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Man Pages


Manual Reference Pages  -  SP_SCAT_RECEIVE (3)

NAME

SP_receive, SP_scat_receive - Receive message from Spread

CONTENTS

Synopsis
Description
Bugs
Author

SYNOPSIS

#include <sp.h>

int SP_receive( mailbox mbox, service *service_type, char sender[MAX_GROUP_NAME], int max_groups, int *num_groups, char groups[][MAX_GROUP_NAME], int16 *mess_type, int *endian_mismatch, int max_mess_len, char *mess);

int SP_scat_receive( mailbox mbox, service *service_type, char sender[MAX_GROUP_NAME], int max_groups, int *num_groups, char groups[][MAX_GROUP_NAME], int16 *mess_type, int *endian_mismatch, scatter *scat_mess);

DESCRIPTION

SP_receive is the general purpose receive function for the toolkit. This receives not only data messages but also membership messages for the connection. Messages for all groups joined on this connection will arrive to the same mailbox so a call to SP_receive will get a single ’message’ from any one of the groups. After the receive completes a number of fields are set to values indicating meta information about the message (such as groups, mess_type, endianness, type, etc).

This function is the most complex used in Spread because it is the only way for the system to return information to the application. The meaning of many of the fields changes depending on whether the message is a data message or a membership message.

The SP_receive function will block if no messages are available.

The mbox gives which connection to receive a message on. Service_type is a pointer to a variable of type ’service’ which will be set to the message type of the message just received. This will be either a REG_MESSAGE or MEMBERSHIP_MESS, and the specific type. The variable that service_type points to is also an input parameter and should be set to either 0, for normal semantics, or DROP_RECV (a defined constant) if you want to receive the ’non-reliable’ semantics that will truncate messages if the provided buffers for message data and group lists are too small. More information about DROP_RECV semantics are provided later in this man page.

The rest of the parameters differ in meaning depending on the service_type. If the service_type is a REG_MESSAGE (i.e. data message) then:

The sender is a pointer to an array of characters of at least MAX_GROUP_NAME size. This will be set to the name of the sending connection(its private group name).

The max_groups is the maximum number of groups you have allocated space for in the ’groups’ array passed to the receive function.

Num_groups is a pointer to an int which will be set to the number of groups set in the ’groups’ array.

The groups array holds upto max_groups group names, each of which is a string of at most MAX_GROUP_NAME characters. All of the groups which are receiving this message will be listed here, unless the array is too small and then as many as can fit will be listed and the num_groups value will be set to be negative. For example, if your groups array could store 5 group names, but a message for 7 groups arrived, the first five group names would appear in the groups array and num_groups would be set to -7.

The mess_type field will be set to the message type field the application sent with the original message, this is only a short int (16bits). This value is already endian corrected before the application receives it.

Endian_mismatch will be set to true (1) if the endianness of the sending machine differs from that of this receiving machine. If the BUFFER_TOO_SHORT error is returned then the endian_mismatch field will be set to the size of the incoming message as a negative value. So if the message requires 300 bytes of buffer space and only 200 bytes was provided in the mess buffer, endian_mismatch will be set to -300.

The actual message body being received will be passed into the buffer given by mess which is at least max_mess_len bytes in size. If the message being received is larger then this buffer the default behaivor will be to return a BUFFER_TOO_SHORT error and provide the required length in the endian_mismatch field. If the DROP_RECV flag was passed in the service_type field, then as much data as possible will be returned and the extra data will be discarded by the system and the return value of SP_receive will indictate an error. If the SP_scat_receive form is used then instead of the mess buffer and length fields, a single scat_mess scatter structure should be passed to receive filled in with whatever buffers you wish to receive into and their lengths. These buffers must be valid memory areas. They will be filled in by the receive call in the order they are listed.

If this is a MEMB_MESSAGE (i.e. membership message) and it specifically is a TRANS_MESS, then:

The sender char array will be set to the name of the group for which the membership change is occurring.

The max_groups and max_mess_len fields are not used and the num_groups is 0 and groups since there are no normal groups for a transitional membership, the sender field is used instead. The mess_type field is set to -1. The endian_mismatch field will be zero since the transitional does not have any endian issues. The mess field contains only the group id of the group for which the transitional message applies. So the layout of the message body is

group_id;
 

So in essence the only information you get is about what group the signal applies to which is provided by the sender field and the group_id value.

The importance of the TRANS_MEMB_MESS is that it tells the application that all messages received after it and before the REG_MEMB_MESS for the same group are ’clean up’ messages to put the messages in a consistant state before actually changing memberships. For more explanations of this please see other documentation and research papers.

If This is a MEMB_MESSAGE (i.e. membership message) and it specifically is a REG_MEMB_MESS, then:

The sender char array will be set to the name of the group for which the membership change is occuring.

The max_groups and max_mess_len fields have the same meaning as before, and the mess_type field will be set to the index of this process in the array of group members. The endian_mismatch field will again be set to 0 since there are no endian issues with regular memberships.

The groups array and mess body are used to provide two kinds of membership information about the change that just occured in this group. The num_groups field will be set to the number of members in the group in the new membership (i.e. after the change occured). Correspondingly the groups array will be set to the private group names of all members of this group in the new membership. This list of names is always in the same order at all receipients and thus can be used to deterministically pick a group representative if one is needed by the application. The actual ordering has changed with different versions of Spread, but is currently ordered first by the daemon the member is connected to and then by their private name. The ordering of the daemons is the same as the order they are listed in the spread.conf file.

The second set of information is stored in the message body and provides a list of all the private group names of those processes which came together from the old group membership into this new membership.

All fields are contiguous in the message buffer. Since the layout of the membership message can change between versions, the best way to process them is to use the functions we provide that translate the message body into well-defined structures of membership information. The parsing functions have two variants, one for regular message bodies that are one contiguous array of bytes, and one for message bodies set up as scatters.

SP_get_memb_info
 
SP_get_vs_sets_info
 
SP_get_vs_set_members
 

SP_scat_get_memb_info
 
SP_scat_get_vs_sets_info
 
SP_scat_get_vs_set_members
 

These three functions fill in standard structures that the application provides with all of the gid and vs set information about the membership.

So if an application wants only to know about it’s own vs_set (identical to the information Spread provided prior to version 4) then that can be achieved by two function calls. First to SP_get_memb_info and then to SP_get_vs_set_members.

Each vs_set is defined as a number of members and a character array of the members names. The type for the array is char members[][MAX_GROUP_NAME].

Each membership message may contain a number of vs_sets, as when a network merge occurs, several different partitions could be merging at the same time, with each partition having it’s own set of members who came together between the old and new memberships.

If you want to parse the contents of the membership message body yourself, the following paragraphs specify the current layout of the fields. This layout can definitely change (and has several times in the past) and we make not guarantees about future changes. So if you choose to parse it yourself you may have to update your code when new Spread releases are made.

The membership message body includes the following fields in order. Each is either a defined structure or if it is an integer, then it is an unsigned 32 bit integer. The vs sets are defined below.

group_id gid;
unsigned int num_vs_sets;
unsigned int local_vs_set_offset;
and a list of
  vs_sets;

All of the vs_sets are stored continuously in the membership messages body, so after reading one vs_set, the next vs_set begins at the next byte. Each vs_set starts with the integer num_vs_members value so that the application can determine the length of the members array to read.

unsigned int num_vs_members;
char members[][MAX_GROUP_NAME]
 

The vs_set members array will have num_vs_members group names, each of which is a fixed length string. The content of the vs_set members array is dependent upon the type of the membership change:

CAUSED_BY_JOIN:
  vs_set contains the private group of the joining process.
CAUSED_BY_LEAVE:
  vs_set contains the private group of the leaving process.
CAUSED_BY_DISCONNECT:
  vs_set contains the private group of the disconnecting process.
CAUSED_BY_NETWORK:
  vs_set contains the group names of the members of the new membership who came together to the new membership. For each set of members who came together an additional vs_set will exist. The vs_set containing the local application private group name will contain those members who came with me.

If this is a MEMB_MESSAGE (i.e. membership message) and it is neither a REG_MEMB_MESS or a TRANS_MEMB_MESS, then it represents exactly the situtation where the member receiving this message has left a group and this is notification that the leave has occured, thus it is sometimes called a self-leave message. The simplest test for this is if a message is CAUSED_BY_LEAVE and REG_MEMB_MESS is FALSE then it is a self-leave message. TRANS_MEMB_MESS never have a CAUSED_BY_ type because they only serve to signal upto where SAFE delivery and AGREED delivery (with no holes) is guaranteed in the complete old group membership.

The other members of the group this member just left will receive a normal TRANS_MEMB_MESS, REG_MEMB_MESS pair as described above showing the membership change.

The fields of SP_receive in this case will be as follows:

The sender char array will be set to the name of the group for which the membership change is occuring.

The max_groups and max_mess_len fields have the same meaning as before, and the mess_type and endian_mismatch fields will again be set to 0.

The groups array and mess body are used to provide two kinds of membership information about the change that just occured in this group. The num_groups field will be set to 0 and the groups array will be empty, since this member is no longer part of the group and thus has no knowledge of it. The mess body will also be empty.

RETURN VALUES

Returns the size of the message received on success or one of the following errors ( < 0 ):
ILLEGAL_SESSION
  The mbox given to receive on was illegal.
ILLEGAL_MESSAGE
  The message had an illegal structure, like a scatter not filled out correctly.
CONNECTION_CLOSED
  During communication to receive the message communication errors occured and the receive could not be completed.
GROUPS_TOO_SHORT
  If the groups array is too short to hold the entire list of groups this message was sent to then this error is returned and the num_groups field will be set to the negative of the number of groups needed.
BUFFER_TOO_SHORT
  If the message body buffer mess is too short to hold the message being received then this error is returned and the endian_mismatch field is set to the negative value of the required buffer length.

BUGS

None.

AUTHOR

Yair Amir <yairamir@cnds.jhu.edu>
Jonathan Stanton <jonathan@cnds.jhu.edu>

SEE ALSO

libspread(3)

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Spread SP_RECEIVE (3) February 2006

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