There's a key piece of magic in the engineering of the Internet which you rely on every single day. It happens in the TCP protocol, one of the fundamental building blocks of the Internet.
TCP is a way to transmit data that is reliable. By this I mean: if you send a message over a network using TCP, it will arrive, and it won't be garbled or corrupted.
By comparison, there is another method of transmitting data called IP which is unreliable. Nobody promises that your data will arrive, and it might get messed up before it arrives. If you send a bunch of messages with IP, don't be surprised if only half of them arrive, and some of those are in a different order than the order in which they were sent, and some of them have been replaced by alternate messages [...]
Here's the magic part: TCP is built on top of IP. In other words, TCP is obliged to somehow send data reliably using only an unreliable tool.
It is what computer scientists like to call an abstraction: a simplification of something much more complicated that is going on under the covers. As it turns out, a lot of computer programming consists of building abstractions. What is a string library? It's a way to pretend that computers can manipulate strings just as easily as they can manipulate numbers. What is a file system? It's a way to pretend that a hard drive isn't really a bunch of spinning magnetic platters that can store bits at certain locations, but rather a hierarchical system of folders-within-folders containing individual files that in turn consist of one or more strings of bytes.
Back to TCP. Earlier for the sake of simplicity I told a little fib, and some of you have steam coming out of your ears by now because this fib is driving you crazy. I said that TCP guarantees that your message will arrive. It doesn't, actually. If your pet snake has chewed through the network cable leading to your computer, and no IP packets can get through, then TCP can't do anything about it and your message doesn't arrive. If you were curt with the system administrators in your company and they punished you by plugging you into an overloaded hub, only some of your IP packets will get through, and TCP will work, but everything will be really slow.
This is what I call a leaky abstraction. TCP attempts to provide a complete abstraction of an underlying unreliable network, but sometimes, the network leaks through the abstraction and you feel the things that the abstraction can't quite protect you from. This is but one example of what I've dubbed the Law of Leaky Abstractions:
All non-trivial abstractions, to some degree, are leaky.
Abstractions fail. Sometimes a little, sometimes a lot. There's leakage. Things go wrong. It happens all over the place when you have abstractions. Here are some examples.
Something as simple as iterating over a large two-dimensional array can have radically different performance if you do it horizontally rather than vertically, depending on the "grain of the wood" -- one direction may result in vastly more page faults than the other direction, and page faults are slow. Even assembly programmers are supposed to be allowed to pretend that they have a big flat address space, but virtual memory means it's really just an abstraction, which leaks when there's a page fault and certain memory fetches take way more nanoseconds than other memory fetches.
The SQL language is meant to abstract away the procedural steps that are needed to query a database, instead allowing you to define merely what you want and let the database figure out the procedural steps to query it. But in some cases, certain SQL queries are thousands of times slower than other logically equivalent queries. A famous example of this is that some SQL servers are dramatically faster if you specify "where a=b and b=c and a=c" than if you only specify "where a=b and b=c" even though the result set is the same. You're not supposed to have to care about the procedure, only the specification. But sometimes the abstraction leaks and causes horrible performance and you have to break out the query plan analyzer and study what it did wrong, and figure out how to make your query run faster.
Even though network libraries like NFS and SMB let you treat files on remote machines "as if" they were local, sometimes the connection becomes very slow or goes down, and the file stops acting like it was local, and as a programmer you have to write code to deal with this. The abstraction of "remote file is the same as local file" leaks. Here's a concrete example for Unix sysadmins. If you put users' home directories on NFS-mounted drives (one abstraction), and your users create .forward files to forward all their email somewhere else (another abstraction), and the NFS server goes down while new email is arriving, the messages will not be forwarded because the .forward file will not be found. The leak in the abstraction actually caused a few messages to be dropped on the floor.
C++ string classes are supposed to let you pretend that strings are first-class data. They try to abstract away the fact that strings are hard and let you act as if they were as easy as integer. Almost all C++ string classes overload the + operator so you can write s + "bar" to concatenate. But you know what? No matter how hard they try, there is no C++ string class on Earth that will let you type "foo" + "bar", because string literals in C++ are always char*'s, never strings. The abstraction has sprung a leak that the language doesn't let you plug. (Amusingly, the history of the evolution of C++ over time can be described as a history of trying to plug the leaks in the string abstraction. Why they couldn't just add a native string class to the language itself eludes me at the moment.)
And you can't drive as fast when it's raining, even though your car has windshield wipers and headlights and a roof and a heater, all of which protect you from caring about the fact that it's raining (they abstract away the weather), but lo, you have to worry about hydroplaning (or aquaplaning in England) and sometimes the rain is so strong you can't see very far ahead so you go slower in the rain, because the weather can never be completely abstracted away, because of the law of leaky abstractions.
One reason the law of leaky abstractions is problematic is that it means that abstractions do not really simplify our lives as much as they were meant to.
The law of leaky abstractions means that whenever somebody comes up with a wizzy new code-generation tool that is supposed to make us all ever-so-efficient, you hear a lot of people saying "learn how to do it manually first, then use the wizzy tool to save time." Code generation tools which pretend to abstract out something, like all abstractions, leak, and the only way to deal with the leaks competently is to learn about how the abstractions work and what they are abstracting. So the abstractions save us time working, but they don't save us time learning.
-- Joel Spolsky
Quoted on Sat Sep 7th, 2013