[erlang-questions] How to think and reason when designing process- and message-centric systems?

Sergey Safarov <>
Thu Dec 15 17:56:07 CET 2016


As guide for first step in erlang I use
http://learnyousomeerlang.com/contents

Think it will help you.

чт, 15 дек. 2016, 18:33 Loïc Hoguin <>:

> Welcome,
>
> On 12/15/2016 01:04 PM, IRLeif wrote:
> > Coming from an object-oriented and data-centric background, I have
> > cognitive difficulties when it comes to conceptualizing, thinking about
> > and designing systems consisting of modules, processes and key-value
> > data stores.
> >
> > My brain reverts to thinking about classes, objects, inheritance trees,
> > encapsulation and SQL-style relational data models. I'm afraid this
> > could lead to unidiomatic Erlang system architectures and
> > implementations, which would be undesirable.
>
> That's what brains do, unfortunately.
>
> Let me stop you there by correcting one small thing: SQL-style
> relational data models can still apply depending on the system you are
> building. PostgreSQL usage is not uncommon in the Erlang world, at least
> for small to medium systems.
>
> Classes, objects, and so on, on the other hand, do not exist, and
> thinking in terms of objects will have a negative impact on the design
> of your system.
>
> How do you solve this? Well it depends on your experience outside of OO.
> Here are a few early tips, apply the relevant ones:
>
> - Have you used a non-OO imperative language? C for example. In these
> languages, you only have functions. In Erlang, you only have functions.
> Erlang is closer to C than OO, so switch your brain to C-style
> programming instead of OO.
>
> If necessary, spend a few hours writing a short C program (or another
> language as long as you don't use/write OO code) and then switch to
> Erlang. This will set your mind on the right path.
>
> - Have you ever heard of the best practice that dictates that variables
> should not be reused? Erlang basically enforces that.
>
> When writing that short program, make sure you do not reuse variables.
>
> - Have you ever written code that recurses over directories to for
> example file files matching a pattern? You can't do that with only
> loops, regardless of the language.
>
> Have the short program use recursion instead of loops.
>
> After doing this your mind will be very close to how it needs to be to
> write sequential Erlang programs. The rest is minor syntax differences
> like commas and semicolons or uppercase first letter for variable names.
>
> 95% of the Erlang code you write is short sequential programs. If you
> have experience with microservices, then think of Erlang processes as
> microservices. If you have experience with client/server development,
> then think of them all as servers that communicate to each other in a
> mostly client/server fashion. The only difference is that they
> communicate using Erlang messages.
>
> It definitely helps to reimplement something like a gen_server or
> gen_statem as you will better grasp links, monitors and timeouts. These
> are useful to have solid systems; although you can do mostly without and
> still get results.
>
> > Here are some of the essential complexities I have difficulties grasping:
> >
> > A) Identifying discrete modules and processes and finding good names for
> > them.
>
> There's basically two kinds of modules in Erlang: those implementing a
> process, and those containing useful functions. For the latter it should
> be obvious: group all the related functions together. For the former,
> you are almost required to put them in a module as you implement
> behaviours, so no big trouble there.
>
> As for naming, well, good luck with that.
>
> I generally follow this pattern: <ns>_<name>[_<suffix>] where <ns> is a
> namespace I put in all modules of my application (often the name of the
> application), <name> is the descriptive name (for example 'router' or
> 'http') and <suffix> is the type of the module, for example 'server',
> 'sup' for supervisor, 'h' for Cowboy handler, 't' for a module
> containing functions for converting from/to and manipulating a type, and
> so on.
>
> > D) Designing a sensible persistent data model with Mnesia or other NoSQL
> > data models (e.g. using CouchDB).
>
> Depending on what you are doing you might not need those. And if you do,
> I would advise not jumping into it early. Start with what you know.
> Erlang is a big enough change as it is.
>
> > E) Deciding which processes should read and write persistent data
> records.
>
> Start simple, measure, then decide what to do about it. Pools of
> connections are not uncommon, and already written for you in most cases.
>
> > F) Incorporating global modules/"shared facilities" like event handlers,
> > loggers, etc.
>
> Don't know what you mean with event handlers exactly, but as far as
> logging goes, use either error_logger or lager depending on your needs.
> Again not much to think about there.
>
> > H) Organizing source code files into separate projects and directory
> > structures.
>
> A few simple rules:
>
> - A module covers one small topic
> - A process does one thing
> - An application covers one topic and does one thing that involves
> multiple modules and processes
>
> > B) Appointing supervisor and worker modules; defining process
> hierarchies.
> > C) Deciding which processes should communicate with each other and how.
> > G) Visualizing the system architecture, processes and communication
> > lines; what kind of graphics to use.
>
> I'll leave that one for others.
>
> > Questions:
> >
> > 1) How do you unlearn "bad habits" from object-oriented way of thinking?
>
> The only way to unlearn something is to not use it for extended periods
> of time. I'm afraid that's not much help. Instead, try manipulating your
> brain into finding the right state of mind when you work on Erlang
> projects.
>
> It's the same problem you have when you play tennis and then switch to
> ping pong. The only difference is that you don't play them in the same
> environment or with the same tools so it's easier to switch from one to
> the other.
>
> Actually changing environment could help you into switching your OO
> brain off: change desk, change editor, and so on.
>
> > 2) How do you think and reason about process-centric systems designs?
> > 3) When designing a new system, how do you approach the above activities?
>
> Start small and naive, measure, then take decisions as to how the
> processes should be organized. Erlang is very easy to refactor compared
> to OO, so take advantage of that. Go for a working prototype, then
> improve upon it. How long you take to go from prototype to production
> will mostly depend on your experience.
>
> I've been interrupted about a thousand times when writing this, so
> hopefully I make some sort of sense. :-)
>
> Cheers,
>
> --
> Loïc Hoguin
> https://ninenines.eu
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> 
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