I have recently attended Amazon Interview and got rejected after design round, its happened three times with Amazon in 2 year, i always getting rejected because of design round.
please help me to know how should i answer an design question.
questions like
1) design elevator control system
2) design whisper-sync feature (used in Amazon instant video platform)
they ask me to design End to end
HLD
LLD
i did not get what exactly differ in both, i explain as a algorithm wise, but could not properly by HLD and LLD.
i mean i searched alot, but could not found a way to answer such question.
Any help will be good for me.
Thanks,
HLD: High Level Design consist of sub-system design to define. It target architect, Technical leads etc.
- Solution Technical Overview
- interfaces
- primary classes
- Database entities with
- Assumptions
LLD: Lower level design consist of individual module/section design for the sub-system. It primarily extension of HLD. It targets core developers & covers following items:
- All classes entities & interfaces
- DB Model (Tables, column attributes, indexes, constraints)
- Algorithms for methods used in classes
- Logging implementation
I hope it helps!!
Related
I am beginning in UML and software analyse and i do not understand how UML and diagrams can influence coding and software architecture while we can directly build the code and its data base without diagrams.
I read lot of tutorials abouat the subject but not enough to understand the utility of UML in coding.
I understand everey diagram and its role. That is not my problem but i do not yet understand their roles after the analyse and design phase.
So what is the role of UML in coding phase of a software ?
Thank you.
The comment by #xmojmr already puts it right. UML creates a model (hence the M in UML) of a system. A model reduces information of a system to a level so it is a) manageable and b) complete. Human brains are not computers and you need a means of communication what the system is all about. You can do that as pure code, as paper document and as UML model. A combination of all is not uncommon. As long as you have tiny systems you can live with pur code and tools like Doxygen. But once it starts getting complex you need some handles. UML offers these to end users, architects, testers, developers, managers, etc. Along with UML you will also need a methodology. UML delivers the syntax how to document a system. But you need some structure above to write a nice novel.
UML-based models play an essential role for coding/implementing a software system in model-based (or model-driven) development. The basic idea is that you start making a model of your problem domain (the domain model), then you derive from it a platform-independent design model, which can be transformed into platform-specific implementation models (e.g. for Java- or C#-based platforms) that are finally encoded in the target languages.
The most prominent part of model-based development is the encoding of model classes (forming the model layer in an MVC architecture for apps) based on a data model (a UML class model) that has been derived from an information design model, which was obtained from a domain information model (where all these information/data models are UML class models).
You can find an instructive example of model-based development in my tutorial book Engineering Front-End Web Apps with Plain JavaScript.
This one is in my point of view a duplicate of that other question. It can't be flagged because there is no accepted answer. The related question on meta stackexchange does not provide a clear solution to that situation.
I think my personal answer was relevant and is applicable to the current question.
To be synthetic, Martin Fowler considers current uses of UML. I think he describe the current practices. Perhaps should these evolve ?
Perhaps would the initial question be the right place to discuss ?
I'm not sure if this is the right place to ask this question but here goes.
I'm currently looking into some different techniques to model a business proces. I need to find a suitable option for my company which develops all kind of web applications.
What i have found so far:
UML, specificly the activity diagrams
Flow charts
Windows workflow foundation
Business process modeling
I had a dive into the world of workflows but it's mainly about automating a process of a company and thats not what I'm looking for. My focus is on software and the process within.
If anyone else knows some other technique or can tell me the advantages or disadvanteges of the techniques I allready found that would be much appreciated because I'm a little stuck right now.
Right now BPMN (Business Process Modeling Notation) and UML activity diagrams are the two most popular options for that.
I think of BPMN as the right choice when you are modeling the business aspects of the organization and move to Activity Diagrams as soon as you drill down to the technical design of the software system (as a core component of the UML language, activity diagrams are a better fit when having to combine workflow information with other views of the system, expressed also as UML diagrams like class diagrams or sequence diagrams).
Note that now a UML profile for BPMN is being created which means that you will be able to mix BPMN and UML diagrams in the same project
I suggest you consider ISO/IEC 24744. It will give you a very different perspective, since it does not use the ubiquitous "organisation as machine" metaphor, going for a more opportunistic, people-oriented viewpoint.
In other words, ISO/IEC 24744 does not represent a business process as a workflow where the process to follow is the driver. Instead, a business process is represented through the work products that are involved and the people that act upon them. The process performed plays an important but secondary role.
If you are interested in why this is so, or what the advantages are, let me know and I'll be happy to elaborate.
I think you are asking about transforming your business requirements/rules into technical requirements and then into a design? After that you will implement this design into code.
Am not sure if this is what you are asking about..
I've been searching the web for some time now. I am looking for small sample exercises for OOD practice (& for some internal TDD workshops).
If there is one single place, where this need is being served, please point me to it.. and close this question
Constraints:
Language-agnostic real world problem
Small : Something that takes an hour to two at max to solve (or has sub-parts that can fit this constraint).
Not Algorithm centred : Not be focussed on just solving a computational task. (There are multiple sites that serve this category.) Involve > 2 interacting entities.
Solved by multiple people, preferably yourself : Goodness verified. Links preferred. Please do not post something that may be a good exercise... subjective
Similar SO question 60109, but the answers dont meet my need here. I found that I've lost my touch (was thrashing ideas) with OOD after prolonged exposure to a day-job. Need to get it back..
Update: Are we collectively out of short OOP exercises ? I was hoping that I'd have a bunch to pick from. However my web-searches (this is a diff exercise in formulating the right search string) and the lack of responses here seem to indicate otherwise. Maybe I posted to SO at a bad time.. in which case bumping this thread for more responses.
http://butunclebob.com/ArticleS.UncleBob.TheBowlingGameKata
http://schuchert.wikispaces.com/Monopoly%28r%29
Jeff Bay's Object Calisthenics. Following these will improve your OO skills.
Bill Wake's spreadsheet TDD challenge
Dave Thomas' CodeKata
Kindness,
Dan
From the AGPPnP book by Robert Martin aka UncleBob
CoffeeMaker Mark IV - Page 2 has the problem statement
Questionnaire Practice Problem
A problem I've worked on in a couple of different jobs is that of writing some generic, data-driven survey/questionnaire functionality. It's not majorly complex, but has enough interesting avenues to be a good OOD practice problem I think. It's definitely real-world and crops up in a lot of places.
You can start off thinking about how to structure a Survey. It is obviously made up of Questions, but do you also want Categories? Can a Question have subquestions? Can a subquestion have subquestions? How deep can you go?
A question probably needs to have potential Scores. What types of scores can you have? What types of questions can you have (multiple choice, multiple answer, freetext, etc.)?
Once you've got the basic business logic, you can also think about how you display a survey . Maybe you have a SurveyRenderer and a QuestionRenderer? How do you decide how to render different types of questions? (Maybe you use a Strategy pattern... as in this SO question.) How do you render a read-only version of the survey?
You can also think about persistence. How do you record responses to a blank questionnaire? How does your object graph of a survey get mapped into a database (or some other backing store), and vice versa?
I am currently trying to draw a set of UML diagrams to represent products, offers, orders, deliveries and payments. These diagrams have probably been invented by a million developers before me.
Are there any efforts to standardize the modeling of such common things? Or even the modeling of specific domains (for example car-manufacturing).
Do you know if there is some sort of repository containing UML diagrams (class diagrams, sequence diagrams, state diagrams...)?
There is a movement for documenting (as opposed to standardizing) models for certain domains. These are called analysis patterns and is a term Martin Fowler came up with. He actually wrote a book called Analysis patterns. Also, he has a dedicated section on his website where he presents some of these patterns accompanied by UML diagrams.
Maybe you'll find some inspiration that will help you in modeling your domain. I've stressed the word inspiration as I think different businesses have different requirements although they operate the same domain so the solutions you might read about may not be appropriate for your problem.
There are many tools out there that do both - but they're generally not free!
Microsoft Visio does both and is extensible. For UML artefacts they come with auto generators into VB/Java template code - but you can modify them to auto-generate any code. There are many users of Visio that have created models from which to use as templates.
Artisan Enterprize is by far the most powerful UML tool (but it's not cheap).
Some would argue that Rational Rose or RUP is the better tool
But for Car-Manufacturing and other similar real world modelling, by far the best tool is Mathworks Simulink (not because it's one of the most expensive). It is by far the best tool beccause you can animate the model - you can prove the model working before generating the slik code (in whatever grammar/language/other Models you care to push it)!
You can obtain a student license for around £180; with the 'real thing' pushing £4000 (for car-related artefacts). The full product with all the trimmings is about £15k. Simulink is also extensible with a C like language though there is a .Net addin and APIs to use a plethora of other langhuages. And, just like Visio there is a world-wide forum creating saleable, shareware & freeware real world model templates. Many world-wide Auto-Manufacturers are already using Simulink.
I think that MiniQuark question is really good and will sooner or later be provided by vendors such as Omondo, Rational IBM etc... Users doesn't just need tools, they need models out of the box and just add their business rules inside an existing well defined architecture. Why to develop from scratch a new architecture if the job has already be done ? In Java we use plenty of frameworks, existing methods etc...so why not to go one level higher and reuse architecture ? It is today impossible to guess how a project will evole and new demands are coming every day. We therefore need a stable architecture which has been tested previously and is extensible. I have seen so many projects starting with a nice architecture then realizing in the middle of the project that this is not what is the best and then changing their architecture. Renaming classes, splitting classes, creating packages etc...after the first iteration it is getting a real mess. Could you imagine what we found after 10 iterations !! a total mess !!
This mess would had been avoided if using a predefined model which has been tested previously because the missing class, or package etc..would have already been created and only a class rename would be sufficient for architecture purposes. Adding business rules methods will end the codding stage before deployment test.
I think there is a confusion between patterns and the initial question which is related to UML model re usability.
There is no today any reusable model out of the box which has been developped. This is really strange but the job has never been done or never been shared.
Omondo has tried to launch an initiative without real success. I have heard that they are working on hundred of out of box models which will be open source and given for free to the community. I hope this will be done because this is really important for me and would save me a lot of time at the beginning of a project.
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So...
I teach formal methods in software engineering. I also teach "agile methodologies". Most people seem to think this is contradictory. I think it makes a lot of sense... I also work for a company, where we need to actually get things done :) While I can apply my earned skill points on "specification" in a day-to-day basis, my colleagues typically flee away from the word "formal".
I used to think that this was due to the intrinsic way we learn how to program: we are usually driven to find a working solution, not to understand the problem. Then I thought this was due to the fact that most people in the formal community are not engineers, but mathematicians or computer scientists. Nowadays, I wonder if it just because the formal-methods community hide behind some kind of "obfuscation" law to use all the available UNICODE symbols, actively develop rude, unesthetic tools, and laugh in the face of standards.
Yes, I've been moving from a "blame them" to a "blame us" perspective ;-)
So, my question is: do you use any kind of formal methods in your company? Have you introduced them, or were they pre-requisites? What techniques do you use to clear the fog of mathematics from people's fears and incite them to use formal methods? What do you think current tools are lacking for a more general usage?
The key to getting people to buy into any methods or methodologies is to show them how it solves problems they are having. If they can see it will make their lives better you have a much improved chance of getting them to adopt the techniques.
And if you can't show them that, perhaps you wanted to adopt the methods based on philosophy rather than practicality. Unless the others share your philosophy then you're not going to get anywhere. And perhaps you shouldn't.
Over the decades there have been a great many methodologies. Newer ones always address the shortcomings of the old ones, yet projects still get in trouble and fail. Why? Because the rock stars that come up with new methodologies are rock stars, and have made a new methodology precisely because they understand the underlying issues and how to apply them. Those who come after tend to blindly follow the recipe, and it doesn't work so well.
So I think the best thing is to teach about the underlying problems and then show how various methods attempt to deal with those problems. The differences in companies, projects, and teams is so great that no one methodology can be applied successfully to all combinations. Learning to choose an appropriate tool and apply it well is crucial.
Thank you for all contributions. They are very insightful. Allow me to flame a bit (don't take it personal, though :-)
Most people seem to think that formal methods are just about program verification. Or critical systems. This may be true if we pursue the ultimate cliche: to prove we are doing the program right (v.s. validation, which asks, as a contributor said, if we are doing the right program).
But consider model finding/checking tools, such as Alloy. Learning to use a tool like this takes a negligable ammount of time for anyone used to UML and OO. Still, it can give you immediate insight over your model. It usually takes no more than 10 minutes to find a counter-example over a small enough subset of the model one's trying to use (and that includes describing the model in Alloy in the first place).
Take requirements engineering as an example. One usually draw a lot of UML. Few people use OCL, though, and many business rules are informally annoted in natural language. Why? Time constraints?
Now consider the fact that the majority just uses her/his gut-feeling to prove that a model is satisfiable. Again, why? I can take the same amount of time (probably even less, since I don't need to care about drawing aesthetics) to write that model in Alloy, and just check for satisfiability? And what kind of mathematics do I need to now? "Predicates"? Fancy name for IFs and booleans ;-) Quantifiers? Fancy names for ForEachs()...
What about big information systems? They don't need to be critical... Just try to analyze in your head a conceptual (not implementation!) diagram with over 600 classes. I see many people banging their head in the wall with easy-to-make model mistakes because they missed some constraint, or the model allows stupid things to happen.
The fact is, one does not need to use formal approaches from head to tail. Granted, I could prove a whole application in Coq, and certify that it is 100% compliant with some specification. This may be the Computer Scientist/Mathematician approach.
Still, with a GTD philisophy, why can't I delegate some tasks for the computer and allow it to help improving my development? Is it really a matter of "time", or plain, simple lack of technical abilities and will to learn/inovate?
Working with line of business IT development in an enterprise means having to transfer knowledge about the business from actual business people into the heads of developers. While I myself find abstract maths to be one of the greatest pastimes there is, it's a terrible communications tool. And communications is what it's all about. While I might conceivably have some success convincing IT people to embrace more abstract notations, I basically have no chance with the business people.
While there are some areas where I can see a role for formal methods in an enterprise (math- and logic-heavy specialist software, significant need for provable properties as in safety critical software) they provide little help with getting correct requirements on e.g. how to fulfil a customer order by issuing one or more supply orders to a set of possible external or internal providers.
I think the jury is still out on model based approaches and domain specific languages. I think they will succeed or fail depending on whether they provide quicker feedback from IT to the wishes and needs of the business side, and whether they presume business people will have to do any significant studying.
Technology is easy. Communication is hard. Formal methods may help us do things right, but those I've seen do nothing to help us do the right things. (Yes, these are cliches, but that's because they're inescapably and painfully true.)
I'm taking a course on 'Specification and Verification'. As part of the course structure we are doing the following-
1. Learning tools like PVS(Prototype Verification System) http://pvs.csl.sri.com/ and SMV(Software Modeling and Verification) http://www.cs.cmu.edu/~modelcheck/smv.html
2. Apart from that we do dissect accidents which happened because of software failures. For e.g. - Failure of Ariane V
I feel formal methods are more applicable to scenarios where the failure cost is more than the design cost. And it seems apt to use them for softwares being used in critical systems. I guess it is used in avionics, chip design etc. and the current automobile industry is also drafting it into practice.
I have tried to get people to embrace formal specification methods a few times (Z and Alloy) and have made the same expirience that you have: Most people, while feeling that they serve a useful purpose, are very uncomfortable using them for actual work.
Funny enough, the same people are more than happy to produce utterly useless UML diagrams in ginormous quantities.
I think there are two main reasons for this:
a.) Many developers are uncomfortable with the level of abstraction required by a formal approach. The fact that most entry-level mathematics education is all calculus and non discrete-mathematics might have to do something with this.
b.) Formal methods require a very bottom up design aproach where you design your core model from the ground up and make it airtight and then connect it up to the actual user requirements by providing an interface on top of it. Since we tend to have requirements drive development efforts, a top-down approach feels more natural although it often leads to inconsistent models. It's like retrofitting a basement underneath your house after it has already been built.
Formal methods make no sense in systems where the cost of failure is low.
In a production web application, you've got multiple front-end boxes, multiple back-end boxes, multiple database boxes - if a program on any one of them fails, it's a non-event. Hardware is so cheap that you can build these systems for far less than the cost of formally specifying all your software.