SE: restore default handlers?

Bob Sneidar bobsneidar at iotecdigital.com
Wed Feb 7 18:58:40 EST 2018


OIC I thought the question was, could 2 processes have the same sqlite db open. I don't think the LC engine opens the file over and over in different threads. I also think that it closes the connection when it is idle, hence the new DBID. 

I just checked for sqlite V3, and it appears multiple processes *can* have the db opened but only one can write at the same time. I do not think that was true for older versions. I think too that Spiceworks tickles the db to keep the connection open continually. The following is from the sqlite FAQ:

Bob S

(5) Can multiple applications or multiple instances of the same application access a single database file at the same time?

Multiple processes can have the same database open at the same time. Multiple processes can be doing a SELECT at the same time. But only one process can be making changes to the database at any moment in time, however.

SQLite uses reader/writer locks to control access to the database. (Under Win95/98/ME which lacks support for reader/writer locks, a probabilistic simulation is used instead.) But use caution: this locking mechanism might not work correctly if the database file is kept on an NFS filesystem. This is because fcntl() file locking is broken on many NFS implementations. You should avoid putting SQLite database files on NFS if multiple processes might try to access the file at the same time. On Windows, Microsoft's documentation says that locking may not work under FAT filesystems if you are not running the Share.exe daemon. People who have a lot of experience with Windows tell me that file locking of network files is very buggy and is not dependable. If what they say is true, sharing an SQLite database between two or more Windows machines might cause unexpected problems.

We are aware of no other embedded SQL database engine that supports as much concurrency as SQLite. SQLite allows multiple processes to have the database file open at once, and for multiple processes to read the database at once. When any process wants to write, it must lock the entire database file for the duration of its update. But that normally only takes a few milliseconds. Other processes just wait on the writer to finish then continue about their business. Other embedded SQL database engines typically only allow a single process to connect to the database at once.

However, client/server database engines (such as PostgreSQL, MySQL, or Oracle) usually support a higher level of concurrency and allow multiple processes to be writing to the same database at the same time. This is possible in a client/server database because there is always a single well-controlled server process available to coordinate access. If your application has a need for a lot of concurrency, then you should consider using a client/server database. But experience suggests that most applications need much less concurrency than their designers imagine.

When SQLite tries to access a file that is locked by another process, the default behavior is to return SQLITE_BUSY. You can adjust this behavior from C code using the sqlite3_busy_handler() or sqlite3_busy_timeout() API functions.


> On Feb 7, 2018, at 10:56 , Ralph DiMola via use-livecode <use-livecode at lists.runrev.com> wrote:
> 
> Mike,
> 
> Yes, the DBID will increment and the DB will be opened multiple time. I have
> SQLite DBs open in the IDE while testing and open in MS Access via ODBC at
> the same time. If both apps are reading then no problem. There is no real
> good transactional record locking in SQLite. LC will throw an error if you
> are  even reading the record in another program at the same time as an
> update happens in LC. You can use these LC errors to do a retry for the
> update.
> 
> Ralph DiMola





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