Spread wrapper for Python
=========================

The spread module is an intentionally thin layer on top of the Spread
client library.  It defines functions, exceptions, types and
constants.  For additional documentation, see the Spread man pages,
especially SP_connect, SP_disconnect, SP_join, SP_leave, SP_multicast,
SP_receive, and SP_poll.  The Spread website (www.spread.org) gives
additional information; especially the Spread Users Guide available
there is helpful.


Functions
---------

connect([spread_name[, private_name[, priority[, membership]]]]) -
Connect to the Spread daemon and return an object of type MailboxType.
Upon failure, SpreadError is raised.  See the SP_connect() man page
for more information.

Arguments:  all arguments are optional:

    spread_name
        a string specifying the port and host of the spread daemon to use,
        of the form "<port>@<hostname>" or "<port>", where <port> is an
        integer port number (typically the constant DEFAULT_SPREAD_PORT)
        represented as a string, and <hostname> specifies the host where
        the daemon runs, typically "localhost".  The default is the
        default Spread port and host.

    private_name
        a string specifying the name under which this client is to be
        known; the default is a private name made up by Spread

    priority
        0 or 1, currently unused; default 0

    membership
        1 to receive membership messages, and 0 to decline receiving
        those; default 1


Exceptions
----------

SpreadError - The exception raised for Spread-related errors.  The
arguments are usually a tuple of an int and a string, indicating the
error constant and the corresponding error message.  Sometimes the
argument is just a string indicating some higher-level error
condition.


Types
-----

MailboxType

This object represents a connection to a spread daemon.  Different
mailboxes are completely independent.  Methods are described below.

Instance variables:

    private_group
        the private group name assigned to this connection by the spread
        daemon (see the SP_connect man page).


RegularMsgType

This object represents a data message as returned by the receive() method.
There are no methods.

Instance variables:

    sender
        the private name of the connection that sent the message

    groups
        a list of the group names to which the message was multicast
        (only groups are listed of which the receiving connection is
        a member)

    message
        a string giving the data associated with the message

    msg_type
        an int with the value of the message_type argument passed
        to multicast() (see the SP_receive manpage; this is a signed
        16-bit int whose meaning (if any) is wholly up to the
        application)

    endian
        an int that is 0 if there are no endian issues with the message


MembershipMsgType

This object represents a membership message as returned by the receive()
method.  There are no methods.

Instance variables:

    reason
        the message type, which is exactly one of the constants
            CAUSED_BY_JOIN
            CAUSED_BY_LEAVE
            CAUSED_BY_DISCONNECT
            CAUSED_BY_NETWORK
        or zero if this is a transitional membership message

    group
        the group to which this message pertains

    group_id
        the unique identifier assigned to this group (not a string but
        an opaque object)

    members
        a list of private names giving the new set of members of the
        group

    extra
        a list of private names of members that have joined or left
        the group (which it is depends on the reason instance variable).

Note:  for transitional messages (indicated by a reason of zero), the
members and extra lists are both empty.


Constants
---------

Constants defined by Spread:

LOW_PRIORITY MEDIUM_PRIORITY HIGH_PRIORITY DEFAULT_SPREAD_PORT
SPREAD_VERSION MAX_GROUP_NAME MAX_PRIVATE_NAME MAX_PROC_NAME
UNRELIABLE_MESS RELIABLE_MESS FIFO_MESS CAUSAL_MESS AGREED_MESS
SAFE_MESS REGULAR_MESS SELF_DISCARD DROP_RECV REG_MEMB_MESS
TRANSITION_MESS CAUSED_BY_JOIN CAUSED_BY_LEAVE CAUSED_BY_DISCONNECT
CAUSED_BY_NETWORK MEMBERSHIP_MESS ENDIAN_RESERVED RESERVED REJECT_MESS
ACCEPT_SESSION

Error constants defined by Spread (all negative numbers):

ILLEGAL_SPREAD COULD_NOT_CONNECT REJECT_QUOTA REJECT_NO_NAME
REJECT_ILLEGAL_NAME REJECT_NOT_UNIQUE REJECT_VERSION CONNECTION_CLOSED
REJECT_AUTH ILLEGAL_SESSION ILLEGAL_SERVICE ILLEGAL_MESSAGE
ILLEGAL_GROUP BUFFER_TOO_SHORT GROUPS_TOO_SHORT MESSAGE_TOO_LONG

Constants defined by the Python wrapper:

DEFAULT_BUFFER_SIZE - The initial data buffer size used by receive().

DEFAULT_GROUPS_SIZE - The initial group buffer size used by receive().


Methods of MailboxType objects
------------------------------

All these can raise SpreadError upon Spread-related failures.

disconnect() - Disconnect from the mailbox.  The mailbox object should
not be used after this call.

fileno() - Return the integer file descriptor for this mailbox
object.  This can be used for reading in a select call to check for
receivable messages.

join(group) - Join the group with the given name.  Return None.

leave(group) - Leave the group with the given name.  Return None.

multicast(service_type, group, message[, message_type=0]) - Send a
message to all members of a group.  Return the number of bytes sent.
Arguments:

    service_type
        one of the integer constants (see the SP_multicast man page
        for their meaning):
            UNRELIABLE_MESS
            RELIABLE_MESS
            FIFO_MESS
            CAUSAL_MESS
            AGREED_MESS
            SAFE_MESS

    group
        the name of the group to send to (the Python wrapper currently
        doesn't support sending data to more than one group at a time)

    message
        the data to be sent, as a Python string

    message_type
        a signed integer, which should fit in 16 bits; the application
        can use this for any purpose it likes, or ignore it; see the
        SP_multicast manpage


receive() - Block (if necessary) until a message is received, and
return an object representing the received message.  The return value
is of type RegularMsgType or MembershipMsgType (see above).  The
caller does not have to worry about buffer sizes; when the underlying
SP_receive() call fails due to a BUFFER_TOO_SHORT or GROUPS_TOO_SHORT
error, the Python wrapper method allocates a buffer of the requested
size and retries the call.

poll() - Return 1 if the receive() method can return a message without
blocking; 0 if no message is available immediately.
