t my system get flooded with arguments passed around? NO Smell#3: Law of Demeterviolation Law of Demeter: “Only talk to your immediate friends” • If an object needs links to too many objects, there may be something wrong with the object • Revealed by Dependency Injection • “Our code often smells because we have a few objects doing too much work, which requires them to know about too many other objects.” [Brandon Keepers] • A nice rule of thumb is to check if we are able to describe the purpose of each class and method without using AND and OR. © Miško Hevery [4], Brandon Keepers [3] Single Responsibility Principle Smell#4:MisplacedandHardCodednewOperator To avoid misplace, clearly separate: • “Code with a whole bunch of new operators and no if statement” = code responsible for starting and wiring things, i.e. Factories. • “Code with a whole bunch of if statements and no new operator” = code that is actually doing something, i.e. Services. To avoid hard coding, make sure that: • Constructor only constructs the object and its dependencies. • Doing any other work in the constructor can significantly hinder testing. • You can end up doing unrelated work (e.g. sending emails) every time you need the object in your test. © Miško Hevery [4] Outlineofthelecture • Bad code smells for • Performance • Scalability • Reliability • Testability • Maintainability • Tactics for • Discussed quality attributes • Conflicts between them © B. Bühnová, PV260 Software Quality BadcodesmellsforMaintainability • Smell #1: EarlyTuning • Never compromise code clarity for premature code optimization. • Smell #2: Super-Flexibility • “Flexibility breeds complexity.” • Do not shoot for something that is flexible from the early beginning. Shoot for something that is simple and build flexibility upon that. • Smell #3: Simple = Stupid, Complex = Smart • “Too complicated answers are always wrong, no matter what the question was.” • Even very smart systems can be based on simple structures. Look at embedded systems or human brain! © B. Bühnová, PV260 Software Quality Outlineofthelecture • Bad code smells for • Performance • Scalability • Reliability • Testability • Maintainability • Tactics for • Discussed quality attributes • Conflicts between them © B. Bühnová, PV260 Software Quality TacticsforPerformance • Tactic #1:Take a profiler into action • Do not guess where the performance problem is. Start your profiler and find the bottlenecks objectively. • It helps you to understand what is happening in the background. • Tactic #2: Examine complexity and frequency of your computations • Complexity – Maybe you can do the thing more efficiently. • Frequency – Maybe you can do the thing less often. • Tactic #3: Concurrency • Only if you understand all aspects and consequences of parallel execution. • Tactic #4: Control the use of resources • Balance the load, control access, cache, replicate, etc. © B. Bühnová, PV260 Software Quality TacticsforReliability • Tactic #1: Monitor what is going on • Acceptance checking for individual methods and code fragments, events collection, processing and logging. • Tactic #2: Handle exceptions carefully • Think twice about exception handling strategy and responsibilities inside the system. • Tactic #3: Make your system fault tolerant • Redundancy and self-healing, e.g. seamless rebinding to a new service provider. • Tactic #4: Implement restart/recovery capabilities • Redirection to a filled-in form when the form submission fails. • System diagnostics and clean-up after major failure. Note 1:We only care about SW reliability (because this is a Software Quality course), not HW, although HW fault tolerance is a very interesting topic. Note 2: We assume that we do not deal with an ultra-reliable system. If so, other mechanisms would need to be in place (e.g. n-version programming). © B. Bühnová, PV260 Software Quality TacticsforTestability • Tactic #1:Write CLEAN code • Simplicity matters. • Tactic #2: Avoid global state • Including its hidden forms. • Tactic #3: Separate interfaces from implementation • Make it possible to exchange implementations during testing. • Tactic #4: Make your dependencies explicit • It makes the life of developers/testers easier, and then even compiler can help to inspect it. • Tactic #5: Separate factories from business logic • During testing it is important to have access to each of these parts without mixing it with the other. © B. Bühnová, PV260 Software Quality TacticsforMaintainability • Tactic #1:Write CLEAN code • “Premature optimization is the root of all evil.” • Clean code is not only easier to change, but also easier to optimize (e.g. for performance, scalability). • Tactic #2: Get ready for change • “Change is the only constant.” • Understand – Interfaces, Inheritance, Polymorphism, Design Patterns. • Tactic #3: Design your SW Architecture carefully • Proper modularization of your system is one of the keys for maintainability. • Tactic #4:Watch all dependencies • Check – Law of Demeter, High Cohesion, Low Coupling. © B. Bühnová, PV260 Software Quality Conflictsbetweenqualityattributes © B. Bühnová, PV260 Software Quality Takeaways • Bad Code Smells apply also to quality attributes. • They are just not that easy to Google. • Tactics in comparison to Bad Code Smells are usually defined on a higher level of abstraction. • Each tactic for a specific quality attribute can act as an antipattern for a different quality attribute. • That is where conflicts between quality attributes emerge. Barbora Bühnová, FI MU Brno buhnova@fi.muni.cz www.fi.muni.cz/~buhnova contact me thanks for listening © B. Bühnová, PV260 Software Quality References • [1] Martin Fowler et al. Refactoring: Improving the Design of Existing Code, Addison-Wesley, Mar 2012. ISBN 978-0133065268. • [2] Patrycja Wegrzynowicz. Automated Refactoring of Performance and Concurrency AntiPatterns. YouTube, Jan 2013. Available at https://www.youtube.com/watch?v=XLCbb6dcsJQ. • [3] Brandon Keepers. Why Our Code Smells.YouTube, June 2012. Available at https://www.youtube.com/watch?v=JxPKljUkFQw. • [4] Miško Hevery.The Clean CodeTalks - Global State and Singletons.YouTube, Nov 2008. Available at https://www.youtube.com/watch?v=-FRm3VPhseI. • [5] Miško Hevery. Guide:WritingTestable Code, Google, Nov 2008. Available in the int. syllabus in IS. • [6] Slava Imeshev.Architecture for Scaling Java Applications to Multiple Servers.YouTube, Aug 2012. Available at https://www.youtube.com/watch?v=DhKpqGDXRCk. • [7] Lars Lundberg et al. (editors). Software quality attributes and trade-offs, Blekinge Institute of Technology, June 2005. • [8] Mikael Svahnberg et al. A Method for Understanding Quality Attributes in SoftwareArchitecture Structures. In Proc. of SEKE'02, pages 819-826. ACM NewYork, 2002. ISBN:1-58113-556-4. • [9] Michael Feathers. Escaping theTechnical Debt Cycle.YouTube, Oct 2014. Available at https://www.youtube.com/watch?v=7hL6g1aTGvo. © B. Bühnová, PV260 Software Quality