Re: Best (simplest) way to share data between processes

If resource usage isn't an issue, then the _easy_ thing to do, that
would also be easily correct is to have a server doing all the
h/w-related reading and clients talking to that server. Use for the
server the technology you feel most confident with. Eg. you may use
Python's http package. I believe that the server from this package
runs in a single thread, and thus processes all requests
synchronously. So, you'll get synchronization for free.

Then, the rest of the scripts that need to talk to h/w will instead be
talking to this server.

Again, this isn't an _efficient_ solution... but, sometimes you don't
need one. And this one is easy to make, easy to debug, easy to expand.
But, if instead you were looking for a more efficient solution, then,
the general idea that allows the http server to work in this case
would still apply: have a single synchronization program that takes
requests asynchronously, and orders them. So, a basic TCP server would
also work as well as a UNIX socket. Your idea with holding a lock on a
file would also work (in fact, plenty of Linux utilities work that
way, eg. apt-get or yum).

----

If you don't want to change the existing script, then instead of
running them directly, you could run them through batch:
https://man7.org/linux/man-pages/man1/batch.1p.html this is a very
simply queuing program that's available for Linux. It will take care
of synchronization by putting the scripts you want to run in a queue
and executing them one at a time.

On Sun, Jul 7, 2024 at 11:12 PM Chris Green via Python-list
<python-list@python.org> wrote:

>
I have a Raspberry Pi in my boat that uses I2C to read a number of
voltages and currents (using ADS1115 A2D) so I can monitor the battery
condition etc.
>
At present various different scripts (i.e. processes) just read the
values using the I2C bus whenever they need to but I'm pretty sure
this (quite rarely) results in false readings because two processes
try to read at the same time.
>
Thus I'm looking for ways to prevent simultaneous access.
>
One fairly obvious way is to have single process/script which reads
the A2D values continuously and writes them to a file.  All other
scripts then read from the file as needed, a simple file lock can then
be used to prevent simultaneous access (well, simultaneous access when
the writing process is writing).
>
Is this the simplest approach?  Are there better ways using
multiprocess?  (They look more complicated though).
>
The I2C bus itself has a mutex but I don't think this guarantees that
(for example) an A2D reading is atomic because one reading takes more
than one I2C bus access.
>
Would a mutex of some sort around each I2C transaction (i.e. complete
A2D reading) be a better way to go?
>
--
Chris Green
·
--
https://mail.python.org/mailman/listinfo/python-list