How are the processes supposed to interact with the database? Without
extra synchronization logic, SQLite.jl gives (occasionally)
ERROR: LoadError: On worker 2:
SQLite.SQLiteException("database is locked")
which on the face of it suggests that all workers are using the same
connection, although I opened the DB separately in each process.
(I think we should get "busy" instead of "locked", but then still have no
good way to test for this and wait for a wake-up signal.)
So we seem to be at least as badly off as the original post, except with DB
calls instead of simple writes.
We shouldn't have to stand up a separate multithreaded DB server just for
this. Would you be kind enough to give us an example of simple (i.e. not
client-server) multiprocess DB access in Julia?
On Saturday, October 15, 2016 at 9:40:17 AM UTC-4, Steven Sagaert wrote:
>
> It still surprises me how in the scientific computing field people still
> refuse to learn about databases and then replicate database functionality
> in files in a complicated and probably buggy way. HDF5 is one example,
> there are many others. If you want to to fancy search (i.e. speedup search
> via indices) or do things like parallel writes/concurrency you REALLY
> should use databases. That's what they were invented for decades ago.
> Nowadays there a bigger choice than ever: Relational or non-relational
> (NOSQL), single host or distributed, web interface or not, disk-based or
> in-memory,... There really is no excuse anymore not to use a database if
> you want to go beyond just reading in a bunch of data in one go in memory.
>
> On Monday, October 10, 2016 at 5:09:39 PM UTC+2, Zachary Roth wrote:
>>
>> Hi, everyone,
>>
>> I'm trying to save to a single file from multiple worker processes, but
>> don't know of a nice way to coordinate this. When I don't coordinate,
>> saving works fine much of the time. But I sometimes get errors with
>> reading/writing of files, which I'm assuming is happening because multiple
>> processes are trying to use the same file simultaneously.
>>
>> I tried to coordinate this with a queue/channel of `Condition`s managed
>> by a task running in process 1, but this isn't working for me. I've tried
>> to simiplify this to track down the problem. At least part of the issue
>> seems to be writing to the channel from process 2. Specifically, when I
>> `put!` something onto a channel (or `push!` onto an array) from process 2,
>> the channel/array is still empty back on process 1. I feel like I'm
>> missing something simple. Is there an easier way to go about coordinating
>> multiple processes that are trying to access the same file? If not, does
>> anyone have any tips?
>>
>> Thanks for any help you can offer.
>>
>> Cheers,
>> ---Zachary
>>
>
