I am working on a large web site, and we're moving a lot of functionality to the client side (Require.js, Backbone and Handlebars stack). There are even discussions about possibly moving all rendering to the client side.
But reading some articles, especially ones about Twitter moving away from client side rendering, which mention that server side is faster / more reliable, I begin to have questions. I don't understand how rendering fairly simple HTML widgets in JS from JSON and templates is a contemporary browser on a dual core CPU with 4-8 GB RAM is any slower than making dozens of includes in your server side app. Are there any actual real life benchmarking figures regarding this?
Also, it seems like parsing HTML templates by server side templating engines can't be any faster than rendering same HTML code from a Handlebars template, especially if this is a precomp JS function?
There are many reasons:
JavaScript is interpreted language and is slower than server side
(usually done in compiled language)
DOM manipulation is slow, and if you are manipulating it in JS it
results in poor performance. There are ways to overcome this like
preparing your rendering in text then evaluating it, this might in fact gets you as close to server side rendering.
Some browsers are just too slow, especially old IE
Performance of compiled language versus interpreted javascript
Caching, ie - serving up the exact same page another user has already requested, this removes the need for each client to render it. Great for sites with huge traffic - ie news sites. Micro-caching can even provide near real-time updates, yet serve significant traffic from the cache. No need to wait for client rendering
Less reliance on users with old computers or slow / crippled browsers
Only need to worry about rendering, less reliance on how different browsers manage DOM (reliability)
But for a complex UI, client side rendering of interactions will provide a snappier user experience.
It really depends on what performance you're trying to optimise, and for how many users.
To run code on the client side it first has to be loaded. Server side code is just loaded when the server start, whereas the client code must potentially be loaded every time the page is. In any case the code must be interpreted when loading the page, even if the file is already cached. You might also have caching of JS parse trees in the browser, but I think those are not persisted, so they won't live long.
This means that no matter how fast JavaScript is (and it is quite fast) work has to be performed while the user waits. Many studies have shown that page loading time greatly affects the users perception of the sites quality and relevance.
Bottom line is that you have 500ms at the most to get your page rendered from a clean cache on your typical developer environment. Slower devices and networks will make that lag just barely acceptable to most users.
So you probably have 50-100ms to do things in JavaScript during page load, all of it, grand total, which means that rendering a complex page, well, not easy.
Related
I've done some web-based projects, and most of the difficulties I've met with (questions, confusions) could be figured out with help. But I still have an important question, even after asking some experienced developers: When functionality can be implemented with both server-side code and client-side scripting (JavaScript), which one should be preferred?
A simple example:
To render a dynamic html page, I can format the page in server-side code (PHP, python) and use Ajax to fetch the formatted page and render it directly (more logic on server-side, less on client-side).
I can also use Ajax to fetch the data (not formatted, JSON) and use client-side scripting to format the page and render it with more processing (the server gets the data from a DB or other source, and returns it to the client with JSON or XML. More logic on client-side and less on server).
So how can I decide which one is better? Which one offers better performance? Why? Which one is more user-friendly?
With browsers' JS engines evolving, JS can be interpreted in less time, so should I prefer client-side scripting?
On the other hand, with hardware evolving, server performance is growing and the cost of sever-side logic will decrease, so should I prefer server-side scripting?
EDIT:
With the answers, I want to give a brief summary.
Pros of client-side logic:
Better user experience (faster).
Less network bandwidth (lower cost).
Increased scalability (reduced server load).
Pros of server-side logic:
Security issues.
Better availability and accessibility (mobile devices and old browsers).
Better SEO.
Easily expandable (can add more servers, but can't make the browser faster).
It seems that we need to balance these two approaches when facing a specific scenario. But how? What's the best practice?
I will use client-side logic except in the following conditions:
Security critical.
Special groups (JavaScript disabled, mobile devices, and others).
In many cases, I'm afraid the best answer is both.
As Ricebowl stated, never trust the client. However, I feel that it's almost always a problem if you do trust the client. If your application is worth writing, it's worth properly securing. If anyone can break it by writing their own client and passing data you don't expect, that's a bad thing. For that reason, you need to validate on the server.
Unfortunately if you validate everything on the server, that often leaves the user with a poor user experience. They may fill out a form only to find that a number of things they entered are incorrect. This may have worked for "Internet 1.0", but people's expectations are higher on today's Internet.
This potentially leaves you writing quite a bit of redundant code, and maintaining it in two or more places (some of the definitions such as maximum lengths also need to be maintained in the data tier). For reasonably large applications, I tend to solve this issue using code generation. Personally I use a UML modeling tool (Sparx System's Enterprise Architect) to model the "input rules" of the system, then make use of partial classes (I'm usually working in .NET) to code generate the validation logic. You can achieve a similar thing by coding your rules in a format such as XML and deriving a number of checks from that XML file (input length, input mask, etc.) on both the client and server tier.
Probably not what you wanted to hear, but if you want to do it right, you need to enforce rules on both tiers.
I tend to prefer server-side logic. My reasons are fairly simple:
I don't trust the client; this may or not be a true problem, but it's habitual
Server-side reduces the volume per transaction (though it does increase the number of transactions)
Server-side means that I can be fairly sure about what logic is taking place (I don't have to worry about the Javascript engine available to the client's browser)
There are probably more -and better- reasons, but these are the ones at the top of my mind right now. If I think of more I'll add them, or up-vote those that come up with them before I do.
Edited, valya comments that using client-side logic (using Ajax/JSON) allows for the (easier) creation of an API. This may well be true, but I can only half-agree (which is why I've not up-voted that answer yet).
My notion of server-side logic is to that which retrieves the data, and organises it; if I've got this right the logic is the 'controller' (C in MVC). And this is then passed to the 'view.' I tend to use the controller to get the data, and then the 'view' deals with presenting it to the user/client. So I don't see that client/server distinctions are necessarily relevant to the argument of creating an API, basically: horses for courses. :)
...also, as a hobbyist, I recognise that I may have a slightly twisted usage of MVC, so I'm willing to stand corrected on that point. But I still keep the presentation separate from the logic. And that separation is the plus point so far as APIs go.
I generally implement as much as reasonable client-side. The only exceptions that would make me go server-side would be to resolve the following:
Trust issues
Anyone is capable of debugging JavaScript and reading password's, etc. No-brainer here.
Performance issues
JavaScript engines are evolving fast so this is becoming less of an issue, but we're still in an IE-dominated world, so things will slow down when you deal with large sets of data.
Language issues
JavaScript is weakly-typed language and it makes a lot of assumptions of your code. This can cause you to employ spooky workarounds in order to get things working the way they should on certain browsers. I avoid this type of thing like the plague.
From your question, it sounds like you're simply trying to load values into a form. Barring any of the issues above, you have 3 options:
Pure client-side
The disadvantage is that your users' loading time would double (one load for the blank form, another load for the data). However, subsequent updates to the form would not require a refresh of the page. Users will like this if there will be a lot of data fetching from the server loading into the same form.
Pure server-side
The advantage is that your page would load with the data. However, subsequent updates to the data would require refreshes to all/significant portions of the page.
Server-client hybrid
You would have the best of both worlds, however you would need to create two data extraction points, causing your code to bloat slightly.
There are trade-offs with each option so you will have to weigh them and decide which one offers you the most benefit.
One consideration I have not heard mentioned was network bandwidth. To give a specific example, an app I was involved with was all done server-side and resulted in 200Mb web page being sent to the client (it was impossible to do less without major major re-design of a bunch of apps); resulting in 2-5 minute page load time.
When we re-implemented this by sending the JSON-encoded data from the server and have local JS generate the page, the main benefit was that the data sent shrunk to 20Mb, resulting in:
HTTP response size: 200Mb+ => 20Mb+ (with corresponding bandwidth savings!)
Time to load the page: 2-5mins => 20 secs (10-15 of which are taken up by DB query that was optimized to hell an further).
IE process size: 200MB+ => 80MB+
Mind you, the last 2 points were mainly due to the fact that server side had to use crappy tables-within-tables tree implementation, whereas going to client side allowed us to redesign the view layer to use much more lightweight page. But my main point was network bandwidth savings.
I'd like to give my two cents on this subject.
I'm generally in favor of the server-side approach, and here is why.
More SEO friendly. Google cannot execute Javascript, therefor all that content will be invisible to search engines
Performance is more controllable. User experience is always variable with SOA due to the fact that you're relying almost entirely on the users browser and machine to render things. Even though your server might be performing well, a user with a slow machine will think your site is the culprit.
Arguably, the server-side approach is more easily maintained and readable.
I've written several systems using both approaches, and in my experience, server-side is the way. However, that's not to say I don't use AJAX. All of the modern systems I've built incorporate both components.
Hope this helps.
I built a RESTful web application where all CRUD functionalities are available in the absence of JavaScript, in other words, all AJAX effects are strictly progressive enhancements.
I believe with enough dedication, most web applications can be designed this way, thus eroding many of the server logic vs client logic "differences", such as security, expandability, raised in your question because in both cases, the request is routed to the same controller, of which the business logic is all the same until the last mile, where JSON/XML, instead of the full page HTML, is returned for those XHR.
Only in few cases where the AJAXified application is so vastly more advanced than its static counterpart, GMail being the best example coming to my mind, then one needs to create two versions and separate them completely (Kudos to Google!).
I know this post is old, but I wanted to comment.
In my experience, the best approach is using a combination of client-side and server-side. Yes, Angular JS and similar frameworks are popular now and they've made it easier to develop web applications that are light weight, have improved performance, and work on most web servers. BUT, the major requirement in enterprise applications is displaying report data which can encompass 500+ records on one page. With pages that return large lists of data, Users often want functionality that will make this huge list easy to filter, search, and perform other interactive features. Because IE 11 and earlier IE browsers are are the "browser of choice"at most companies, you have to be aware that these browsers still have compatibility issues using modern JavaScript, HTML5, and CSS3. Often, the requirement is to make a site or application compatible on all browsers. This requires adding shivs or using prototypes which, with the code included to create a client-side application, adds to page load on the browser.
All of this will reduce performance and can cause the dreaded IE error "A script on this page is causing Internet Explorer to run slowly" forcing the User to choose if they want to continue running the script or not...creating bad User experiences.
Determine the complexity of the application and what the user wants now and could want in the future based on their preferences in their existing applications. If this is a simple site or app with little-to-medium data, use JavaScript Framework. But, if they want to incorporate accessibility; SEO; or need to display large amounts of data, use server-side code to render data and client-side code sparingly. In both cases, use a tool like Fiddler or Chrome Developer tools to check page load and response times and use best practices to optimize code.
Checkout MVC apps developed with ASP.NET Core.
At this stage the client side technology is leading the way, with the advent of many client side libraries like Backbone, Knockout, Spine and then with addition of client side templates like JSrender , mustache etc, client side development has become much easy.
so, If my requirement is to go for interactive app, I will surely go for client side.
In case you have more static html content then yes go for server side.
I did some experiments using both, I must say Server side is comparatively easier to implement then client side.
As far as performance is concerned. Read this you will understand server side performance scores.
http://engineering.twitter.com/2012/05/improving-performance-on-twittercom.html
I think the second variant is better. For example, If you implement something like 'skins' later, you will thank yourself for not formatting html on server :)
It also keeps a difference between view and controller. Ajax data is often produced by controller, so let it just return data, not html.
If you're going to create an API later, you'll need to make a very few changes in your code
Also, 'Naked' data is more cachable than HTML, i think. For example, if you add some style to links, you'll need to reformat all html.. or add one line to your js. And it isn't as big as html (in bytes).
But If many heavy scripts are needed to format data, It isn't to cool ask users' browsers to format it.
As long as you don't need to send a lot of data to the client to allow it to do the work, client side will give you a more scalable system, as you are distrubuting the load to the clients rather than hammering your server to do everything.
On the flip side, if you need to process a lot of data to produce a tiny amount of html to send to the client, or if optimisations can be made to use the server's work to support many clients at once (e.g. process the data once and send the resulting html to all the clients), then it may be more efficient use of resources to do the work on ther server.
If you do it in Ajax :
You'll have to consider accessibility issues (search about web accessibility in google) for disabled people, but also for old browsers, those who doesn't have JavaScript, bots (like google bot), etc.
You'll have to flirt with "progressive enhancement" wich is not simple to do if you never worked a lot with JavaScript. In short, you'll have to make your app work with old browsers and those that doesn't have JavaScript (some mobile for example) or if it's disable.
But if time and money is not an issue, I'd go with progressive enhancement.
But also consider the "Back button". I hate it when I'm browsing a 100% AJAX website that renders your back button useless.
Good luck!
2018 answer, with the existence of Node.js
Since Node.js allows you to deploy Javascript logic on the server, you can now re-use the validation on both server and client side.
Make sure you setup or restructure the data so that you can re-use the validation without changing any code.
I have just learned react recently and intend to use it for my next project. I have come across react server side rendering for a few times, but wonders why do we still need it in "modern age".
In this article, it argues that with server side rendering, user does not have to wait to load those js from CDN or somewhere to see the initial static page, and the page will resume functionality when js arrives.
But after building with webpack production configuration, and gzip, the whole bundle (with react, my code and a lot other stuff) only takes 40kb, and I have aws CDN for it. I don't quite see the reason to use server side rendering for my situation.
So the question is why people still use server side rendering if the resulting javascript bundle is so tiny after gzip?
Load Times
A rendered view of the application can be delivered in the response of the initial HTTP request. In a traditional single page web app, the first request would come back, the browser would parse the HTML, then make subsequent requests for the scripts — which would eventually render the page. Those requests will still happen, but they won't ever get in the way of the user seeing the initial data.
This doesn't make much difference on quick internet connections, but for users on mobiles in low network coverage areas, this initial rendering of data can make apps render 20-30 seconds faster.
Views with static data can also be cached at a network level, meaning that a rendered view of a React application can be served with very little computational overhead
SEO
When a search engine crawler arrives at a web page, the HTML is served and the static content is inspected and indexed. In a purely client side Javascript application, there is no static content. It is all created and injected dynamically once the appropriate scripts load and run.
Unlike React, most frameworks have no way of serialising their component graph to HTML and then reinflating it. They have to use a more convoluted approach, which often involves rendering their page in a headless browser at the server, then serving up that version whenever a crawler requests it.
React can just render the component tree to a HTML string from a server side JS environment such as Node.js, then serve that straight away. No need for headless browsers or extra complications.
Noscript
It also allows you to write applications which gracefully degrade and ultimately, can be used as thin clients. This means that the processing goes on at the server and the application can be used in a browser with Javascript disabled. Whether or not that's an important market to consider is a debate for another time.
All-or-none rendering
This is a UX concern, some designers decide they don't like incremental rendering. The designer wants the page to show up complete and perfect, without the loading spinner, insertion of fetched text here and there, and displacement of layout.
Scroll bar restoration is a difficult issue for client side rendering. See Keep scroll position when navigating back (When navigating forward and then back, the scroll position is lost). Server side rendering does not suffer from this issue.
I think if you're chasing SEO will better rendered on server.
so all contents will read by bot SEO.
In addition to SEO mentioned above, with SSR browser could present the page right away even before all Java Script files are loaded. I have a tutorial explaining this.
I've been reading some very interesting articles about the whole client vs. server rendering lately.
http://www.onebigfluke.com/2015/01/experimentally-verified-why-client-side.html
http://www.quirksmode.org/blog/archives/2015/01/angular_and_tem.html
http://tomdale.net/2015/02/youre-missing-the-point-of-server-side-rendered-javascript-apps/
Now I've been a bit of a fan boy when it comes to client side but after I read these articles some points started to show up in favor of the server side rendering, to my surprise... The main points were:
1) You can upgrade your server, but not your users device - This means, well, yes... you are in control of the server, so if it's under performing you may opt to upgrade/scale. You can't force users to upgrade their devices.
2) First paint vs. last paint - Now on the experimentally verified... link above it shows when the users first see the page (first paint) and when the users may use the page 100% (last paint). Now from what I can think of when the user sees the page, it takes their brain some time to process the signals from the visual cortex to the frontal cortex and then to the premoter cortex where the user actually starts clicking his/her finger, that is of course if the html is rendered first so the brain has something to process while loading is happening in the background (js files, binding etc.).
What really got me was the bit were twitter reported people of having up to 10 seconds of loading time for client side rendering, no one should ever experience that! It's kind of saying, "Well if you don't have a good enough device, sorry, you'll just have to wait.".
I've been thinking, isn't there a good way of using both client-side and server-side templating engines and which both client and server use the same template engine and code. In that case it's only to figure out if it's benefactor to supply the client with the rendered page or let the client render it themselves.
In any case, share your thoughts on my sayings and the articles if you want. I'm all ears!
UPD: do it only if you really need it
(4 years and 2 isomorphic enterprise apps later)
If you're required to do SSR, fine. If you can go with a simple SPA - go with it.
Why so? SPAs are easier to develop, easier to debug and cheaper and easier to run.
The development and debugging complications are evident. What do I mean by "cheaper and easier to run", though? Well, guess what, if 10K users try to open your app at the same time your static HTML website (i.e. a built SPA) you won't even feel it. If you're running an isomorphic webapp though, the TTFB will go up, RAM usage will go up and eventually you'll have to run a cluster of those.
So, unless you are required to show some super-low TTFB times (which will likely come through aggressive caching), don't overcomplicate your life.
Original answer from 2015:
Basically you're looking for an isomorphic web app that shares the same code for frontend and backend.
Isomorphic JavaScript
JavaScript applications which run both client-side and server-side. Isomorphic JavaScript frameworks are the next step in the evolution of JavaScript frameworks. These new libraries and frameworks are solving the problems associated with traditional JavaScript frameworks.
I bet this guy explains that much better that me.
So, when a user comes to the page, the server renders the full page with contents. So it loads faster and requires no extra ajax requests to load data, etc. Then, when a user navigates to another page, the usual techniques for single page applications are used.
So, WHY WOULD I CARE?
Old browsers / Weak devices / Disabled Javascript
SEO
Some page load improvements
Old browsers / Weak devices / Disabled Javascript
For example, IE9 does not support History API. So, for old browsers (and if user disables javascript too), they would just navigate through pages just like they did it it in good old days.
SEO
Google says it supports SPA's but SPA's aren't likely to appear in the top results of google search, are they?
Page speed
As it was stated, the first page loads with one HTTP request, and that's all.
OK, so
There are lots of articles on that:
http://nerds.airbnb.com/isomorphic-javascript-future-web-apps/
http://www.sitepoint.com/isomorphic-javascript-applications/
https://www.lullabot.com/articles/what-is-an-isomorphic-application
But SHOULD I CARE?
It's up to you, of course.
Yeah, that's cool, but it takes much work to rewrite/adapt the existing app. And if your backend is in PHP/Ruby/Python/Java/Whatever, I've got bad news for you (it's not necessarily impossible, but close to that).
It depends on the website, you can try to collect some stats and if the percentage of users with old devices is small, it's not worth the trouble, so why not...
LET THEM SUFFER
If you care only about users with old devices, then c'mon, it 2015, and it's your user's problem if he's using IE8 of browsing websites with a iPod Touch 2. For example, Angular dropped IE8 support in 1.3 approximately a year ago, so why wouldn't you just alert the users that they need to upgrade ;)
Cheers!
All of the conversations on this topic miss one point. Bytes sent to the client. Pages rendered as HTML on the server are a lot smaller. Less bytes transmitted is better for everyone, both server and client. I've seen the bandwidth costs on cloud sites and even a 10% reduction can be a huge saving. Client side JS pages are always fat.
A friend of mine is programming a web page. Instead of writing any HTML in the .html file, he is outputting the entire website with javascript. Excluding SEO, or the fact that it will be a pain to debug, what are the downsides to this? Also, if there is an upside, what would an upside be?
As you are already aware, SEO is a big deal. SEO is usually the elephant in the room (e.g. the big deal) for javascript-based web-sites and you either forgo SEO or have to somehow design an alternate path that the search engines can index you via and that doesn't run afoul of their rules about serving different content to search engines.
Some potential downsides:
Accessibility - access via screen readers and other tools that may be geared to the page HTML.
Mobile - Sometimes rendering the page will require larger downloads, more javascript code and more data and more CPU to build the page than a simpler server-rendered page. This may cause compromises on small devices without a lot of bandwidth or horsepower.
Performance - The initial page display time may be slower as you can't render anything until all data and code has been downloaded and run. Subsequent page times might be faster or slower depending upon how the app is written. Sometimes you save server trips by doing client-side rendering (which could be faster), but sometimes it's slower to do things on the client.
Security - Somethings simply can't be secured client-side the way they can server-side.
Code secrecy - Code in browsers is open to the world to see. Code on servers can be kept secret.
Same-origin limitations - Browsers are much more limited in who they can contact than servers are because of the same-origin limitation.
Memory - Lots of code and lots of data can consume a lot more memory for your app than a server-generated HTML page. This probably isn't meaningful on a desktop, but could be on a smaller device.
Some of the upsides:
The content can be dynamically rendered with lots of smarts based on the type of user, the screen size, the capabilities of the device, etc... and this local rendering can generally be done more effectively than trying to do it on the server.
You can often do lots of the functions of the app without ever reloading the page, just fetching data from the server or issuing commands to the server with ajax and never reloading the page.
One big downside may be mobile.
Depending on the functionality, a javascript-only page may be slow for those on mobile devices.
The pages may be resource heavy, if you are using one or more libraries and / or accessing lots of information. This could also cost a fair amount for those on mobile devices.
Another downside could be accessibility. Not sure how enabling software for low/no vision users would work with a js only site.
My opinion that such kind of coding is more appropriate for members only areas, that of course are not reachable by search engines.
Provided that you use a good library, that is able to do the layout for you, such as ExtJS, it's an interesting coding. You can build web applications that look similar to desktop applications. Browser differences are smoothed out by the library and expect to have very few problems, if any.
For public websites, in general the SEO argument is a pretty big one. If nobody can find you...
I want to design a web application whose only interface is json i.e. all the http requests receive responses only in json format and dont render any html on the server side. All the form posts convert the form data into a json object and then post it as a string. All the rendering is done by client side javascript.
The one downside of this approach I know is that browsers without javascript wont be able to do much with this architecture but the interaction on the site is rich enough to be useless to non-javascript browsers anyway.
Are there any other downsides of this approach of designing a web application?
It's an increasingly-common pattern, with tools such as GWT and ext-js. Complex web apps such as GMail have been over 90% JS-created DOM for some time. If you are developing a traditional 'journal' type website with mainly written content to be read this approach will be overkill. But for a complex app that wishes to avoid page refreshes it may well be appropriate.
One downside is that not only does it require a browser that supports JavaScript, it is also easy for the computing resources required by the app to creep up to the point where it needs quite a powerful browser. If you develop in Chrome on a top-end PC you might come to run the app on a less powerful machine such as a netbook or mobile device and find it has become quite sluggish.
Another downside is you lose the opportunity to use HTML tools when working on your pages, and that viewing your application's pages' DOM trees under Firebug or Chrome Developer Tools may be hard work because the relationship between the elements and your code may not be clear.
Edit: another thing to consider is that it is more work to make pages more accessible, as keyboard shortcuts may have to be added (you may not be able to use the browser built in behavior here) and users with special needs may find it more difficult to vary the appearance of the app, for instance by increasing font size.
Another edit: it's unlikely now text content on your website will be successfully crawled by search engines. For this reason you sometimes see server created text-only pages representing the same content, that refer browsers to the JS-enabled version of the page.
Other than the issue you point out, there's another: Speed. But it's not necessarily a big issue, and in fact using JSON rather than HTML may (over slower connections) improve rather than hamper speed.
Web browsers are highly optimised to render HTML, both whole pages (e.g., normally) and fragments (e.g., innerHTML and the various wrappers for it, like jQuery's html or Prototype's update). There's a lot you can do to minimize the speed impact of working through your returned data and rendering the result, but nothing is going to be quite as fast as grabbing some HTML markup from the server and dumping it into the browser for display.
Now, that said, it's not necessarily going to be a big problem at all. If you use efficient techniques (some notes in this article), and if you primarily render the results by building up HTML strings you're then going to hand to the brower (again, via innerHTML or wrappers for it), or if you're rending only a few elements at a time, it's unlikely that there will be any perceptible speed difference.
If, instead, you build up substantial trees by creating and appending individual elements via the DOM API or wrappers for it, you're very likely to notice a performance impact. That seems like the right thing to do, but it involves lots of trips across the DOM/JavaScript boundary and means the browser has to present the DOM version of things to your code at all intermediate steps; in contrast, when you hand it an HTML string, it can do its thing and romp through it full-speed-ahead. You can see the difference in this performance test. It's substantial.
Over slower connections, the speed impact may be made up for or even overcome if the JSON data is more compact than the HTML would have been, because of the smaller size on the wire.
You've got to be more mindful of high-latency, low-bandwidth connections when you're building something like this. The likelihood is, you're going to be making a lot of Ajax calls to sync data and grab new data from the server, and the lag can be noticeable if there's a lot of latency. You need a strategy in place to keep the user informed about the progress of any communication between client and server.
In development, it's something that can be overlooked, especially if you're working with a local web server, but it can be killer in production. It means looking into prefetching and caching strategies.
You also need an effective way to manage HTML fragments/templates. Obviously, there are some good modules out there for rendering templates - Mustache.js, Underscore template, etc. - but keeping on top of the HTML fragments can cause some maintenance headaches. I tend to store the HTML templates in separate files, and load them dynamically via Ajax calls (plus caching to minimise HTTP requests).
Edit - another con:
Data syncing - if you use a server database as your data "authority" then it's important to keep data in sync between the server and client. This is even more relevant if changes to data on one client affects multiple clients. You then get into the realms of dealing with realtime, asynchronous updates, which can cause some interesting conceptual challenges. (Fortunately, using frameworks and libraries such as Socket.IO and Backbone.js can really make things easier).
Edit - pros:
There are some huge advantages to this type of application - it's far more responsive, and can really enhance the user experience. Trivial actions that would normally require a round-trip to the server and incur network overhead can now be performed quickly and seamlessly.
Also, it allows you to more effectively couple data to your views. Chances are, if you're handling the data on the client-side, you will have a framework in place that allows you to organise the data and make use of an ORM - whether its Backbone.js, Knockout.js or something similar. You no longer have to worry about storing data in html attributes or in temporary variables. Everything becomes a lot more manageable, and it opens the door for some really sophisticated UI development.
Also also, JavaScript opens up the possibility for event-driven interaction, which is the perfect paradigm for highly interactive applications. By making use of the event loop, you can hook your data directly to user-initiated and custom events, which opens up great possibilities. By hooking your data models directly to user-driven events, you can robustly handle updates and changes to data and render the appropriate output with minimal fuss. And it all happens at high speed.
I think the most important thing is what is your requirement, if you want to build a interactive application, giving desktop like feel then go for client side development. Using Javascript framework like backbone.js or knockout.js will really help in organizing and maintaining the code. The advantages are already detailed out in previous answers. As respect to the performance in rendering with respect to server side rendering is concerned here is a nice blog post which made thinking.
http://openmymind.net/2012/5/30/Client-Side-vs-Server-Side-Rendering/