Does the MATLAB Interface to SQLite support concurrency? - matlab

Question

SQLite as a database supports varying degrees of concurrency depending on version and settings, therefore I was expecting the MATLAB Interface to SQLite in the Database Toolbox would support some level of concurrency. And when a database access fails, it should at the very least show errors.
However, when I used the following sniplet,
conn=sqlite("sqlite_concurr_test.db","create");
conn.close();
ppool=parpool(4);
ff=parallel.Future.empty();
disp("Write 500 numbers")
for ii=1:500
ff(ii)=parfeval(ppool,#writeOne,0,ii);
end
for ii=1:500
ff(ii).wait()
end
delete(ppool);
conn=sqlite("sqlite_concurr_test.db");
readback=conn.sqlread("test");
disp("Readback "+num2str(size(readback,1))+" numbers");
function writeOne(ii)
conn=sqlite("sqlite_concurr_test.db");
conn.sqlwrite("test",array2table(ii));
conn.close();
end
I got the unexpected result of
Starting parallel pool (parpool) using the 'Processes' profile ...
Connected to the parallel pool (number of workers: 4).
Write 500 numbers
Parallel pool using the 'Processes' profile is shutting down.
Readback 74 numbers
This indicates that some database write did not occur, and there are no error reports.
What can I do to change this behavior? Is there anything I should do to assure parallel access, or at the very least get notifications if anything goes wrong?

Answer 1

Whatever the configuration of your sqlite3 database connection you can only have one simultaneous writer. With wal mode you can have a single writer running concurrently with multiple reader. In your code sample whatever this statement does conn.sqlwrite("test",array2table(ii));, you don't check for returned error if any. So you probably have errors on concurrent writes, but you actully don't detect them.