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Joe Armstrong, creator of the Erlang programming language, said a couple of times that, though the Erlang system is the perfect tool for writing programs designed for concurrent execution, it is not the best tool for compute-intensive tasks, such as numerical calculations. Those tasks, he said, would be best served by calling an external program written in Fortran.
The problem, however, is that he never told us how to run such a program from inside the Erlang machine.
Code for this article is available on my Github
The plan
We are going to write a Fortran program that receives input and does some calculations with the values provided by the user. Because our problem here is not how to write Fortran, but rather how to run the program from inside the Erlang runtime, a simple program will do just fine. For this exercise, we will write a program that calculates the force according to Newton's 2nd law.
In order to interface with the Fortran program, we use a genserver process that manages an Erlang port. Ports are Erlang processes that allow running external programs and interact with them in the same way we interact with processes in the Erlang machine, by sending and receiving messages.
Our objective is to call the genserver API in Erlang, have the function transparently execute in the Fortran program (that is, as if it had been called locally), and return the correct result.
Eshell V13.1.2 (abort with ^G)
1> f90:start_link("./newton").
{ok,<0.84.0>}
2> f90:force(90,80).
7200
The Fortran program
First, we focus on the Fortran program that will effectively perform the calculations. As we said, we are going to calculate the force using newton's formula, which is, as we all know, defined as: f = m * a.
Let's start with the simplest program possible:
! newton0.f90
program newton
implicit none
integer mass, accel
mass = 90
accel = 80
write (*,'(10i0)') force(mass, accel)
contains
function force(m, a) result (f)
implicit none
integer m, a, f
f = m * a
end function force
end program newton
The program consists of a simple function definition for the Newtonian formula, a couple of variable declarations and assignments, and a write statement that prints the result of applying the function to the variables as arguments.
We compile the program with gfortran newton0.f90 -o newton. Running it in the terminal produces the correct result.
For the next step, we need to process user input:
! newton.f90
program newton
implicit none
integer rstat
integer mass, accel
character(20) :: input
do while(.true.)
do while(.true.)
read(*,'(A)',iostat=rstat) input
if (rstat /= 0) then
stop
else
if (input(1:4) == 'mass') then
call maybe_read_int(mass, 4 + 2)
else if (input(1:5) == 'accel') then
call maybe_read_int(accel, 5 + 2)
else if (input(1:6) == 'return') then
exit
else
write(*,*) 'bad input'
end if
end if
end do
write (*,'(10i0)') force(mass, accel)
mass = 0
accel = 0
end do
contains
subroutine maybe_read_int(field, index)
implicit none
integer field, index
read(input(index:), *, iostat=rstat) field
if (rstat == 0) then
write(*,'(10i0)') field
else
write(*,*) 'bad input'
endif
end subroutine maybe_read_int
function force(m, a) result (f)
implicit none
integer m, a, f
f = m * a
end function force
end program newton
The program starts with a loop, which reads the standard input, and assigns its value to a string. The input is matched to the expected patterns:
- mass [value]: stores the value in the mass variable
- accel [value]: stores the value in the accel variable
- return: exits the loop and proceeds to the next step
Next, the program prints the result of the force function over the variables, just like in the previous snippet. Let's compile the program and run it in the terminal to see it in action.
> ./newton
> mass 90
90
> accel 80
80
> return
7200
Great, we have successfully written a Fortran program that calculates the force using the Newtonian formula. Now, we need to write an Erlang program that is able to run it.
The Erlang program
As we said, we want to be able to call an Erlang function that produces the result from the Fortran program's execution. The appropriate way to do this in Erlang is using ports. Acording to the documentation:
Ports provide the basic mechanism for communication with the external world, from Erlang's point of view. The ports provide a byte-oriented interface to an external program. When a port is created, Erlang can communicate with it by sending and receiving lists of bytes (not Erlang terms). This means that the programmer might have to invent a suitable encoding and decoding scheme.
When to use: Ports can be used for all kinds of interoperability situations where the Erlang program and the other program runs on the same machine. Programming is fairly straight-forward.
Most things in the Erlang system are processes, and ports are no exception. A port is a processes that sends and receives messages from other processes in the Erlang runtime, and sends and receives messages from the external program.
One key difference of message passing from ports to the external program, compared with message passing between processes inside the Erlang runtime, is that internal processes understand Erlang terms (e.g. atoms, tuples, records, maps), while the external program absolutely does not. Everything we can do is send and receive binaries.
This is very similar to what happens in the distributed architecture of microservices implemented in different programming languages. For example, three services running Node, JVM, and Python cannot share objects native to their respective languages. They must instead resort to a common binary (string) protocol (e.g. JSON) for the API, and implement encoding/decoding internally.
Luckily for us, our Fortran program is designed to expect very simple binary patterns. Let's see how we create a port and send and receive messages.
Eshell V13.1.2 (abort with ^G)
1> Port = open_port({spawn, "./newton"}, [use_stdio, exit_status]).
#Port<0.5>
2> Port ! {self(), {command, "mass 90\n"}}.
{<0.82.0>,{command,"mass 90\n"}}
3> flush().
Shell got {#Port<0.5>,{data,"90\n"}}
ok
4> Port ! {self(), {command, "accel 90\n"}}.
{<0.82.0>,{command,"accel 90\n"}}
5> flush().
Shell got {#Port<0.5>,{data,"90\n"}}
ok
6> Port ! {self(), {command, "return\n"}}.
{<0.82.0>,{command,"return\n"}}
6> flush().
Shell got {#Port<0.5>,{data,"8100\n"}}
ok
We create a port with the Erlang library function open_port/2, which receives the command to the external program and some options, and returns the port Pid. We use the Pid to send messages to the port, which it will relay to the external program using the stdin.
Here, we are creating a port to the Fortran program "./newton", and sending it three messages. The flush() call reads the messages received by the Erlang shell process. Notice the third message: it is the result of the force() function, executed by the Fortran program!
We are very close to wrapping this up. All that is left is to put this functionality in a genserver that manages the port and provides a clear interface in accordance with the OTP standards.
%% f90.erl
-module(f90).
-behaviour(gen_server).
-export([start_link/1, force/2, stop/0]).
-export([init/1, handle_call/3, handle_cast/2]).
%% API
start_link(Filename) ->
gen_server:start_link({local, ?MODULE}, ?MODULE, Filename, []).
stop() ->
gen_server:cast(?MODULE, stop).
force(Mass, Accel) ->
gen_server:call(?MODULE, {force, Mass, Accel}).
%% callbacks
init(Filename) ->
process_flag(trap_exit, true),
Port = open_port({spawn, Filename}, [use_stdio, exit_status]),
{ok, {port, Port}}.
handle_call({force, Mass, Accel}, _From, {port, Port} = State) ->
call_port(mass(Mass), Port),
call_port(accel(Accel), Port),
Res = call_port(result(), Port),
{reply, Res, State}.
handle_cast(stop, {port, Port}) ->
port_close(Port),
{stop, normal, []}.
call_port(X, Port) ->
port_command(Port, X),
receive
{_, {data, Data}} ->
TrimmedData = string:trim(Data),
list_to_integer(TrimmedData)
after 500 -> nil
end.
accel(X) -> "accel " ++ integer_to_list(X) ++ "\n".
mass(X) -> "mass " ++ integer_to_list(X) ++ "\n".
result() -> "return\n".
We start the genserver with start_link/1 by providing the external program command as argument. The init/1 function runs as a callback, spawns the port, and stores the port Pid in the genserver state.
The force/2 function takes two integers, mass and acceleration, and will callback a handler that sends messages to the port and receives the response from the external program.
With this, we have implemented exactly the functionality we wanted in the beginning: a server that provides a simple Erlang function and produces the result of a calculation done in an external Fortran program.
Conclusion
The computer is an awesome machine, and it provides so many different tools and patterns with which we can create programs. It is a waste of intelligence to be confined to a single programming language.
In this exercise, we showed that with a simple construct, the Erlang port, we can interface with external programs in other languages. Once we set up a common binary protocol between the two programs, we can send and receive messages just like we do inside the Erlang machine, requesting execution and getting back results.
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