Distributed Computing with Emacs

I got an Elisp idea today and even went as far as implementing a proof of concept for it: distributed computing with Emacs Lisp. As usual for me the idea takes advantage of Lisp features to make the task pretty simple, very specifically Elisp's implementation. In this case it's the Lisp reader, printer, and the fact that Elisp functions have a printed representation, both byte-compiled and not.

Here's the proof of concept code: dist-emacs.el

A central server listens for TCP connections. Clients offering their CPU for use connect to the server and await instructions. The server sends a single, no-argument, anonymous function to the client. The client calls the function, returning the resulting form back to the server. In order to transmit the function it's encoded into a string using the Lisp printer, and the client turns it back into an executable function with the Lisp reader.

For some simple security there is a shared password between the client and server. When the server sends a function it includes a signature, and the client only runs code that matches the signature. To create a signature the string encoded version of the function is appended with the password (both strings) and hashed with a secure hashing algorithm. Only someone who knows the password — including other clients — can create the signature.

(defun sign-sexp (password sexp)
  "Return signature of the given s-exp."
  (sha1 (format "%s%s" password sexp))

To make it easy for the client to read in both the signature and the function we just cons them together before encoding them as text.

(defun encode (password sexp)
  "Encode a s-exp for transmission to client."
  (prin1-to-string (cons (sign-sexp password sexp) sexp)))

The client calls the Lisp reader on the string, then checks the signature in the car cell against the s-expression in the cdr cell. This will return the function if it's legitimate, otherwise nil.

(defun decode (password str)
  "Decode string into s-exp, checking the signature in the process."
  (let* ((cons (read str))
         (sig  (car cons))
         (sexp (cdr cons)))
    (if (equal sig (sign-sexp password sexp))

And that's the core of it. It just needs some network code to move the string between computers. That part can be found in the linked source above.

To demo this, I'll use the whiten function from my previous post. I'll run it with three different strings on three different computers. Assume we started the dist-emacs server (dist-start) and connected three clients (dist-connect) from three computers to it. The clients were fired up from scratch so there's no whiten function on them yet, but there is one defined on the server. First we'll send the function definition to the clients. The dist-dist function takes a list of functions and passes each one to a client. Ideally I'd want this function to be more intelligent, managing a work queue so that an arbitrary length list of functions will be fed one at a time to each client. That's not the case here.

(dist-dist (mapcar (lambda (p)
                          `(lambda ()
                             (fset 'whiten ,(symbol-function 'whiten))))

Also like in the previous post, this is an abstraction leak with the Emacs implementation. But I like this trick so I'm going to use it anyway. :-) Next we call it on each client with a different string.

(dist-dist (list (lambda () (whiten "good"))
                 (lambda () (whiten "news"))
                 (lambda () (whiten "everyone"))))

The way I have it set up for my proof of concept the results are just spit back into the server's *Messages* buffer. If we watch that buffer we can see each results come back in one at a time as each machine finishes. I can watch Emacs saturate the CPU on every client machine simultaneously as it works.


This isn't the same order as the clients, but the order in which the jobs were completed.

As for the practicality, I doubt there really is one. It's really only a neat concept (or maybe not even that). For almost the exact same reasons as my distributed JavaScript idea, this is a solution looking for a problem. The problem needs to be able to be broken into small computation units, because Emacs has no threading, and it has to be low bandwidth, because it has to be parsed all at once from a string. If you want to pass large data sets it needs to be done out-of-band, which probably defeats the purpose. There seem to be few to no problems that fit these limitations.

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Chris Wellons

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