The high cost of indirection

 A lot has been written about how complex computers and electronics have become.  I think a lot of this complexity derives from the conflict between a particular engineering principle and a psychological phenomenon.  As far as I know no one has explicitly discussed this phenomenon, so I’ll do it here:


Pete’s Rule: Every layer of indirection is exponentially mind-boggling. 


I think this rule is at least partly responsible for the vastly diminishing returns in productivity that new software technology brings to users and programmers.  At the least, indirection severely inhibits casual new users who are trying to join the online community; and if designers don’t curtail their use of it, indirection could force baby-boomers to give up their cars in a decade or so.

What is a layer of indirection? 

 When you press a button that is labeled with what you are trying to do, like print a document, you may find that pressing it reveals another layer of choices, one of which actually prints the document.  This style of “drill-down” menu is one layer of indirection.

Or consider a much more elaborate example: the Domain Name System (DNS).  In the 70’s and early 80’s, computers on the internet were known by IP address numbers.  There was file called the hosts.txt file that gave the IP addresses of many of the more important computers by name.  If you ran a computer you could get a copy of the global hosts.txt file and even add more names to your copy of it for your own private use.  This was a new level of indirection: instead of using IP addresses, you could use names, which some programs understood as needing to be looked up in the file and converted to addresses.  In the 80’s this system was replaced with the hierarchical Domain Name System.  The old central administration, which used to maintain the global hosts.txt address file, instead started maintaining a global list of addresses of name servers.  The name servers in turn stored localized versions of something like the old hosts.txt file to map from name to IP address.  To get at a particular address book which might contain the name of a computer you wanted to reach, you had to give the name of the domain, like IBM, after the name of the computer, like ‘research1’.  The domains themselves were grouped according to purpose, such as commercial, educational, government, etc.

It’s hard to remember how mind-boggling this was.  People wondered why the name of the computer came first, and then the name of the company, and finally the name of the abstract high-level domain.  Shouldn’t it be the other way around?  And how could companies and schools be counted on to keep their address books in good order?  Wouldn’t the whole system fill up with garbage once it was out of the hands of the central organization?

Instead, the whole system has worked amazingly well.  However, it is far more complex for computers to look up names using DNS instead of a local file.  And now another layer has been added: instead of configuring your computer to know the address of a reliable DNS name server, you computer likely gets the name server’s address from a DHCP server. 

I’m not saying that DNS or DHCP are bad.  I’m just pointing out that they are mind-boggling, and that it had taken quite a while for those of us who lived through the change to adapt to it, including re-writing a whole lot of software and changing how IT organizations ran.  Normally, you don’t still see this discomfort, but if you work in a corporate or consumer software support organization, you know that troubleshooting problems with DNS is beyond the average user.   In this case, the value provided by DNS is worth the complexity to the software developer and end user because it enabled the explosion of internet connectivity.

In any case, DNS is not long for the world, in its current visibly hierarchical form.  It’s becoming less necessary for anybody to know about domain names and hierarchies, especially obscure details like ‘.com’ vs. ‘.net’  Sometime not too long from now, a better method will be developed for finding web sites and sending emails.  Already I am no longer remembering any DNS names because I just use google to go straight to the website I want based on what is at the site, not based on its name.  And for email, I use the auto-generated address book that accumulates in my mailer as people send mail to me.  I don’t know the email addresses for the majority of people I correspond with.  Pretty soon, DNS addresses will be a hidden layer of the internet, the way DNS hid IP addresses in the 90’s. 

Popping the stack


Where was I, before that digression on the evolution of addresses, names, DNS, and the sub-digression on the future of naming vs. searching?  Let’s get back to the thesis: levels of indirection are exponentially mind-boggling.  To take a diversion while you are trying to reach somewhere else is a type of indirection.  To navigate it mentally you need to take note of where you are, tuck it away, deal with the diversion, and then return to where you were and continue on.  In computer science this is a fundamental operation called pushing and then popping the stack.  It is natural for humans and computers do this.  However, computers are WAY better at it.  If a conversation with someone goes off the track by one or two topics you might never get back on track, and if you go three levels deep and then get all the way back your brain will probably react by laughing when it notices what happened.  That’s its way of saying that you’re reaching its limit. 

A computer has no inherent limit to how deep it can go in dealing with sub-tasks.   Programmers, who are usually better at breaking down subtasks than most people, are comfortable when computers do it too.  The problem is that this has a high cost in terms of mental complexity for the end user, because we have to push and pop our mental stacks to keep track of what is going on as we navigate through layers of indirection. The software development community, and increasingly the entire design community, is not paying enough attention to this cost because they personally are comfortable coping with it and because they see how well it solves so many of their problems. 

Where do levels of indirection come from?

How does indirection arise in software?  Indirection lets a designer get around a limitation without utterly redoing the original solution, by adding another layer on top of the original solution.  This might make the system able to handle more operations, or work in new situations that weren’t envisioned earlier.  For example, early file systems kept all the files organized by name, like a big phonebook.  But, like a phone book, the list could get unwieldy, and if the same name appeared more than once it could be confusing.  A solution was to add a layer of indirection.  The directory would contain not the names of files but the names of other directories.  This would let you organize the many filenames into separate compartments, just like having a cabinet full of folders, and the folders full of documents. 

Some of the greatest computer scientists ever eventually recognized that if you can put directories under the master directory there is no reason why you couldn’t put other directories inside of those directories as well.  To anyone reading this article, this idea is obvious, but it is usually acknowledged that Multics in 1969 was the first OS to use the concept.  To me this is proof by example of Pete’s rule: the concept of filename indirection was so mind-boggling that hundreds of software geniuses missed it for years.  And if you still think that hierarchical file systems are obvious and inevitable, try to walk a true novice computer user through the Microsoft Office “Save as” dialog over the phone.   The program will help you by proposing an obscure location to start.  You can then try to explain how to “navigate up and down” through folders and disks.  Or, you can just accept the default, which puts all the documents together into a single directory, like the first operating systems.[pnp1] 

Menus: 20 years from blessing to curse


The trajectory of software menu development shows how levels of indirection can be great at solving technical problems but carry a growing cost that is being ignored.  Everyone would agree that software menus, pioneered by Xerox and Apple in the late 70’s and early 80’s, have hugely empowered the average person to use computers productively.  Instead of being forced to learn and remember the syntax of specific commands and codes, you could look in a menu of commands and select the one that looked like it would do what you wanted to do.  You could begin using a computer productively with very little orientation. 

As the underlying programs became more powerful, more menus were necessary and each menu became bigger.   Where should all these menus and items go?  One solution: menus that contain other menus, known as “walking menus”.  Once programmers made this jump they took it to infinity by nesting menus inside of menus of menus.  Each menu pops up to the right of the parent menu until they reach the far right of the screen, and then … the menus start walking back the other way? 

One problem with this is the dexterity required to control a mouse as you travel down all these paths, without letting the menus disappear, especially when you make the wrong selection somewhere in the process and want to back up partially without starting all over.

The real problem, however, is that underlying complexity of the program is not overcome by placing the operations into hierarchical menus.  A user who wants to change a particular setting in Microsoft Outlook now has to search all the menus not for the setting but for a sub-menu that might contain another menu or set of tabbed panels for controlling the setting.  The search literally grows exponential in size as the depth of the menus and tabbed panels increases, and the brain is correspondingly unable to cope with it.  The underlying exponential growth in alternatives introduced by each layer is what gives rise to Pete’s rule.

What happens is that feature-creep in programs like Word and Outlook outstrips our ability not only to understand or intuit the behavior of the features but even to keep track of them in a depth-first hunt through the trees of settings and panels.  The problem is that our brains can no longer confront such complexity directly by simply looking at all the options at once, but must rely on very limited short-term memory to hunt through an exponential, non-linear forest of options.  Or, we can train ourselves to remember all the relevant options, but this is a costly long-term option.  There seems to be almost no industry competition based on reducing the underlying complexity itself, however.

Instead, features are creeping into other electronic appliances, not just PC software, to the point that the designers are moving to menus and other levels of indirection like shift-keys and mode-toggle switches.  My universal remote control has a 5-way switch to control different devices using the same buttons, plus a blue shift button to activate alternate meanings of the buttons for a given appliance.

Cell-phones have already blossomed with menus and soft-keys that make it harder to use basic features like the phone’s directory.  But what I dread most is the introduction of menus to car electronics.  The temptation is strong, because there just isn’t a lot of room for more direct-action buttons and knobs, and the features keep exploding: climate control, cruise-control, ride-control, seat position, radio, phone, navigation, security, and on and on.  The electrical engineers and designers are finding that once the dash is covered in buttons and displays, each button, knob, and display must do double and triple-duty at least, and menus are the best-known way to do this.

With short-term memory already in heavy demand just to drive a car, how deep can our brains go into these menus while driving?  Probably not more than one level deep.  I know that this sounds like an old man complaining about how hectic modern life is.  I’m sure that children growing up now will master the complexity, even if it means that driving in the future will be like playing an intense video game and we old timers will have to stay on the side roads if we venture out at all.  But I also think that there is opportunity for the electronics and auto industries to compete by developing simpler systems and especially by avoiding levels of indirection in user interfaces of devices that already should be getting as much of our attention to operate as we can give.

Indirection afflicts programmers, not just users

The software community, which is so quick to embrace indirection, is finding it to be increasingly irritating.  Looking at a large program, programmers ask themselves: when is this forsaken code going to actually do something?  When you trace down the calls from a function like write() in C or, worse, Java, it seems like the data never gets written to disk.  At the lowest level, if you can find it, you will see some layers that represent relatively concrete things like the disk hardware registers and the file system.  Those might be hidden under representations of the operating system and, on top of that, things like text streams, data buffers, and ultimately the platonic ideal of “output” itself.

It’s reached the point where new programmers don’t need to spend time learning how to use a programming language to write a particular algorithm.  What they need to learn is how to understand and extend a vast existing software “framework”.  Software frameworks are the current trend in the continuing process of shrouding the functional nuggets in ever more elaborate cocoons.  Frameworks, APIs, toolkits, application servers; they all promise to improve productivity by doing almost all of “the work” for you.  But all this work is mainly just the overhead introduced by the framework itself. 

The terminology of these frameworks: packages, struts, cocoon, foundation classes, etc. suggest things that contain other things or prop them up; not things that actually do anything themselves.  Largely they perform functions that are crucial to the existence of the framework itself, not to the operation of the application using it.

A huge learning curve presented by commercial software environments follows from the growth of the frameworks.  Productivity is not rocketing up the way it should be when the framework “does all the work for you.”  Instead, productivity goes down while you master the framework, and then just when you start to get productive, the framework is revolutionized on you, as it has, for example, with server-side Java about five times in the first 8 years of its existence. 

Pete’s rule could explain why the mental circumscription provided by object-oriented software classes provides relief, but the open-ended, hidden multiple layers of indirection from class inheritance feel more like a burden.

Brooks showed us how the complexity of communications in a team grows quadratically with the size of the team and leads to diminishing and then negative returns when the team grows into the thousands. The exponential complexity growth introduced by adding more layers of indirection will overwhelm an engineer’s ability to understand a system fully in only a dozen layers or so.  And if you are trying to use and learn such a system, even two layers will use up a good chunk of your short-term capacity.

What can be done?


There are at least two ways to avoid boggling the minds of users and programmers with levels of indirection.  Both of these ideas are being explored but they haven’t made much of an impact yet.  The first idea is the harder one: keep it simple.  It’s not impossible.  For example, a small business like a dry cleaner or oil change shop can handle all of its IT needs, including keeping a good size customer database, with a 70’s vintage DOS-style program that has at most two layers, implemented with different screen layouts that you switch in and out of with a button, and a special purpose keyboard template.  Why does a retiree with a PC at home need a hierarchical file system and a professional-style WYSIWYG publishing system in order to read email from her family and write short letters?  

We could also use technology to try to overcome Pete’s Rule.  Just as google eliminates the need to know the web address of any particular site, we could use search to end the need for explicitly naming almost every item in a computer.  We don’t give names to our shoes or clothes.  We shouldn’t have to name everything in our computers.  We also don’t need to define a particular place where every household item is going to stay.  Similarly, every computer item should just be in the computer, not nested into a hierarchical structure, and when we want one, we can look for it. If I can find a particular song lyric on the web using search I should be able to find any paper or picture on my computer the same way.  

Layers of indirection are to software and product design as alcohol is to Homer Simpson: the cause of, and solution to, all of life’s problems.   A professor of mine told me that his professor liked to say that the solution to every problem in computer science has always been another layer of indirection.  It remains just as alluring today.  But now both the public and the engineers are at the point of diminishing returns.  When the answer to your technical problem is obviously another layer of indirection, maybe you have a problem.  Spare yourself, your colleagues, and your users and at least consider taking the harder path of simplifying or even starting over. 


Copyright 2003 Peter Prokopowicz


 [pnp1]Symlinks are another indirection that are still challenging the experts and boggling everyday users.  See Rob Pike's
"Lexical file names in Plan 9: Getting .. right"