Archive for October 2006
Notes on Aspect Oriented Programming: Radical Research in Modularity, by Gregor Kiczales, for Google TechTalks on 16 May 2006.
Pointcut Language and Overall Program Structure
Ableson and Sussman: We have a language when we have primitives, a way to compose those primitives to perform useful work, and a mechanism to abstract and refer to compositions elsewhere.
Pointcut syntax is a little language, with syntax to select a set of join points (primitives), construct logical expressions with several sets (composition), and label an expression (abstraction).
This is the sole advantage of modularity: abstract a set of primitives in one distinct place, and combine them with tools in the language. Using the language, instead of hand-weaving, provides a more structured way to consider how changing the base concern may change the pointcuts.
Dominant model applies crosscutting concerns to join points in base concerns structured as block models within a hierarchy, instead of composing orthogonal sets of crosscutting concerns with base concerns structured as crosscutting concerns.
Procedural, Object, and Aspect Programming Methods
Say we describe the ordinary process of functional/procedural programming as a problem expert (eg, the task at hand) collecting knowledge from other disciplines (secure, optimize, distribute, synchronize, persist, debug/log/trace, quality of service) to apply to the problem. Obviously the programmer can integrate all of this knowledge on a deep level, since all the knowledge appears before the analytic capability of the human mind. However, there is a limit to the size and number of disparate disciplines which can be integrated, before the scale and complexity produces an error rate unacceptable to customers.
Object-oriented programming does not alleviate this situation, since each class must still integrate all the crosscutting concerns with methods, or statements within methods, in the class definition.
Aspect-oriented programming allows a team of specialists to each apply expertise to many basic problems. One person could do everything the team does, but as mentioned can only work on a few very large problems at once, before reaching human limits; whereas a specialist can dispense information as needed across many large systems. This optimizes resources at the cost of the mentioned deep integration of specialist knowledge with other specialist knowledge. That integration is left up to the resource manager (the weaver), who schedules and reconciles the contributions of the specialists and the basic problem’s manager. The basic problem’s manager, in aspect-oriented programming, can represent the customer, verify correctness of the integration, and validate the integrated solution.
Consider Pointcuts in Aspects, and Pointcut Specifiers in Base Concerns, When Changing Base Concerns
We must be careful, when editing base concerns, to not break pointcuts; or, breaking them, to remember to update the pointcut. This is less likely to cause error than the original method of weaving crosscutting concerns into the base concerns by hand.
With respect to my weaver for shell-scripts and makefiles, I am hesitant to use explicit comments in the base scripts as search patterns for the sed scripts. However, these make a contract with any transforming program: we can consider these points reliable enough to anchor transforms. Without this, especially if the search patterns refer to code statements, additional pointcuts may not find the points they specify still in the program by the time the weaver gets to them (but at least they will fail-safe and apply no advice).
The base concern’s comments should only refer to the base concern, not any crosscutting concern, and should certainly not say “at this point, insert that statement from the other crosscutting concern.” Those kinds of comments defeat the whole purpose of modularization, and do not recognize that we may eventually weave completely different crosscutting concerns into the base concern.
We should not try to apply a single comment across many base concerns, to reduce the effort to write the join point specifier. This binds, in a single relation, all the base concerns, and all the crosscutting concerns which use the comment. If the advice changes for half the base concerns, then half the instances of the single comment must change. If instead the pointcut is a combination of several comments, then we more easily split off a subset of these into a new pointcut, to receive new advice.
The Weaving-Loom Analogy
weave applies one crosscutting concern to one base concern. The variation lace customizes the base concern for an application (eg, by laying sed changes over the file).
loom weaves many crosscutting concerns into many base concerns, adding each crosscutting concern to the base concern as modified by previous crosscutting concerns. Under user control, the loom orders crosscutting concerns applied to each base concern. User control is essential since the user writes the crosscutting concerns, and knows which crosscutting concerns apply to other crosscutting concerns, so should weave last.
warp finds crosscutting concerns which modify a given base concern. It also finds crosscutting concerns and advice which modify a given base concern’s given code statement.
weft finds base concerns modified by a given crosscutting concern. It also finds base concerns’ code statements modified by a given crosscutting concern’s given advice. woof is already taken as a s(n)ide-acronym for another function in my build system, and is slightly silly anyway, although it does have the advantage of turning loom upside-down and backward.
Weaving New Aspects into Reused Legacy Code
AOP removes all code for crosscutting concerns from base concerns. This is better than macros or libraries, which require at least calls in the base concern. The base concern can be completely oblivious to crosscutting concerns, which allows us to weave unanticipated crosscutting concerns as easily as we add the ones we had in mind when we wrote the base concern.
This is key to reusability, and in fact aspects should unlock the entire previous pre-aspect code base to modification, without editing the current working, tested, and delivered code files. I don’t believe in silver bullets, but the separation of concerns here is the first opportunity I’ve seen to keep the old code separate from the new code, yet use them together in a new program, while easily incorporating any further changes to the old code.
For new code, aspects promote writing code for reuse, since each concern is a separate file, and can be bundled and released separately. Reuse is available for base concerns since they do not refer to crosscutting concerns (low coupling), and completely implement one concern of the program (high cohesion). Reuse is easier for crosscutting concerns because: (1) if the aspect has no join point to match, it fails safe and applies no advice to the base concern; and (2) the advice code refers only to methods and fields exposed by the pointcut, which hides (low coupling) the remainder of the base concern.
Integrating Proprietary and Open-Source Code
This method could be very useful in incorporating open-source code into proprietary codebases. If we can show that we did not directly modify the original open-source code, then we may not need to release our proprietary changes to the open-source code.
Before compilation, the reused open-source code is in its files with its copyrights, in exactly the same state as when downloaded. Proprietary code is in its files with its copyrights. Since we must redistribute the open-source code files with any modifications to those files (the files under copyright), we just redistribute the original, unmodified open-source files, since they were not changed.
I am not a lawyer, so this analysis could be way off: specifically, we could still consider the distributed, weaved version a derived work, which means that all code that went into producing it must be distributed. Definitely must speak to an IP lawyer before basing any decisions on this.
Dynamic Join Points
Since I’m writing a weaver, I should consider join points other than code statements (including comments) in shell scripts and makefiles. These implement a static join point model, which change the code before it runs. Dynamic join point models create advice which executes only upon certain runtime behavior: for example, if a sub-script or particular makefile is called with certain parameters or targets, or a certain shell variable or makefile macro receives a particular value.
We could of course explicitly write more complicated advice to implement this. But it would be simpler to write the advice if the pointcut matched the runtime state of the program. To do this, we could either (1) monitor the execution state at runtime (better suited to modeling languages) or (2) extend the pointcut language to describe a runtime state. With (2), the weaver translates the runtime state description into static code to detect the state and apply the simpler advice.
Since the crosscutting concern in my implementation “owns” the final executable form, I can prototype new changes to the base concern and affect only the product or platform which needs the change. Once it’s proven, I can move any new variables or code in the interface back to the base concern, to become a default part of the interface for other products or platforms. At this point, I may need to write advice to change the interface for products and platforms which must differently define its values.
We can extend this thinking to consider prototyping itself a crosscutting concern, and maintain several prototypes concurrently, weaving them in according to control options. Of course, this begs me to make control options crosscutting concerns.
Effect on Software Configuration Management
AOP introduces the weaving step between code checkin and compilation. For C/C++, this step constructs woven header and source code files by combining (1) base header and source code files with (2) header and source code file content in advice gathered in files which encapsulate each crosscutting concern. The order of application of crosscutting concerns should be specified in the target makefile.
Alternatively, we could make the Aspect C and C++ compilers available as alternative platforms (eg, *_aspect), and let the compilers sort it out. The caveat here is that I haven’t used one of these languages yet. I do know that they would integrate most cleanly if they were actually preprocessors which created new *.c/*.cpp (C/C++ source code) or *.i/*.ii (preprocessed C/C++ source code) files suitable for compilation with the build system’s cross-compilers.
The only problem with Microsoft is they just have no taste, they have absolutely no taste, and what that means is – I don’t mean that in a small way I mean that in a big way. In the sense that they they don’t think of original ideas and they don’t bring much culture into their product ehm and you say why is that important – well you know proportionally spaced fonts come from type setting and beautiful books, that’s where one gets the idea – if it weren’t for the Mac they would never have that in their products and ehm so I guess I am saddened, not by Microsoft’s success – I have no problem with their success, they’ve earned their success for the most part. I have a problem with the fact that they just make really third rate products.