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

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