Hi Richard, Richard Hipp <d...@sqlite.org> writes:
> In shared-cache mode, SQLite acquires a mutex on the shared cache, thus > serializing access. Oh, that's quite a surprise. I would have never said that something like this is going on just by reading the documentation page on shared cache: http://www.sqlite.org/sharedcache.html Specifically, quoting Section 2.2, "Table Level Locking": "When two or more connections use a shared-cache, locks are used to serialize concurrent access attempts on a per-table basis. Tables support two types of locks, "read-locks" and "write-locks". Locks are granted to connections - at any one time, each database connection has either a read-lock, write-lock or no lock on each database table. At any one time, a single table may have any number of active read-locks or a single active write lock. To read data [from] a table, a connection must first obtain a read-lock. To write to a table, a connection must obtain a write-lock on that table." >From this I at the time concluded that there could be multiple connections reading from a table concurrently. But it appears I was wrong. Perhaps this page should be updated to reflect this. Specifically, the last paragraph in Section 2.0: "There are three levels to the shared-cache locking model, transaction level locking, table level locking and schema level locking. They are described in the following three sub-sections." Should probably read something along these lines: "There are four levels to the shared-cache locking model: transaction level locking, table level locking, cache level locking, and schema level locking. They are described in the following three sub-sections." Then a new section should be inserted between 2.2 and 2.3 that reads along these lines: "2.3 Cache Level Locking When a connection needs to access (read or write) data from a table, it must acquire an exclusive mutex on the shared cache. In particular, this means that in the shared cache mode actual access to the database pages, regardless of whether for reading or writing, is always serialized." If you find it suitable, feel free to use this to update the documentation. Boris -- Boris Kolpackov, Code Synthesis http://codesynthesis.com/~boris/blog Compiler-based ORM system for C++ http://codesynthesis.com/products/odb Open-source XML data binding for C++ http://codesynthesis.com/products/xsd XML data binding for embedded systems http://codesynthesis.com/products/xsde _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
