[erlang-questions] Glossary

Dave Pawson dave.pawson@REDACTED
Mon Jul 20 16:39:12 CEST 2009

2009/7/20 Richard Andrews <bflatmaj7th@REDACTED>:
> Apologies. This got long. I think it is reasonably accurate.

Since it's just that bit different from other system+language setup,
I think it's worth getting the understanding right. Tks for the patience.

>>>> Node

>> Node = (approx) a single processor/computer/machine?
>> That's how I had it.  Then one of those is 'the erlang node', i.e. where
>> whole programs are started from.
> Hmmm. A node is a program/process started in the operating system on a
> machine. You would see it as a task under Windows task manager or a
> process in UNIX top.

So not necessarily 'one per CPU'? With your definition I can
see I could start several 'programs' from one machine. OK.

> It is a virtual machine program for running
> erlang code in.

That's clear!

 When a node program starts it is configured to
> internally start various erlang processes based on what that node is
> designed to achieve. Erlang nodes can start other nodes, but systems
> don't always emanate from a nucleus. It can be detrimental to fault
> tolerance (which is a big reason for using erlang).

OK, I'm happy  with that one.

>>>> Process
>>> A light-weight state machine with a mailbox for receiving messages
>>> from other processes or system IO resources. In java you might know
>>> them as green threads. The node schedules processes to run when there
>>> is something for them to do (like a message in the mailbox).
>> This seems the key bit. Yet least natural to get hold of.
>> Receiving messages is a key part.
> An erlang program as a whole is event driven.

[A program being a number of processes?]

An external event (eg.
> IO or timer) will cause a message to arrive at a process (eg. data
> from a socket) which will cause other messages to flow between
> processes, aggregating data, checking what should be done. Some
> processes are connected to the outside world (eg. network socket, file
> handle) and data will flow out of the erlang node via those.

 I'm clear with the messaging idea as implementing events.

> Erlang was designed for highly asynchronous applications with lots of
> partially completed tasks running concurrently and safely (not
> interfering with each other). Because erlang processes CANNOT access
> the information stored in any other process, the only way to get to
> that info is to ask nicely and wait for the response message.

I like that clean interface. One of the Erlang differences I guess.

> might seem inefficient to a C coder

I've done similar things in Assembler, and suffered the failures ;-)

>  * Because state cannot be shared, when a process suffers a terminal
> fault (eg. unhandled situation), only that small process is killed.
> The rest of the system can be guaranteed to be unaffected and keeps on
> ticking. So what would typically be a segfault or abort in a C program
> becomes an internal process restart. Errors are contained and the
> system keeps running.

I read that in the O'Reilly book. I've yet to play with it. Propogating
errors to an appropriate handler level makes sense.

>> Is it true that a process is generally a single module ('a chunk of code')?
>> Even if code from other modules is used?
>> And what's the relationship between scheduling, the VM and processes/modules?
> A process is started by specifying to the erlang node the
> module+function+arguments to start the process running (called
> spawning). This is often abbreviated to MFA
> (Module/Function/Arguments). A process runs until the code calls the
> exit function (or gets killed).

Or hangs waiting for a message?

OTP (which you are probably using)
> provides some common boiler-plate process templates like gen_server.
> These use one module to drive core process behaviour. So in this
> respect you are correct. And yes from that core module the code can
> run code from other modules.


> The erlang VM like an OS kernel it receives events when there is work
> for a process to perform (eg. timer expires, file descriptor/handle
> gets data, etc) and manages swapping CPU time between the internal
> erlang processes to achieve this goal. Modules are just containers of
> code which processes call as they run. You can think of modules as
> shared libraries that all the erlang processes can use.

I'm good with that. My background I've written schedulers for real time systems
which implement that 'kernel' type functionality. I guess I can ignore the VM
when thinking about design, just let it do it's thing.

> Unsolicited advice: Erlang programming syntax can seem just plain
> wrong to a newcomer.

I'll settle for 'odd' :-) Having played with Scheme and Lisp, it isn't
much worse!

 If you hit one of these issues where you think
> erlang must be the dumbest most inefficient language on earth, ask
> about it. Erlang is different, but it works the way it does for very
> good reasons.

It seems to have that quirkiness that comes from doing a specific job?

The O'Reilly book contains quite a bit of 'experience' put forth as advice
and some solid logic as to why some things are as they are.

I'm still quite intrigued by it.

Again. Tks Richard.


Dave Pawson
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