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### 3.4 Establishing cluster connections

Loadable Function: connections = pconnect (hosts)
Loadable Function: connections = pconnect (hosts, options)

  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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