corosync/README.devmap

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-------------------------------------------------------------------------------
This file provides a map for developers to understand how to contribute
to the openais project.  The purpose of this document is to prepare a
developer to write a service for openais, or understand the architecture
of openais.

The following is described in this document:

 * all files, purpose, and dependencies
 * architecture of openais
 * taking advantage of virtual synchrony
 * adding libraries
 * adding services

-------------------------------------------------------------------------------
 all files, purpose, and dependencies.
-------------------------------------------------------------------------------

*----------------*
*- AIS INCLUDES -*
*----------------*

include/ais_amf.h
-----------------
	Definitions for AMF interface.

include/ais_ckpt.h
------------------
	Definitions for CKPT interface.

include/ais_clm.h
-----------------
	Definitions for CLM interface.

include/ais_msg.h
-----------------
	All the stuff that is used to specify how lib and executive communicate
	including message identifiers, message request data, and mesage response
	data.

include/ais_types.h
-------------------
	Base type definitions for AIS interface.

include/list.h
-------------
	Doubly linked list inline implementation.

include/queue.h
---------------
	FIFO queue inline implementation.

	depends on list.

include/sq.h
------------
	Sort queue where items are sorted according to a sequence number.  Avoids
	Sort, hence, install of a new element takes is O(1).  Inline implementation.

	depends on list.

*---------------*
* AIS LIBRARIES *
*---------------*
lib/amf.c
---------
	AMF user library linked into user application.

lib/ckpt.c
----------
	CKPT user library linked into user application.

lib/clm.c
---------
	CLM user library linked into user application.

lib/util.c
----------
	Utility functions used by all libraries.

*-----------------*
*- AIS EXECUTIVE -*
*-----------------*

exec/amf.{h|c}
-------------
	Server side implementation of Availability Management Framework (AMF API).

exec/ckpt.{h|c}
	Server side implementation of Checkpointing (CKPT API).

exec/clm.{h|c}
	Server side implementation of Cluster Membership (CLM API).


exec/gmi.{h|c}
--------------
	group messaging interface supporting reliable totally ordered group multicast
	using ring topology.  Supports extended virtual synchrony delivery semantics
	with strong membership guarantees.

	depends on cglpoll.
	depends on queue.
	depends on sq.
	depends on list.

exec/handlers.h
---------------
	Functional specification of a service that connects into AIS executive.
	If all functions are implemented, new services can easily be added.

exec/main.{h|c}
--------------
	Main dispatch functionality and global data types used to connect AIS
	services into one component.

exec/mempool.{h|c}
------------------
	Memory pool implementation that supports preallocated memory blocks to
	avoid OOM errors.

exec/parse.{h|c}
----------------
	Parsing functions for parsing /etc/ais/groups.conf and
	/etc/ais/network.conf into internally used data structures.

exec/poll.{h|c}
---------------
	poll abstraction with support for nearly unlimited large poll handlers
	and timer handlers.

	depends on tlist.

exec/print.{h|c}
----------------
	Logging implementation meant to replace syslog.  syslog has nasty side
	effect of causing a signal every time a message is logged.

exec/tlist.{h|c}
-----------------
	Timer list interface for supporting timer addition, removal, expiry, and 
	determination of timeout period left for next timer to expire.

	depends on list.

exec/log/print.{h|c}
--------------------
	Prototype implementation of logging to syslog without using syslog C
	library call.

loc
---
Counts the lines of code in the AIS implementation.

-------------------------------------------------------------------------------
 architecture of openais
-------------------------------------------------------------------------------

The openais project is a client server architecture.  Libraries implement the
SA Forum APIs and are linked into the end-application.  Libraries request
services from the ais executive.  The ais executive uses the group messaging
protocol to provide cluster communication between multiple processors (nodes).
Once the group makes a decision, a response is sent to the library, which then
responds to the user API.

               ----------------------------------------
               |AIS CLM, AMF, CKPT library (openais.a)|
               ----------------------------------------
               |      Interprocess Communication      |
               ----------------------------------------
               |           openais Executive          |
               |                                      |
               |     --------- --------- ---------    |
               |     |  AMF  | |  CLM  | | CKPT  |    |
               |     |Service| |Service| |Service|    |
               |     --------- --------- ---------    |
               |                                      |
               |       ----------- -----------        |
               |       |  Group  | |  Poll   |        |
               |       |Messaging| |Interface|        |
               |       |Interface| -----------        |
               |       -----------                    |
               |                                      |
               ----------------------------------------

                    Figure 1: openais Architecture

Every application that intends to use openais links with the libais library.
This library uses IPC, or more specifically BSD unix sockets, to communicate
with the executive.  The library is a small program responsible only for
packaging the request into a message.  This message is sent, using IPC, to
the executive which then processes it.  The library then waits for a response.

The library itself contains very little intelligence.  Some utility services
are provided:

 * create a connection to the executive
 * send messages to the executive
 * retrieve messages from the executive
 * Queue message for out of order delivery to library (used for async calls)
 * Poll on a fd
 * request the executive send a dummy message to break out of dispatch poll
 * create a handle instance
 * destroy a handle instance
 * get a reference to a handle instance
 * release a reference to a handle instance

When a library connects, it sends via a message, the service type.  The 
service type is stored and used later to reference the message handlers
for both the library message handlers and executive message handlers.
Every message sent contains an integer identifier, which is used to index
into an array of message handlers to determine the correct message handler
to execute.

When a library sends a message via IPC, the delivery of the message occurs
to the library message handler for the service specified in the service type.
The library message handler is responsible for sending the message via the 
group messaging interface to all other processors (nodes) in the system via
the API gmi_mcast().  In this way, the library handlers are also very simple
containing no more logic then what is required to repackage the message into
an executive message and send it via the group messaging interface.

The group messaging interface sends the message according to the extended
virtual synchrony model.  The group messaging interface also delivers the
message according to the extended virtual synchrony model.  This has several
advantages which are described in the virtual synchrony section.  One
advantage that must be described now is that messages are self-delivered;
if a node sends a message, that same message is delivered back to that 
node.

When the executive message is delivered, it is processed by the executive
message handler.  The executive message handler contains the brains of
AIS and is responsible for making all decisions relating to the request
from the libais library user.

-------------------------------------------------------------------------------
 taking advantage of virtual synchrony
-------------------------------------------------------------------------------

definitions:
processor: a system responsible for executing the virtual synchrony model
configuration: the list of processors under which messages are delivered
partition: one or more processors leave the configuration
merge: one or more processors join the configuration
group messaging: sending a message from one sender to many receivers

Virtual synchrony is a model for group messaging.  This is often confused
with particular implementations of virtual synchrony.  Try to focus on
what virtual syncrhony provides, not how it provides it, unless interested
in working on the group messaging interface of openais.

Virtual synchrony provides several advantages:

 * integrated membership
 * strong membership guarantees
 * agreed ordering of delivered messages
 * same delivery of configuration changes and messages on every node
 * self-delivery
 * reliable communication in the face of unreliable networks
 * recovery of messages sent within a configuration where possible
 * use of network multicast using standard UDP/IP

Integrated membership allows the group messaging interface to give
configuration change events to the API services.  This is obviously beneficial
to the cluster membership service (and its respective API0, but is helpful
to other services as described later.

Strong membership guarantees allow a distributed application to make decisions
based upon the configuration (membership).  Every service in openais registers
a configuration change function.  This function is called whenever a
configuration change occurs.  The information passed is the current processors,
the processors that have left the configuration, and the processors that have
joined the configuration.  This information is then used to make decisions
within a distributed state machine.  One example usage is that an AMF component
running a specific processor has left the configuration, so failover actions
must now be taken with the new configuration (and known components).

Virtual synchrony requires that messages may be delivered in agreed order.
FIFO order indicates that one sender and one receiver agree on the order of
messages sent.  Agreed ordering takes this requirement to groups, requiring that
one sender and all receivers agree on the order of messages sent.

Consider a lock service.  The service is responsible for arbitrating locks
between multiple processors in the system.  With fifo ordering, this is very
difficult because a request at about the same time for a lock from two seperate
processors may arrive at all the receivers in different order.  Agreed ordering
ensures that all the processors are delivered the message in the same order.
In this case the first lock message will always be from processor X, while the
second lock message will always be from processor Y.   Hence the first request
is always honored by all processors, and the second request is rejected (since
the lock is taken).  This is how race conditions are avoided in distributed
systems.

Every processor is delivered a configuration change and messages within a
configuration in the same order.  This ensures that any distributed state
machine will make the same decisions on every processor within the
configuration.  This also allows the configuration and the messages to be
considered when making decisions.

Virtual synchrony requires that every node is delivered messages that it
sends.  This enables the logic to be placed in one location (the handler
for the delivery of the group message) instead of two seperate places.  This
also allows messages that are sent to be ordered in the stream of other
messages within the configuration.

Certain guarantees are required of virtually synchronous systems.  If
a message is sent, it must be delivered by every processor unless that
processor fails.  If a particular processor fails, a configuration change
occurs creating a new configuration under which a new set of decisions
may be made.  This implies that even unreliable networks must reliably
deliver messages.   The implementation in openais works on unreliable as
well as reliable networks.

Every message sent must be delivered, unless a configuration change occurs.
In the case of a configuration change, every message that can be recovered
must be recovered before the new configuration is installed.  Some systems
during partition won't continue to recover messages within the old
configuration even though those messages can be recovered.  Virtual synchrony
makes that impossible, except for those members that are no longer part
of a configuration.

Finally virtual syncrhony takes advantage of hardware multicast to avoid
duplicated packets and scale to large transmit rates.  On 100mbit network,
openais can approach wire speeds depending on the number of messages queued
for a particular processor.

What does all of this mean for the developer?

 * messages are delivered reliably
 * messages are delivered in the same order to all nodes
 * configuration and messages can both be used to make decisions

-------------------------------------------------------------------------------
 adding libraries
-------------------------------------------------------------------------------

The first stage in adding a library to the system is to develop the library.

Library code should follow these guidelines:

 * use SA Forum coding style for APIs to aid in debugging
 * implement all library code within one file named after the api.
   examples are ckpt.c, clm.c, amf.c.
 * use parallel structure as much as possible between different APIs
 * make use of utility services provided by the library
 * if something is needed that is generic and useful by all services,
   submit patches for other libraries to use these services.
 * use the reference counting handle manager for handle management.

------------------
 Version checking
------------------

struct saVersionDatabase {
	int versionCount;
	SaVersionT *versionsSupported;
};

The versionCount number describes how many entries are in the version database.
The versionsSupported member is an array of SaVersionT describing the acceptable
versions this API supports.

An api developer specifies versions supported by adding the following C
code to the library file:

/*
 * Versions supported
 */
static SaVersionT clmVersionsSupported[] = {
	{ 'A', 1, 1 },
	{ 'a', 1, 1 }
};

static struct saVersionDatabase clmVersionDatabase = {
	sizeof (clmVersionsSupported) / sizeof (SaVersionT),
	clmVersionsSupported
};

After this is specified, the following API is used to check versions:

SaErrorT
saVersionVerify (
	struct saVersionDatabase *versionDatabase,
	const SaVersionT *version);

An example usage of this is
	SaErrorT error;

	error = saVersioNVerify (&clmVersionDatabase, version);

	where version is a pointer to an SaVersionT passed into the API.

error will return SA_OK if the version is valid as specified in the
version database.

------------------
 Handle Instances
------------------

Every handle instance is stored in a handle database.  The handle database
stores instance information for every handle used by libraries.  The system
includes reference counting and is safe for use in threaded applications.

The handle database structure is:

struct saHandleDatabase {
	unsigned int handleCount;
	struct saHandle *handles;
	pthread_mutex_t mutex;
	void (*handleInstanceDestructor) (void *);
};

handleCount is the number of handles
handles is an array of handles
mutex is a pthread mutex used to mutually exclude access to the handle db
handleInstanceDestructor is a callback that is called when the handle
	should be freed because its reference count as dropped to zero.

The handle database is defined in a library as follows:

static void clmHandleInstanceDestructor (void *);

static struct saHandleDatabase clmHandleDatabase = {
	handleCount: 0,
	handles: 0,
	mutex: PTHREAD_MUTEX_INITIALIZER,
	handleInstanceDestructor: clmHandleInstanceDestructor
};

There are several APIs to access the handle database:

SaErrorT
saHandleCreate (
	struct saHandleDatabase *handleDatabase,
	int instanceSize,
	int *handleOut);

Creates an instance of size instanceSize in the handleDatabase paraemter
returning the handle number in handleOut.  The handle instance reference
count starts at the value 1.

SaErrorT
saHandleDestroy (
	struct saHandleDatabase *handleDatabase,
	unsigned int handle);

Destroys further access to the handle.  Once the handle reference count
drops to zero, the database destructor is called for the handle.  The handle
instance reference count is decremented by 1.

SaErrorT
saHandleInstanceGet (
	struct saHandleDatabase *handleDatabase,
	unsigned int handle,
	void **instance);

Gets an instance specified handle from the handleDatabase and returns
it in the instance member.  If the handle is valid SA_OK is returned
otherwise an error is returned.  This is used to ensure a handle is
valid.  Eveyr get call increases the reference count on a handle instance
by one.

SaErrorT
saHandleInstancePut (
	struct saHandleDatabase *handleDatabase,
	unsigned int handle);

Decrements the reference count by 1.  If the reference count indicates
the handle has been destroyed, it will then be removed from the database
and the destructor called on the instance data.  The put call takes care
of freeing the handle instance data.

Create a data structure for the instance, and use it within the libraries
to store state information about the instance.  This information can be
the handle, a mutex for protecting I/O, a queue for queueing async messages
or whatever is needed by the API.

-----------------------------------
 communicating with the executive
-----------------------------------

A service connection is created with the following API;

SaErrorT
saServiceConnect (
	int *fdOut,
	enum req_init_types init_type);


The fdOut parameter specifies the address where the file descriptor should
be stored.  This file descriptor should be stored within an instance structure
returned by saHandleCreate.
The init_type parameter specifies the service number to use when connecting.


A message is sent to the executive with the function:

SaErrorT
saSendRetry (
	int s,
	const void *msg,
	size_t len,
	int flags);

the s member is the socket to use retrieved with saServiceConnect
the msg member is a pointer to the message to send to the service
the len member is the length of the message to send
the flags parameter is the flags to use with the sendmsg system call

A message is received from the executive with the function:

SaErrorT
saRecvRetry (
	int s,
	void *msg,
	size_t len,
	int flags);

the s member is the socket to use retrieved with saServiceConnect
the msg member is a pointer to the message to receive to the service
the len member is the length of the message to receive
the flags parameter is the flags to use with the sendmsg system call

A message is sent using io vectors with the following function:

SaErrorT saSendMsgRetry (
	int s,
	struct iovec *iov,
	int iov_len);

the s member is the socket to use retrieved with saServiceConnect
the iov is an array of io vectors to send
iov_len is the number of iovectors in iov

Waiting for a file descriptor using poll systemcall is done with the api:

SaErrorT
saPollRetry (
	struct pollfd *ufds,
	unsigned int nfds,
	int timeout);

where the parameters are the standard poll parameters.

Messages can be received out of order searching for a specific message id with:

SaErrorT
saRecvQueue (
	int s,
	void *msg,
	struct queue *queue,
	int findMessageId);
Where s is the socket to receive from
where msg is the message address to receive to
where queue is the queue to store messages if the message doens't match
findMessageId is used to determine if a message matches (if its equal,
it is received, if it isn't equal, it is stored in the queue)

An API can activate the executive to send a dummy message with:

SaErrorT
saActivatePoll (int s);

This is useful in dispatch functions to cause poll to drop out of waiting
on a file descriptor when a connection is finalized.

Looking at the lib/clm.c file is invaluable for showing how these APIs
are used to communicate with the executive.

----------
 messages
----------
Please follow the style of the messages.  It makes debugging much easier
if parallel style is used.

An init message should be added to req_init_types.

enum req_init_types {
	MESSAGE_REQ_CLM_INIT,
	MESSAGE_REQ_AMF_INIT,
	MESSAGE_REQ_CKPT_INIT,
	MESSAGE_REQ_CKPT_CHECKPOINT_INIT,
	MESSAGE_REQ_CKPT_SECTIONITERATOR_INIT
};

These are the request CLM message identifiers:

Every library request message is defined in ais_msg.h and should look like this:

enum req_clm_types {
	MESSAGE_REQ_CLM_TRACKSTART = 1,
	MESSAGE_REQ_CLM_TRACKSTOP,
	MESSAGE_REQ_CLM_NODEGET
};

These are the response CLM message identifiers:

enum res_clm_types {
	MESSAGE_RES_CLM_TRACKCALLBACK = 1,
	MESSAGE_RES_CLM_NODEGET,
	MESSAGE_RES_CLM_NODEGETCALLBACK
};

index 0 of the message is special and is used for the activate poll message in
every API.  That is why req_clm_types and res_clm_types starts at 1.

This is the message header that should start every message:

struct message_header {
	int magic;
	int size;
	int id;
};

This is described later:

struct message_source {
	struct conn_info *conn_info;
	struct in_addr in_addr;
};

This is the MESSAGE_REQ_CLM_TRACKSTART message id above:

struct req_clm_trackstart {
	struct message_header header;
	SaUint8T trackFlags;
	SaClmClusterNotificationT *notificationBufferAddress;
	SaUint32T numberOfItems;
};

The saClmClusterTrackStart api should create this message and send it to the 
executive.

responses should be of:

struct res_clm_trakcstart

------------
 some notes
------------
* Avoid doing anything tricky in the library itself.  Let the executive 
  handler do all of the work of the system.  minimize what the API does.
* Once an api is developed, it must be added to the makefile.  Just add
  a line for the file to EXECOBJS build line.
* protect I/O send/recv with a mutex.
* always look at other libraries when there is a question about how to 
  do something.  It has likely been thought out in another library.

-------------------------------------------------------------------------------
 adding services
-------------------------------------------------------------------------------
Services are defined by service handlers and messages described in
include/ais_msg.h.  These two peices of information are used by the executive
to dispatch the correct messages to the correct receipients.

-------------------------------
 the service handler structure
-------------------------------

A service is added by defining a structure defined in exec/handlers.h.  The
structure is a little daunting:

struct service_handler {
	int (**libais_handler_fns) (struct conn_info *conn_info, void *msg);
	int libais_handler_fns_count;
	int (**aisexec_handler_fns) (void *msg);
	int aisexec_handler_fns_count;
	int (*confchg_fn) (
		struct sockaddr_in *member_list, int member_list_entries,
		struct sockaddr_in *left_list, int left_list_entries,
		struct sockaddr_in *joined_list, int joined_list_entries);
	int (*libais_init_fn) (struct conn_info *conn_info, void *msg);
	int (*libais_exit_fn) (struct conn_info *conn_info);
	int (*aisexec_init_fn) (void);
};

libais_handler_fns are a list of functions that are dispatched by
the executive when the library requests a service.

libais_handler_fns_count is the number of functions in the handler list.

aisexec_handler_fns are a list of functions that are dispatched by the
group messaging interface when a message is delivered by the group messaging
interface.

aisexec_handler_fns_count is the number of functions in the aisexec_handler_fns
list.

confchg_fn is called every time a configuration change occurs.

libais_init_fn is called every time a library connection is initialized.

libais_exit_fn is called every time a library connection is terminated by
the executive.

aisexec_init_fn is called once during startup to initialize service specific
data.

---------------------------
 look at a service handler
---------------------------

A typical declaration of a full service is done in a file exec/service.c.  
Looking at exec/clm.c:

static int (*clm_libais_handler_fns[]) (struct conn_info *conn_info, void *) = {
	message_handler_req_lib_activatepoll,
	message_handler_req_clm_trackstart,
	message_handler_req_clm_trackstop,
	message_handler_req_clm_nodeget
};

static int (*clm_aisexec_handler_fns[]) (void *) = {
	message_handler_req_exec_clm_nodejoin
};
	
struct service_handler clm_service_handler = {
	libais_handler_fns:		clm_libais_handler_fns,
	libais_handler_fns_count:	sizeof (clm_libais_handler_fns) / sizeof (int (*)),
	aisexec_handler_fns:		clm_aisexec_handler_fns ,
	aisexec_handler_fns_count:	sizeof (clm_aisexec_handler_fns) / sizeof (int (*)),
	confchg_fn:			clmConfChg,
	libais_init_fn:			message_handler_req_clm_init,
	libais_exit_fn:			clm_exit_fn,
	aisexec_init_fn:		clmExecutiveInitialize
};

if a library sends a message with id 0, message_handler_req_lib_activatepoll
is called by the executive.  If a message id of 1 is sent,
message_handler_req_clm_trackstart is called.  

When a message is sent via the group messaging interface with the id of 0,
message_handler_req_exec_clm_nodejoin is called.

Whenever a new connection occurs from a library, message_handler_req_clm_init
is called.

Whenever a connection is terminated by the executive, clm_exit_fn is called.

On startup, clmExecutiveInitialize is called.

This service handler is exported via exec/clm.h as follows:

extern struct service_handler clm_service_handler;

----------------------
 service handler list
----------------------

Then the service handler is linked into the executive by adding an include
for the clm.h to the main.c file and including the service in the service
handlers array:

/*
 * All service handlers in the AIS
 */
struct service_handler *ais_service_handlers[] = {
    &clm_service_handler,
    &amf_service_handler,
    &ckpt_service_handler,
    &ckpt_checkpoint_service_handler,
    &ckpt_sectioniterator_service_handler
};

and including the definition (it is included already above).

Make sure:

#define AIS_SERVICE_HANDLERS_COUNT 5

is defined to the number of entries in ais_service_handlers


Within the main.h file is a list of the service types in the enum:

enum socket_service_type {
	SOCKET_SERVICE_INIT,
	SOCKET_SERVICE_CLM,
	SOCKET_SERVICE_AMF,
	SOCKET_SERVICE_CKPT,
	SOCKET_SERVICE_CKPT_CHECKPOINT,
	SOCKET_SERVICE_CKPT_SECTIONITERATOR
};

SOCKET_SERVICE_CLM = service handler 0, SOCKET_SERVICE_AMF = service
handler 1, etc.

-------------------------
 the conn_info structure
-------------------------

information about a particular connection is stored in the connection
information structure.  

struct conn_info {
	int fd;				/* File descriptor for this connection */
	int active;			/* Does this file descriptor have an active connection */
	char *inb;			/* Input buffer for non-blocking reads */
	int inb_nextheader;	/* Next message header starts here */
	int inb_start;		/* Start location of input buffer */
	int inb_inuse;		/* Bytes currently stored in input buffer */
	struct queue outq;		/* Circular queue for outgoing requests */
	int byte_start;			/* Byte to start sending from in head of queue */
	enum socket_service_type service;/* Type of service so dispatch knows how to route message */
	struct saAmfComponent *component;	/* Component for which this connection relates to  TODO shouldn't this be in the ci structure */
	int authenticated;		/* Is this connection authenticated? */
	struct list_head conn_list;
	struct ais_ci ais_ci;	/* libais connection information */
};


This structure is daunting, but don't worry it rarely needs to be manipulated.
The only two members that should ever be accessed by a service are service
(which is set during the library init call) and ais_ci which is used to store
connection specific information.

The connection specific information is:

struct ais_ci {
	struct sockaddr_un un_addr;	/* address of AF_UNIX socket, MUST BE FIRST IN STRUCTURE */
	union {
		struct aisexec_ci aisexec_ci;
		struct libclm_ci libclm_ci;
		struct libamf_ci libamf_ci;
		struct libckpt_ci libckpt_ci;
	} u;
};

If adding a service, a new structure should be defined in main.h and added
to the union u in ais_ci.  This union can then be used to access connection
specific information and mantain state securely.

------------------------------
 sending responses to the api
------------------------------

A message is sent to the library from the executive message handler using
the function:

extern int libais_send_response (struct conn_info *conn_info, void *msg,
	int mlen);

conn_info is passed into the library message handler or stored in the
executive message.  This member describes the connection to send the response.

msg is the message to send
mlen is the length of the message to send

--------------------------------------------
 deferring response to an executive message
--------------------------------------------

THe source structure is used to store information about the source of a
message so a later executive message can respond to a library request.  In
a library handler, the source field should be set up with:

msg.source.conn_info = conn_info;
msg.source.s_addr = this_ip.sin_addr.s_addr;
gmi_mcast (msg)

In this case conn_info is passed into the library message handler

Then the executive message handler determines if this processor is responsible
for responding:

if (req_exec_amf_componentregister->source.in_addr.s_addr ==
	this_ip.sin_addr.s_addr) {

	libais_send_response ();

}

Not pretty, but it works :)

----------------------------
 sending messages using gmi
----------------------------
To send a message to every processor and the local processor for self
delivery according to virtual synchrony semantics use:

#define GMI_PRIO_HIGH		0
#define GMI_PRIO_MED		1
#define GMI_PRIO_LOW		2

int gmi_mcast (
	struct gmi_groupname *groupname,
	struct iovec *iovec,
	int iov_len,
	int priority);

groupname is a global and should always be aisexec_groupname

An example usage of this function is:

	struct req_exec_clm_nodejoin req_exec_clm_nodejoin;
	struct iovec req_exec_clm_iovec;
	int result;

	req_exec_clm_nodejoin.header.magic = MESSAGE_MAGIC;
	req_exec_clm_nodejoin.header.size =
		sizeof (struct req_exec_clm_nodejoin);
	req_exec_clm_nodejoin.header.id = MESSAGE_REQ_EXEC_CLM_NODEJOIN;
	memcpy (&req_exec_clm_nodejoin.clusterNode, &thisClusterNode,
		sizeof (SaClmClusterNodeT));

	req_exec_clm_iovec.iov_base = &req_exec_clm_nodejoin;
	req_exec_clm_iovec.iov_len = sizeof (req_exec_clm_nodejoin);

	result = gmi_mcast (&aisexec_groupname, &req_exec_clm_iovec, 1,
		GMI_PRIO_HIGH);

Notice the priority field.  Priorities are used when determining which
queued messages to send first.  Higher priority messages (on one processor)
are sent before lower priority messages.

-----------------
 library handler
-----------------
Every library handler has the prototype:

static int message_handler_req_clm_init (struct conn_info *conn_info,
        void *message);

The start of the handler function should look something like this:

int message_handler_req_clm_trackstart (struct conn_info *conn_info,
	void *message)
{
        struct req_clm_trackstart *req_clm_trackstart =
		(struct req_clm_trackstart *)message;

 { package up library handler message into executive message }
}

This assigns the void *message to a structure that can be used by the
library handler.

The conn_info field is used to indicate where the response should respond to.
Use the tricks described in deferring a response to the executive handler to
have the executive handler respond to the message.

avoid doing anything tricky in a library handler.  Do all the work in the
executive handler at first.  If later, it is possible to optimize, optimize
away.

-------------------
 executive handler
-------------------
Every executive handler has the prototype:

static int message_handler_req_exec_clm_nodejoin (void *message);

The start of the handler function should look something like this:

static int message_handler_req_exec_clm_nodejoin (void *message);
{
        struct req_exec_clm_nodejoin *req_exec_clm_nodejoin = (struct req_exec_clm_nodejoin *)message;

 { do real work of executing request, this is done on every node }
}

The conn_info structure is not available.  If it is needed, it can be stored
in the message sent by the library message handler in a source structure.

The message field contains the message sent by the library handler

--------------------
 the libais_init_fn
--------------------
This function is responsible for authenticating the connection.  If it is
not properly implemented, no further communication to the executive on that
connection will work.  Copy the init function from some other service
changing what looks obvious.

--------------------
 the libais_exit_fn
--------------------
This function is called every time a service connection is disconnected by
the executive.  Free memory, change structures, or whatever work needs to
be done to clean up.

----------------
 the confchg_fn
----------------
This function is called whenever a configuration change occurs.  Some 
services may not need this function, while others may.  This is a good way
to sync up joining nodes with the current state of the information stored
on a particular processor.

-------------------------------------------------------------------------------
Final comments
-------------------------------------------------------------------------------
GDB is your friend, especially the "where" command.  But it stops execution.
This has a nasty side effect of killing the current configuration.  In this
case GDB may become your enemy.

printf is your friend when GDB is your enemy.  

If stuck, ask on the mailing list, send your patches.  Alot of time has been
spent designing openais, and even more time debugging it.  There are people
that can help you debug problems, especially around things like message
delivery.

Submit patches early to get feedback, especially around things like parallel
style.  Parallel style is very important to ensure maintainability by the
openais community.

If this document is wrong or incomplete, complain so we can get it fixed
for other people.

Have fun!