Difference between frontend backend and middleware in web development

Question

I was wondering if anyone can compare contrast the differences between frontend  backend  and middleware   middle-end    succinctly   Are there cases where they overlap  Are there cases where they MUST overlap  and frontend backend cannot be separated  In terms of bottlenecks  which end is associated with which type of bottlenecks

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Generally speaking  people refer to an application s presentation layer as its front end  its persistence layer  database  usually  as the back end  and anything between as middle tier  This set of ideas is often referred to as 3-tier architecture   They let you separate your application into more easily comprehensible  and testable   chunks  you can also reuse lower-tier code more easily in higher tiers   Which code is part of which tier is somewhat subjective  graphic designers tend to think of everything that isn t presentation as the back end  database people think of everything in front of the database as the front end  and so on   Not all applications need to be separated out this way  though   It s certainly more work to have 3 separate sub-projects than it is to just open index php and get cracking  depending on  1  how long you expect to have to maintain the app  2  how complex you expect the app to get  you may want to forgo the complexity

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Here is one breakdown   Front-end tier -  User Interface layer usually consisting of a mix of HTML  Javascript  CSS  Flash  and various server-side code like ASP Net  classic ASP  PHP  etc   Think of this as being closest to the user in terms of code   Middleware  middle-tier -  One tier back  generally referred to as the  plumbing  part of a system   Java and C  are common languages for writing this part that could be viewed as the glue between the UI and the data and can be webservices or WCF components or other SOA components possibly   Back-end tier -  Databases and other data stores are generally at this level   Oracle  MS-SQL  MySQL  SAP  and various off-the-shelf pieces of software come to mind for this piece of software that is the final processing of the data   Overlap can exist between any of these as you could have everything poured into one layer like an ASP Net website that uses the built-in AJAX functionality that generates Javascript while the code behind may contain database commands making the code behind contain both middle and back-end tiers   Alternatively  one could use VBScript to act as all the layers using ADO objects and merging all three tiers into one   Similarly  taking middleware and either front or back-end can be combined in some cases   Bottlenecks generally have a few different levels to them   1  Database or back-end processing -  This can vary from payroll or sales or other tasks where the throughput to the database is bogging things down   2  Middleware bottlenecks -  This would be where some web service may be hitting capacity but the front and back ends have bandwidth to handle more traffic   Alternatively  there may be some server that is part of a system that isn t quite the UI part or the raw data that can be a bottleneck using something like Biztalk or MSMQ   3  Front-end bottlenecks -  This could client or server-side issues   For example  if you took a low-end PC and had it load a web page that consisted of a lot of data being downloaded  the client could be where the bottleneck is   Similarly  the server could be queuing up requests if it is getting hammered with requests like what Amazon com or other high-traffic websites may get at times   Some of this is subject to interpretation  so it isn t perfect by any means and YMMV     EDIT  Something to consider is that some systems can have multiple front-ends or back-ends   For example  a content management system will likely have a way for site visitors to view the content that is a front-end but what about how content editors are able to change the data on the site   The ability to pull up this data could be seen as front-end since it is a UI component or it could be seen as a back-end since it is used by internal users rather than the general public viewing the site   Thus  there is something to be said for context here

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In terms of networking and security  the Backend is by far the most  should be  secure node   The middle-end portion  usually being a web server  will be somewhat in the wild and cut off in many respects from a company s network   The middle-end node is usually placed in the DMZ and segmented from the network with firewall settings   Most of the server-side code parsing of web pages is handled on the middle-end web server   Getting to the backend means going through the middle-end  which has a carefully crafted set of rules allowing disallowing access to the vital nummies which are stored on the database  backend  server

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There are in fact 3 questions in your question     Define frontend  middle and back end How and when do they overlap   Their associated usual bottlenecks    What JB King has described is correct  but it is a particular  simple version  where in fact he mapped front  middle and bacn to an MVC layer  He mapped M to the back  V to the front  and C to the middle   For many people  it is just fine  since they come from the ugly world where even MVC was not applied  and you could have direct DB calls in a view   However in real  complex web applications  you indeed have two or three different layers  called front  middle and back  Each of them may have an associated database and a controller   The front-end will be visible by the end-user  It should not be confused with the front-office  which is the UI for parameters and administration of the front  The front-end will usually be some kind of CMS or e-commerce Platform  Magento  etc    The middle-end is not compulsory and is where the business logics is  It will be based on a PIM  a MDM tool  or some kind of custom database where you enrich your produts or your articles  for CMS   It ll also be the place where you code business functions that need to be shared between differents frontends  for instance between the PC frontend and the API-based mobile application   Sometimes  an ESB or tool like ActiveMQ will be your middle-end  The back-end will be a 3rd layer  surrouding your source database or your ERP  It may be jsut the API wrting to and reading from your ERP  It may be your supplier DB  if you are doing e-commerce  In fact  it really depends on web projects  but it is always a central repository  It ll be accessed either through a DB call  through an API  or an Hibernate layer  or a full-featured back-end application  This description means that answering the other 2 questions is not possible in this thread  as bottlenecks really depend on what your 3 ends contain   what JB King wrote remains true for simple MVC architectures  at the time the question was asked  5 years ago   maybe the MVC pattern was not yet so widely adopted  Now  there is absolutely no reason why the MVC pattern would not be followed and a view would be tied to DB calls  If you read the question  Are there cases where they MUST overlap  and frontend backend cannot be separated   in a broader sense  with 3 different components  then there times when the 3 layers architecture is useless of course  Think of a simple personal blog  you ll not need to pull external data or poll RabbitMQ queues

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Here is a real world example which shows front mid back end   General description    Frontend is responsible for presenting data to user  Please note interesting quirk that you may have two different front ends associated with single backend Backend provides business logic data persistence  Middleware  activemq in the picture  is responsible for system to system  integration between backends  Usually it is installed as separate application    Overlapping   It is possible to have overlapping between frontend and backend  This usually leaads to long-term issues with application maintenance and scalability  Fairly common in legacy applications    Most modern technology stacks encourage developers to have strict separation  For example in the picture you can see that backend of the first system has rest web service which is a clear separation line   Bottlenecks  Most bottlenecks in large are caused by database network  Databases are located in backend  As for network issues every connection goes through netowrk  so every connection has potential for being slow  With good application design these issues are avoidable to large extend

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Frontend refers to the client-side  whereas backend refers to the server-side of the application  Both are crucial to web development  but their roles  responsibilities and the environments they work in are totally different  Frontend is basically what users see whereas backend is how everything works

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