The serial nature of lock management screams performance bottleneck A big consideration here is lock in Java

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The serial nature of lock management screams performance bottleneck A big consideration here is lock
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granularity, as the fewer locks required the less of a bottleneck you'll have A Coarse-Grained Lock (438) can address lock table contention
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With a system transaction pessimistic locking scheme, such as "SELECT FOR UPDATE " or entity EJBs, deadlock is a distinct possibility because these locking mechanisms will wait until a lock becomes available Think of deadlock this way Two users need resources A and B If one gets the lock on A and the other gets the lock on B, both transactions might sit and wait forever for the other lock Given that we're spanning multiple system transactions, waiting for a lock doesn't make much sense, especially since a business transaction might take 20 minutes Nobody wants to wait for those locks And this is good because coding for a wait involves timeouts and quickly gets complicated Simply have your lock manager throw an exception as soon as a lock is unavailable This removes the burden of coping with deadlock
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A final requirement is managing lock timeouts for lost sessions If a client machine crashes in the middle of a transaction that lost transaction is unable to complete and release any owned locks This is a big deal for a Web application where sessions are regularly abandoned by users Ideally you'll have a timeout mechanism managed by your application server rather than make your application handle timeouts Web application servers provide an HTTP session for this Timeouts can be implemented by registering a utility object that releases all locks when the HTTP session becomes invalid Another option is to associate a timestamp with each lock and consider invalid any lock older than a certain age
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Pessimistic Offline Lock is appropriate when the chance of conflict between concurrent sessions is high A user should never have to throw away work Locking is also appropriate when the cost of a conflict is too high regardless of its likelihood Locking every entity in a system will almost surely create tremendous data contention problems, so remember that Pessimistic Offline Lock is very complementary to Optimistic Offline Lock (416) and only use Pessimistic Offline Lock where it's truly required
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If you have to use Pessimistic Offline Lock, you should also consider a long transaction Long transactions are never a good thing, but in some situations they may be no more damaging than Pessimistic Offline Lock and much easier to program Do some load testing before you choose
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Don't use these techniques if your business transactions fit within a single system transaction Many system transaction pessimistic locking techniques ship with the application and database servers you're already using, among them the "SELECT FOR UPDATE" SQL statement for database locking and the entity EJB for application server locking Why worry about timeouts, lock visibility, and such, when there's no need to Understanding these locking types can certainly add a lot of value to your implementation of Pessimistic Offline Lock Understand, though, that the inverse isn't true! What you read here won't prepare you to write a database manager or transaction monitor All the offline locking techniques presented in this book depend on your system having a real transaction monitor of its own
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Example: Simple Lock Manager (Java)
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In this example we'll first build a lock manager for exclusive read locks remember that you need these locks to read or edit an object Then we'll demonstrate how the lock manager might be used for a business transaction that spans multiple system transactions
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The first step is to define our lock manager interface
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interface ExclusiveReadLockManager public static final ExclusiveReadLockManager INSTANCE = (ExclusiveReadLockManager) PluginsgetPlugin(ExclusiveReadLockManagerclass); public void acquireLock(Long lockable, String owner) throws ConcurrencyException; public void releaseLock(Long lockable, String owner); public void relaseAllLocks(String owner);
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Notice that we're identifying lockable with a long and owner with a string Lockable is a long because each table in our database uses a long primary key that's unique across the entire system and so serves as a nice lockable ID (which must be unique across all types handled by the lock table) The owner ID is a string because the example will be a Web application, and the HTTP session ID makes a good lock owner within it
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We'll write a lock manager that interacts directly with a lock table in our database rather than with a lock object Note that this is our own table called lock, like any other application table, and not part of the database's internal locking mechanism Acquiring a lock is a matter of successfully inserting a row into the lock table Releasing it is a matter of deleting that row Here's the schema for the lock table and part of the lock manager implementation:
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table lock create table lock(lockableid bigint primary key, ownerid bigint) class ExclusiveReadLockManagerDBImpl implements ExclusiveLockManager private static final String INSERT_SQL = "insert into lock values( , )"; private static final String DELETE_SINGLE_SQL = "delete from lock where lockableid = and ownerid = "; private static final String DELETE_ALL_SQL = "delete from lock where ownerid = "; private static final String CHECK_SQL = "select lockableid from lock where lockableid = and ownerid = "; public void acquireLock(Long lockable, String owner) throws ConcurrencyException { if (!hasLock(lockable, owner)) { Connection conn = null; PreparedStatement pstmt = null; try { conn = ConnectionManagerINSTANCEgetConnection(); pstmt = connprepareStatement(INSERT_SQL); pstmtsetLong(1, lockablelongValue()); pstmtsetString(2, owner); pstmtexecuteUpdate(); } catch (SQLException sqlEx) { throw new ConcurrencyException("unable to lock " + lockable); } finally { closeDBResources(conn, pstmt); } } } public void releaseLock(Long lockable, String owner) { Connection conn = null; PreparedStatement pstmt = null; try { conn = ConnectionManagerINSTANCEgetConnection(); pstmt = connprepareStatement(DELETE_SINGLE_SQL); pstmtsetLong(1, lockablelongValue()); pstmtsetString(2, owner); pstmtexecuteUpdate(); } catch (SQLException sqlEx) { throw new SystemException("unexpected error releasing lock on " + lockable); } finally { closeDBResources(conn, pstmt); }
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Not shown in the lock manager are the public releaseAllLocks() and the private hasLock() methods releaseAllLocks() does exactly as its name implies and releases all locks for an owner hasLock() queries the database to check if an owner already owns a lock It's not uncommon for session code to attempt to acquire a lock it already owns This means that acquireLock() must first check that the owner doesn't already have the lock before attempting to insert the lock row As the lock table is usually a point of resource contention, these repetitive reads can degrade application performance It may be necessary for you to cache owned locks at the session level for the ownership checks Be careful doing this
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Now let's put together a simple Web application to maintain customer records First we'll set up a bit of infrastructure to facilitate business transaction processing Some concept of a user session will be required by the layers beneath the Web tier, so we won't be able to rely solely on the HTTP session Let's refer to this new session as the application session to distinguish it from the HTTP session Application sessions will store their ID, a user name, and an Identity Map (195) to cache objects loaded or created during the business transaction They'll be associated with the currently executing thread in order that they be found
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class AppSession private String user; private String id; private IdentityMap imap; public AppSession(String user, String id, IdentityMap imap) { thisuser = user; thisimap = imap; thisid = id; } class AppSessionManager private static ThreadLocal current = new ThreadLocal(); public static AppSession getSession() { return (AppSession) currentget(); } public static void setSession(AppSession session) { currentset(session); }
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We're going to use a Front Controller (344) to handle requests, so we'll need to define a command The first thing each command must do is indicate its intention to either start a new business transaction or continue one that already exists This is a matter of setting up a new application session or finding the current one Here we have an abstract command that provides convenience methods for establishing business transaction context
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interface Command public void init(HttpServletRequest req, HttpServletResponse rsp); public void process() throws Exception; abstract class BusinessTransactionCommand implements Command public void init(HttpServletRequest req, HttpServletResponse rsp) { thisreq = req; thisrsp = rsp; } protected void startNewBusinessTransaction() { HttpSession httpSession = getReq()getSession(true); AppSession appSession = (AppSession) httpSessiongetAttribute(APP_SESSION); if (appSession != null) { ExclusiveReadLockManagerINSTANCErelaseAllLocks(appSessiongetId()); }
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appSession = new AppSession(getReq()getRemoteUser(), httpSessiongetId(), new IdentityMap()); AppSessionManagersetSession(appSession); httpSessionsetAttribute(APP_SESSION, appSession); httpSessionsetAttribute(LOCK_REMOVER, new LockRemover(appSessiongetId())); } protected void continueBusinessTransaction() { HttpSession httpSession = getReq()getSession(); AppSession appSession = (AppSession) httpSessiongetAttribute(APP_SESSION); AppSessionManagersetSession(appSession); } protected HttpServletRequest getReq() { return req; } protected HttpServletResponse getRsp() { return rsp; }
Notice that when we establish a new application session we remove locks for any existing one We also add a listener to the HTTP session's binding events that will remove any locks owned by an application session when the corresponding HTTP session expires
class LockRemover implements HttpSessionBindingListener private String sessionId; public LockRemover(String sessionId) { thissessionId = sessionId; } public void valueUnbound(HttpSessionBindingEvent event) { try { beginSystemTransaction(); ExclusiveReadLockManagerINSTANCErelaseAllLocks(thissessionId); commitSystemTransaction(); } catch (Exception e) { handleSeriousError(e); } }
Our commands contain both standard business logic and lock management, and each command must execute within the bounds of a single system transaction To ensure this we can decorate [Gang of Four] it with a transactional command object Be sure that all locking and standard domain business for a single request occur within a single system transaction The methods that define system transaction boundaries depend on your deployment context It's mandatory to roll back the system transaction when a concurrency exception, and any other exception in this case, is detected, as that will prevent any changes from entering the permanent record data when a conflict occurs
class TransactionalComamnd implements Command public TransactionalCommand(Command impl) { thisimpl = impl; } public void process() throws Exception { beginSystemTransaction(); try { implprocess(); commitSystemTransaction(); } catch (Exception e) { rollbackSystemTransaction(); throw e; } }
Now it's a matter of writing the controller servlet and concrete commands The controller servlet has the responsibility of wrapping each command with transaction control The concrete commands are required to establish business transaction context, execute domain logic, and acquire and release locks where appropriate
class ControllerServlet extends HttpServlet protected void doGet(HttpServletRequest req, HttpServletResponse rsp) throws ServletException, IOException { try { String cmdName = reqgetParameter("command"); Command cmd = getCommand(cmdName); cmdinit(req, rsp); cmdprocess(); } catch (Exception e) { writeException(e, rspgetWriter()); } } private Command getCommand(String name) { try { String className = (String) commandsget(name); Command cmd = (Command) ClassforName(className)newInstance(); return new TransactionalCommand(cmd); } catch (Exception e) { eprintStackTrace(); throw new SystemException("unable to create command object for " + name); } } class EditCustomerCommand implements Command public void process() throws Exception { startNewBusinessTransaction(); Long customerId = new Long(getReq()getParameter("customer_id")); ExclusiveReadLockManagerINSTANCEacquireLock( customerId, AppSessionManagergetSession()getId()); Mapper customerMapper = MapperRegistryINSTANCEgetMapper(Customerclass); Customer customer = (Customer) customerMapperfind(customerId); getReq()getSession()setAttribute("customer", customer); forward("/editCustomerjsp"); } class SaveCustomerCommand implements Command public void process() throws Exception { continueBusinessTransaction(); Customer customer = (Customer) getReq()getSession()getAttribute("customer"); String name = getReq()getParameter("customerName"); customersetName(name); Mapper customerMapper = MapperRegistryINSTANCEgetMapper(Customerclass); customerMapperupdate(customer); ExclusiveReadLockManagerINSTANCEreleaseLock(customergetId(), AppSessionManagergetSession()getId()); forward("/customerSavedjsp"); }
The commands just shown will prevent any two sessions from working with the same customer at the same time Any other command in the application that works with a customer object must be sure either to acquire the lock or to work only with a customer locked by a previous command in the same business transaction Given that we have a hasLock() check in the lock manager we could simply acquire the lock in every command This might be bad for performance, but it would certainly guarantee that we have a lock Implicit Lock (449) discusses other foolproof approaches to locking mechanics
The amount of framework code might seem a bit out of proportion to the amount of domain code Indeed, Pessimistic Offline Lock requires at a minimum choreographing an application session, a business transaction, a lock manager, and a system transaction, which is clearly a challenge This example serves more as an inspiration than as an architecture template, as it lacks robustness in many areas