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I've read about SPA and it advantages. I find most of them unconvincing. There are 3 advantages that arouse my doubts.
Question: Can you act as advocate of SPA and prove that I am wrong about first three statements?
=== ADVANTAGES ===
1. SPA is extremely good for very responsive sites:
Server-side rendering is hard to implement for all the intermediate
states - small view states do not map well to URLs.
Single page apps are distinguished by their ability to redraw any part
of the UI without requiring a server roundtrip to retrieve HTML. This
is achieved by separating the data from the presentation of data by
having a model layer that handles data and a view layer that reads
from the models.
What is wrong with holding a model layer for non-SPA? Does SPA the only compatible architecture with MVC on client side?
2. With SPA we don't need to use extra queries to the server to download pages.
Hah, and how many pages user can download during visiting your site? Two, three? Instead there appear another security problems and you need to separate your login page, admin page etc into separate pages. In turn it conflicts with SPA architecture.
3.May be any other advantages? Don't hear about any else..
=== DISADVANTAGES ===
Client must enable javascript.
Only one entry point to the site.
Security.
P.S. I've worked on SPA and non-SPA projects. And I'm asking those questions because I need to deepen my understanding. No mean to harm SPA supporters. Don't ask me to read a bit more about SPA. I just want to hear your considerations about that.
Let's look at one of the most popular SPA sites, GMail.
1. SPA is extremely good for very responsive sites:
Server-side rendering is not as hard as it used to be with simple techniques like keeping a #hash in the URL, or more recently HTML5 pushState. With this approach the exact state of the web app is embedded in the page URL. As in GMail every time you open a mail a special hash tag is added to the URL. If copied and pasted to other browser window can open the exact same mail (provided they can authenticate). This approach maps directly to a more traditional query string, the difference is merely in the execution. With HTML5 pushState() you can eliminate the #hash and use completely classic URLs which can resolve on the server on the first request and then load via ajax on subsequent requests.
2. With SPA we don't need to use extra queries to the server to download pages.
The number of pages user downloads during visit to my web site?? really how many mails some reads when he/she opens his/her mail account. I read >50 at one go. now the structure of the mails is almost the same. if you will use a server side rendering scheme the server would then render it on every request(typical case).
- security concern - you should/ should not keep separate pages for the admins/login that entirely depends upon the structure of you site take paytm.com for example also making a web site SPA does not mean that you open all the endpoints for all the users I mean I use forms auth with my spa web site.
- in the probably most used SPA framework Angular JS the dev can load the entire html temple from the web site so that can be done depending on the users authentication level. pre loading html for all the auth types isn't SPA.
3. May be any other advantages? Don't hear about any else..
these days you can safely assume the client will have javascript enabled browsers.
only one entry point of the site. As I mentioned earlier maintenance of state is possible you can have any number of entry points as you want but you should have one for sure.
even in an SPA user only see to what he has proper rights. you don't have to inject every thing at once. loading diff html templates and javascript async is also a valid part of SPA.
Advantages that I can think of are:
rendering html obviously takes some resources now every user visiting you site is doing this. also not only rendering major logics are now done client side instead of server side.
date time issues - I just give the client UTC time is a pre set format and don't even care about the time zones I let javascript handle it. this is great advantage to where I had to guess time zones based on location derived from users IP.
to me state is more nicely maintained in an SPA because once you have set a variable you know it will be there. this gives a feel of developing an app rather than a web page. this helps a lot typically in making sites like foodpanda, flipkart, amazon. because if you are not using client side state you are using expensive sessions.
websites surely are extremely responsive - I'll take an extreme example for this try making a calculator in a non SPA website(I know its weird).
Updates from Comments
It doesn't seem like anyone mentioned about sockets and long-polling.
If you log out from another client say mobile app, then your browser
should also log out. If you don't use SPA, you have to re-create the
socket connection every time there is a redirect. This should also
work with any updates in data like notifications, profile update etc
An alternate perspective: Aside from your website, will your project
involve a native mobile app? If yes, you are most likely going to be
feeding raw data to that native app from a server (ie JSON) and doing
client-side processing to render it, correct? So with this assertion,
you're ALREADY doing a client-side rendering model. Now the question
becomes, why shouldn't you use the same model for the website-version
of your project? Kind of a no-brainer. Then the question becomes
whether you want to render server-side pages only for SEO benefits and
convenience of shareable/bookmarkable URLs
I am a pragmatist, so I will try to look at this in terms of costs and benefits.
Note that for any disadvantage I give, I recognize that they are solvable. That's why I don't look at anything as black and white, but rather, costs and benefits.
Advantages
Easier state tracking - no need to use cookies, form submission, local storage, session storage, etc. to remember state between 2 page loads.
Boiler plate content that is on every page (header, footer, logo, copyright banner, etc.) only loads once per typical browser session.
No overhead latency on switching "pages".
Disadvantages
Performance monitoring - hands tied: Most browser-level performance monitoring solutions I have seen focus exclusively on page load time only, like time to first byte, time to build DOM, network round trip for the HTML, onload event, etc. Updating the page post-load via AJAX would not be measured. There are solutions which let you instrument your code to record explicit measures, like when clicking a link, start a timer, then end a timer after rendering the AJAX results, and send that feedback. New Relic, for example, supports this functionality. By using a SPA, you have tied yourself to only a few possible tools.
Security / penetration testing - hands tied: Automated security scans can have difficulty discovering links when your entire page is built dynamically by a SPA framework. There are probably solutions to this, but again, you've limited yourself.
Bundling: It is easy to get into a situation when you are downloading all of the code needed for the entire web site on the initial page load, which can perform terribly for low-bandwidth connections. You can bundle your JavaScript and CSS files to try to load in more natural chunks as you go, but now you need to maintain that mapping and watch for unintended files to get pulled in via unrealized dependencies (just happened to me). Again, solvable, but with a cost.
Big bang refactoring: If you want to make a major architectural change, like say, switch from one framework to another, to minimize risk, it's desirable to make incremental changes. That is, start using the new, migrate on some basis, like per-page, per-feature, etc., then drop the old after. With traditional multi-page app, you could switch one page from Angular to React, then switch another page in the next sprint. With a SPA, it's all or nothing. If you want to change, you have to change the entire application in one go.
Complexity of navigation: Tooling exists to help maintain navigational context in SPA's, like history.js, Angular 2, most of which rely on either the URL framework (#) or the newer history API. If every page was a separate page, you don't need any of that.
Complexity of figuring out code: We naturally think of web sites as pages. A multi-page app usually partitions code by page, which aids maintainability.
Again, I recognize that every one of these problems is solvable, at some cost.
But there comes a point where you are spending all your time solving problems which you could have just avoided in the first place. It comes back to the benefits and how important they are to you.
Disadvantages
1. Client must enable javascript. Yes, this is a clear disadvantage of SPA. In my case I know that I can expect my users to have JavaScript enabled. If you can't then you can't do a SPA, period. That's like trying to deploy a .NET app to a machine without the .NET Framework installed.
2. Only one entry point to the site. I solve this problem using SammyJS. 2-3 days of work to get your routing properly set up, and people will be able to create deep-link bookmarks into your app that work correctly. Your server will only need to expose one endpoint - the "give me the HTML + CSS + JS for this app" endpoint (think of it as a download/update location for a precompiled application) - and the client-side JavaScript you write will handle the actual entry into the application.
3. Security. This issue is not unique to SPAs, you have to deal with security in exactly the same way when you have an "old-school" client-server app (the HATEOAS model of using Hypertext to link between pages). It's just that the user is making the requests rather than your JavaScript, and that the results are in HTML rather than JSON or some data format. In a non-SPA app you have to secure the individual pages on the server, whereas in a SPA app you have to secure the data endpoints. (And, if you don't want your client to have access to all the code, then you have to split apart the downloadable JavaScript into separate areas as well. I simply tie that into my SammyJS-based routing system so the browser only requests things that the client knows it should have access to, based on an initial load of the user's roles, and then that becomes a non-issue.)
Advantages
A major architectural advantage of a SPA (that rarely gets mentioned) in many cases is the huge reduction in the "chattiness" of your app. If you design it properly to handle most processing on the client (the whole point, after all), then the number of requests to the server (read "possibilities for 503 errors that wreck your user experience") is dramatically reduced. In fact, a SPA makes it possible to do entirely offline processing, which is huge in some situations.
Performance is certainly better with client-side rendering if you do it right, but this is not the most compelling reason to build a SPA. (Network speeds are improving, after all.) Don't make the case for SPA on this basis alone.
Flexibility in your UI design is perhaps the other major advantage that I have found. Once I defined my API (with an SDK in JavaScript), I was able to completely rewrite my front-end with zero impact on the server aside from some static resource files. Try doing that with a traditional MVC app! :) (This becomes valuable when you have live deployments and version consistency of your API to worry about.)
So, bottom line: If you need offline processing (or at least want your clients to be able to survive occasional server outages) - dramatically reducing your own hardware costs - and you can assume JavaScript & modern browsers, then you need a SPA. In other cases it's more of a tradeoff.
One major disadvantage of SPA - SEO. Only recently Google and Bing started indexing Ajax-based pages by executing JavaScript during crawling, and still in many cases pages are being indexed incorrectly.
While developing SPA, you will be forced to handle SEO issues, probably by post-rendering all your site and creating static html snapshots for crawler's use. This will require a solid investment in a proper infrastructures.
Update 19.06.16:
Since writing this answer a while ago, I gain much more experience with Single Page Apps (namely, AngularJS 1.x) - so I have more info to share.
In my opinion, the main disadvantage of SPA applications is SEO, making them limited to kind of "dashboard" apps only. In addition, you are going to have a much harder times with caching, compared to classic solutions. For example, in ASP.NET caching is extreamly easy - just turn on OutputCaching and you are good: the whole HTML page will be cached according to URL (or any other parameters). However, in SPA you will need to handle caching yourself (by using some solutions like second level cache, template caching, etc..).
I would like to make the case for SPA being best for Data Driven Applications. gmail, of course is all about data and thus a good candidate for a SPA.
But if your page is mostly for display, for example, a terms of service page, then a SPA is completely overkill.
I think the sweet spot is having a site with a mixture of both SPA and static/MVC style pages, depending on the particular page.
For example, on one site I am building, the user lands on a standard MVC index page. But then when they go to the actual application, then it calls up the SPA. Another advantage to this is that the load-time of the SPA is not on the home page, but on the app page. The load time being on the home page could be a distraction to first time site users.
This scenario is a little bit like using Flash. After a few years of experience, the number of Flash only sites dropped to near zero due to the load factor. But as a page component, it is still in use.
For such companies as google, amazon etc, whose servers are running at max capacity in 24/7-mode, reducing traffic means real money - less hardware, less energy, less maintenance. Shifting CPU-usage from server to client pays off, and SPAs shine. The advantages overweight disadvantages by far.
So, SPA or not SPA depends much on the use case.
Just for mentioning another, probably not so obvious (for Web-developers) use case for SPAs:
I'm currently looking for a way to implement GUIs in embedded systems and browser-based architecture seems appealing to me. Traditionally there were not many possibilities for UIs in embedded systems - Java, Qt, wx, etc or propriety commercial frameworks. Some years ago Adobe tried to enter the market with flash but seems to be not so successful.
Nowadays, as "embedded systems" are as powerful as mainframes some years ago, a browser-based UI connected to the control unit via REST is a possible solution. The advantage is, the huge palette of tools for UI for no cost. (e.g. Qt require 20-30$ per sold unit on royalty fees plus 3000-4000$ per developer)
For such architecture SPA offers many advantages - e.g. more familiar development-approach for desktop-app developers, reduced server access (often in car-industry the UI and system muddles are separate hardware, where the system-part has an RT OS).
As the only client is the built-in browser, the mentioned disadvantages like JS-availability, server-side logging, security don't count any more.
2. With SPA we don't need to use extra queries to the server to download pages.
I still have to learn a lot but since I started learn about SPA, I love them.
This particular point may make a huge difference.
In many web apps that are not SPA, you will see that they will still retrieve and add content to the pages making ajax requests. So I think that SPA goes beyond by considering: what if the content that is going to be retrieved and displayed using ajax is the whole page? and not just a small portion of a page?
Let me present an scenario. Consider that you have 2 pages:
a page with list of products
a page to view the details of a specific product
Consider that you are at the list page. Then you click on a product to view the details. The client side app will trigger 2 ajax requests:
a request to get a json object with the product details
a request to get an html template where the product details will be inserted
Then, the client side app will insert the data into the html template and display it.
Then you go back to the list (no request is done for this!) and you open another product. This time, there will be only an ajax request to get the details of the product. The html template is going to be the same so you don't need to download again.
You may say that in a non SPA, when you open the product details, you make only 1 request and in this scenario we did 2. Yes. But you get the gain from an overall perspective, when you navigate across of many pages, the number of requests is going to be lower. And the data that is transferred between the client side and the server is going to be lower too because the html templates are going to be reused. Also, you don't need to download in every requests all those css, images, javascript files that are present in all the pages.
Also, let's consider that you server side language is Java. If you analyze the 2 requests that I mentioned, 1 downloads data (you don't need to load any view file and call the view rendering engine) and the other downloads and static html template so you can have an HTTP web server that can retrieve it directly without having to call the Java application server, no computation is done!
Finally, the big companies are using SPA: Facebook, GMail, Amazon. They don't play, they have the greatest engineers studying all this. So if you don't see the advantages you can initially trust them and hope to discover them down the road.
But is important to use good SPA design patterns. You may use a framework like AngularJS. Don't try to implement an SPA without using good design patterns because you may end up having a mess.
Disadvantages:
Technically, design and initial development of SPA is complex and can be avoided. Other reasons for not using this SPA can be:
a) Security: Single Page Application is less secure as compared to traditional pages due to cross site scripting(XSS).
b) Memory Leak: Memory leak in JavaScript can even cause powerful Computer to slow down. As traditional websites encourage to navigate among pages, thus any memory leak caused by previous page is almost cleansed leaving less residue behind.
c) Client must enable JavaScript to run SPA, but in multi-page application JavaScript can be completely avoided.
d) SPA grows to optimal size, cause long waiting time. Eg: Working on Gmail with slower connection.
Apart from above, other architectural limitations are Navigational Data loss, No log of Navigational History in browser and difficulty in Automated Functional Testing with selenium.
This link explain Single Page Application's Advantages and Disadvantages.
Try not to consider using a SPA without first defining how you will address security and API stability on the server side. Then you will see some of the true advantages to using a SPA. Specifically, if you use a RESTful server that implements OAUTH 2.0 for security, you will achieve two fundamental separation of concerns that can lower your development and maintenance costs.
This will move the session (and it's security) onto the SPA and relieve your server from all of that overhead.
Your API's become both stable and easily extensible.
Hinted to earlier, but not made explicit; If your goal is to deploy Android & Apple applications, writing a JavaScript SPA that is wrapped by a native call to host the screen in a browser (Android or Apple) eliminates the need to maintain both an Apple code base and an Android code base.
I understand this is an older question, but I would like to add another disadvantage of Single Page Applications:
If you build an API that returns results in a data language (such as XML or JSON) rather than a formatting language (like HTML), you are enabling greater application interoperability, for example, in business-to-business (B2B) applications. Such interoperability has great benefits but does allow people to write software to "mine" (or steal) your data. This particular disadvantage is common to all APIs that use a data language, and not to SPAs in general (indeed, an SPA that asks the server for pre-rendered HTML avoids this, but at the expense of poor model/view separation). This risk exposed by this disadvantage can be mitigated by various means, such as request limiting and connection blocking, etc.
In my development I found two distinct advantages for using an SPA. That is not to say that the following can not be achieved in a traditional web app just that I see incremental benefit without introducing additional disadvantages.
Potential for less server request as rendering new content isn’t always or even ever an http server request for a new html page. But I say potential because new content could easily require an Ajax call to pull in data but that data could be incrementally lighter than the itself plus markup providing a net benefit.
The ability to maintain “State”. In its simplest terms, set a variable on entry to the app and it will be available to other components throughout the user’s experience without passing it around or setting it to a local storage pattern. Intelligently managing this ability however is key to keep the top level scope uncluttered.
Other than requiring JS (which is not a crazy thing to require of web apps) other noted disadvantages are in my opinion either not specific to SPA or can be mitigated through good habits and development patterns.
This article makes a pretty convincing argument that because URLs are long-lived (they get bookmarked and passed around), they should be meaningful, and that using the hash for real routing (determining what data is shown on the page and/or the state of the application) is thus improper. When I try to actually do that in my single-page application, though, I run up against a problem: how do I render my links so that all browsers can use the application? As I see it, there are three options:
all hrefs have a #/ prefix. This works great in HTML4 browsers. In HTML5 browsers, I can add a Sammy route that redirects to the hash-less version, which also works great. There might be a problem with browsers marking links as visited when they're not or not marking them visited when they are. The other problem is that it's... wrong. Anyone who shares a link by right-clicking it and selecting "Copy Link URL" will be sending a working but kludgy URL around.
no hrefs have a #/ prefix. As far as I can tell, HTML4 browsers will have no way of intercepting these link clicks, which means that every one will cause a page refresh. Though the application would probably still function, since I could use Sammy routes to rewrite the hashless versions to hashy ones on page load, the page loads would kill the performance of the single-page application.
I dynamically determine whether to prefix with #/ or not. This means that all of my links have to have dynamic markup and dramatically complicates the application.
The hash value of a URL never caused and entire re-load of page. Not in HTML4 and before that. A hash value has always been an internal link, and therefore it can be used perfectly (take a look at twitter for example). Of course, when you refresh the page, you will reload the page. But that is obvious.
With JavaScript you can actually read this hash value (see also this question/answer: How can you check for a #hash in a URL using JavaScript?) using window.location.hash
Using a more recent browser, you can also detect a hash change, which is useful if users actually change the URL: On - window.location.hash - Change?
But when you, as the website, change the URL you don't need read this, you already know because you just changed it.
This way, using hashes, people can exchange the URLs, and you can actually read which URL they are requesting, and therefore it should work perfectly.
Today a lot of content on Internet is generated using JavaScript (specifically by background AJAX calls). I was wondering how web crawlers like Google handle them. Are they aware of JavaScript? Do they have a built-in JavaScript engine? Or do they simple ignore all JavaScript generated content in the page (I guess quite unlikely). Do people use specific techniques for getting their content indexed which would otherwise be available through background AJAX requests to a normal Internet user?
JavaScript is handled by both Bing and Google crawlers. Yahoo uses the Bing crawler data, so it should be handled as well. I didn't look into other search engines, so if you care about them, you should look them up.
Bing published guidance in March 2014 as to how to create JavaScript-based websites that work with their crawler (mostly related to pushState) that are good practices in general:
Avoid creating broken links with pushState
Avoid creating two different links that link to the same content with pushState
Avoid cloaking. (Here's an article Bing published about their cloaking detection in 2007)
Support browsers (and crawlers) that can't handle pushState.
Google later published guidance in May 2014 as to how to create JavaScript-based websites that work with their crawler, and their recommendations are also recommended:
Don't block the JavaScript (and CSS) in the robots.txt file.
Make sure you can handle the load of the crawlers.
It's a good idea to support browsers and crawlers that can't handle (or users and organizations that won't allow) JavaScript
Tricky JavaScript that relies on arcane or specific features of the language might not work with the crawlers.
If your JavaScript removes content from the page, it might not get indexed.
around.
Most of them don't handle Javascript in any way. (At least, all the major search engines' crawlers don't.)
This is why it's still important to have your site gracefully handle navigation without Javascript.
I have tested this by putting pages on my site only reachable by Javascript and then observing their presence in search indexes.
Pages on my site which were reachable only by Javascript were subsequently indexed by Google.
The content was reached through Javascript with a 'classic' technique or constructing a URL and setting the window.location accordingly.
Precisely what Ben S said. And anyone accessing your site with Lynx won't execute JavaScript either. If your site is intended for general public use, it should generally be usable without JavaScript.
Also, related: if there are pages that you would want a search engine to find, and which would normally arise only from JavaScript, you might consider generating static versions of them, reachable by a crawlable site map, where these static pages use JavaScript to load the current version when hit by a JavaScript-enabled browser (in case a human with a browser follows your site map). The search engine will see the static form of the page, and can index it.
Crawlers doesn't parse Javascript to find out what it does.
They may be built to recognise some classic snippets like onchange="window.location.href=this.options[this.selectedIndex].value;" or onclick="window.location.href='blah.html';", but they don't bother with things like content fetched using AJAX. At least not yet, and content fetched like that will always be secondary anyway.
So, Javascript should be used only for additional functionality. The main content taht you want the crawlers to find should still be plain text in the page and regular links that the crawlers easily can follow.
crawlers can handle javascript or ajax calls if they are using some kind of frameworks like 'htmlunit' or 'selenium'