Lock Modes in Java

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Lock Modes
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Lock mode (or lock type) is the term for the type of lock a DBMS has arranged for a transaction When DBMS makers pretend to offer a bizarre plethora of lock modes, we must force the truth from them: there are only three lock modes, and usually two are enough The rest are fine distinctions and nuances, and we won't discuss them here The three lock modes are shared, update, and exclusive
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A lock's mode determines (a) what other locks can coexist with this lock and (b) whether other transactions have read and/or write permission when this lock exists (For the record, a latch is an exclusive lock on anything In this section, though, we're discussing only the non-latch locks those used for locking rows, pages, or tables and we'll use the generic word "object" to mean a row, a page, or a table) The coexistence rules for locks are simple
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A shared lock may coexist with any number of other shared locks, or with one update lock on the same object An update lock may coexist with any number of shared locks, but not with another update lock nor with an exclusive lock on the same object An exclusive lock on an object may not coexist with any other lock on that object
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Table 15-1 shows these rules graphically The lock rules for read and/or write permission are not so simple It's often said that you need a shared or update lock to read, and you need an exclusive lock to write, but that's not always true In this context, "read" means either "access" or "add to result set," while "write" means "data change" it doesn't imply "physical write to disk," though that can certainly happen In simple terms, though, these are the rules Table 15-1 Lock Modes and Coexistence Locks on One Object Comment Shared Shared Shared Shared Shared n shared locks may coexist with one another Shared Shared Update Shared Shared n shared locks may coexist with one update lock Exclusive one exclusive lock may not coexist with any other lock
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Shared locks are for reading At most levels you can't read an object unless you have a shared lock on it Update locks are for reading when updating is planned They're not strictly necessary, but are used by most DBMSs Despite the name, if you have an update lock on an object you can not write the object Update locks are like shared locks for permissions, but like exclusive locks for coexistence Exclusive locks are for writing If you have an exclusive lock on an object, you can write that object Because of this, some people call exclusive locks "intent-to-update" locks, a term you should avoid, because there are other locks with similar names You need an exclusive lock to write, and an exclusive lock will prevent other transactions from acquiring shared locks in order to read Exclusive locks are thus the barriers to concurrency
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Usually a transaction reads an object before it tries to write the object, so an exclusive lock is normally an upgrade from a shared or update lock For example, assume a transaction reads a table and gets a shared lock on Row #1 Later on, the transaction does a data change, so it changes the shared lock on Row #1 to an exclusive lock on Row #1 We'll look at how locks affect read/write permissions in greater detail later in this chapter
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Versioning
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The description of lock modes applies to all of the Big Eight except InterBase and Oracle These DBMSs do use exclusive locks, and they do use shared locks for tables, but they have a variation: sometimes they don't use shared locks for rows Instead, they use versioning, also known as Multi Version Concurrency Control (MVCC) Here's how Oracle handles MVCC
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The DBMS keeps a log of data changes A changed page has two versions: the original is in the log file, and the new version is in the main file The DBMS also keeps a global big-integer serial number called the System Change Number (SCN) When a new transaction starts, Oracle increments the SCN and assigns the incremented integer as the SCN for that transaction When a page is written, Oracle stores the transaction's SCN in the page, and the transaction's SCN becomes the SCN for that page
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