What Is a Lock in Java

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What Is a Lock
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Physically, a lock is one of three things: a latch, a mark on the wall, or a RAM record
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Latches
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A latch is a low-level on-off mechanism that ensures two processes or threads can't access the same object at the same time You can make latches with, for example, the MS Windows NT API CreateMutex or CreateSemaphore functions Consider the following situation (we'll call it Situation #1)
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Transaction #1 reads the first byte of a row in shared memory Transaction #2 interrupts and clears the row Transaction #1 continues to read, and probably crashes unacceptable!
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To avoid the crash, Situation #1 must change Here's what should happen
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Transaction #1 gets a latch associated with the row it's going to read Transaction #1 reads the first byte of the row in shared memory Transaction #2 interrupts and tries to get the same latch Transaction #2 fails to acquire the latch so it goes back to sleep Transaction #1 continues to read, and does not crash Transaction #1 releases the latch Transaction #2 interrupts again, gets the latch, and clears the row Transaction #2 releases the latch
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More concisely, the second situation uses mutexes or semaphores or EnterCriticalSection calls so that, when one thread starts to read a row, other threads are prevented from interrupting the read and changing the end of the row MySQL uses a single statement latch for all processes, which means that two SQL statements cannot execute at the same time The rest of the Big Eight are less extreme; they latch only while reading and writing bits of shared memory A latch must be on for every shared bit of memory and must go off as soon as the memory has been read/written Generally a transaction holds a maximum of one latch at any time, and that time is less than one millisecond A second transaction that tries to get the same latch is blocked, so it must either spin (ie, retry after an interval), or it must be enqueued (ie, get in line) The words "blocked," "spin," and "enqueued" are forgettable jargon, but "latch" is a vital word because all multi-thread or multi-process DBMSs use latches
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Marks on the wall
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A mark on the wall (properly known as an ITL slot) is used occasionally, primarily by Oracle, to mark a row in a page of a data file By putting a mark right beside the row accessed by a transaction, the DBMS ensures that no other transaction has to spend much time checking whether the row is locked This mechanism only works when the number of locks in a page is small enough to fit in the page, so marks on the wall are appropriate mostly for exclusive locks (see "Lock Modes" later in this chapter)
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RAM records
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A RAM record (or lock record) is the usual mechanism for locks of rows, pages, or tables The RAM record is a permanent memory area that contains data describing what's locked, and how For example, Microsoft's 32-byte lock records contain the information shown in Figure 15-1 Figure 15-1 A Microsoft lock record
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Keeping lock records in memory can cause high overhead For example, if lock records are in order by time of creation rather than in order by RID or indexed by RID, they're quite a jumble When the DBMS wants to lock a row, it must first scan all the lock records to see whether another transaction already has a lock on that row So if Transaction #1 has done a scan and locked all of a table's rows (assume there are 50,000 rows), and then Transaction #2 does a scan of the same table, Transaction #2 must, on average, check 50,000/2 locks for each page for a total of 50,000 * (50,000/2) = 1,250,000,000 checks To prevent such unacceptable situations, a DBMS will reduce lock longevity and lock count through practices like escalation We'll discuss escalation later in this chapter When the DBMS encounters a lock that it can't pass, it has two choices: it can return an error to the user (NO WAIT), or it can try spinning and enqueuing (WAIT) Oracle lets you choose whether you want NO WAIT or WAIT Other DBMSs insist on WAIT, but they will time out if a specific period expires (say, one minute) Generally the DBMS vendors think it's better to WAIT, because users who receive an error message will probably just try again from the beginning of the transaction
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