[Spread-cvs] commit: r301 - trunk/docs

jonathan at spread.org jonathan at spread.org
Mon Feb 20 11:29:29 EST 2006

Author: jonathan
Date: 2006-02-20 11:29:28 -0500 (Mon, 20 Feb 2006)
New Revision: 301

New documentation for writing Multi-threaded and Multi-process spread client applications.
Minor typo fix in SP_kill.

Added: trunk/docs/MultithreadedClients.txt
--- trunk/docs/MultithreadedClients.txt	                        (rev 0)
+++ trunk/docs/MultithreadedClients.txt	2006-02-20 16:29:28 UTC (rev 301)
@@ -0,0 +1,33 @@
+Spread specific help with writing Multi-Threaded and Multi-Process client applications
+The Spread system supports both single-threaded client applications and multi-threaded client applications. As much as possible it tries to provide both types with high-efficiency interfaces and API's. However, writing multi-threaded applications entails some unavoidable increase in complexity, both at the API level and with performance considerations and so this document attempts to explain how best to utilize Spread from multi-threaded applications.
+Spread exports several different APIs for client applications. The core Spread API provides extended virtual synchrony semantics and is defined in the sp.h header and consists of functions beginning with the SP_ prefix. The Flush Spread API provides Virtual Synchrony semantics and is defined in the fl.h header and consists of functions beginning with the FL_ prefix. Both APIs can be used by multithreaded applications and are written to be 'thread-safe' (taking not of some specific semantic limitations explained below). 
+Linking and Libraries:
+For applications that are single-threaded and do not need, or want, threaded locks around shared datastructures, a 'non-thread-safe' version of the core Spread API's is available in the libspread-core library. All multi-threaded applications should use either the main libspread library which contains all of the API's in their thread-safe form, or the libtspread-core library for only the core Spread API's in thread-safe form. (Note the "t" character in the library name)
+The rest of this document will assume you do want the multi-thread safe libraries and will use either the VS or EVS Spread API's.
+To link your application code with Spread you will need to compile your code with the platform appropriate _REENTRANT define enabled, and will need to link with the pthreads library on unix systems as well as the libspread or libtspread-core library. On Windows you need to link with the multi-thread safe windows system libraries just like any normal Windows threaded program.
+Spread Locking Semantics:
+Multi-threaded programs can use separate threads to receive messages on different connections, send messages to connections, process application messages, etc. The Spread library's internal locking will protect the library data structures from concurrent access, will provide serialization of SP_receive family calls so only one thread receives each message, and will enforce that each message send occurs atomically on a connection. If more then one thread is sending or receiving from the same connection, then the ordering of messages sent or received can not be determined because it depends on the order in which the processes are granted the shared lock for the connection.
+If several threads call SP_recv on the same connection, then they will all be blocked until messages arrive and then each one will read one message from the connection, the order in which they receive the messages is undetermined as they are all blocked on a shared lock. Thus normally if you care about the message ordering or safety semantics, only one thread will be assigned to call SP_receive for each connection. After it receives the messages, it can make an application level decision to dispatch other threads to process the message contents based on the type of message and what semantics are enforced. 
+In the same way if multiple threads call SP_mcast functions on the same connection, each message will be sent out correctly, but the order in which they are sent is indeterminate.
+Multiple Processes Semantics:
+Multiple processes, on the other hand, should NEVER call SP_receive on the same spread connection as the locking does not protect against accesses from different processes and thus can cause corrupted messages, deadlocks and other problems.  What a program can do when using multiple processes is pass a spread connection from one process to another through a fork call, and as long as only one of the processes uses the connection, then the behavior will be correct. For example a parent process can pass the connection through a fork to a child and as long as only the parent or the child (but not both) use the connection it will work correctly, even if a parent is also multithreaded. Note, this does NOT allow a process to send the spread connection to another process through mechanisms such as Unix Domain Socket File Descriptor passing, as although the spread connection is a file descriptor and will correctly be passed, the library state of the connection that is stored in the libspread will not be shared with the receiving process, and thus the spread library will not be able to work with the received connection. 
+When calling fork in a multi-threaded application using Spread, the Spread library uses an at_fork handler to release any locks that are held in the child process at the time of the fork. This is strictly speaking, unsafe, because a parent thread may have a Mutex lock on the mailbox because it is reading from the socket and once the locks in the child are reset, the child process could also attempt a read -- resulting in garbage in both processe.  However -- We document and require that spread mailboxes cannot be read from different processes at the same time, so the application is responsible for not using them this way.
+To enable a multi-threaded application to who is the parent of a forked child to cleanly remove their libspread state about the connection passed to the client, the function SP_kill has now been exposed in the Spread API. This function will close the Spread connection and remove any library state about the connection WITHOUT notifying the daemon about the close. If, as in the forked parent/child case, the client connection is still connected and open in a different process (such as the parent calling SP_kill after a fork and the child not closing it) then the daemon will not notice any change and will continue to communicate with the process who currently has the connection still open. 
+However, generally the SP_disconnect function should be used to disconnect from the daemon as it correctly and expediently notifies the daemon about the disconnection. If a client calls SP_kill instead of SP_disconnect when no other process has the client side mbox open (because of fork) then the network connection to the daemon will be closed and the daemon will detect the client as disconnected as soon as the network layer (TCP) registers a closed socket on the daemon side. Thus it will appear to the daemon as if a true network fault occurred or the client crashed as opposed to cleanly disconnecting. 

Modified: trunk/docs/SP_kill.3
--- trunk/docs/SP_kill.3	2006-02-19 21:04:21 UTC (rev 300)
+++ trunk/docs/SP_kill.3	2006-02-20 16:29:28 UTC (rev 301)
@@ -13,7 +13,7 @@
 should be called when the application needs to close 
 a connection to the Spread daemon, but does not want to notify the
 daemon about the close. The most common case of needing this is when a
-Spread clienta application calls the unix function 
+Spread client application calls the unix function 
 .B fork()
 and needs to close the connection in either the parent or child since the other
 will handle the communcation. 

More information about the Spread-cvs mailing list