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Software Configuration Management

Software Configuration Management. Walter F. Tichy University of Karlsruhe. Definition. Software Configuration Management (SCM) is the discipline of controlling the evolution of software systems. SCM is indispensable for the development and maintenance of large, long-lived software.

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Software Configuration Management

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  1. Software Configuration Management Walter F. Tichy University of Karlsruhe Blanko ‘98 22-23 Oct. 1998

  2. Definition • Software Configuration Management (SCM) is the discipline of controlling the evolution of software systems. • SCM is indispensable for the development and maintenance of large, long-lived software. Blanko ‘98 22-23 Oct. 1998

  3. Why is it needed? • Software changes constantly. • Change causes chaos. • SCM helps prevent change chaos: • Tracks development history, • Prevents or reconciles conflicting changes, • Tracks configurations and baselines, • Tracks error reports and fixes, • Automates configuration selection, • keeps software in well-defined state. • All this in a distributed environment Blanko ‘98 22-23 Oct. 1998

  4. Symptoms of the Change Chaos • Problems of identification and tracking: • “This program worked yesterday. What happened?” • “I fixed this error last week. Why is it back?” • “Where are all my changes from last week?” • “This seems like an obvious fix. Has it been tried before?” • “Who is responsible for this change?” Blanko ‘98 22-23 Oct. 1998

  5. Change Chaos Continued • Problems of version selection • “Has everything been compiled? Tested?” • “How do I configure for test, with my updates and no others?” • “How do I exclude this incomplete/faulty change?” • “I can’t reproduce the error in this copy!” • “Exactly which fixes went into this configuration?” • “Oh shoot. I need to merge 250 files!” Blanko ‘98 22-23 Oct. 1998

  6. More Chaos …. • Software Delivery Problems • “Which configuration does this customer have?” • “Did we deliver a consistent configuration?” • “Did the customer modify the code?” • “The customer skipped the previous two releases. What happens if we send him the new one?” • These problems sound familiar to anyone who has worked in software development. Blanko ‘98 22-23 Oct. 1998

  7. History of SCM • Configuration management was first developed for the aerospace industry in the 1950’s. It was used among others in the Apollo Space Program for tracking thousands of changes. • SCM began in the 1970’s with SCCS and MAKE. • SCM is now a successful part of the software tools market, with at least 50 tools available. • SCM tools deliver recognized benefits to users. • SCM is a technically challenging area. Blanko ‘98 22-23 Oct. 1998

  8. Literature about SCM • International Workshop on Software Configuration Management • 6th to 8th meeting in 1996, 1997, 1998. • Conference proceedings by Springer Verlag. • Various books. • “Applying RCS and SCCS”, Bolinger & Bronson, O’Reilly, 1995. • “Configuration Management”, Tichy, Wiley, 1994. • Survey of many SCM systems: www.cs.colorado.edu/ ~andre/configuration_management.html Blanko ‘98 22-23 Oct. 1998

  9. Contents 1. Introduction Ö 2. Requirements Space of SCM Systems 3. Versions 4. Distribution 5. Configurations and Building 6. Change Tracking 7. Process 8. Summary Blanko ‘98 22-23 Oct. 1998

  10. 2. SCM Requirements Space Process Support Distribution (LAN, WAN) Change Request Tracking Building Version Management Blanko ‘98 22-23 Oct. 1998

  11. Requirements Space • Version Management • Traceable version history • branch & merge • stable workspaces for developers and integrators • control over incorporation of concurrent changes Blanko ‘98 22-23 Oct. 1998

  12. Requirements Space Cont. • Configurations • build anywhere, anytime, with correct components • several builds running in parallel • each build can run itself in parallel • Change Request Tracking • link between change requests, solutions, releases • state changes (tested, accepted, released, etc.) • accurate status reports Blanko ‘98 22-23 Oct. 1998

  13. Requirements Space Cont. • Process Support • process support for submit/pickup/build&test/accept • parallel work is the rule • change set tracking • dependency tracking Blanko ‘98 22-23 Oct. 1998

  14. Requirements Space Cont. • Distribution • distributed teams need to be coordinated. • in case of time zone differences: difficult to do SCM informally. • databases must be connected and up-to-date. Blanko ‘98 22-23 Oct. 1998

  15. 3. Version Management • What is covered by Version Management? Software object/item: Any identifiable, machine-readable document. Examples:requirements documents, design documents, specifications, code, documentation, test programs, test data, test results, user manuals, CAD drawings, etc. Integration with CAD/CAM and engineering data management is becoming possible today. Blanko ‘98 22-23 Oct. 1998

  16. Software Objects A software object has a unique name and a persistent body (contents). A software object is immutable: • every change creates a new object with new name; • it cannot be changed once creation process is finished (freezing prevents misidentification). Classification according to • creation: source or derived • structure: atomic or configuration Blanko ‘98 22-23 Oct. 1998

  17. Software Objects Source Object: A manually created software object. Normally “sacred”, i.e., cannot be deleted. Derived Object: a software object generated fully automatically. Can be deleted to make room. Has a history attribute recording exactly how it was generated (input+generator). Blanko ‘98 22-23 Oct. 1998

  18. Structure of Software Objects The body of a software object is either • atomic (there are no parts that can be varied independently, without changing the object as a whole). • a configuration (there are several independently variable or replaceable parts, which are either atomic or themselves configurations). Blanko ‘98 22-23 Oct. 1998

  19. Example Configurations Table top TT1 Instructions:Attach Legs L1 with Bolts B1 to TT1. Legs L1 Bolts B1 Which one is a derived configuration? Blanko ‘98 22-23 Oct. 1998

  20. Two Types of Versions: • Revision: A software object that was created by modifying an existing one. • Variant: Two software objects that have an important property in common (e.g., the functional specification, interface), but differ in others (e.g. user interface, platform, algorithms, data structures, user groups). Blanko ‘98 22-23 Oct. 1998

  21. More on Versions • Both atoms and configurations have versions. • Source objects always have versions (except perhaps bug reports). • Version group: a set of software objects related via the relations revision-of and variant-of. Blanko ‘98 22-23 Oct. 1998

  22. checkout 2.3 2.3 2.3 checkin Organization of Version Groups: Reservation Model Data base (vault) Workspace (sandbox) Fix.1 1.1 1.2 1.3 2.1 2.2 Par.1 Par.2 Revision-of Variant-of Change cycle: 1. checkout (create copy in workspace, reserve template in vault) 2. modify copy in workspace 3. checkin (move workspace copy into template) Blanko ‘98 22-23 Oct. 1998

  23. checkout 2.3 2.3 checkin 2.3’ Optimistic Model Data base (vault) Workspace 1 Fix.1 1.1 1.2 1.3 2.1 2.2 Par.1 Par.2 Revision-of Variant-of Workspace 2 Blanko ‘98 22-23 Oct. 1998

  24. R.1 R.1 L.1 R.2 R.2 L.1 M.2 M.2 R.3 L.2 M.3 M.3 The Branch&Merge Models with reservation without reservation M.1 M.1 Blanko ‘98 22-23 Oct. 1998

  25. GUI.1 Kanji.1 GUI.2 Kanji.2 Kanji.3 Kanji.3 Kanji.4 GUI.3 GUI.3 GUI.5 GUI.4 Kanji.5 Kanji.6 Kanji.6 Distributed Version Control Boston GUI.0 GUI.0 Tokyo Blanko ‘98 22-23 Oct. 1998

  26. checkout 2.3 2.3 2.3 2.2 checkin Using deltas to reduce space Data base (vault) Workspace (sandbox) Fix.1 2.2 1.1 1.2 1.3 2.1 Par.1 Par.2 A delta is a difference between two revisions. Can be computed in forward or backward direction on checkin. Used to regenerate one revision from another. Blanko ‘98 22-23 Oct. 1998

  27. Recent improvements to versioning a. Versioning of arbitrary objects, not just ASCII text • binaries • text processor documents • spreadsheets • CAD files Improvements in delta generators make this possible (also with factor 5 better compression ratio than Unix diff.) Blanko ‘98 22-23 Oct. 1998

  28. Recent improvements to versioning b. Direct vault access, without checkout, for various tools (compile, print, etc.) Possible by intercepting file system calls or with special DLLs. No checkout for tools. c. Hyperlinks to other documents (design, documentation, change requests) d. Graphical user interfaces. e. Version control built into tools (editors). Blanko ‘98 22-23 Oct. 1998

  29. 4. Distributed Version Control Tokyo: Kanji locale Boston: new GUI London: main program Blanko ‘98 22-23 Oct. 1998

  30. Distributed Version Control Geographically separated, cooperative work requires: • remote access to vaults, or • replication of vaults and update propagation. • branch mastership by distributed groups. • merging of parallel branches. Blanko ‘98 22-23 Oct. 1998

  31. Solutions to Distributed Version Control 1. Cross-mounted files systems (LAN); 2. Remote checkout/checkin (client/server), central builds (central model); 3. Remote access plus local caching, local builds (decentralized pull model) 4. Periodic database replication, local builds (decentralized push model); Blanko ‘98 22-23 Oct. 1998

  32. Merging is essential for distributed development • Current merge tools are line based and produce too many false conflicts. • Full semantic merging not practical. • Structural merges (smart merges) in the research stage. (Example: developer A changes identifiers, developer B changes code using the old identifiers. Now what?) Blanko ‘98 22-23 Oct. 1998

  33. 5. Configurations Three major models: a. Composition model (sets of software objects) b. Change set model (bundles of changes) c. Long Transaction Model (all changes are isolated into transactions) Blanko ‘98 22-23 Oct. 1998

  34. a. Composition Model (Parts List) Configuration is a set of software objects (either atoms or other configurations) or version groups. When version groups appear in a configuration, then version selection is needed. Best done with rules. Blanko ‘98 22-23 Oct. 1998

  35. Version Groups Baseline Version Selection Compiler Lexical Analysis Back End Semantic Analysis Syntactic Analysis Optimization Code Generation System Model Blanko ‘98 22-23 Oct. 1998

  36. Version selection by rules 1. Baseline: no versions, no selection needed 2. Developer: select all version checked out by me, + the newest revision on main branch for others. Blanko ‘98 22-23 Oct. 1998

  37. Version selection by rules 3. Cautious Developer: select last baseline + all version checked out by me + all versions checked in by me. Blanko ‘98 22-23 Oct. 1998

  38. Version selection by rules 4. Maintainer: select latest released baseline + all versions checked out by me + newest versions on corrective branches 5. Cautious Maintainer: select latest released baseline + all versions checked out by me + all versions checked in by me Blanko ‘98 22-23 Oct. 1998

  39. Version selection by rules 6. Reconfiguration: select according to 1 through 5 + variants by attribute (e.g. platform=Unix) 7. Time machine: select according to 1 through 6 + ignore everything after a cutoff date. Blanko ‘98 22-23 Oct. 1998

  40. Version selection by rules 8. New Release: select according to 1 through 7 + the newest, stable versions that are associated with a given configuration of change requests. 9. Automatic configuration: select according to 1 through 8, but search for objects whose interfaces fit those of a small set of key software objects. Blanko ‘98 22-23 Oct. 1998

  41. b. Change Set Model A change set is a set of deltas relative to a baseline. A new configuration is selected by specifying a baseline and a change set to be applied. Arbitrary combination of changes is not normally possible, because of change interaction. Blanko ‘98 22-23 Oct. 1998

  42. enh A.enh merge A.f/e fix A.fix fix B.fix B.fix B enh C.enh C.enh C enh fix fix/enh Change Set Model A B C Blanko ‘98 22-23 Oct. 1998

  43. c. Long Transaction Model Each change happens in a long-running transaction, starting with a configuration. Subtransactions are started for explorative work. Versions are not visible except when committing transactions. Transactions generate work spaces that are completely isolated. Blanko ‘98 22-23 Oct. 1998

  44. Long Transaction Model Blanko ‘98 22-23 Oct. 1998

  45. Long Transaction Model • Concurrency control schemes coordinate concurrent transactions. • Locking • Optimistic concurrency control • Long transactions represent workspaces and development paths. • Disadvantage: Concurrency control limits change propagation in the repository. Blanko ‘98 22-23 Oct. 1998

  46. Recent improvements A. Configuration management on the Web • World-wide accessibility • Rich infrastructure • Platform independence • (Anytime, anywhere configuration mgmt.) B. Automatic configuration, starting with a small set of software objects. Blanko ‘98 22-23 Oct. 1998

  47. 6. Change Tracking • Maintain a database of change requests; • Trace their states (submitted, accepted, in progress, in test, accepted, etc.); • Collect change configurations; • Provide a multitude of reports; • Link change requests to solutions. Blanko ‘98 22-23 Oct. 1998

  48. Bug report27 Bug report26 Bug report25 Design Part 1: class C1 { method c11(){ } method c12 (){ } } C1 C2 Design Part 2: D1 D2 Traceability Links Requirements: This system shall…. ………… ……. ………. … .. Blanko ‘98 22-23 Oct. 1998

  49. Change Tracking Solutions • Use a database (relational or special purpose), • Are typically not distributed and permit little customization. • Next generation change trackers use standardized database interface (SQL), so customer can choose the database, even a distributed one. Blanko ‘98 22-23 Oct. 1998

  50. 7. Process Support • Checkout/checkin with reservation model is too primitive for large teams. • Some tool vendors provide a process engine which the customer can program. This is difficult, time consuming and error prone for most production teams. • Ready-made parallel process models are usable, but still somewhat simple-minded. Blanko ‘98 22-23 Oct. 1998

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