--- /dev/null
+Samba Architecture
+------------------
+
+First preliminary version Dan Shearer Nov 97
+Quickly scrabbled together from odd bits of mail and memory. Please update.
+
+This document gives a general overview of how Samba works
+internally. The Samba Team has tried to come up with a model which is
+the best possible compromise between elegance, portability, security
+and the constraints imposed by the very message SMB and CIFS
+protocol.
+
+It also tries to answer some of the frequently asked questions such as:
+
+ * Is Samba secure when running on Unix? The xyz platform?
+ What about the root priveliges issue?
+
+ * Pros and cons of multithreading in various parts of Samba
+
+ * Why not have a separate process for name resolution, WINS,
+ and browsing?
+
+
+Multithreading and Samba
+------------------------
+
+People sometimes tout threads as a uniformly good thing. They are very
+nice in their place but are quite inappropriate for smbd. nmbd is
+another matter, and multi-threading it would be very nice.
+
+The short version is that smbd is not multithreaded, and alternative
+servers that take this approach under Unix (such as Syntax, at the
+time of writing) suffer tremendous performance penalties and are less
+robust. nmbd is not threaded either, but this is because it is not
+possible to do it while keeping code consistent and portable across 35
+or more platforms. (This drawback also applies to threading smbd.)
+
+The longer versions is that there are very good reasons for not making
+smbd multi-threaded. Multi-threading would actually make Samba much
+slower, less scalable, less portable and much less robust. The fact
+that we use a separate process for each connection is one of Samba's
+biggest advantages.
+
+Threading smbd
+--------------
+
+A few problems that would arise from a threaded smbd are:
+
+0) It's not only to create threads instead of processes, but you
+ must care about all variables if they have to be thread specific
+ (currently they would be global).
+
+1) if one thread dies (eg. a seg fault) then all threads die. We can
+immediately throw robustness out the window.
+
+2) many of the system calls we make are blocking. Non-blocking
+equivalents of many calls are either not available or are awkward (and
+slow) to use. So while we block in one thread all clients are
+waiting. Imagine if one share is a slow NFS filesystem and the others
+are fast, we will end up slowing all clients to the speed of NFS.
+
+3) you can't run as a different uid in different threads. This means
+we would have to switch uid/gid on _every_ SMB packet. It would be
+horrendously slow.
+
+4) the per process file descriptor limit would mean that we could only
+support a limited number of clients.
+
+5) we couldn't use the system locking calls as the locking context of
+fcntl() is a process, not a thread.
+
+Threading nmbd
+--------------
+
+This would be ideal, but gets sunk by portability requirements.
+
+Andrew tried to write a test threads library for nmbd that used only
+ansi-C constructs (using setjmp and longjmp). Unfortunately some OSes
+defeat this by restricting longjmp to calling addresses that are
+shallower than the current address on the stack (apparently AIX does
+this). This makes a truly portable threads library impossible. So to
+support all our current platforms we would have to code nmbd both with
+and without threads, and as the real aim of threads is to make the
+code clearer we would not have gained anything. (it is a myth that
+threads make things faster. threading is like recursion, it can make
+things clear but the same thing can always be done faster by some
+other method)
+
+Chris tried to spec out a general design that would abstract threading
+vs separate processes (vs other methods?) and make them accessible
+through some general API. This doesn't work because of the data
+sharing requirements of the protocol (packets in the future depending
+on packets now, etc.) At least, the code would work but would be very
+clumsy, and besides the fork() type model would never work on Unix. (Is there an OS that it would work on, for nmbd?)
+
+A fork() is cheap, but not nearly cheap enough to do on every UDP
+packet that arrives. Having a pool of processes is possible but is
+nasty to program cleanly due to the enormous amount of shared data (in
+complex structures) between the processes. We can't rely on each
+platform having a shared memory system.
+
+nbmd Design
+-----------
+
+Originally Andrew used recursion to simulate a multi-threaded
+environment, which use the stack enormously and made for really
+confusing debugging sessions. Luke Leighton rewrote it to use a
+queuing system that keeps state information on each packet. During the
+1.9.18 alpha series it was decided that this was too unwieldy to
+manage. If the protocol was cleaner than it is then it would be OK
+but with the way the protocol works you really need some data hiding.
+The mistake we made was to transfer all the info from the packets to
+more specialised structures. It bit us badly when we then found we
+needed some detail of the original packet to handle some special
+case. The specialised structures kept growing till they almost
+contained all the info of the original packet! The code became
+extremely hairy, which became particularly evident when Jeremy fixed
+browsing on multiple subnets for 1.9.17.
+
+Then Jeremy rewrote nmbd. The packet data in nmbd isn't what's on the
+wire. It's a nice format that is very amenable to processing but still
+keeps the idea of a distinct packet. See "struct packet_struct" in
+nameserv.h. It has all the detail but none of the on-the-wire
+mess. This makes it ideal for using in disk or memory-based databases
+for browsing and WINS support.
+
+nmbd now consists of a series of modules. It...
+
+
+Samba Design and Security
+-------------------------
+
+Why Isn't nmbd Multiple Daemons?
+--------------------------------
+
git.samba.org / metze / old / samba4-sync / samba4-sync.git / .git / commitdiff