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Client Session State contains a number of advantages In particular, it reacts well in supporting stateless server objects with maximal clustering and failover resiliency Of course, if the client fails all is lost, but often the user expects that anyway
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The arguments against Client Session State vary exponentially with the amount of data involved With just a few fields everything works nicely With large amounts of data the issues of where to store the data and the time cost of transferring everything with every request become prohibitive This is especially true if your stars include an http client
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There's also the security issue Any data sent to the client is vulnerable to being looked at and altered Encryption is the only way to stop this, but encrypting and decrypting with each request are a performance burden Without encryption you have to be sure you aren't sending anything you would rather hide from prying eyes Fingers can pry too, so don't assume that what got sent out is the same as what gets sent back Any data coming back will need to be completely revalidated
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You almost always have to use Client Session State for session identification Fortunately, this should be just one number, which won't burden any of the above schemes You should still be concerned about session stealing, which is what happens when a malicious user changes his session ID to see if he can snag someone else's session Most platforms come up with a random session ID to reduce this risk; if not run, a simple session ID through a hash
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Server Session State
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Keeps the session state on a server system in a serialized form
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In the simplest form of this pattern a session object is held in memory on an application server You can have some kind of map in memory that holds these session objects keyed by a session ID; all the client needs to do is to give the session ID and the session object can be retrieved from the map to process the request
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This basic scenario assumes, of course, that the application server carries enough memory to perform this task It also assumes that there's only one application server that is, no clustering and that, if the application server fails, it's appropriate for the session to be abandoned and all work done so far to be lost in the great bitbucket in the sky
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For many applications this set of assumptions is actually not a problem However, for others it may be problematic There are ways of dealing with cases where the assumptions are no longer valid, and these introduce common variations that add complexity to an essentially simple pattern
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The first issue is that of dealing with memory resources held by the session objects Indeed, this is the common objection to Server Session State The answer, of course, is not to keep resources in memory but instead serialize all the session state to a memento [Gang of Four] for persistent storage This presents two questions: In what form do you persist the Server Session State, and where do you persist it
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The form to use is usually as simple a form as possible, since the accent of Server Session State is its simplicity in programming Several platforms provide a simple binary serialization mechanism that allows you to serialize a graph of objects quite easily Another route is to serialize into another form, such as text fashionably as an XML file
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The binary form is usually easier, since it requires little programming, while the textual form usually requires at least a little code Binary serializations also require less disk space; although total disk space is rarely a concern, large serialized graphs will take longer to activate into memory
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There are two common issues with binary serialization First, the serialized form is not human readable which is a problem if humans want to read it Second, there may be problems with versioning If you modify a class by, say, adding a field after you've serialized it, you may not be able to read it back Of course, not many sessions are likely to span an upgrade of the server software unless it's a 24/7 server where you may have a cluster of machines running, some upgraded and some not
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This brings us to the question of where to store the Server Session State An obvious possibility is on the application server itself, either in the file system or in a local database This is the simple route, but it may not support efficient clustering or failover To support these the passivated Server Session State needs to be somewhere generally accessible, such as on shared server This will support clustering and failover at the cost of a longer time to activate the server although caching may well eliminate much of this cost
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This line of reasoning may lead, ironically to storing the serialized Server Session State in the database using a session table indexed by the session ID This table would require a Serialized LOB (272) to hold the serialized
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Server Session State Database performance varies when it comes to handling large objects, so the performance aspects of this one are very database dependent
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At this point we're right at the boundary between Server Session State and Database Session State (462) This boundary is completely arbitrary, but I've drawn the line at the point where you convert the data in the Server Session State into tabular form
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If you're storing Server Session State in a database, you'll have to worry about handling sessions going away, especially in a consumer application One route is to have a daemon that looks for aged sessions and deletes them, but this can lead to a lot of contention on the session table Kai Yu told me about an approach he used with success: partitioning the session table into twelve database segments and every two hours rotating the segments, deleting everything in the oldest segment and then directing all inserts to it While this meant that any session that was active for twenty-four hours got unceremoniously dumped, that would be sufficiently rare to not be a problem
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All these variations take more and more effort to do, but the good news is that application servers increasingly support these capabilities automatically Thus, it may well be that application server vendors can worry their ugly little heads about them Java Implementation
The two most common techniques for Server Session State are using the http session and using a stateful session bean The http session is the simple route and causes the session data to be stored by the Web server In most cases this leads to server affinity and it can't cope with failover Some vendors are implementing a shared http session capability that allows you to store http session data in a database that's available to all application servers (You can also do this manually, of course)
The other common route is via a stateful session bean, which requires an EJB server The EJB container handles all persistence and passivation, so this makes it very easy to program to The main disadvantage is that the specification doesn't ask the application server to avoid server affinity However, some application servers provide this capability One, IBM's WebSphere, can serialize a stateful session bean into a BLOB in DB2, which allows multiple application servers to get at its state
A lot of people say that, since stateless session beans perform better, you should always use them instead of stateful beans Frankly, that's hogwash Load-test with your environment first to see if you fall into the range of speed difference between stateful and stateless that makes any difference to your application ThoughtWorks has load-tested apps with a couple of hundred of concurrent users and not found any performance problems due to stateful beans on that size of user load If the performance advantage isn't significant for your loads, and stateful beans are easier, then you should use them There are other reasons to be wary of stateful beans failover may be more problematic depending on your vendor, but the performance difference only appears under a heavy load
Another alternative is to use an entity bean On the whole, I've been pretty dismissive of entity beans, but you can use one to store a Serialized LOB (272) of session data This is pretty simple and less likely to raise many of the issues that usually surround entity beans NET Implementation
Server Session State is easy to implement with the built-in session state capability By default NET stores session data in the server process itself You can also adjust the storage using a state service, which can reside on the local machine or on any other machine on the network With a separate state service you can reset the Web server and still retain the session state You make the change between in-process state and a state service in the configuration file, so you don't have to change the application