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Design and Validation of Cloud Storage Systems using Maude Peter Csaba Olveczky University of Oslo University of Illinois at Urbana-Champaign Based on joint work with Jon Grov and members of UIUCs Center for Assured Cloud Computing

  1. Maude: Software Engineering Perspective (cont.) What about performance analysis? (Randomized) simulations 1 Probabilistic analysis (using PVeStA) 2 ◮ statistical model checking Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 19 / 58

  2. Maude: Software Engineering Perspective (cont.) Same artifact for: precise system description rapid prototyping extensive testing correctness analysis performance estimation Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 20 / 58

  3. Case Study I Modeling, Analyzing, and Extending Megastore Joint work with Jon Grov (U. Oslo) Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 21 / 58

  4. Megastore Megastore: Google’s wide-area replicated data store 3 billion write and 20 billion read transactions daily (2011) Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 22 / 58

  5. Megastore: Key Ideas (I) (Figure from http://cse708.blogspot.jp/2011/03/megastore-providing-scalable-highly.html ) Data divided into entity groups Peter’s email Books on rewriting logic Narciso’s documents Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 23 / 58

  6. Megastore: Key Ideas (II) Consistency for transactions accessing a single entity group ◮ no guarantee if transaction reads multiple entity groups Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 24 / 58

  7. Our Work [Developed and] formalized [our version of the] Megastore [approach] in Maude Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 25 / 58

  8. Our Work [Developed and] formalized [our version of the] Megastore [approach] in Maude ◮ first (public) formalization/detailed description of Megastore Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 25 / 58

  9. Our Work [Developed and] formalized [our version of the] Megastore [approach] in Maude ◮ first (public) formalization/detailed description of Megastore 56 rewrite rules (37 for fault tolerance features) Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 25 / 58

  10. Performance Estimation Key performance measures: ◮ average transaction latency ◮ number of committed/aborted transactions Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 26 / 58

  11. Performance Estimation Key performance measures: ◮ average transaction latency ◮ number of committed/aborted transactions Randomly generated transactions (rate 2.5 TPS) 30% 30% 30% 10% Madrid ↔ Paris 10 15 20 50 Network delays: Madrid ↔ New York 30 35 40 100 Paris ↔ New York 30 35 40 100 Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 26 / 58

  12. Performance Estimation Key performance measures: ◮ average transaction latency ◮ number of committed/aborted transactions Randomly generated transactions (rate 2.5 TPS) 30% 30% 30% 10% Madrid ↔ Paris 10 15 20 50 Network delays: Madrid ↔ New York 30 35 40 100 Paris ↔ New York 30 35 40 100 Simulating for 200 seconds: Avg. latency (ms) Commits Aborts Madrid 218 109 38 New York 336 129 16 Paris 331 116 21 Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 26 / 58

  13. Megastore-CGC: extending Megastore Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 27 / 58

  14. Motivation Some transactions must access multiple entity groups Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 28 / 58

  15. Motivation Some transactions must access multiple entity groups Our work: extend Megastore with consistency for transactions accessing multiple entity groups Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 28 / 58

  16. Motivation Some transactions must access multiple entity groups Our work: extend Megastore with consistency for transactions accessing multiple entity groups Megastore-CGC piggybacks ordering and validation onto Megastore’s coordination protocol ◮ no additional messages for validation/commit! ◮ maintains Megastore’s performance and fault tolerance Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 28 / 58

  17. Performance Comparison using Real-Time Maude Simulating for 1000 seconds (no failures) Megastore: Commits Aborts Avg. latency (ms) Madrid 652 152 126 Paris 704 100 118 New York 640 172 151 Megastore-CGC: Commits Aborts Val. aborts Avg.latency (ms) Madrid 660 144 0 123 Paris 674 115 15 118 New York 631 171 10 150 Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 29 / 58

  18. Model Checking Megastore-CGC Model checking scenarios 5 transactions , no failures, message delay 30 ms or 80 ms − → 108,279 reachable states, 124 seconds 3 transactions, one site failure and fixed message delay − → 1,874,946 reachable states, 6,311 seconds 3 transactions, fixed message delay and one message failure − → 265,410 reachable states, 858 seconds Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 30 / 58

  19. Case Study II Work by Si Liu, Muntasir Raihan Rahman, Stephen Skeirik, Indranil Gupta, Jos´ e Meseguer, Son Nguyen, Jatin Ganhotra (ICFEM’14, QEST’15) Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 31 / 58

  20. Apache Cassandra Key-value data store originally developed at Facebook Used by Amadeus, Apple, CERN, IBM, Netflix, Facebook/Instagram, Twitter, . . . Open source Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 32 / 58

  21. Cassandra Overview Read consistency either one, quorum, or all Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 33 / 58

  22. Cassandra Overview Read consistency either one, quorum, or all Write consistency either zero, one, quorum, or all [Figures from http://www.slideshare.net/nuboat/cassandra-distributed-data-store ] Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 33 / 58

  23. Motivation Formal model from 345K LOC 1 ◮ allows experimenting with different optimizations/variations Analyze basic property: eventual consistency 2 When/how often does Cassandra give stronger guarantees? 3 ◮ strong consistency ◮ read-your-writes Performance evaluation: 4 ◮ compare PVeStA analyses with real implementations Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 34 / 58

  24. Formal Analysis with Multiple Clients Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 35 / 58

  25. Performance Estimation Formal model + PVeStA vs. actual implementation Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 36 / 58

  26. P-Store P-Store [N. Schiper, P. Sutra, and F. Pedone; IEEE SRDS’10] Replicated and partitioned data store Serializability Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 37 / 58

  27. P-Store P-Store [N. Schiper, P. Sutra, and F. Pedone; IEEE SRDS’10] Replicated and partitioned data store Serializability Atomic multicast orders concurrent transactions Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 37 / 58

  28. P-Store P-Store [N. Schiper, P. Sutra, and F. Pedone; IEEE SRDS’10] Replicated and partitioned data store Serializability Atomic multicast orders concurrent transactions Group commitment for atomic commit Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 37 / 58

  29. Atomic Multicast Definition Atomic Multicast: Consistent reception order of messages (a): any pair of nodes receive the same atomic-multicast messages in the same order (b): induced “global read order” must be acyclic Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 38 / 58

  30. Atomic Multicast Definition Atomic Multicast: Consistent reception order of messages (a): any pair of nodes receive the same atomic-multicast messages in the same order (b): induced “global read order” must be acyclic Example A reads m 1 < m 2 B reads m 2 < m 3 C reads m 3 < m 1 satisfies (a) but not (b) Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 38 / 58

  31. Atomic Multicast in Maude (I) Fundamental problem in distributed systems Impose order on conflicting concurrent transactions Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 39 / 58

  32. Atomic Multicast in Maude (I) Fundamental problem in distributed systems Impose order on conflicting concurrent transactions Many algorithms for atomic multicast Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 39 / 58

  33. Atomic Multicast in Maude (I) Fundamental problem in distributed systems Impose order on conflicting concurrent transactions Many algorithms for atomic multicast Define generic atomic multicast primitive in Maude ◮ abstract ◮ covers all possible receiving orders Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 39 / 58

  34. Atomic Multicast in Maude (I) Fundamental problem in distributed systems Impose order on conflicting concurrent transactions Many algorithms for atomic multicast Define generic atomic multicast primitive in Maude ◮ abstract ◮ covers all possible receiving orders Infrastructure stores (un)read AM messages Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 39 / 58

  35. My Work: Atomic Multicast in Maude (II) Atomic-multicast message M : rl [atomic-multicast] : < O : Node | msgToSend : M , receivers : OS > => < O : Node | ... > (atomic-multicast M from O to OS) . Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 40 / 58

  36. My Work: Atomic Multicast in Maude (II) Atomic-multicast message M : rl [atomic-multicast] : < O : Node | msgToSend : M , receivers : OS > => < O : Node | ... > (atomic-multicast M from O to OS) . Read: crl [receiveAtomicMulticast] : (msg M from O2 to O) < O : Node | ... > AM-TABLE => < O : Node | ... > updateAM(MC, O, AM-TABLE) if okToRead(MC, O, AM-TABLE) . Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 40 / 58

  37. Analyzing P-Store Find all reachable final states from init3 : Maude> (search init3 =>! C:Configuration .) Solution 1 C:Configuration --> ... < c1 : Client | pendingTrans : t1, txns : emptyTransList > < c2 : Client | pendingTrans : t2, txns : emptyTransList > < r1 : PStoreReplica | aborted : none, committed : < t1 : Transaction | ... > < r2 : PStoreReplica | aborted : none, committed : < t2 : Transaction | ... > ... Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 41 / 58

  38. Analyzing P-Store Find all reachable final states from init3 : Maude> (search init3 =>! C:Configuration .) Solution 1 C:Configuration --> ... < c1 : Client | pendingTrans : t1, txns : emptyTransList > < c2 : Client | pendingTrans : t2, txns : emptyTransList > < r1 : PStoreReplica | aborted : none, committed : < t1 : Transaction | ... > < r2 : PStoreReplica | aborted : none, committed : < t2 : Transaction | ... > ... sites validate transactions but client never gets result Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 41 / 58

  39. Analyzing P-Store (cont.) Solution 5 ... < r1 : PStoreReplica | aborted : none, committed : none, submitted : < t1 : Transaction | ... >, ... > < r2 : PStoreReplica | aborted : none, committed : < t2 : Transaction| ... > ... > Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 42 / 58

  40. Analyzing P-Store (cont.) Solution 5 ... < r1 : PStoreReplica | aborted : none, committed : none, submitted : < t1 : Transaction | ... >, ... > < r2 : PStoreReplica | aborted : none, committed : < t2 : Transaction| ... > ... > Host does not validate t1 even when needed info known Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 42 / 58

  41. Fixing P-Store Found the source of the errors ◮ all replicas must be involved in voting and notification ⋆ not just write replicas Modeled and analyzed proposed corrected version Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 43 / 58

  42. P-Store Summary “P-Store verified” 3 significant errors found one confusing definition key assumption missing Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 44 / 58

  43. Our Conclusions Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 45 / 58

  44. Our Conclusions I Developed formal models of large industrial data stores ◮ Google’s Megastore (from brief description) ◮ Apache Cassandra (from 345K LOC and description) ◮ P-Store (academic) Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 46 / 58

  45. Our Conclusions I Developed formal models of large industrial data stores ◮ Google’s Megastore (from brief description) ◮ Apache Cassandra (from 345K LOC and description) ◮ P-Store (academic) Automatic model checking analysis of consistency properties Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 46 / 58

  46. Our Conclusions I Developed formal models of large industrial data stores ◮ Google’s Megastore (from brief description) ◮ Apache Cassandra (from 345K LOC and description) ◮ P-Store (academic) Automatic model checking analysis of consistency properties Designed own transactional data stores ◮ Megastore-CGC ◮ variation of Cassandra Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 46 / 58

  47. Our Conclusions I Developed formal models of large industrial data stores ◮ Google’s Megastore (from brief description) ◮ Apache Cassandra (from 345K LOC and description) ◮ P-Store (academic) Automatic model checking analysis of consistency properties Designed own transactional data stores ◮ Megastore-CGC ◮ variation of Cassandra Errors, ambiguities, missing assumptions found in “verified” P-Store Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 46 / 58

  48. Our Conclusions I Developed formal models of large industrial data stores ◮ Google’s Megastore (from brief description) ◮ Apache Cassandra (from 345K LOC and description) ◮ P-Store (academic) Automatic model checking analysis of consistency properties Designed own transactional data stores ◮ Megastore-CGC ◮ variation of Cassandra Errors, ambiguities, missing assumptions found in “verified” P-Store Maude/PVeStA performance estimation close to real implementations Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 46 / 58

  49. Our “Software Engineering” Conclusions Quickly develop formal models/prototypes of complex systems ◮ experiment with different design choices Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 47 / 58

  50. Our “Software Engineering” Conclusions Quickly develop formal models/prototypes of complex systems ◮ experiment with different design choices Simulation and model checking throughout design phase ◮ model-checking-based-testing for subtle “corner cases” ◮ replaces days of whiteboard analysis ◮ too many scenarios for standard test-based development ◮ catch bugs early! Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 47 / 58

  51. Our “Software Engineering” Conclusions Quickly develop formal models/prototypes of complex systems ◮ experiment with different design choices Simulation and model checking throughout design phase ◮ model-checking-based-testing for subtle “corner cases” ◮ replaces days of whiteboard analysis ◮ too many scenarios for standard test-based development ◮ catch bugs early! Single artifact for ◮ system description ◮ rapid prototyping ◮ model checking ◮ performance estimation Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 47 / 58

  52. Our “Software Engineering” Conclusions Quickly develop formal models/prototypes of complex systems ◮ experiment with different design choices Simulation and model checking throughout design phase ◮ model-checking-based-testing for subtle “corner cases” ◮ replaces days of whiteboard analysis ◮ too many scenarios for standard test-based development ◮ catch bugs early! Single artifact for ◮ system description ◮ rapid prototyping ◮ model checking ◮ performance estimation Megastore and Megastore-CGC modeler had no formal methods experience Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 47 / 58

  53. Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 48 / 58

  54. Amazon Web Services Amazon Web Services (AWS): ◮ world’s largest cloud computing service provider ◮ more profitable than Amazon’s retail business Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 49 / 58

  55. Amazon Web Services Amazon Web Services (AWS): ◮ world’s largest cloud computing service provider ◮ more profitable than Amazon’s retail business Amazon Simple Storage Service (S3) ◮ stores > 3 trillion objects ◮ 99.99% availability of objects ◮ > 1 million requests per second DynamoDB data store Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 49 / 58

  56. Amazon Web Services and Formal Methods Formal methods used extensively at AWS during design of S3, DynamoDB, . . . Used Lamports TLA+ ◮ model checking Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 50 / 58

  57. Experiences at Amazon WS Model checking finds “corner case” bugs that would be hard to find with standard industrial methods: Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 51 / 58

  58. Experiences at Amazon WS Model checking finds “corner case” bugs that would be hard to find with standard industrial methods: “We have found that standard verification techniques in industry are necessary but not sufficient. We routinely use deep design reviews, static code analysis, stress testing, and fault-injection testing but still find that subtle bugs can hide in complex fault-tolerant systems.” Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 51 / 58

  59. Experiences at Amazon WS Model checking finds “corner case” bugs that would be hard to find with standard industrial methods: “the model checker found a bug that could lead to losing data [...]. This was a very subtle bug; the shortest error trace exhibiting the bug included 35 high-level steps. [...] The bug had passed unnoticed through extensive design reviews, code reviews, and testing.” Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 51 / 58

  60. Experiences at Amazon WS II A formal specification is a valuable precise description of an algorithm: Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 52 / 58

  61. Experiences at Amazon WS II A formal specification is a valuable precise description of an algorithm: “the author is forced to think more clearly, helping eliminating “hand waving,” and tools can be applied to check for errors in the design, even while it is being written. In contrast, conventional design documents consist of prose, static diagrams, and perhaps psuedo-code in an ad hoc untestable language.” Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 52 / 58

  62. Experiences at Amazon WS II A formal specification is a valuable precise description of an algorithm: “Talk and design documents can be ambiguous or incomplete, and the executable code is much too large to absorb quickly and might not precisely reflect the intended design. In contrast, a formal specification is precise, short, and can be explored and experimented on with tools.” Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 52 / 58

  63. Experiences at Amazon WS III Formal methods are surprisingly feasible for mainstream software development and give good return on investment: Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 53 / 58

  64. Experiences at Amazon WS III Formal methods are surprisingly feasible for mainstream software development and give good return on investment: “In industry, formal methods have a reputation for requiring a huge amount of training and effort to verify a tiny piece of relatively straightforward code. Our experience with TLA+ shows this perception to be wrong. [...] Amazon engineers have used TLA+ on 10 large complex real-world systems. In each, TLA+ has added significant value. [...] Engineers have been able to learn TLA+ from scratch and get useful results in two to three weeks.” Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 53 / 58

  65. Experiences at Amazon WS III Formal methods are surprisingly feasible for mainstream software development and give good return on investment: “Using TLA+ in place of traditional proof writing would thus likely have improved time to market, in addition to achieving greater confidence in the system’s correctness.” Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 53 / 58

  66. Experiences at Amazon WS III Quick and easy to experiment with different design choices: Peter Csaba ¨ Olveczky (U. Oslo/UIUC) Cloud Storage Systems in Maude UCM, February 20, 2017 54 / 58

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