Stem Cell Design Notes

Stem Cells are the fundamental working unit in a Stem application.
Cells are Perl objects that have 3 primary characteristics: First, they
must be registered with a Stem address. Second, they must have public
methods that can take an incoming message as an argument. And third, a
cell must be able to generate messages.  There are three major types of
Stem Cells: class, object and cloned Cells.  These are described in
further detail below.

Class Cells are created by a Stem class which registers itself (using
the Stem::Route::register_class routine at module load time) and are
always registered using its class name.  Class Cells are typically
created by modules which manage some global resource and don't need to
have multiple object instances created. A common reason for this is a
module which has a 'status_cmd' (or similar) method that is used to
get the status of the whole module. The Class Cell registration makes
those methods accessible via messages. Some Stem classes such as
Stem::Conf, Stem::Portal, Stem::Msg are Class Cells. Some modules can be
a Class Cell and also create Object Cells.  Class Cells can optionally
be registered with aliases. The aliases make it easier to send a command
message from the terminal (using Stem::TtyMsg) to a class Cell
(Stem::Route is aliased to 'reg', Stem::Cron is aliased to 'cron').

Object Cells are objects that are created by a class's constructor and
are then registered with the Stem::Route::register_cell routine. The
registration takes the object and a required name (unique to this Hub).
Most often an Object Cell is created by a configuration but any module
can construct an object and register it. Since configurations can be
loaded from files and executed anywhere, Stem Cells can be configured at
any time during the existance of the current Stem system.

Cloned Cells are only created by existing parent Object Cells. (Parent
Cells are Object Cells set up to create Cloned Cells). When the parent
Cell gets some form of trigger (typically a socket connection or a
special command message), it makes a clone of itself and does whatever
special work the cloned object needs. The parent Cell owns a Stem::Id
object which it calls to generate a unique (within this Cell) Target
name which it uses to register the cloned Cell. So the new Cell has a
unique Cell/Target name pair which can be used in messages directed at
it. In a typical case, the new Cloned Cell will send a message elsewhere
informing some other object about its existance; e.g., The Stem::SockMsg
class can be configured to clone itself when a socket connection is made and
then it will send a 'pipe_start' command message out. In an Inetd
configuration that message would be directed to a parent Stem::Proc
Object Cell which will clone itself and fork a process. This clone will
respond to the SockMsg message with its new target address, thereby
setting up a Stem pipe between the socket and process. When either the
process exits or the socket is closed, the cloned Cells are notified and
they clean up and unregister themselves.

You can always find the current set of Cells in a Hub by sending a
'status' command message to the Class Cell Stem::Route. This is also
registered with the alias of 'reg'. So from the terminal (if your Hub
has configured in Stem::TtyMsg) you can type:

reg status

to get the registered Cells in this Hub or

:hubname:reg status

to get the Cells in a remote Hub.


When a Stem message is delivered in a Hub, its 'to' address is looked up
in the Hub's registry and the message is delivered to the destination
Cell via a method.  A Cell must have some well known methods that handle
incoming messages. These method names have well defined formats and
uses. In general there are three groups of incoming message methods. All
of these delivery methods get passed the message itself as their sole
argument.

The first group of delivery methods are those that handle command
messages. These are named for the command (the 'cmd' field of a command
message) with '_cmd' appended. So a foo command message will be
delivered to the foo_cmd method if it exists in the destination Cell. If
a command message method returns a value, it is automatically sent back
to the originating Cell in a response message.

The second group handles all other message types. They are named for the
message type with a suffix of '_in'. So a 'foo' type message would be
delivered to the 'foo_in' method if it exists in the destination Cell. A
very common message type is 'data' and it gets delivered to the
'data_in' method.

The final group has the single method 'msg_in' which is used if no other
method can handle the message. This is the default message delivery
method. You can have a Cell with just this method and it should be
designed to handle all expected message types.

The use of specific delivery methods is not critical but it encourages
cleaner Cell design by having methods focus on doing work and not on
deciding how to handle different message types. This is in keeping with
the Stem design philosophy of doing as much common work as possible
behind the scenes, while leaving only the problem specific work to the
Cell.

There are no design considerations for sending messages from a Cell. It
just creates a message object with the class call Stem::Msg->new and
then dispatches it.  If the message doesn't have the 'from' address set,
it will default to the address of the current Cell (if it is known).  If
the code that generates a new message is not a registered Cell, then you
must specify the 'from' address as one can't be deduced.

For more on Cell addresses see registry_notes and message_notes.