Hmm. Seems this term has also been snagged by Microsoft... I'll put up a description of what I understand by a stateless object:
More precisely an object without mutators - a state-changeless object.
Compare these two date implementations:
# stateful class Date: def __init__( self, year, month, day ): self.year = year self.month = month self.day = day def advanceday( self ): self.day = self.day + 1 if self.day > daysinmonth( month, year ): self.day = 1 self.month = self.month + 1 if self.month > 12: self.month = 1 self.year = self.year + 1 # stateless class Date: def __init__( self, year, month, day ): self.year = year self.month = month self.day = day def nextday( self ): day = self.day + 1 if day > daysinmonth( month, year ): day = 1 month = self.month + 1 if month > 12: year = self.year + 1 month = 1 else: year = self.year else: month = self.month year = self.year return Date( year, month, day )
A StatelessObject is a collection of [hopefully] related subroutines.
...which is why it's not a real object; it's just a collection of related functions., See UtilityClasses.
Not strictly true. An MTS object can hold state between client calls. See Enable/Disable Commit.
Nevertheless stateless is THE one and only way to get high performance, and is why MS hold most of the TPC-C records.
It can be copied and cached without losing semantics.
Stateless objects that are nothing but buckets for subroutines are one of the causes of RavioliCode.
'that are nothing but buckets for subroutines' is the key. You can get quite some useful effects from StatelessObjects. Consider e.g. the object True and False deriving from boolean (as in SmallTalk).
I know at least one VB developers who thinks that because MTS demand StatelessObjects, they must be a good thing in themselves. So he writes two types of classes, Stateless objects that contain all the processing logic and classes with just get and let properties that might as well be Types. He seems to be rather proud of this.
Just to get away from the Microsoft bashing, please look at Java Servlets. They are more stateless than MTS objects.
But servlets are (or should be) communication utilities that link the incoming message/request to the application. They don't exist except as intermediaries for the message.
In small or narrow applications they can be used to perform logic, as the SimplestThingThatCouldPossiblyWork but this restricts growth.
A stateless object is a subroutine library. See Java Math for an egregious example.
Oops! Java Math isn't so much a stateless object as an objectless object.