I have a question about the structure of an OTP application. Given a fictitious employee time tracking application for my example, I am trying to reason which application layout would make the most sense using OTP. Note: This is just an example fictiicous application, nothing I am working on. Just an example to better understand OTP orchestration. The fictitious employee time tracking application will have employees, schedules, time tracking (clock in/clock out), vacation requests, and overtime submissions. Following OTP, each one should be a process, gen_server for example. Which application makes the most sense: Option Number One Each employee is a gen_server process. The employee process holds the employees name, id number, etc. Each schedule is a gen_server process. A schedule holds an employee’s schedule, which days they work, etc. The schedule process is linked to an employee, or a worker process under an employee gen_server? Each time tracking event is a gen_server process. This process holds a clock-in or clock-out event. Also linked to an employee, or supervised by an employee gen_server, not sure which is better. This does seem like too many processes may accumulate over time though. Each vacation request is a gen_server process This process holds information regarding an employees vacation request, dates, and approval information. Also linked, or supervised by an employee gen_server process. Each overtime submission is a gen_server process This process holds overtime information, and who may have approved it. Also linked to an employee, or supervised by an employee gen_server process. This setup seems like it might spawn too many gen_server processes especially for the clock-in/clock-out. So would the second option be more appropriate? Option Number Two Each employee is a gen_server process. The employee process holds the employees name, id number, etc. Each schedule is a gen_server process. A schedule holds an employee’s schedule, which days they work, etc. The schedule process is linked to an employee, or a worker process under an employee gen_server? Each time events is a gen_server process. This process holds a list of clock-in or click-out events. One process for each employee. So, instead of one process for each clock-in or click-out even, just one process holding a list of clock-in and click-out events. Each vacation requests is a gen_server process Same as the above, one process per employee, containing a list of vacation requests instead of each request being a process itself. Each overtime submissions is a gen_server process Same as the above, one process per employee, containing a list of overtime requests instead of each request being a process itself. I’m very curious to hear your thoughts. Please remember this application is not real, not a web app question and not a persistence question. Just simply curious about people’s opinion on structuring processes.

It’s a common misconception that you should use processes to structure your application. Instead use modules to organize your application and use processes when you need concurrency or shared state between processes. I think you should start by considering the external interface of your application, right now it’s a black box so you can design it however you want. But if you add an HTTP interface you probably want to handle requests in parallel, so the web server will start a process per connection or request. So now you need shared state between these processes which can be solved as you proposed, but your solutions are overly complex. Why a process per employee, why not a single process for all employees, or a single process for your whole database? You should also be careful about using processes to store information that is not transient. You don’t want to lose employee information because the employee process crashed.

lack of persistence, comes to mind.. what happens in catastrophic events? and then the “to spawn or not to spawn” The Erlangelist - To spawn, or not to spawn? eg. your usage of processes could be construed as OO programming with all the side effects and “spaghetti code” of that.. Wouldn’t the employees also work in different locations, and then in different departments, shifts, etc. and that is perhaps also were a genserver boundary starts to make sense.. I like this talk as a good introduction to context mapping/DDD etc. [ElixirDaze 2018 - Building beautiful systems with Phoenix contexts... by Andrew Hao]

It’s just an example, to ask about processed. Persistence isn’t in question. But I know what you’re saying.

My question is just about processes only. Not concerned about persistence, nor Phoenix— the application isn’t even real :smile: just curious to hear about people’s opinion about structuring processes.

Perhaps I didn’t form my question correctly. This application isn’t real, my focus isn’t the interface, nor disk bases persistence. I’m not even saying such an application is practical. Just structuring an application with processes — a pure Elixir OTP application using gen servers. The persistence in my example are gen servers is all. Is it really a misconception to structure an app with processes when you need state? Is that not the whole idea of thinking in processes? I think you’re right, there’s pretty much no standard it’s all up in the air as to how you want to do it.

ETS tables are stored in memory, so I didn’t suggest it for persistence – I suggested it because having a single “source of truth” for state is simpler (at least in my experience) to manage than if it is distributed over multiple processes. Plus :ets has many other helpful functions for iterating over the data which you’d probably have to reimplement yourself if you go with bare genservers.

[image] Matt: Is it really a misconception to structure an app with processes when you need state? Is that not the whole idea of thinking in processes? Yes, that’s the misconception. You should start by structuring your application using modules and then add processes where they are actually needed to solve your business problems.

Yes, I get it. Thank you for your suggestion. What I’m really after and it’s hard to get answers like this because we are all speed readers :laughing: is I’m just curious about people’s opinion structuring an all gen_server application, essentially. One process for each element, one process holding a list of elements, etc.

Yes, ets is awesome. I use it, and mnesia. I don’t know why people hate on mnesia so much. Again, my question isn’t about easy — just: if you were limited to just gens how would you structure this app. Maybe that’s how I should have asked the question :smile:

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Structuring an OTP application

Questions & Help Questions

idi527 June 21, 2018, 8:32am 2

Have you considered using ETS? It is part of OTP after all.