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Connects to a network of parallel cluster servers.
As a precondition, a server must have been started at each machine of
the cluster, see pserver. Connections are not guaranteed to
work if client and server are from parallel packages of
different versions, so versions should be kept equal.
hosts is a cell-array of strings, holding the names of all
server machines. The machines must be unique, and their names must be
resolvable to the correct addresses also at each server machine, not
only at the client. This means e.g. that the name localhost is
not acceptable (exception: localhost is acceptable as the first
of all names).
Alternatively, but deprecated, hosts can be given as previously, as a character array with a machine name in each row. If it is given in this way, the first row must contain the name of the client machine (for backwards compatibility), so that there is one row more than the now preferred cell-array hosts would have entries.
pconnect returns an opaque variable holding the network
connections. This variable can be indexed to obtain a subset of
connections or even a single connection. (For backwards compatibility,
a second index of : is allowed, which has no effect). At the
first index position is the client machine, so this position does not
correspond to a real connection. At the following index positions are
the server machines in the same order as specified in the cell-array
hosts. So in the whole the variable of network connections has
one position more than the number of servers given in hosts
(except if hosts was given in the above mentioned deprecated
way). You can display the variable of network connections to see what
is in it. The variable of network connections, or subsets of it, is
passed to the other functions for parallel cluster execution
(reval, psend, precv, sclose,
select_sockets among others – see documentation of these
functions).
options: structure of options; field use_tls is
true by default (TLS with SRP authentication); if set to
false, there will be no encryption or authentication. Field
password_file can be set to an alternative path to the file
with authentication information (see below). Field user can
specify the username for authentication; if the username is so
specified, no file with authentication information will be used at the
client, but the password will be queried from the user.
The client and the server must both use or both not use TLS. If TLS is switched off, different measures must be taken to protect ports 12501 and 12502 at the servers and the client against unauthorized access; e.g. by a firewall or by physical isolation of the network.
For using TLS, authorization data must be present at the server
machine. These data can conveniently be generated by
parallel_generate_srp_data. By default, the client
authentication file is created in the same run. The helptext of
parallel_generate_srp_data documents the expected locations of
the authentication data.
The SRP password will be sent over the encrypted TLS channel from the client to each server, to avoid permanently storing passwords at the server for server-to-server data connections. Due to inevitable usage of external libraries, memory with sensitive data can, however, be on the swap device even after shutdown of the application, both at the client and at the server machines.
Example (let data travel through all machines), assuming
pserver was called on each remote machine and authentication
data is present (e.g. generated with
parallel_generate_srp_data):
sockets = pconnect ({'remote.machine.1', 'remote.machine.2'});
reval ('psend (precv (sockets(2)), sockets(1))', sockets(3));
reval ('psend (precv (sockets(1)), sockets(3))', sockets(2));
psend ('some data', sockets(2));
precv (sockets(3))
--> ans = some data
sclose (sockets);
See also: pserver, reval, psend, precv, sclose, parallel_generate_srp_data, select_sockets, rfeval.
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