>Hi,I looked at a few of the multithreading docs and posts and it looks >like SQLite3 supports reads and writes from multiple threads provided >SQLite3 is compile with THREADSAFE option.
The SQLite code is threadsafe. It is singly entrant on a per connection basis. The three THREADSAFE options invoke various levels of protection for programmers who do not understand multithreaded programming. THREADSAFE=0 disables all thread checks. It is intended where the library will only be used from a single thread. Ever. THe same thread. Always. THREADSAFE=1 enables SERIALIZATION within the SQLite library itself to prevent programmers from violating the single-entrancy rule per connection. If you attempt multiple entrances, then one will succeed and the others will go into a scheduler WAIT until the first one is completed. THey will then proceed one after another until they are all complete, with only one ever executing at a time, per connection. THREADSAFE=2 disables the mutexes used for SERIALIZATION. If you violate the single entrancy per connection requirement then it is likely that you will experience the rapture. >In this case each thread has it own DB connection. In which case? >My question is if I have a single DB connection which is used >by multiple threads then are simultaneous reads and writes supported >from these threads. Yes. But only one thread per connection will be permitted to execute code within the library. With THREADSAFE=1 the library itself will ensure this is enforced. With THREADSAFE=2 you must enforce this requirement. >I understand writes will lock the DB but other than this will >this work. Yes. However all operations on a single connection operate on a single connection. That is, there is ONE transaction context per connection and "multiple threads" are irrelevant to this fact. On a single connection requests are MULTI-LEAVED. Context is by connection, not by statement or thread. >Basically are there issues if a read is happening on the DB >connection can another read happen in the same DB connection from another >thread. Define "issues". Generally no. >On the same thread this is not an issue. I was wondering whether there could >be issues when doing from multiple threads. No. Only one thread is executing within the library at a time, per connection. There is no difference between multiple threads and multi-leaving the same sequence of requests on a single thread. >The reason I am sharing a single >connection with multiple threads is because I need to listen to the DB >change notification. Currently SQLite3 supports listening to >notifications >from a single connection only. For this reason I am maintaining a single >connection shared by multiple threads.ThanksPrashant _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
