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I was reading an interesting blog post about Erlang/OTP and the actor model. I also hear that Scala supports the actor model. From the little I gathered so far, the actor model breaks down processing into components that communicate with each other by passing messages. Typically, those processes are immutable.
Are those features language-specific though or more at the architecture level? more specifically, can't you just implement the same actor model in almost any language, and just use some form of message-queue to pass messages between worker processes? (for example, use something like celery). Or is it that those languages like Erlang and Scala simply do this transparently and much faster?
Certainly you can define an "Actor Library" in virtually any language, but in Erlang the model is baked-in to the language, and is really the only concurrency model available.
While Scala's actors system is well implemented, at the end of the day, it still vulnerable to some hazards that Erlang is immune from. I'll draw your attention to this paper.
This would be the case for any Actor library implemented in any imperative language that supports shared mutable state.
An interesting exception to this is Nodes.js. Some work is being done with actors between Nodes that probably exhibit the same isolation properties as Erlang, simply because there is no shared mutable state.
Actor model is not limited to any specific platform or programming language, it's just a model after all.
Erlang and Scala have really good and useful implementations of this model, which fits nicely in typical technology stack of these platforms and helps to effectively solve certain kinds of tasks.
To add to the points mentioned above, the fact that in Erlang actor model is the only way you can program, makes your code scalable from the get-go. Erlang processes are lightweight, and you can spawn 10-100K on one machine (I don't think you can do it with python), this changes the way you approach problems. For example, in our product we parse web server logs with Erlang and spawn an Erlang process to handle each line. That way, if one log line is corrupted, or the process that handles it crashes, nothing happens to the other ones.
Another difference is when you start using OTP you get processes supervisors and you can make processes connected so if one terminates all the others do.
Other than that, Erlang has some other nice feature (which can be found in other languages through libraries, but again here it's baked in) like pattern matching and hot deploy.
No, there is nothing language-specific about the Actor Model. In fact, you already mention Scala in your question, where actors are not part of the language but are instead implemented as a library. (Three competing libraries, actually.)
However, just like Functional Programming or Object-Oriented Programming, having direct support for Actor Programming, or at least support for some abstractions that make it easier to implement, in the language will lead to a very different programming experience. Anyone who has ever done Functional Programming or Object-Oriented Programming in C will probably understand this.
This has baffled me for a long time.
Given basic atomic primitives like compare & swap, I can see how to implement a spin lock (from which I can build mutexes).
However, I don't see how I can build condition variables out of this. How is this done?
It's not particularly simple. The following is a link to a paper by Douglas Schmidt (who is also largely responsible for the ACE libraries) that details several approaches for implementing condition variables on Windows using the synchronization primitives available in Win32 (pre-Vista). The approaches include using only the basic, generally available on any OS primitives, and discusses the various limitations of the approaches:
http://www.cs.wustl.edu/~schmidt/win32-cv-1.html
The bottom line (concluding remarks):
This article illustrates why developing condition variables on Win32 platforms is tricky and error-prone. There are several subtle design forces that must be addressed by developers. In general, the different implementations we've examined vary according to their correctness, efficiency, fairness, and portability. No one solution provides all these qualities optimally.
The SignalObjectsAndWait solution in Section 3.4 is a good approach if fairness is paramount. However, this approach is not as efficient as other solutions, nor is it as portable. Therefore, if efficiency or portability are more important than fairness, the SetEvent approach described in Section 3.2 may be more suitable. Naturally, the easiest solution would be for Microsoft to simply provide condition variables in the Win32 API.
Note that starting in Vista, Windows supports condition variables using native APIs:
http://msdn.microsoft.com/en-us/library/ms686903.aspx
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Inspired after watching Michael Feather's SCNA talk "Self-Education and the Craftsman", I am interested to hear about practical examples in software development where discrete mathematics have proved helpful.
Discrete math has touched every aspect of software development, as software development is based on computer science at its core.
http://en.wikipedia.org/wiki/Discrete_math
Read that link. You will see that there are numerous practical applications, although this wikipedia entry speaks mainly in theoretical terms.
Techniques I learned in my discrete math course from university helped me quite a bit with the Professor Layton games.
That counts as helpful... right?
There are a lot of real-life examples where map coloring algorithms are helpful, besides just for coloring maps. The question on my final exam had to do with traffic light programming on a six-way intersection.
As San Jacinto indicates, the fundamentals of programming are very much bound up in discrete mathematics. Moreover, 'discrete mathematics' is a very broad term. These things perhaps make it harder to pick out particular examples. I can come up with a handful, but there are many, many others.
Compiler implementation is a good source of examples: obviously there's automata / formal language theory in there; register allocation can be expressed in terms of graph colouring; the classic data flow analyses used in optimizing compilers can be expressed in terms of functions on lattice-like algebraic structures.
A simple example the use of directed graphs is in a build system that takes the dependencies involved in individual tasks by performing a topological sort. I suspect that if you tried to solve this problem without having the concept of a directed graph then you'd probably end up trying to track the dependencies all the way through the build with fiddly book-keeping code (and then finding that your handling of cyclic dependencies was less than elegant).
Clearly most programmers don't write their own optimizing compilers or build systems, so I'll pick an example from my own experience. There is a company that provides road data for satnav systems. They wanted automatic integrity checks on their data, one of which was that the network should all be connected up, i.e. it should be possible to get to anywhere from any starting point. Checking the data by trying to find routes between all pairs of positions would be impractical. However, it is possible to derive a directed graph from the road network data (in such a way as it encodes stuff like turning restrictions, etc) such that the problem is reduced to finding the strongly connected components of the graph - a standard graph-theoretic concept which is solved by an efficient algorithm.
I've been taking a course on software testing, and 3 of the lectures were dedicated to reviewing discrete mathematics, in relation to testing. Thinking about test plans in those terms seems to really help make testing more effective.
Understanding of set theory in particular is especially important for database development.
I'm sure there are numerous other applications, but those are two that come to mind here.
Just example of one of many many...
In build systems it's popular to use topological sorting of jobs to do.
By build system I mean any system where we have to manage jobs with dependency relation.
It can be compiling program, generating document, building building, organizing conference - so there is application in task management tools, collaboration tools etc.
I believe testing itself properly procedes from modus tollens, a concept of propositional logic (and hence discrete math), modus tollens being:
P=>Q. !Q, therefore !P.
If you plug in "If the feature is working properly, the test will pass" for P=>Q, and then take !Q as given ("the test did not pass"), then, if all these statements are factually correct, you have a valid, sound basis for returning the feature for a fix. By contrast, many, maybe most testers operate by the principle:
"If the program is working properly, the test will pass. The test passed, therefore the program is working properly."
This can be written as: P=>Q. Q, therefore P.
But this is the fallacy of "affirming the consequent" and does not show what the tester believes it shows. That is, they mistakenly believe that the feature has been "validated" and can be shipped. When Q is given, P may in fact either be true or it may be untrue for P=>Q, and this can be shown with a truth table.
Modus tollens is core to Karl Popper's notion of science as falsification, and testing should proceed in much the same way. We're attempting to falsify the claim that the feature always works under every explicit and implicit circumstance, rather than attempting to verify that it works in the narrow sense that it can work in some proscribed way.
The Pragmatic Programmer advocates the use of code generators.
Do you create code generators on your projects? If yes, what do you use them for?
In "Pragmatic Programmer" Hunt and Thomas distinguish between Passive and Active code generators.
Passive generators are run-once, after which you edit the result.
Active generators are run as often as desired, and you should never edit the result because it will be replaced.
IMO, the latter are much more valuable because they approach the DRY (don't-repeat-yourself) principle.
If the input information to your program can be split into two parts, the part that changes seldom (A) (like metadata or a DSL), and the part that is different each time the program is run (B)(the live input), you can write a generator program that takes only A as input, and writes out an ad-hoc program that only takes B as input.
(Another name for this is partial evaluation.)
The generator program is simpler because it only has to wade through input A, not A and B. Also, it does not have to be fast because it is not run often, and it doesn't have to care about memory leaks.
The ad-hoc program is faster because it's not having to wade through input that is almost always the same (A). It is simpler because it only has to make decisions about input B, not A and B.
It's a good idea for the generated ad-hoc program to be quite readable, so you can more easily find any errors in it. Once you get the errors removed from the generator, they are gone forever.
In one project I worked on, a team designed a complex database application with a design spec two inches thick and a lengthy implementation schedule, fraught with concerns about performance. By writing a code generator, two people did the job in three months, and the source code listings (in C) were about a half-inch thick, and the generated code was so fast as to not be an issue. The ad-hoc program was regenerated weekly, at trivial cost.
So active code generation, when you can use it, is a win-win. And, I think it's no accident that this is exactly what compilers do.
Code generators if used widely without correct argumentation make code less understandable and decrease maintainability (the same with dynamic SQL by the way). Personally I'm using it with some of ORM tools, because their usage here mostly obvious and sometimes for things like searcher-parser algorithms and grammatic analyzers which are not designed to be maintained "by hands" lately. Cheers.
In hardware design, it's fairly common practice to do this at several levels of the 'stack'. For instance, I wrote a code generator to emit Verilog for various widths, topologies, and structures of DMA engines and crossbar switches, because the constructs needed to express this parameterization weren't yet mature in the synthesis and simulation tool flows.
It's also routine to emit logical models all the way down to layout data for very regular things that can be expressed and generated algorithmically, like SRAM, cache, and register file structures.
I also spent a fair bit of time writing, essentially, a code generator that would take an XML description of all the registers on a System-on-Chip, and emit HTML (yes, yes, I know about XSLT, I just found emitting it programatically to be more time-effective), Verilog, SystemVerilog, C, Assembly etc. "views" of that data for different teams (front-end and back-end ASIC design, firmware, documentation, etc.) to use (and keep them consistent by virtue of this single XML "codebase"). Does that count?
People also like to write code generators for e.g. taking terse descriptions of very common things, like finite state machines, and mechanically outputting more verbose imperative language code to implement them efficiently (e.g. transition tables and traversal code).
We use code generators for generating data entity classes, database objects (like triggers, stored procs), service proxies etc. Anywhere you see lot of repititive code following a pattern and lot of manual work involved, code generators can help. But, you should not use it too much to the extend that maintainability is a pain. Some issues also arise if you want to regenerate them.
Tools like Visual Studio, Codesmith have their own templates for most of the common tasks and make this process easier. But, it is easy to roll out on your own.
It is often useful to create a code generator that generates code from a specification - usually one that has regular tabular rules. It reduces the chance of introducing an error via a typo or omission.
Yes ,
I developed my own code generator for AAA protocol Diameter (RFC 3588).
It could generate structures and Api's for diameter messages reading from an XML file that described diameter application's grammar.
That greatly reduced the time to develop complete diameter interface (such as SH/CX/RO etc.).
in my opinion a good programming language would not need code generators because introspection and runtime code generation would be part of language e.g. in python metaclasses and new module etc.
code generators usually generate more unmanageable code in long term usage.
however, if it is absolutely imperative to use a code generator (eclipse VE for swing development is what I use at times) then make sure you know what code is being generated. Believe me, you wouldn't want code in your application that you are not familiar with.
Writing own generator for project is not efficient. Instead, use a generator such as T4, CodeSmith and Zontroy.
T4 is more complex and you need to know a .Net programming language. You have to write your template line by line and you have to complete data relational operations on your own. You can use it over Visual Studio.
CodeSmith is an functional tool and there are plenty of templates ready to use. It is based on T4 and writing your own temlate takes too much time as it is in T4. There is a trial and a commercial version.
Zontroy is a new tool with a user friendly user interface. It has its own template language and is easy to learn. There is an online template market and it is developing. Even you can deliver templates and sell them online over market.
It has a free and a commercial version. Even the free version is enough to complete a medium-scale project.
there might be a lot of code generators out there , however I always create my own to make the code more understandable and suit the frameworks and guidelines we are using
We use a generator for all new code to help ensure that coding standards are followed.
We recently replaced our in-house C++ generator with CodeSmith. We still have to create the templates for the tool, but it seems ideal to not have to maintain the tool ourselves.
My most recent need for a generator was a project that read data from hardware and ultimately posted it to a 'dashboard' UI. In-between were models, properties, presenters, events, interfaces, flags, etc. for several data points. I worked up the framework for a couple data points until I was satisfied that I could live with the design. Then, with the help of some carefully placed comments, I put the "generation" in a visual studio macro, tweaked and cleaned the macro, added the datapoints to a function in the macro to call the generation - and saved several tedious hours (days?) in the end.
Don't underestimate the power of macros :)
I am also now trying to get my head around CodeRush customization capabilities to help me with some more local generation requirements. There is powerful stuff in there if you need on-the-fly decision making when generating a code block.
I have my own code generator that I run against SQL tables. It generates the SQL procedures to access the data, the data access layer and the business logic. It has done wonders in standardising my code and naming conventions. Because it expects certain fields in the database tables (such as an id column and updated datetime column) it has also helped standardise my data design.
How many are you looking for? I've created two major ones and numerous minor ones. The first of the major ones allowed me to generate programs 1500 line programs (give or take) that had a strong family resemblance but were attuned to the different tables in a database - and to do that fast, and reliably.
The downside of a code generator is that if there's a bug in the code generated (because the template contains a bug), then there's a lot of fixing to do.
However, for languages or systems where there is a lot of near-repetitious coding to be done, a good (enough) code generator is a boon (and more of a boon than a 'doggle').
In embedded systems, sometimes you need a big block of binary data in the flash. For example, I have one that takes a text file containing bitmap font glyphs and turns it into a .cc/.h file pair declaring interesting constants (such as first character, last character, character width and height) and then the actual data as a large static const uint8_t[].
Trying to do such a thing in C++ itself, so the font data would auto-generate on compilation without a first pass, would be a pain and most likely illegible. Writing a .o file by hand is out of the question. So is breaking out graph paper, hand encoding to binary, and typing all that in.
IMHO, this kind of thing is what code generators are for. Never forget that the computer works for you, not the other way around.
BTW, if you use a generator, always always always include some lines such as this at both the start and end of each generated file:
// This code was automatically generated from Font_foo.txt. DO NOT EDIT THIS FILE.
// If there's a bug, fix the font text file or the generator program, not this file.
Yes I've had to maintain a few. CORBA or some other object communication style of interface is probably the general thing that I think of first. You have object definitions that are provided to you by the interface you are going to talk over but you still have to build those objects up in code. Building and running a code generator is a fairly routine way of doing that. This can become a fairly lengthy compile just to support some legacy communication channel, and since there is a large tendency to put wrappers around CORBA to make it simpler, well things just get worse.
In general if you have a large amount of structures, or just rapidly changing structures that you need to use, but you can't handle the performance hit of building objects through metadata, then your into writing a code generator.
I can't think of any projects where we needed to create our own code generators from scratch but there are several where we used preexisting generators. (I have used both Antlr and the Eclipse Modeling Framework for building parsers and models in java for enterprise software.) The beauty of using a code generator that someone else has written is that the authors tend to be experts in that area and have solved problems that I didn't even know existed yet. This saves me time and frustration.
So even though I might be able to write code that solves the problem at hand, I can generate the code a lot faster and there is a good chance that it will be less buggy than anything I write.
If you're not going to write the code, are you going to be comfortable with someone else's generated code?
Is it cheaper in both time and $$$ in the long run to write your own code or code generator?
I wrote a code generator that would build 100's of classes (java) that would output XML data from database in a DTD or schema compliant manner. The code generation was generally a one time thing and the code would then be smartened up with various business rules etc. The output was for a rather pedantic bank.
Code generators are work-around for programming language limitations. I personally prefer reflection instead of code generators but I agree that code generators are more flexible and resulting code obviously faster during runtime. I hope, future versions of C# will include some kind of DSL environment.
The only code generators that I use are webservice parsers. I personally stay away from code generators because of the maintenance problems for new employees or a separate team after hand off.
I write my own code generators, mainly in T-SQL, which are called during the build process.
Based on meta-model data, they generate triggers, logging, C# const declarations, INSERT/UPDATE statements, data model information to check whether the app is running on the expected database schema.
I still need to write a forms generator for increased productivity, more specs and less coding ;)
I've created a few code generators. I had a passive code generator for SQL Stored procedures which used templates. This generated generated 90% of our stored procedures.
Since we made the switch to Entity Framework I've created an active codegenerator using T4 (Text Template Transformation Toolkit) inside visual studio. I've used it to create basic repository partial classes for our entities. Works very nicely and saves a bunch of coding. I also use T4 for decorating the entity classes with certain Attributes.
I use code generation features provided by EMF - Eclipse Modeling Framework.
Code generators are really useful in many cases, especially when mapping from one format to another. I've done code generators for IDL to C++, database tables to OO types, and marshalling code just to name a few.
I think the point the authors are trying to make is that if you're a developer you should be able to make the computer work for you. Generating code is just one obvious task to automate.
I once worked with a guy who insisted that he would do our IDL to C++ mapping manually. In the beginning of the project he was able to keep up, because the rest of us were trying to figure out what to do, but eventually he became a bottleneck. I did a code generator in Perl and then we could pretty much do his "work" in a few minutes.
See our "universal" code generator based on program transformations.
I'm the architect and a key implementer.
It is worth noting that a significant fraction of this generator, is generated using this generator.
We uses Telosys code generator in our projects : http://www.telosys.org/
We have created it to reduce the development duration in recurrent tasks like CRUD screens, documentation, etc...
For us the most important thing is to be able to customize the generator's templates, in order to create new generation targets if necessary and to customize existing templates. That's why we have also created a template editor (for Velocity .vm files).
It works fine for Java/Spring/AngularJS code generator and can be adapt for other targets (PHP, C#, Python, etc )
An initial draft of requirements specification has been completed and now it is time to take stock of requirements, review the specification. Part of this process is to make sure that there are no sizeable gaps in the specification. Needless to say that the gaps lead to highly inaccurate estimates, inevitable scope creep later in the project and ultimately to a death march.
What are the good, efficient techniques for pinpointing missing and implicit requirements?
This question is about practical techiniques, not general advice, principles or guidelines.
Missing requirements is anything crucial for completeness of the product or service but not thought of or forgotten about,
Implicit requirements are something that users or customers naturally assume is going to be a standard part of the software without having to be explicitly asked for.
I am happy to re-visit accepted answer, as long as someone submits better, more comprehensive solution.
Continued, frequent, frank, and two-way communication with the customer strikes me as the main 'technique' as far as I'm concerned.
It depends.
It depends on whether you're being paid to deliver what you said you'd deliver or to deliver high quality software to the client.
If the former, simply eliminate ambiguity from the specifications and then build what you agreed to. Try to stay away from anything not measurable (like "fast", "cool", "snappy", etc...).
If the latter, what Galwegian said + time or simply cut everything not absolutely drop-dead critical and build that as quickly as you can. Production has a remarkable way of illuminating what you missed in Analysis.
evaluate the lifecycle of the elements of the model with respect to a generic/overall model such as
acquisition --> stewardship --> disposal
do you know where every entity comes from and how you're going to get it into your system?
do you know where every entity, once acquired, will reside, and for how long?
do you know what to do with each entity when it is no longer needed?
for a more fine-grained analysis of the lifecycle of the entities in the spec, make a CRUDE matrix for the major entities in the requirements; this is a matrix with the operations/applications as the rows and the entities as the columns. In each cell, put a C if the application Creates the entity, R for Reads, U for Updates, D for Deletes, or E for "Edits"; 'E' encompasses C,R,U, and D (most 'master table maintenance' apps will be Es). Then check each column for C,R,U, and D (or E); if one is missing (except E), figure out if it is needed. The rows and columns of the matrix can be rearranged (manually or using affinity analysis) to form cohesive groups of entities and applications which generally correspond to subsystems; this may assist with physical system distribution later.
It is also useful to add a "User" entity column to the CRUDE matrix and specify for each application (or feature or functional area or whatever you want to call the processing/behavioral aspects of the requirements) whether it takes Input from the user, produces Output for the user, or Interacts with the user (I use I, O, and N for this, and always make the User the first column). This helps identify where user-interfaces for data-entry and reports will be required.
the goal is to check the completeness of the specification; the techniques above are useful to check to see if the life-cycle of the entities are 'closed' with respect to the entities and applications identified
Here's how you find the missing requirements.
Break the requirements down into tiny little increments. Really small. Something that can be built in two weeks or less. You'll find a lot of gaps.
Prioritize those into what would be best to have first, what's next down to what doesn't really matter very much. You'll find that some of the gap-fillers didn't matter. You'll also find that some of the original "requirements" are merely desirable.
Debate the differences of opinion as to what's most important to the end users and why. Two users will have three opinions. You'll find that some users have no clue, and none of their "requirements" are required. You'll find that some people have no spine, and things they aren't brave enough to say out loud are "required".
Get a consensus on the top two or three only. Don't argue out every nuance. It isn't possible to envision software. It isn't possible for anyone to envision what software will be like and how they will use it. Most people's "requirements" are descriptions of how the struggle to work around the inadequate business processes they're stuck with today.
Build the highest-priority, most important part first. Give it to users.
GOTO 1 and repeat the process.
"Wait," you say, "What about the overall budget?" What about it? You can never know the overall budget. Do the following.
Look at each increment defined in step 1. Provide a price-per-increment. In priority order. That way someone can pick as much or as little as they want. There's no large, scary "Big Budgetary Estimate With A Lot Of Zeroes". It's all negotiable.
I have been using a modeling methodology called Behavior Engineering (bE) that uses the original specification text to create the resulting model when you have the model it is easier to identify missing or incomplete sections of the requirements.
I have used the methodolgy on about six projects so far ranging from less than a houndred requirements to over 1300 requirements. If you want to know more I would suggest going to www.behaviorengineering.org there some really good papers regarding the methodology.
The company I work for has created a tool to perform the modeling. The work rate to actually create the model is about 5 requirements for a novice and an expert about 13 requirements an hour. The cool thing about the methodolgy is you don't need to know really anything about the domain the specification is written for. Using just the user text such as nouns and verbs the modeller will find gaps in the model in a very short period of time.
I hope this helps
Michael Larsen
How about building a prototype?
While reading tons of literature about software requirements, I found these two interesting books:
Problem Frames: Analysing & Structuring Software Development Problems by Michael Jackson (not a singer! :-).
Practical Software Requirements: A Manual of Content and Style by Bendjamen Kovitz.
These two authors really stand out from the crowd because, in my humble opinion, they are making a really good attempt to turn development of requirements into a very systematic process - more like engineering than art or black magic. In particular, Michael Jackson's definition of what requirements really are - I think it is the cleanest and most precise that I've ever seen.
I wouldn't do a good service to these authors trying to describe their aproach in a short posting here. So I am not going to do that. But I will try to explain, why their approach seems to be extremely relevant to your question: it allows you to boil down most (not all, but most!) of you requirements development work to processing a bunch of check-lists* telling you what requirements you have to define to cover all important aspects of the entire customer's problem. In other words, this approach is supposed to minimize the risk of missing important requirements (including those that often remain implicit).
I know it may sound like magic, but it isn't. It still takes a substantial mental effort to come to those "magic" check-lists: you have to articulate the customer's problem first, then analyze it thoroughly, and finally dissect it into so-called "problem frames" (which come with those magic check-lists only when they closely match a few typical problem frames defined by authors). Like I said, this approach does not promise to make everything simple. But it definitely promises to make requirements development process as systematic as possible.
If requirements development in your current project is already quite far from the very beginning, it may not be feasible to try to apply the Problem Frames Approach at this point (although it greatly depends on how your current requirements are organized). Still, I highly recommend to read those two books - they contain a lot of wisdom that you may still be able to apply to the current project.
My last important notes about these books:
As far as I understand, Mr. Jackson is the original author of the idea of "problem frames". His book is quite academic and theoretical, but it is very, very readable and even entertaining.
Mr. Kovitz' book tries to demonstrate how Mr. Jackson ideas can be applied in real practice. It also contains tons of useful information on writing and organizing the actual requirements and requirements documents.
You can probably start from the Kovitz' book (and refer to Mr. Jackson's book only if you really need to dig deeper on the theoretical side). But I am sure that, at the end of the day, you should read both books, and you won't regret that. :-)
HTH...
I agree with Galwegian. The technique described is far more efficient than the "wait for customer to yell at us" approach.