Parallel Changes in Large-Scale Software Development: An Observational Case Study ACM Transactions on Software Engineering and Methodology, Vol. 10, No. 3, July 2001, Pages 308 – 337 Michelle Thomson Agnieszka Jankowska Shiona Webster

Authors Strategy The authors strategy for understanding the problem of parallel changes was to look at the problem from a number of different angles and viewpoints in the context of a large-scale, real-time system and a large-scale development. The authors strategy for understanding the problem of parallel changes was to look at the problem from a number of different angles and viewpoints in the context of a large-scale, real-time system and a large-scale development.

Characteristics An essential characteristic of large-scale software development is parallel development by a team of developers An essential characteristic of large-scale software development is parallel development by a team of developers How this development is structured and supported has a profound effect on both the quality and timeliness of the product How this development is structured and supported has a profound effect on both the quality and timeliness of the product

Why parallel development Release preparation Release preparation Post-release maintenance (segregated from new development) Post-release maintenance (segregated from new development) Tailored or customer-specific software Tailored or customer-specific software Segregation of work by different development teams or individuals Segregation of work by different development teams or individuals Segregation of work on different features Segregation of work on different features Deployment of different software variants into different environments Deployment of different software variants into different environments

Issues Four essential problems in software development: 1) evolution 1) evolution 2) scale 2) scale 3) multiple dimensions of system organisation 3) multiple dimensions of system organisation 4) distribution of knowledge. 4) distribution of knowledge.

Evolution Evolution compounds the problems of parallel development because not only is there parallel development within each version, but also among the releases as well Evolution compounds the problems of parallel development because not only is there parallel development within each version, but also among the releases as well

Scale Scale compounds the problems by increasing the degree of parallel development and therefore increasing both the interactions and interdependencies among the developers Scale compounds the problems by increasing the degree of parallel development and therefore increasing both the interactions and interdependencies among the developers

Multiple dimensions of system organisation Multiple dimensions of system organisation compounds the problems by preventing tidy separations of development into independent work units Multiple dimensions of system organisation compounds the problems by preventing tidy separations of development into independent work units By System organisation, we are looking at hardware and software which make up the product and not the developers organisation By System organisation, we are looking at hardware and software which make up the product and not the developers organisation

Distribution of Knowledge Distribution of knowledge compounds the problem by decreasing the degree of awareness that is distributed Distribution of knowledge compounds the problem by decreasing the degree of awareness that is distributed

File checkin/checkout, branching and merging

Serial development using exclusive checkouts

Branching

Merging

Management of Parallel Changes One of the standard features that enables developers to create parallel versions of the code is the branching mechanism One of the standard features that enables developers to create parallel versions of the code is the branching mechanism Every time a developer needs to create a new version of the code, they request the configuration management system (CMS) to create a new branch Every time a developer needs to create a new version of the code, they request the configuration management system (CMS) to create a new branch The different versions of the code are then all stored in the same physical file The different versions of the code are then all stored in the same physical file

Benefit of CMS in parallel development

Classic vs. Modern Configuration Management Systems In the classic configuration management system the merging process has to be done manually. There are no mechanisms to collapse branches back together In the classic configuration management system the merging process has to be done manually. There are no mechanisms to collapse branches back together Modern configuration system provide mechanisms for automatically merging several versions back together Modern configuration system provide mechanisms for automatically merging several versions back together

About ClearCase: A Software Configuration Management System (SCMS) originally from Rational Software (now IBM/Rational)

Levels of parallel development 5ESS was maintained as a series of releases each offering new features on top of existing ones 5ESS was maintained as a series of releases each offering new features on top of existing ones Releases were being build in parallel with varying amount of overlapping development Releases were being build in parallel with varying amount of overlapping development Features were being developed in parallel (both with a simple release and multiple releases) Features were being developed in parallel (both with a simple release and multiple releases) * - Wikipidia.

The 5ESS change management process Two layer approach ECMS IMR MR SCCS Change management layer Configuration management layer Deltas

Process of implementing an MR Make private copy of file Make private copy of file Try out change Try out change Retrieve original from SCCS – lock for editing Retrieve original from SCCS – lock for editing Makes changes [deltas] in the SCCS – release locks Makes changes [deltas] in the SCCS – release locks Retrieve amended files again from SCCS for reading Retrieve amended files again from SCCS for reading Put code through inspection & testing Put code through inspection & testing Submitted back for load integration Submitted back for load integration MR closed when changes made and approved MR closed when changes made and approved IMR is closed IMR is closed

IMR vs. MR activity

Lucent Technologies subsystem: 5ESS

Facts: Case study data found that: The higher degree of parallelism the higher the number of defects! The higher degree of parallelism the higher the number of defects! 12.5% of all deltas are made to the same file by different developers within the same 24h period Across the subsystems, 3% of the deltas made within 24h by different developers physically overlap each others changes.

Case study observations The parallel development plays a vital role in large- scale software development The parallel development plays a vital role in large- scale software development As study showed that the current tools (2001) available for the level or parallelism involved in large-scale development were inadequate As study showed that the current tools (2001) available for the level or parallelism involved in large-scale development were inadequate The number of defects was proportional to the level of parallelism The number of defects was proportional to the level of parallelism

Team work In parallel development team work plays a vital role to insure good integration of the system In parallel development team work plays a vital role to insure good integration of the system Communication, effort and interaction need to be effectively organised and executed for parallel development effort to succeed Communication, effort and interaction need to be effectively organised and executed for parallel development effort to succeed Team members need be aware of what other team members are doing and why Team members need be aware of what other team members are doing and why

Workflow Organization Microsoft Solution Framework

It is not a question of how well each process works, the question is how well they all work together. Lloyd Dobens

References Dewayne E. Perry et al. ACM Transactions on Software Engineering and Methodology, Vol. 10, No. 3, July 2001, Pages Brad Appleton, Stephen Berczuk, Ralph Cabrera, and Robert Orenstein. Permission is granted to copy for the PLoP '98 conference Tom Demarco, Timothy Lister. Peopleware: Productive Projects and Teams. Dorset House Plastic SCM Platform Parallel Development. Pablo Santos Luaces. Codice Software. February Context switching. contect-switching.html contect-switching.html