| # Wal-Mode Blocking Locks | |
| On some Unix-like systems, SQLite may be configured to use POSIX blocking locks | |
| by: | |
| * building the library with SQLITE\_ENABLE\_SETLK\_TIMEOUT defined, and | |
| * configuring a timeout in ms using the sqlite3\_busy\_timeout() API. | |
| Blocking locks may be advantageous as (a) waiting database clients do not | |
| need to continuously poll the database lock, and (b) using blocking locks | |
| facilitates transfer of OS priority between processes when a high priority | |
| process is blocked by a lower priority one. | |
| Only read/write clients use blocking locks. Clients that have read-only access | |
| to the \*-shm file nevery use blocking locks. | |
| Threads or processes that access a single database at a time never deadlock as | |
| a result of blocking database locks. But it is of course possible for threads | |
| that lock multiple databases simultaneously to do so. In most cases the OS will | |
| detect the deadlock and return an error. | |
| ## Wal Recovery | |
| Wal database "recovery" is a process required when the number of connected | |
| database clients changes from zero to one. In this case, a client is | |
| considered to connect to the database when it first reads data from it. | |
| Before recovery commences, an exclusive WRITER lock is taken. | |
| Without blocking locks, if two clients attempt recovery simultaneously, one | |
| fails to obtain the WRITER lock and either invokes the busy-handler callback or | |
| returns SQLITE\_BUSY to the user. With blocking locks configured, the second | |
| client blocks on the WRITER lock. | |
| ## Database Readers | |
| Usually, read-only are not blocked by any other database clients, so they | |
| have no need of blocking locks. | |
| If a read-only transaction is being opened on a snapshot, the CHECKPOINTER | |
| lock is required briefly as part of opening the transaction (to check that a | |
| checkpointer is not currently overwriting the snapshot being opened). A | |
| blocking lock is used to obtain the CHECKPOINTER lock in this case. A snapshot | |
| opener may therefore block on and transfer priority to a checkpointer in some | |
| cases. | |
| ## Database Writers | |
| A database writer must obtain the exclusive WRITER lock. It uses a blocking | |
| lock to do so if any of the following are true: | |
| * the transaction is an implicit one consisting of a single DML or DDL | |
| statement, or | |
| * the transaction is opened using BEGIN IMMEDIATE or BEGIN EXCLUSIVE, or | |
| * the first SQL statement executed following the BEGIN command is a DML or | |
| DDL statement (not a read-only statement like a SELECT). | |
| In other words, in all cases except when an open read-transaction is upgraded | |
| to a write-transaction. In that case a non-blocking lock is used. | |
| ## Database Checkpointers | |
| Database checkpointers takes the following locks, in order: | |
| * The exclusive CHECKPOINTER lock. | |
| * The exclusive WRITER lock (FULL, RESTART and TRUNCATE only). | |
| * Exclusive lock on read-mark slots 1-N. These are immediately released after being taken. | |
| * Exclusive lock on read-mark 0. | |
| * Exclusive lock on read-mark slots 1-N again. These are immediately released | |
| after being taken (RESTART and TRUNCATE only). | |
| All of the above use blocking locks. | |
| ## Summary | |
| With blocking locks configured, the only cases in which clients should see an | |
| SQLITE\_BUSY error are: | |
| * if the OS does not grant a blocking lock before the configured timeout | |
| expires, and | |
| * when an open read-transaction is upgraded to a write-transaction. | |
| In all other cases the blocking locks implementation should prevent clients | |
| from having to handle SQLITE\_BUSY errors and facilitate appropriate transfer | |
| of priorities between competing clients. | |
| Clients that lock multiple databases simultaneously must be wary of deadlock. | |