[Monetdb-developers] [M5] RFC: function shipping semantics

Fabian Groffen Fabian.Groffen at cwi.nl
Thu Jul 12 13:35:03 CEST 2007

Hi all,

I'd like some input on an issue that I can't get a clear vision on for

Recently I added the remote module in MonetDB 5 which basically has 3
functions which perform actions with a remote (MonetDB 5) server.  The
functions get, put and exec retrieve, store and execute an object from,
to or on a remote host.  get and put allow for retrieval and storage of
basically any type, scalars, strings, BATs, etc.  exec only executes
functions available on the remote host.  The idea is to have one to
"put()" a function to the remote host, and then execute it, retrieving
its return value.  exec is currently implemented that way, however, get
and put of functions is not because it is unclear to me how this should
exactly happen.

For the case of put() I can think of the following which seems quite
intuitive and ok to me:

  function user.func(v:int):str;
  	r := "something";
  end func;

  m := remote.put("host", "user.func(:int):str");
  t := 5;
  q := remote.exec("host", m, t);

The example puts function func to the remote host and executes it with
argument t which is 5.  The return string kept in q will contain the
string "something".  Because the identifier of the function on the
remote host is returned and assigned to m, the function can easily be
executed, since exec requires the function name as string.  Currently
that function name is just the function without its signature.  The
signature (which is not required, even not desired currently) can be
deduced from the arguments given, resulting in a normal function
resolution and (slightly misleading) error reporting if that fails.

The other way around (thus to get a remote function) is still unclear to
me how to do it.  I could imagine the following that should work:

  v := remote.get("host", m);
  w := remote.get("host", "user.func(:int):str");

Problem with v is that m is not a full signature, so a conflict may
arise if m is overloaded.  This is not necessarily a problem, since m is
a unique identifier, so no overloading for it exists.  However, it is a
requirement that a get after a put should always succeed and result in
the same object that was put.

The bigger problem here, however, is how to deal with the function being
get.  Since a function cannot be stored in a variable in MAL, it has to
be stored on the local machine, or its contents as a string returned.
In the first case the get() function would return its local identifier.
In the latter case it would return a (big) string.  In both cases, the
function cannot be "used" as function in the program.  A variable
containing a function name cannot be used as in e.g.:

  k := v(t);

where v, a variable, would be used as function, "dereferencing" v's
value or something.  Of course with a human factor one can do:


and use the value of v to construct a call to the locally stored
function.  In the case of a string, the function has to be made, which I
don't know how to do that, but would give explicit control over its
name to the program.

I don't think the concept of eval is clearing up things, as well as
possible at all (if you think about the k := statement where only v
should be expanded, not t neither k).


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