Handling client-side state in round-trip based web app can be fairly tricky. The tricky part is when and how to save changed state during the entire session the user has with your server and how to synchronize state seamlessly between the client and the server along the way without slowing down your pages. The statelessness of the web will most probably challenge you now and then.

There are a number of approaches and techniques that can help you and they all have their sweet spots, but also built-in downsides and often unwanted side effects. As in a lot of other areas in software development, it is often important to know much data, you need to handle and what kind of security constraints, your solution should work under.

Classic approaches to handle state across round-trips

In order to handle state in you application, there are a number of classic techniques you can use, each with their own built-in pros and cons. None of them are perfect, as you will see. Below is a brief discussion on pros and cons for the various techniques.

This section of the post turned out to be fairly dense, but I think it’s important and you can really get into trouble, if you don’t know these things.

Here goes:

• Storing state data in cookies was the original solution to the problem. It’s used on almost every web site out there for storing user ids and session ids.
  
  Pros: (1) good and easy for small amounts of data – 4K is considered the limit, generally, but you should never go that far, (2) data can be encrypted by the server – but is then of course inaccessible to the client-side code. Cookies is also where the server stores user- and session ids between requests in all frameworks I know of. (3) Data is personal and bound to the user and the pc he is logged in to.
  
  Cons: (1) cookies can be stolen by malicious software, (2) the browser submits the cookies related to a given hostname in any request to the server, including when requesting .jpg, .css and .js files, so you will hurt performance, if your cookies are to big.
  
  
• State data can also be stored in hidden HTML input fields.
  
  Pros: (1) This way, you can store data in a way that is invisible to the user. (2) If the field is part of a form, its content is automatically submitted as part of the form, when the user submits his input user bookmarks your page, the current state is not saved with the bookmark.
  
  Cons: (1) Transporting the state back to the server, requires that the page is posted back to the server (for instance by the user pressing a submit button) or that some client side code reads the fields and send them. (2) If the data is not encrypted (and it’s not, unless you do it), it can be manipulated easily by a hacker, so you will have to validate it (and you should do that on every round-trip). You should of course validate the incoming data anyway before you stick anything in your database. (3) Remember, that post-backs does not “play well with the web”, because it’s not “back button compatible” (see below). (4) If the user bookmarks your page, the current state is not saved with the bookmark.
  
  
• A variant of storing data in hidden fields is ASP.NET WebForms’ hidden field named ViewState. It’s used by internal WebForms logic to store the state of the view, which in this case is select properties from controls in the page and also a few other things. 
  
  ViewState has the same pros and cons as normal hidden fields, but makes things worse by encrypting data which makes it grow in size. If you develop using the built-in grid components and don’t watch out, you can easily end up with a page weight of say 300K or the like. Then your stuff is suddenly only suited for Intranet usage. This has caused many WebForms developers to spend lots of time on fine tuning pages by disabling ViewState on individual controls in their pages. To be fair, Microsoft did put a lot of effort in improving this in .NET4, but you are still stuck with the post-back requirement. It should also be noted, that it’s fairly easy for a hacker to open up the encrypted ViewState and start experimenting with application’s internals.
  
  
• You can also store state data in all links in your page as query string parameters. This will cause the browser to send the parameters to the server when the user clicks a link. This method has been used a lot by big sites like Amazon and EBay.
  
  Pros: Simple and robust solution, if you can live with the cons and with the fact that your state is public and easily changeable. In Amazon’s case, where I assume they just use it to determine, say, which sidebar ads they want to display on the next page, it is hardly a big problem, if somebody experiments with the values.
  
  Cons: (1) Results in ugly URLs. (2) Has the same performance downsides as described for cookies because the browser also sends the URL of the referring page to the server in an HTTP header as part of every request. Actually, this will cause the browser to send the state data twice when you click a page link. (3) Web servers generally “only” support URLs up to a size of 2K characters. (4) It’s a lot of work to “patch” every link in the page to include all the state data. Functions for this is not standard anywhere, I think (– at least not in ASP.NET). (5) It’s easy for the user to start hacking on your site by messing around with parameters. (6) Foreign web site will get a copy of your state values, if you link or redirect to somewhere else. (7) Client-side code cannot easily interact with these parameters.
  
  
• Fragment identifiers are are a lot like the above query string parameters with regards to pros and cons. Fragment identifiers are the part of the URI that is after a “#” (hash mark). Like “Examples” at the end of: http://en.wikipedia.org/wiki/Fragment_identifier#Examples. The original idea behind it to allow the user to jump quickly between internal sections of a web page, and hence the browser doesn’t request a new page from the server when the user clicks local fragment identifier links on a page. This also means that communication with the server involves sending data using client side code.
  
  Widespread use of the fragment identifier for keeping state is fairly new and almost exclusively used web apps with large amounts of client side logic, so I wouldn’t really consider it a classic technique for storing state, although it has been doable for a lot of years. I consider this technique one big, clever hack, but it works beautifully and I think that we will see this everywhere in the near future.
  
  
• Last, but not least, you can use server side storage, like ASP.NET’s Session State. This is usually a key-value table that resides in memory or is stored in a database on the server-side.
  
  Pros: (1) virtually unlimited storage compared to the above methods. (2) Only a small identifier is transported back and forth between the browser and the server – usually this a session key in a cookie.
  
  Cons: (1) It doesn’t scale well. Letting users take up extra memory means less users per server, which isn’t good. (2) If you stick with the in-memory solution, you will get server affinity, meaning that your user will have to always return back to the same server to be able to get to his data. If your server sits behind a load-balancer, you must use “sticky sessions”. (3) If/when you server crash or is restarted, you loose your data and kick all your users on that server off. (4) These two cons can be avoided by storing session data in a database, but then you pay by loading and saving data from and to the database on every request, which is generally considered “expensive”. (I wonder if not the new, high-speed no-SQL databases, like MongoDB will change this picture soon.) (5) You can’t keep these data forever and will have to delete them at some point. On ASP.NET, the default expiration time for session data in memory is 20 minutes (less than a lunch break). You will turn some users down no matter which expiry you set. If you stick your session data in a database, you will have to run clean-up jobs regularly. (7) Again: client-side code cannot easily interact with these parameters.

Phew. There is a lot to consider here! As I said, I find it important to know both pros and cons, in order to be able to make right architectural decisions.

REST – specifying everything in the request

It’s perhaps appropriate to also mention REST in this post, because it gets a fair amount of buzz these days (even though it’s been around since 2000). And because is about managing state. REST means REpresentational State Transfer.

When evaluating REST, it’s fair to say, that it has the same pros and cons as I listed with query string parameters above (- although REST is using more transport mechanisms than just query strings). The reason is, that one of REST’s novel goals is to make the server stateless, in order to gain scalability. And making the server stateless, of course means that the client must send all relevant state to the server in every request.

So, if you are dealing with complex application state, REST in combination with round-trips will most probably make your requests too heavy and your app won’t perform well. It is however very usable if your app hasn’t got a lot of client-side state to handle. And it’s also a very good way for a “fat” client to communicate to the server.

REST is however a very sound architectural style and it would pay off for most web developers to know and use it everywhere it’s applicable. In the .NET world, REST has not yet caught on well. It’s a pity, but I think it will. It should; it captures the essence of the web and makes you’re your app scalable and robust.

Future options for keeping state on the client side

In the future you can save state on the client side, but it will take years before this is widespread. Google pioneered this with Google Gears, which gave you an in-browser, client-side database and for example enabled you to use Gmail while you were disconnected. This was a good idea and it has now been moved in under the HTML5 umbrella, so the old Gears is now deprecated and will disappear from the web Dec, 2012. I should also mention, that similar features exists in both Silverlight and Flash, which should be no surprise.

What HTML5 will bring, is a number of options:

• Web Storage & DOM Storage
• Indexed DB (formerly WebSimpleDB)
• Web SQL Database

I won’t go into these at this time; the browser vendors haven’t agreed on these yet and as with other HTML5 technologies, it’s still on the “bleeding edge”.

Also: if you plan to store data more or less permanently on he client-side, you better have a plan for a very robust data synchronization mechanism that must kick in, when the same user uses your app from many different devices, as well as robust protection against evil, client-side tinkering with your app’s state.

Conclusions, perspective and “going client/server”

It’s important to recognize that, as a web app developer, you inherit intrinsic problems from the web’s architecture and dealing with state across round-trips is one of them.

A lot of different methods for dealing with this has been developed over the years and all of them has their built-in downsides. Hence, you must choose wisely. One of the above methods might be just what you need. Or might not.

If you need to deliver a really sophisticated UI, then my take is that basing your solution on the round-trip model won’t cut it. The web was not made for sophisticated UIs, so the classic toolbox doesn’t cut it.

There are ways to get around the limitations however: you simply don’t base your solution on round-tripping. You “go client/server” and create a full fletched JavaScript app that lives on a single web page (or a few perhaps). But that’s an entirely different story… and it will have to wait until another time.

While researching links for this post, I came across this MSDN article that seems like a good read: Nine Options for Managing Persistent User State in Your ASP.NET Application. Especially if you work on the ASP.NET stack.