Re: Supervisor and configuration update — how to do it?

[Michael Truog]

On 5/25/20 11:08 AM, Max Lapshin wrote:
> I'm afraid that I haven't got your idea.
>
>
> Look, I have a live video stream. It collects video frames into big
> chunks. There is a configuration setting that allows to change size of
> these chunks: from 1 seconds to 10 seconds.
>
> This configuration must be done without any restarts.
>
> How do you advice to do this configuration?
I am a fan of using process messages to change configuration, for two 
main reasons:
1) The configuration data stays consistent because the changes are 
controlled (i.e., the state changes go through sequential states, 
keeping the number of possible states small, making things easier to 
test and understand)
2) Specific errors when configuration problems occur are able to be 
handled, probably by reporting the problem to who changed the 
configuration and either avoiding or limiting the configuration change.  
A configuration error during startup should cause the startup to fail, 
to avoid the undefined runtime length pursuing erroneous operation.

Assuming a gen_server process, use a call if the configuration change is 
able to return an error.  If no error is possible, e.g., if a record 
field is updated and that is all that happens, use a cast.  The 
configuration change might be slow when changing 1000s of processes, but 
the messages can be done in parallel and the benefits (everything can be 
tested in a controlled way, all the configuration states are valid, no 
inconsistent configuration data will occur, etc.) are better than 
pursuing the global data approach.

Best Regards,
Michael