principles:law_of_leaky_abstractions
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principles:law_of_leaky_abstractions [2021-09-02 12:45] – old revision restored (2021-05-11 22:02) 65.21.179.175 | principles:law_of_leaky_abstractions [2021-10-18 21:51] (current) – +++ restored +++ christian | ||
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> All non-trivial abstractions, | > All non-trivial abstractions, | ||
- | A solution is bad if | + | A solution is bad if |
- | a) the leakiness of abstractions is ignored (bad usage of an abstraction) or | + | |
- | b) the benefits of the abstraction cannot justify the disadvantages created by its leakiness (bad abstraction) or | + | |
- | c) the abstraction is more leaky than necessary (bad abstraction) | + | |
===== Description ===== | ===== Description ===== | ||
+ | |||
+ | Abstractions are typically not perfect. Especially performance aspects are very hard to abstract away from. So there are cases when using the abstraction properly is not possible without knowing the basics underneath the abstraction. So developing and, even more, fixing defects in a system with leaky abstractions makes it necessary to know about all the details the abstraction is supposed to protect the developers from. Often abstractions reduce the effort to develop a feature while they increase the effort for fixing bugs. | ||
+ | |||
+ | A typical problem with leaky abstractions is that the leakiness is ignored. This does not directly mean that the abstraction itself is bad. Often the situation without the abstraction would be worse so it's good to have it. Nevertheless its unavoidable leaks have to be kept in mind. See [[#Example 1: Distributed Objects|example 1]]. | ||
+ | |||
+ | There are also situations where the abstraction is not crafted well. Sometimes there is a way to make the abstraction better (see [[#Example 2: String Classes In C++|example 2]]) but sometimes the whole abstraction is wrong and not having it would be better. | ||
===== Rationale ===== | ===== Rationale ===== | ||
- | Abstractions | + | |
+ | * Bad usages of abstractions | ||
+ | * Suboptimal abstractions can be made better. There are unnecessary leaks and each leak is a possibility for a future usage fault (see [[Murphy' | ||
+ | * The benefits of an abstraction are only reached at the cost of some liabilities so an abstraction is only good when the liabilities are small enough compared to the benefits. | ||
===== Strategies ===== | ===== Strategies ===== | ||
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Joel Spolsky: // | Joel Spolsky: // | ||
+ | |||
+ | The blog article has another focus as it rather explains LLA as an effect rather than an engineering advice. Because abstractions are leaky, developers should and have to know the details abstractions try to protect them from. The principle aspect is only a side aspect there but the main focus here. | ||
+ | |||
===== Evidence ===== | ===== Evidence ===== | ||
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===== Examples ===== | ===== Examples ===== | ||
- | ==== Example 1: String Classes In C++ ==== | + | ==== Example 1: Distributed Objects ==== |
+ | |||
+ | There is plenty of middleware which centers around the notion of distributed objects: RMI, CORBA, DCOM, ... These technologies abstract away from the fact that the objects are not local but distributed over the network. They create the illusion that calling all objects are local. But all these technologies are leaky abstractions. There is no way to abstract from the fact that calling a remote object may fail. The network connection may break down, the remote machine may not be available, etc. Furthermore there are completely different performance characteristics of remote calls. There is some unavoidable latency and no abstraction what so ever can change this. | ||
+ | |||
+ | This does not mean that these technologies are generally bad. There is a value in these abstractions but the leaks have to be kept in mind. Ignoring the fact that remote calls may fail will result in fragile systems. If the distributed system to develop should be robust, there has to be code handling failing remote calls. And for performance reasons, remote interfaces have to be crafted in a way that remote calls are minimized. So for example [[patterns: | ||
+ | |||
+ | |||
+ | ==== Example 2: String Classes In C++ ==== | ||
In C++ string literals such as ''" | In C++ string literals such as ''" | ||
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(this example is taken from ((Joel Spolsky: // | (this example is taken from ((Joel Spolsky: // | ||
+ | |||
+ | ==== Example 3: List of further examples ==== | ||
+ | |||
+ | * The virtual address space is leaky. Paging leads to a loss of performance. And page faults show up for example while iterating over a two-dimensional array. | ||
+ | * Platform details are leaky. The Java Virtual Machine does a pretty good job abstracting from the underlying operating system. But at least when it comes to accessing files based on file paths, the underlying file system structure leaks through. On Windows you have " | ||
+ | * SQL is leaky. Some queries have a considerably lower performance than others despite being semantically equivalent. | ||
+ | * Network file systems (SMB, NFS, etc) are leaky because they are remote. The connection may break down and the access is much slower than local file access. | ||
+ | * The finiteness of real-world machines leaks. Programming languages are said to be Turing complete. They theoretically are as powerful as a Turing machine. In fact they are not. Turing machines are conceptual automata with infinite memory. But in the real world memory is finite. There can be OutOfMemoryErrors: | ||
===== Description Status ===== | ===== Description Status ===== | ||
/* Choose one of the following and comment out the rest: */ | /* Choose one of the following and comment out the rest: */ | ||
- | [[wiki: | + | /*[[wiki: |
/ | / | ||
- | /*[[wiki: | + | [[wiki: |
===== Further Reading ===== | ===== Further Reading ===== | ||
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Discuss this wiki article and the principle on the corresponding [[talk: | Discuss this wiki article and the principle on the corresponding [[talk: | ||
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principles/law_of_leaky_abstractions.txt · Last modified: 2021-10-18 21:51 by christian