Meaning of Leaky Abstraction?

Question

What does the term "Leaky Abstraction" mean? (Please explain with examples. I often have a hard time grokking a mere theory.)

Answer 1

It simply means that your abstraction exposes some of the implementation details. See the wikipedia article.

Answer 2

Wikipedia has a pretty good definition for this

A leaky abstraction refers to any implemented abstraction, intended to reduce (or hide) complexity, where the underlying details are not completely hidden

Or in other words for software it's when you can observe implementation details of a feature via limitations or side effects in the program.

A quick example would be C# / VB.Net closures and their inability to capture ref / out parameters. The reason they cannot be captured is due to an implementation detail of how the lifting process occurs. This is not to say though that there is a better way of doing this.

Answer 3

Well, in a way it is a purely theoretical thing, though not unimportant.

We use abstractions to make things easier to comprehend. I may operate on a string class in some language to hide the fact that I'm dealing with an ordered set of characters that are individual items. I deal with an ordered set of characters to hide the fact that I'm dealing with numbers. I deal with numbers to hide the fact that I'm dealing with 1s and 0s.

A leaky abstraction is one that doesn't hide the details its meant to hide. If call string.Length on a 5-character string in Java or .NET I could get any answer from 5 to 10, because of implementation details where what those languages call characters are really UTF-16 data-points which can represent either 1 or .5 of a character. The abstraction has leaked. Not leaking it though means that finding the length would either require more storage space (to store the real length) or change from being O(1) to O(n) (to work out what the real length is). If I care about the real answer (often you don't really) you need to work on the knowledge of what is really going on.

More debatable cases happen with cases like where a method or property lets you get in at the inner workings, whether they are abstraction leaks, or well-defined ways to move to a lower level of abstraction, can sometimes be a matter people disagree on.

Answer 4

An example in the django ORM many-to-many example:

Notice in the Sample API Usage that you need to .save() the base Article object a1 before you can add Publication objects to the many-to-many attribute. And notice that updating the many-to-many attribute saves to the underlying database immediately, whereas updating a singular attribute is not reflected in the db until the .save() is called.

The abstraction is that we are working with an object graph, where single-value attributes and mult-value attributes are just attributes. But the implementation as a relational database backed data store leaks... as the integrity system of the RDBS appears through the thin veneer of an object interface.

Answer 5

Here's an example familiar to .NET developers: ASP.NET's Page class attempts to hide the details of HTTP operations, particularly the management of form data, so that developers don't have to deal with posted values (because it automatically maps form values to server controls).

But if you wander beyond the most basic usage scenarios the Page abstraction begins to leak and it becomes hard to work with pages unless you understand the class' implementation details.

One common example is dynamically adding controls to a page - the value of dynamically-added controls won't be mapped for you unless you add them at just the right time: before the underlying engine maps the incoming form values to the appropriate controls. When you have to learn that, the abstraction has leaked.

Answer 6

I'll continue in the vein of giving examples by using RPC.

In the ideal world of RPC, a remote procedure call should look like a local procedure call (or so the story goes). It should be completely transparent to the programmer such that when they call SomeObject.someFunction() they have no idea if SomeObject (or just someFunction for that matter) are locally stored and executed or remotely stored and executed. The theory goes that this makes programming simpler.

The reality is different because there's a HUGE difference between making a local function call (even if you're using the world's slowest interpreted language) and:

• calling through a proxy object
• serializing your parameters
• making a network connection (if not already established)
• transmitting the data to the remote proxy
• having the remote proxy restore the data and call the remote function on your behalf
• serializing the return value(s)
• transmitting the return values to the local proxy
• reassembling the serialized data
• returning the response from the remote function

In time alone that's about three orders (or more!) of magnitude difference. Those three+ orders of magnitude are going to make a huge difference in performance that will make your abstraction of a procedure call leak rather obviously the first time you mistakenly treat an RPC as a real function call. Further a real function call, barring serious problems in your code, will have very few failure points outside of implementation bugs. An RPC call has all of the following possible problems that will get slathered on as failure cases over and above what you'd expect from a regular local call:

• you might not be able to instantiate your local proxy
• you might not be able to instantiate your remote proxy
• the proxies may not be able to connect
• the parameters you send may not make it intact or at all
• the return value the remote sends may not make it intact or at all

So now your RPC call which is "just like a local function call" has a whole buttload of extra failure conditions you don't have to contend with when doing local function calls. The abstraction has leaked again, even harder.

In the end RPC is a bad abstraction because it leaks like a sieve at every level -- when successful and when failing both.