if it's not critical that a view session have the most recent data this strategy will suffice in Java

Encoding QR in Java if it's not critical that a view session have the most recent data this strategy will suffice
if it's not critical that a view session have the most recent data this strategy will suffice
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If it becomes critical that a business transaction must always have the most recent data, regardless of its intention to edit, use the exclusive read lock This requires that a business transaction acquire a lock simply to load the record Clearly such a strategy has the potential to severely restrict a system's concurrency For most enterprise systems the exclusive write lock will afford much more concurrent record access than this lock will
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A third strategy combines the two lock types to provide the restrictive locking of the exclusive read lock as well as the increased concurrency of the exclusive write lock Called the read/write lock, it's a bit more complicated than the first two The relationship of the read and write locks is the key to getting the best of both worlds:
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Read and write locks are mutually exclusive A record can't be write-locked if any other business transaction owns a read lock on it; it can't be read-locked if any other business transaction owns a write lock on it Concurrent read locks are acceptable The existence of a single read lock prevents any business transaction from editing the record, so there's no harm in allowing any number of sessions as readers once one has been allowed to read
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Allowing multiple read locks is what increases system concurrency The downside of this scheme is that it's a bit nasty to implement and presents more of a challenge for domain experts to wrap their heads around when they're modeling the system
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In choosing the correct lock type think about maximizing system concurrency, meeting business needs, and minimizing code complexity Also keep in mind that the locking strategy must be understood by domain modelers and analysts Locking is not just a technical problem; the wrong lock type, simply locking every record, or locking the wrong types of records can result an ineffective Pessimistic Offline Lock strategy An ineffective Pessimistic Offline Lock strategy is one that doesn't prevent conflict at the onset of the business transaction or that degrades the concurrency of your multi-user system such that it seems more like single-user system The wrong locking strategy can't be saved by a proper technical implementation In fact, it's not a bad idea to include Pessimistic Offline Lock in your domain model
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Once you've decided upon your lock type, define your lock manager The lock manager's job is to grant or deny any request by a business transaction to acquire or release a lock To do its job it needs to know what's being locked as well as the intended owner of the lock the business transaction It's quite possible that your concept of a business transaction isn't some thing that can be uniquely identified, which makes it a bit difficult to pass a business transaction to the lock manager In this case consider your concept of a session, as you're more likely to have a session object at your disposal The terms "session" and "business transaction" are fairly interchangeable As long as business transactions execute serially within a session the session will be fine as a Pessimistic Offline Lock owner The code example should shed some light on this idea
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The lock manager shouldn't consist of much more than a table that maps locks to owners A simple one might wrap an in-memory hash table, or it might be a database table Whatever, you must have one and only one lock table, so if it's in memory be sure to use a singleton [Gang of Four] If your application server is clustered, an in-memory lock table won't work unless it's pinned to a single server instance The database-based lock manager is probably more appropriate once you're in a clustered application server environment
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The lock, whether implemented as an object or as SQL against a database table, should remain private to the
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lock manager Business transactions should interact only with the lock manager, never with a lock object
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Now it's time to define the protocol according to which a business transaction must use the lock manager This protocol has to include what to lock and when, when to release a lock, and how to act when a lock can't be acquired
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What to lock depends upon when to lock, so let's look at when first Generally, the business transaction should acquire a lock before loading the data, as there's not much point in acquiring a lock without a guarantee that you'll have the latest version of the locked item Since we're acquiring locks within a system transaction, however, there are circumstances where the order of the lock and load won't matter Depending on your lock type, if you're using serializable or repeatable read transactions, the order in which you load objects and acquire locks might not matter An option is to perform an optimistic check on an item after you acquire the Pessimistic Offline Lock You should be very sure that you have the latest version of an object after you've locked it, which usually translates to acquiring the lock before loading the data
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Now, what are we locking It seems that we're locking objects or records or just about anything, but what we usually lock is actually the ID, or primary key, that we use to find those objects This allows us to obtain the lock before we load them Locking the object works fine so long as it doesn't force you to break the rule about an object's being current after you acquire its lock
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The simplest rule for releasing locks is to do it when the business transaction completes Releasing a lock prior to completion might be allowable, depending on your lock type and your intention to use that object again within the transaction Still, unless you have a very specific reason to release early, such as a particularly nasty system liveliness issue, stick to doing it upon completion of the business transaction
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The easiest course of action for a business transaction that can't acquire a lock is to abort The user should find this acceptable since Pessimistic Offline Lock should result in failure rather early in the transaction The developer and designer can certainly help the situation by not waiting until late in the transaction to acquire a particularly contentious lock If at all possible acquire all of your locks before the user begins work
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For any given item that you intend to lock, access to the lock table must by serialized With an in-memory lock table it's easiest to serialize access to the entire lock manager with whatever constructs your programming language provides If you need concurrency greater than this affords, be aware you are entering complex territory
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If the lock table is stored in a database the first rule, of course, is to interact with it within a system transaction Take full advantage of the serialization capabilities that a database provides With the exclusive read and exclusive write locks serialization is a simple matter of having the database enforce a uniqueness constraint on the column storing the lockable item's ID Storing read/write locks in a database makes things a bit more difficult since the logic requires reads of the lock table in addition to inserts and so it becomes imperative to avoid inconsistent reads A system transaction with an isolation level of serializable provides ultimate safety as it guarantees no inconsistent reads Using serializable transactions throughout our system might get us into performance trouble, but a separate serializable system transaction for lock acquisition and a less strict isolation level for other work might ease this problem Another option is to investigate whether a stored procedure might help with lock management Concurrency management can be tough, so don't be afraid to defer to your database at key moments
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