[erlang-questions] "calling" a gen_fsm

Theepan vasdeveloper@REDACTED
Wed Sep 23 18:50:41 CEST 2015

Hello Craig,

I respect your views, although I have different views. I have used both the
methods in the system that I had developed, of course with a reason. One of
the systems that I developed had to handle more than half a million
transactions per second from a single box, and thus performance and
hardware efficiency was paramount for us. I was very conscious about the
design and each line I wrote. May be for a couple of thousands transactions
per second system, you don't have to worry about these much.

>> "Many workers depending on a single state." I'm trying to see where this
is meaningful without an elaboration.

There are plenty of example for this. How would you model the states of a
stock market (singleton), on a trading system?


On Wed, Sep 23, 2015 at 6:55 PM, zxq9 <zxq9@REDACTED> wrote:

> An opposing view...
> On 2015年9月23日 水曜日 16:26:21 Theepan wrote:
> > I don't think it is a good idea to separate the state maintenance from
> the
> > real work that is happening, unless you have a strong reason for it. What
> > happens if you separate them is that you are keeping a process just for
> > state maintenance, and most of the time the gen_fsm process is going to
> be
> > idle. Other drawback is that the worker process will have to constantly
> > check the state machine to ensure that it doing work at the correct
> state.
> > There is a chance of off state processing and it is hardware hungry.
> I almost completely disagree with this view. I find gen_fsm to be useful
> specifically so that I can use an OTP process "just for state maintenance"
> -- sometimes this means a system composed mostly of tiny fsms, but more
> often it just means fsms are acting as logical gateways to control
> processing flow throughout the system (the valves and gateways of my
> digital kingdom).
> I *like* this separation because it makes the overall system much easier
> to reason about and modify. Some really huge system-wide behaviors can be
> modified very simply this way.
> As for being "hardware hungry"... I've never run into this problem. My
> gen_fsms tend to be really tiny, take up very little memory (generally
> ending with very close to the same number of bits as they started), and
> hardly running at all (sometimes good candidates for hiberation -- but
> usually there isn't any call for this). I really have a hard time
> understanding performance arguments (in terms of memory or processing time)
> against spawning temporary processes to get work done and terminating them
> when they are complete, as I have *never* encountered a case where this was
> actually a performance problem (at least not a noticeable one). Perhaps I'm
> just doing very different things with Erlang than everyone else -- but in
> any case without knowing the context *any* effort to deliberately increase
> instead of decrease the number of responsibilities a process has for
> performance reasons is headed in the wrong direction.
> At the very least when hacking around a problem I find it most useful to
> segregate as many responsibilites as possible early on, and that means
> gen_fsms are *only* finite-state machines, and they control processing flow
> based on their state. Any points of extended work are things I *want* to
> put in other processes, if for no other reason than an "idle" gen_fsm is a
> *highly responsive* gen_fsm. This helps me know more about the way a system
> responds under load (if this is even a concern) because I can just see what
> work is being spawned and how long bits of it is sticking around instead of
> trying to figure out how busy a given pack of fsms is. Sometimes (usually?)
> after some initial development I wind up moving things into a very
> different configuration because I understand the problem better, the needs
> of the system solidify a bit, any bottlenecks become obvious (and finding
> ways to avoid having them to begin with is usually pretty easy once the
> system is better understood), etc. But to start out with, ghah, putting
> more duties into a process than less has just never worked out very well
> for me.
> > There are places where you will separate the state maintenance, for
> > instance many workers depending on a single state. At the low level
> gen_fsm
> > and gen_tcp are just Erlang process, and what you are trying to do right
> > now is good for what you are doing.
> "Many workers depending on a single state." I'm trying to see where this
> is meaningful without an elaboration.
> -Craig
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