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    <p>The general answer to issues of data staleness in a client is either polling, or server push (<a href="http://en.wikipedia.org/wiki/Comet_(programming)" rel="nofollow noreferrer">Comet</a>), depending on how important it is to have up-to-date information.</p> <p>With polling, your client sends a request for fresh information every N minutes and shows the updated data in the UI (preferably not reloading the whole UI, just parts that have updated).</p> <p>With server-pushing, your client has a persistent open connection to the server, reconnecting as soon as the connection times out, and whenever the server has new data to tell the client, it does so immediately. This has the advantage of less staleness in the client, but more overhead in total because of all of these persistent connections being made.</p> <p>Locking is difficult, because it requires a lot of coordination among all parties. The server has to tell the second client to ask for a model that it is locked, then tell it again when it's unlocked, and the client has to know whether it still wants to make a change at all (what if user-A made the same change user-B wanted to make?). In general keeping all clients as up-to-date as is reasonable is the best answer.</p> <p>To work with your example, after user-A updates a model, the server publishes to anyone who cares that the model has been changed, and user-B's client updates accordingly. Same when user-B updates the model, user-A can tell right away (or after their client polls again).</p> <p>The <a href="http://code.google.com/p/rocket-gwt/" rel="nofollow noreferrer">rocket-gwt</a> project includes support for Comet, and I've heard good things about it, though I've never tried it myself.</p> <p>A similar option, though not immediately usable, is <a href="http://dev.w3.org/html5/websockets/" rel="nofollow noreferrer">HTML5 WebSockets</a>, which removes the need for a persistent connection to the server by opening a specialized direct "channel" between the server and the client. Unfortunately, WebSockets are currently only supported on a small subset of browsers. Hopefully as adoption grows this will be a better option. That being said, if you want to use WebSockets in GWT, I have written <a href="http://github.com/ImJasonH/websocket-gwt" rel="nofollow noreferrer">some code</a> to do it (shameless self-promotion)</p>
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    1. POHandling data update/freshness issue in web-app in general (or GWT specifically)
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    1. USJason Hall
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    1. POHandling data update/freshness issue in web-app in general (or GWT specifically)
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    1. COHi Jason, Thank you for the detailed answer (unfortunately I can't +1 you yet according to the reputation rule). I was hoping to avoid either solution (polling or comet) because that would add an extra layer/complexity to the web-app itself. Client-side polling would require a Xms number that plays well with most users. But if some users would perform the update operation faster than the polling time limit, additional handling would be required (telling them that someone else have already updated the info).
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    2. COYou definitely don't want to be polling more than once per second, or even once per minute. So then the problem becomes one of knowing when two write operations conflict, giving one user in that case an error message, and showing them the current state of the data so that they can decide if they still want to make a change. This will probably be more of a question for your server-side architecture, particularly your database layer. Is your server talking to a separate database server, and what kind of database is it?
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    3. COThe database is a typical RDBMS (let's say MySQL). One of the projects I used to work on had to use version for every persisted object. One of the advantages of using version is to detect this particular issue. But it feels like an added complexity.
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