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Overview
Comment: | Add a page that illustrates use of the new experimental sqlite3_unlock_notify() API. |
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f0a5786f014a5dfd791c612e1f1e78d9 |
User & Date: | dan 2009-03-13 15:33:06.000 |
Context
2009-03-13
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16:08 | Minor update to unlock_notify.in. (check-in: 9a56445dbf user: dan tags: trunk) | |
15:33 | Add a page that illustrates use of the new experimental sqlite3_unlock_notify() API. (check-in: f0a5786f01 user: dan tags: trunk) | |
2009-03-06
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04:13 | Added a few more FAQ entries. Minor update and corrections. (check-in: 4072a11193 user: shaneh tags: trunk) | |
Changes
Added pages/unlock_notify.in.
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Before writing to a table, a write (exclusive) lock must be obtained on that table. Before reading, a read (shared) lock must be obtained. A connection releases all held table locks when it concludes its transaction. If a connection cannot obtain a required lock, then the call to [sqlite3_step()] returns SQLITE_LOCKED. <p> This page presents a technique using the SQLite [sqlite3_unlock_notify()] API to create a version of sqlite3_step() that blocks until the required lock is available instead of returning SQLITE_LOCKED immediately, for use in multi-threaded applications. If the sqlite3_blocking_step() function presented to the left returns SQLITE_LOCKED, this indicates that to block would deadlock the system. <p> The [sqlite3_unlock_notify()] API, which is only available if the library is compiled with the pre-processor symbol SQLITE_ENABLE_UNLOCK_NOTIFY defined, is [sqlite3_unlock_notify | documented here]. This page is no substitute for reading the full API documentation! <p><b>The sqlite3_unlock_notify() API</b> <p> After a call to [sqlite3_step()] returns SQLITE_LOCKED, the [sqlite3_unlock_notify()] API may be invoked to register for an unlock-notify callback. The unlock-notify callback is invoked by SQLite after the database connection holding the table-lock that prevented the call to sqlite3_step() from succeeding has finished its transaction and released all locks. For example, if the call to sqlite3_step() is an attempt to read from table X, and some other connection Y is holding a write-lock on table X, then sqlite3_step() will return SQLITE_LOCKED. If sqlite3_unlock_notify() is then called, the unlock-notify callback will be invoked after connection Y's transaction is concluded. The connection that the unlock-notify callback is waiting on, in this case connection Y, is known as the "blocking connection". <p> If a call to sqlite3_step() that attempts to write to a database table returns SQLITE_LOCKED, then more than one other connection may be holding a read-lock on the database table in question. In this case SQLite simply selects one of those other connections at random and issues the unlock-notify callback when that connection's transaction is finished. Whether the call to sqlite3_step() was blocked by one or many connections, when the corresponding unlock-notify callback is issued it is not guaranteed that the required lock is available, only that it may be. <p> When the unlock-notify callback is issued, it is issued from within a call to sqlite3_step() (or sqlite3_close()) associated with the blocking connection. It is illegal to invoke any sqlite3_XXX() API functions from within an unlock-notify callback. The expected use is that the unlock-notify callback will single some other waiting thread or schedule some action to take place later. <p> The algorithm used by the sqlite3_blocking_step() function is as follows: <ol> <li><p> Call sqlite3_step() on the supplied statement handle. If the call returns anything other than SQLITE_LOCKED, then return this value to the caller. Otherwise, continue. <li><p> Invoke sqlite3_unlock_notify() on the database connection handle associated with the supplied statement handle to register for an unlock-notify callback. If the call to unlock_notify() returns SQLITE_LOCKED, then return this value to the caller. <li><p> Call sqlite3_reset() on the statement handle. Since an SQLITE_LOCKED error may only occur on the first call to sqlite3_step() (it is not possible for one call to sqlite3_step() to return SQLITE_ROW and then the next SQLITE_LOCKED), the statement handle may be reset at this point without affecting the results of the query from the point of view of the caller. If sqlite3_reset() were not called at this point, the next call to sqlite3_step() would return SQLITE_MISUSE. <li><p> Block until the unlock-notify callback is invoked by another thread. Then go back to step 1. </ol> <p><b>Writer Starvation</b> <p> Based on the description above, it could be concluded that if there are sufficient database readers reading the same table often enough, it is possible that the table will never become unlocked and that a connection waiting for a write-lock on the table will wait indefinitely. This phenomena is known as writer-starvation. <p> SQLite helps applications avoid this scenario. After any attempt to obtain a write-lock on a table fails (because one or more other connections are holding read-locks), all attempts to open new transactions on the shared-cache fail until one of the following is true: <ul> <li> The current writer concludes its transaction, OR <li> The number of open read-transactions on the shared-cache drops to zero. </ul> <p> Failed attempts to open new read-transactions return SQLITE_LOCKED to the caller. If the caller then calls sqlite3_unlock_notify() to register for an unlock-notify callback, the blocking connection is the connection that currently has an open write-transaction on the shared-cache. This prevents writer-starvation as, if no new read-transactions may be opened and assuming all existing read-transactions are eventually concluded, the writer will eventually have an opportunity to obtain the required o write-lock. <p><b>The pthreads API</b> <p> By the time sqlite3_unlock_notify() is invoked by sqlite3_blocking_step(), it is possible that the blocking connection that prevented the sqlite3_step() call from succeeding has already finished its transaction. In this case, the unlock-notify callback is invoked immediately, before sqlite3_unlock_notify() returns. Or, it is possible that the unlock-notify callback is invoked by a second thread after sqlite3_unlock_notify() is called but before the thread starts waiting to be asynchronously signalled. <p> Exactly how such a potential race-condition is handled depends on the threads and synchronization primitives interface used by the application. This example uses pthreads, the interface provided by modern UNIX-like systems, including Linux. <p> The pthreads interface provides the pthread_cond_wait() function. This function allows the caller to simultaneously release a mutex and start waiting for an asynchronous signal. Using this function, a "fired" flag and a mutex, the race-condition described above may be eliminated as follows: <p> When the unlock-notify callback is invoked, which may be before the sqlite3_blocking_step() thread begins waiting for the asynchronous signal, it does the following: <ol> <li> Obtains the mutex. <li> Sets the "fired" flag to true. <li> Attempts to signal a waiting thread. <li> Releases the mutex. </ol> <p> When the sqlite3_blocking_step() thread is ready to begin waiting for the unlock-notify callback to arrive, it: <ol> <li> Obtains the mutex. <li> Checks if the "fired" flag has been set. If so, the unlock-notify callback has already been invoked. Release the mutex and continue. <li> Atomicly releases the mutex and begins waiting for the asynchronous signal. When the signal arrives, continue. </ol> <p> This way, it doesn't matter if the unlock-notify callback has already been invoked, or is being invoked, when the blocking_step() thread begins blocking. <p><b>Possible Enhancements</b> <p> The code on this page could be improved in at least two ways: <ul> <li> It could manage thread priorities. <li> It could handle a special case of SQLITE_LOCKED that can occur when dropping a table or index. </ul> <p> Even though the sqlite3_unlock_notify() function only allows the caller to specify a single user-context pointer, an unlock-notify callback is passed an array of such context pointers. This is because if when a blocking connection concludes its transaction, if there is more than one unlock-notify registered to call the same C function, the context-pointers are marshalled into an array and a single callback issued. If each thread were assigned a priority, then instead of just signalling the threads in arbitrary order as this implementation does, higher priority threads could be signalled before lower priority threads. <p> If a "DROP TABLE" or "DROP INDEX" SQL command is executed, and the associated database connection currently has one or more actively executing SELECT statements, then SQLITE_LOCKED is returned. If sqlite3_unlock_notify() is called in this case, then the specified callback will be invoked immediately. Unless the other running SELECT statements complete execution in the meantime, re-attempting the "DROP TABLE" or "DROP INDEX" statement will return another SQLITE_LOCKED error. In the implementation of sqlite3_blocking_step() shown to the left, this could cause an infinite loop. <p> The caller could distinguish between this special "DROP TABLE|INDEX" case and other cases by using extended error codes. When it is appropriate to call sqlite3_unlock_notify(), the extended error code is SQLITE_LOCKED_SHAREDCACHE. Otherwise, in the "DROP TABLE|INDEX" case, it is just plain SQLITE_LOCKED. <div style="clear:both"></div> |