Messaging

Multiple smbd processes usually run simultaneously - a main daemon process and couple of connections processes. Samba code has special API that allows different processes send messages each other. There are two implementations of the messaging code - different in current Samba3 and Samba4 code.

Samba 3 implementation

This implementaion uses TDB file for storing messages and UNIX signals for notifying message receiver.

All messages are stored in messages.tdb file (one can find it in var/locks directory). Each message is just a database record that has six fields:

• messaging version (there is only one version now - 1)
• message type
• sender process id
• receiver process id
• message data length
• message data

There are lot of message types in Samba3, you can see them all in include/messages.h file.

Process sends a message to other process with special function message_send_pid and to all processes with message_send_all. Each process should call message_dispatch routinely to check if messages were received and run appropriate handler.

Samba 4 implementation

This implementation uses UNIX domain sockets. Each smbd listens on a unix domain socket with the name based on its pid. All sockets are located in var/locks directory. One message is packed into one datagram, so now we have message size problem - there is no way to send messages with size larger than allowed by a datagram.

Testing

You can test messaging speed with LOCAL-MESSAGING test from Samba4 smbtorture. Present implementations have almost similar speed.