This document describes an abstract Gemstone notification mechanism implemented by Alfred Wullschleger at the Swiss National Bank. It introduces classes like TpzNotification and TpzNotificationReceiver that allow objects to notify other objects of changes in a conflict-free way. TpzNotification objects are used as events to mediate notifications, while TpzNotificationReceiver objects act as handlers to execute necessary actions. This separates the generation and handling of notifications completely. The mechanism uses a notification receiver collection to dispatch notifications to the corresponding receivers. This provides a standardized way for objects to notify about changes without directly using the changed objects themselves.

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Gemstone Notifications
Implementation of an abstract Gemstone
notification mechanism
presented by
Alfred Wullschleger
Swiss National Bank

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The Author
● Smalltalker since 1992
● Project OVID at Fides Informatik (1992-1999)
– OVID currently in production more than 11 years
● Project OASE at Swiss National Bank (since
1999 in production)
– Financial Statistics from Swiss Banks and
Companies
– based on Gemstone/S and VisualWorks
– ongoing development under full production

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Gemstone Notification Basics
● a GS session declares interest in change
notification for anObject by:
– System addToNotifySet: anObject
● whenever anObject changes and is commited (typically
by another session), execution of
– System signaledObjects
● returns an array including anObject and the
signaledObjects are cleared by this operation
● by this you can poll for changed objects if you like

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Gemstone Notification Handler
● using
– System enableSignaledObjectsError
● allows for the following Gemstone exception:
– Exception installStaticException:
[:theException :cat :num :args |
„... do here whatever is interesting... „
System enableSignaledObjectsError]
category: GemStoneError
number: (ErrorSymbols at: #rtErrSignalCommit).

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Goal: Abstract Notification
● We want to have a highly standardized way of
notification
– do not use the objects, that are just there from
application perspective
● it has to be fully conflict free
– any session can commit independently
– there can be as many notifiers as you like

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Mechanisms
„Event Side“:
Objects that are committed
and want to inform
other Objects
about this
„Event Handler Side“:
a Session, that wants to
know, when certain Objects
are changed and commited
use tpzNotify as an „Event“
mediated by a TpzNotification
uses a TpzNotificationReceiver
as „Event handler“
to execute the necessary actions
complete separation of
Generation and Handling

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Event Side: 2 Classes
● TpzNotification
– notifierSymbol: defines a notification by a symbol
– sessionValues:
● an array of size System maxSessionId.
● containing a TpzNotificationElement for each possible
session
● TpzNotificationElement
– has a value (anInteger) and a sessionId. The
sessionId is used for quick lookup in the
sessionValues

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Event Side: Notification
● TpzNotification>>tpzNotify
– (sessionValues at: System session) tpzNotify
● TpzNotificationElement>>tpzNotify
– value := value + 1
● could also use value := value not
● since all parallel GS sessions have different
sessionIds, there cannot occur any collision
● since index lookup in the array is fast, no
problem even for 1000 sessions

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Handler Side: 2 Classes
● Where do we put TpzNotificationReceivers (the
handlers for the notifications)?
– into session state
– can use index 20 (user slot in session state)
● define a holder for all notificationReceivers:
TpzNotificationReceiverCollection
– collects all notification receivers
– implements the notification exception
– dispatches notifications to the corresponding
receivers

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TpzNotificationReceiver
● Defines a process loop, which is activated by its
corresponding TpzNotification
– registerForNotification: aSymbol
● gets the corresponding TpzNotification
● inits the process loop
● inserts itself into the TpzNotificationReceiverCollection
● executes the first update

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TpzNotificationReceiverCollection
● main Methods:
– addNotificationReceiver: aNotificationReceiver
receivers at: aNotificationReceiver class
put: aNotificationReceiver
– notifierSignaled
| notelems |
notelems := System signaledObjects.
receivers do: [:each |
each notifierSignaled: notelems].
● called from the exception

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Dispatch of signaledObjects
● in TpzNotificationReceiver:
● notifierSignaled: signaledObjects
(tpzNotification hasSignaledObjects: signaledObjects)
ifFalse: [signalingInformation := nil. ^self].
signalingInformation := signaledObjects.
processSemaphore signal
● in TpzNotification:
● hasSignaledObjects: signaledObjects
signaledObjects detect: [ :elem |
(sessionValues at: elem sessionId ) == elem ]
ifNone: [ ^false ].
^true
● is quick, since typically few signaledObjects

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Receivers for specific Notifications
● each specific receiver is a subclass of
TpzNotificationReceiver
– must implement 2 methods:
● class side: notifierSymbol
– to declare, which Notification is accepted by this receiver
● instance side: executeUpdateFromNotification
– the activation of the code for which the notification is all about

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Demo
● We have two Handlers for the same
notifierSymbol #SimpleNotification:
– SimpleNotificationHandler
● waits 15 seconds, then returns a string
– SecondSimpleNotificationHandler
● waits 5 seconds, then returns some other string
– during the waiting, the results of the handlers are nil

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Thank you for listening
Questions?