Estimating savings from OSS code reuse, or: where does the money comes from?

We are approaching 100% of usage within other software, that is every software system contains some OSS code inside. Why? There is a perfectly sound reason, and this reason is related to a long standing tenet of software engineering: doing software takes time and money, and code needs to be maintained for a long time- adding additional costs on top. In one of the most widely known article in software engineering (“no silver bullet: essence and accidents of software engineering“), Frederick Brooks exposes some fundamental reasons behind the inherent difficulty of making software, especially large scale software systems. He also coined his law, the “no silver bullet law”:

There is no single development, in either technology or in management technique, that by itself promises even one order of magnitude improvement in productivity, in reliability, in simplicity.

Despite many trials, and many technologies (better languages, OOP, formal methods, automatic programming and many others..) the law has remained true until now. In the same article, however, Brooks marks some potential attacks on the inherent difficulty of making software:

  • buy, don’t build (that is, if possible don’t code at all)
  • requirement refining, rapid prototyping, incremental building
  • great designers

It is quite easy to make a parallel with open source style development, that promotes the same ideas:

  • reuse components and source code from other projects
  • release early/release often (or allow anyone read access to CVS for making their own version)
  • meritocracy (small group of respected core developers, and many smaller contributors)

In the software engineering world the reuse of code coming from the “external” world is commonly called COTS, Commercial Off The Shelf, and has been studied for many years. Boehm and others created a model for mixed development that can be graphically presented as:

As can be seen in the image, there are costs that are related to the integration of COTS (in our case, OSS) within a newly developed product. These costs are related to the evaluation (and searching) of OSS, “tailoring” (the adaptation of the code for the project needs), and development of glue code (the layer of code between OSS modules and between OSS and internally developed code).

I would like to present some results based on the COCOMO II model, adapted to a model where a varying percentage of code is developed or reused from OSS. First of all, some assumptions:

  • The average company cost of a developer is fixed at 25€ per hour. It should be a reasonable approximation of european costs (in particular, costs in mediterranean areas like Spain, France, Italy, Greece); we know that it is considerably lower than other estimates (especially US ones), but this way we provide a “lower bound” for savings instead of averages.
  • The “tailoring” of code is performed on 15% of the OSS code; percentage comes from several separate projects, with estimates ranging from 5% for mature projects with structured and well-documented interfaces to 20% for complex, deeply-interlocked code like that found in embedded systems.
  • Tailoring cost is higher than traditional coding; for this reason, the COCOMO complexity index is increased to 6 compared to new-code development.
  • Volatility is based on our own model for cost estimation and data from literature on COTS (“Empirical observations on COTS software integration effort based on the initial COCOTS calibration database”, Abts C., Boehm B.W., Bailey Clark E.) and it can be approximate with an average effort equivalent to 1.5 to 2.5 full time person-year.

This is the result:

Project size (lines of code) % of OSS total cost (Keuro) Savings duration (years) avg. staffing
100000 0 1703 0% 1.7 20.5
100000 50 975 43% 1.3 15.4
100000 75 487 71% 0.9 8.6
1000000 0 22000 0% 3.3 141.7
1000000 50 12061 45% 2.6 103.2
1000000 75 3012 86% 2 32
10000000 0 295955 0% 7.5 818
10000000 50 160596 46% 5.9 631.2
10000000 75 80845 73% 3.8 421

In the case of 10Mlines of code, the saving is estimated at more than 210M€, that is consistent with previous estimates of savings by Nokia in reusing open source within Maemo. Even for the “small” project of 100000 lines, the savings are estimated at 1.2M€. Another interesting aspect is related to  staffing and time: not only the use of OSS can reduce development time substantially, but it allows for a substantial reduction in the amount of staff necessary for the development. In the smallest example (100000 lines of code, still substantial) the average staffing is reduced from more than 20 developers to slightly less than 9, bringing this project within reach even by small companies, and in my personal view it explains the exceptional take-up of OSS by new and innovative companies, that even before external sources of capital (like VCs) are capable of creating non-trivial projects with very limited resources.

  1. No comments yet.
(will not be published)