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Lehman ‘laws’ of software evolution

Derek Jones from The Shape of Code

The so called Lehman laws of software evolution originated in a 1968 study, and evolved during the 1970s; the book “Program Evolution: processes of software change” by Lehman and Belady was published in 1985.

The original work was based on measurements of OS/360, IBM’s flagship operating system for the computer industries flagship computer. IBM dominated the computer industry from the 1950s, through to the early 1980s; OS/360 was the Microsoft Windows, Android, and iOS of its day (in fact, it had more developer mind share than any of these operating systems).

In its day, the Lehman dataset not only bathed in reflected OS/360 developer mind-share, it was the only public dataset of its kind. But today, this dataset wouldn’t get a second look. Why? Because it contains just 19 measurement points, specifying: release date, number of modules, fraction of modules changed since the last release, number of statements, and number of components (I’m guessing these are high level programs or interfaces). Some of the OS/360 data is plotted in graphs appearing in early papers, and can be extracted; some of the graphs contain 18, rather than 19, points, and some of the values are not consistent between plots (extracted data); in later papers Lehman does point out that no statistical analysis of the data appears in his work (the purpose of the plots appears to be decorative, some papers don’t contain them).

One of Lehman’s early papers says that “… conclusions are based, comes from systems ranging in age from 3 to 10 years and having been made available to users in from ten to over fifty releases.“, but no other details are given. A 1997 paper lists module sizes for 21 releases of a financial transaction system.

Lehman’s ‘laws’ started out as a handful of observations about one very large software development project. Over time ‘laws’ have been added, deleted and modified; the Wikipedia page lists the ‘laws’ from the 1997 paper, Lehman retired from research in 2002.

The Lehman ‘laws’ of software evolution are still widely cited by academic researchers, almost 50-years later. Why is this? The two main reasons are: the ‘laws’ are sufficiently vague that it’s difficult to prove them wrong, and Lehman made a large investment in marketing these ‘laws (e.g., publishing lots of papers discussing these ‘laws’, and supervising PhD students who researched software evolution).

The Lehman ‘laws’ are not useful, in the sense that they cannot be used to make predictions; they apply to large systems that grow steadily (i.e., the kind of systems originally studied), and so don’t apply to some systems, that are completely rewritten. These ‘laws’ are really an indication that software engineering research has been in a state of limbo for many decades.

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Ecosystems as major drivers of software development

Derek Jones from The Shape of Code

During the age of the Algorithm, developers wrote most of the code in their programs. In the age of the Ecosystem, developers make extensive use of code supplied by third-parties.

Software ecosystems are one of the primary drivers of software development.

The early computers were essentially sold as bare metal, with the customer having to write all the software. Having to write all the software was expensive, time-consuming, and created a barrier to more companies using computers (i.e., it was limiting sales). The amount of software that came bundled with a new computer grew over time; the following plot (code+data) shows the amount of code (thousands of instructions) bundled with various IBM computers up to 1968 (an anti-trust case eventually prevented IBM bundling software with its computers):

Instructions contained in IBM computers shipped during the 1960s.

Some tasks performed using computers are common to many computer users, and users soon started to meet together, to share experiences and software. SHARE, founded in 1955, was the first computer user group.

SHARE was one of several nascent ecosystems that formed at the start of the software age, another is the Association for Computing Machinery; a great source of information about the ecosystems existing at the time is COMPUTERS and AUTOMATION.

Until the introduction of the IBM System/360, manufacturers introduced new ranges of computers that were incompatible with their previous range, i.e., existing software did not work.

Compatibility with existing code became a major issue. What had gone before started to have a strong influence on what was commercially viable to do next. Software cultures had come into being and distinct ecosystems were springing up.

A platform is an ecosystem which is primarily controlled by one vendor; Microsoft Windows is the poster child for software ecosystems. Over the years Microsoft has added more and more functionality to Windows, and I don’t know enough to suggest the date when substantial Windows programs substantially depended on third-party code; certainly small apps may be mostly Windows code. The Windows GUI certainly ties developers very closely to a Windows way of doing things (I have had many people tell me that porting to a non-Windows GUI was a lot of work, but then this statement seems to be generally true of porting between different GUIs).

Does Facebook’s support for the writing of simple apps make it a platform? Bill Gates thought not: “A platform is when the economic value of everybody that uses it, exceeds the value of the company that creates it.”, which some have called the Gates line.

The rise of open source has made it viable for substantial language ecosystems to flower, or rather substantial package ecosystems, with each based around a particular language. For practical purposes, language choice is now about the quality and quantity of their ecosystem. The dedicated followers of fashion like to tell everybody about the wonders of Go or Rust (in fashion when I wrote this post), but without a substantial package ecosystem, no language stands a chance of being widely used over the long term.

Major new software ecosystems have been created on a regular basis (regular, as in several per decade), e.g., mainframes in the 1960s, minicomputers and workstation in the 1970s, microcomputers in the 1980s, the Internet in the 1990s, smart phones in the 2000s, the cloud in the 2010s.

Will a major new software ecosystem come into being in the future? Major software ecosystems tend to be hardware driven; is hardware development now basically done, or should we expect something major to come along? A major hardware change requires a major new market to conquer. The smartphone has conquered a large percentage of the world’s population; there is no larger market left to conquer. Now, it’s about filling in the gaps, i.e., lots of niche markets that are still waiting to be exploited.

Software ecosystems are created through lots of people working together, over many years, e.g., the huge number of quality Python packages. Perhaps somebody will emerge who has the skills and charisma needed to get many developers to build a new ecosystem.

Software ecosystems can disappear; I think this may be happening with Perl.

Can a date be put on the start of the age of the Ecosystem? Ideas for defining the start of the age of the Ecosystem include:

  • requiring a huge effort to port programs from one ecosystem to another. It used to be very difficult to port between ecosystems because they were so different (it has always been in vendors’ interests to support unique functionality). Using this method gives an early start date,
  • by the amount of code/functionality in a program derived from third-party packages. In 2018, it’s certainly possible to write a relatively short Python program containing a huge amount of functionality, all thanks to third-party packages. Was this true for any ecosystems in the 1980s, 1990s?

An ecosystems reading list.

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StatsModels: the first nail in R’s coffin

Derek Jones from The Shape of Code

In 2012, when I decided to write a book on evidence-based software engineering, R was the obvious system to use for data analysis. At the time, lots of new books had “using R” or “with R” added at the end of their titles; I chose “using R”.

When developers tell me they need to do some statistical analysis, and ask whether they should use Python or R, I tell them to use Python if statistics is a small part of the program, otherwise use R.

If I started work on the book today, I would till choose R. If I were starting five-years from now, I could be choosing Python.

To understand why I think Python will eventually take over the niche currently occupied by R, we need to understand the unique selling points of both systems.

R’s strengths are that it supports a way of thinking that is a good fit for doing data analysis and has an extensive collection of packages that simplify the task of applying a wide variety of analysis techniques to data.

Python also has packages supporting the commonly used data analysis techniques. But nearly all the Python packages provide a developer-mentality interface (i.e., they provide an API like any other package), R provides data-analysis-mentality interfaces. R supports a way of thinking that data analysts can identify with.

Python’s strengths, over R, are a much larger base of developers and language support for writing large programs (R is really a scripting language). Yes, Python has a package ecosystem supporting the full spectrum of application domains, this is not relevant for analysing a successful invasion of R’s niche market (but it is relevant for enticing new developers who are still making up their mind).

StatsModels is a Python package based around R’s data-analysis-mentality interface. When I discovered this package a few months ago, I realised the first nail had been hammered into R’s coffin.

Yes, today R has nearly all the best statistical analysis packages and a large chunk of the leading edge stuff. But packages can be reimplemented (C code can be copy-pasted, the R code mapped to Python); there is no magic involved. Leading edge has a short shelf life, and what proves to be useful can be duplicated; the market for leading edge code in a mature market (e.g., data analysis) is tiny.

A bunch of bright young academics looking to make a name for themselves will see the major trees in the R forest have been felled. The trees in the Python data-analysis-mentality forest are still standing; all it takes is a few people wanting to be known as the person who implemented the Python package that everybody uses for XYZ analysis.

A collection of packages supporting the commonly (and eventually not so commonly) used data analysis techniques, with a data-analysis-mentality interface, removes a major selling point for using R. Python is a bigger developer market with support for many other application domains.

The flow of developers starting out with R will slow down, casual R users will have nothing to lose from switching when the right project comes along. There will be groups where everybody uses R and will continue to use R because that is what everybody else in the group uses. Ten-Twenty years from now R, developers could be working in a ghost town.

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Evolutionary pressures on C++, Java and Python

Derek Jones from The Shape of Code

The future evolution of C++, Java and Python is being driven by very different interested parties, and it’s going to be interesting watching events unfold over the next 5-10 years.

I have previously written about how the C++ Standard’s committee is past its sell-by date, has taken off its ball and chain and is now in the hands of bored consultants.

Bjarne Stroustrup was once effectively treated as C++’s Benevolent Dictator For Life (during the production of the first C++ Standard some people were labeled as Bjarne groupees); things have moved on since then, but the ‘old-guard’ are trying to make a comeback. Suggesting that people ought to base their thinking on a book published almost 25-years ago (Stroustrup’s “The Design and Evolution of C++”; a very interesting book that is well worth reading) creates a rather backward looking image. Bored consultants are looking to work on exciting new ideas. The old-guard need to appear modern to attract followers (even if the ideas are old ideas with a fresh coat of paint).

The threat to C++ is from bored consultants, each adding their own pet idea to the language standard; a situation that Stroustrup thinks is starting to happen.

Java, the language, is owned by Oracle, the company (let’s not get too involved in exactly what they own, have copyright on, etc). Oracle are not shy about asking people for licensing fees. Java is now on a 6-month release cycle (at least the Oracle version, there are Open Source implementations) and the free support only applies to the current release; paying a license fee buys support for versions older than 6-months. In the short term, the cheapest solution is for companies to pay for support.

Oracle are always happy to send in the lawyers and if too many customers switch to non-Oracle implementations, I’m sure something can be found to introduce enough uncertainty to discourage work/distribution involving Open Source Java implementations.

Will Java survive Oracle’s licensing? It is not in their interest for Java to die; Oracle will adjust their terms to keep the money flowing in, but over the longer term I think willing Java developers are going to be hard to find.

Guido van Rossum recently removed himself from the post of Python’s Benevolent Dictator For Life. One of the jobs of a benevolent dictator is maintaining some degree of language coherence, which involves preventing people’s pet ideas from being added to the language. Does this mean that Python is slowly going to be become more and more bloated? Perhaps, but I think a more likely problem is a language fork, multiple implementations of slightly different (at first) languages all claiming to be Python.

These days, the strength of Python is its large collection of very useful, commercial grade, packages, and future language details may turn out to be irrelevant. There is a lot to learn from the Python 2/3 transition, but true believers like to think that things will turn out differently for them.

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The age of the Algorithm is long gone

Derek Jones from The Shape of Code

I date the age of the Algorithm from roughly the 1960s to the late 1980s.

During the age of the Algorithms, developers spent a lot of time figuring out the best algorithm to use and writing code to implement algorithms.

Knuth’s The Art of Computer Programming (TAOCP) was the book that everybody consulted to find an algorithm to solve their current problem (wafer thin paper, containing tiny handwritten corrections and updates, was glued into the library copies of TAOCP held by my undergraduate university; updates to Knuth was news).

Two developments caused the decline of the age of the Algorithm (and the rise of the age of the Ecosystem and the age of the Platform; topics for future posts).

  • The rise of Open Source (it was not called this for a while), meant it became less and less necessary to spend lots of time dealing with algorithms; an implementation of something that was good enough, was available. TAOCP is something that developers suggest other people read, while they search for a package that does something close enough to what they want.
  • Software systems kept getting larger, driving down the percentage of time developers spent working on algorithms (the bulk of the code in commercially viable systems deals with error handling and the user interface). Algorithms are still essential (like the bolts holding a bridge together), but don’t take up a lot of developer time.

Algorithms are still being invented and some developers spend most of their working with algorithms, but peak Algorithm is long gone.

Perhaps academic researchers in software engineering would do more relevant work if they did not spend so much time studying algorithms. But, as several researchers have told me, algorithms is what people in their own and other departments think computing related research is all about. They remain shackled to the past.