COM Events for Late-bound Delivery of Information
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COM+ is an evolution of Microsoft Component Object Model (COM) and Microsoft Transaction Server (MTS). COM+ builds on and extends applications written using COM, MTS, and other COM-based technologies.
COM Events for Late-bound Delivery of Information
- 1. COM+ EVENTS LATE-BOUND DELIVERY OF INFORMATION Computer Call April 08, 1999
- 2. PUBLISHING INFORMATION • One of the classic programming problems of distributed computing is how to advertise and deliver information to one or more interested parties without a priori knowledge of their identity • In this context there are only two types of programs: those who possess info, and those who need it
- 3. Application client IStockEvents Component THE COMPONENT PROGRAMMING MODEL (REQUEST/REPLY) • Early bound • Class Identity (CLSID) usually compiled in • 1 : 1 • Client calls a single component • Blocking • Client thread generally blocks until the call completes
- 4. Subscriber Publisher PUBLISH/SUBSCRIBE SYSTEMS (PUSH) • Late bound • Subscriber identity not known at development time • 1 : many • “Events” are sent to all registered subscribers • Non-blocking • Publishers generally do not wait for a response from subscribers Subscriber Subscriber
- 5. EVENT PROGRAMMING MODELS • Standard event interface • FireEvent(VARIANT StockEvent) • Standard event object • StockPriceChange(Event StockEvent) • Arbitrary interface • StockPriceChange(char* Company, double StockPrice)
- 6. COM EVENTS TODAY (CONNECTION POINTS) • No activation • Shared life-cycles between publisher and subscriber • Interface-only (no implementation) • Binding, dispatching, error semantic, etc., left to developer • Subscriptions aren’t persistent • No opportunity for interception between publisher and subscriber
- 7. option explicit ' -------------------------------------------------- ' Call a simple component ' -------------------------------------------------- Dim obj set obj = CreateObject(”StockComponent.1") obj.StockPriceChange ”MSFT", 185.00 set obj = Nothing COM+ PROGRAMMING MODEL • Simple programming model • Create Object, Invoke method, Release Client IStockEvents StockComponent.1
- 8. option explicit ' ------------------------------------------ ' Fire an event ' ------------------------------------------ Dim obj set obj = CreateObject(”StockEvents.1") obj.StockPriceChange ”MSFT", 185.00 set obj = Nothing COM EVENTS PROGRAMMING MODEL • Same basic programming model • Create Object, Invoke method, Release Subscriber Publisher IStockEvents Event class (StockEvents.1) IStockEvents IStockEvents IStockEvents Subscriber Subscriber
- 9. EVENT CLASS AS INTERMEDIARY • Indirection between the publisher and subscriber • Meta-data defined (CLSID, ProgID, IID) • Requires no coding on part of developer • Exposes arbitrary interface • May be Secured to control event delivery • Subscriptions are wired to methods within the EventClass’ firing interface
- 10. TRANSFORMATIONS VIA THE “EVENTCLASS” • Introduction of the EventClass caused two transformations: • Ordinary, vanilla COM client became a publisher • Ordinary, vanilla COM component became a subscriber • Publisher = COM client • Subscriber = COM component
- 11. EVENT CLASS INTERFACE EXAMPLE interface IStockEvents { HRESULT StockPriceChange ( BSTR StockSymbol, double Price ); HRESULT NewStock ( BSTR StockSymbol, BSTR CompanyName ); HRESULT StockSymbolChanged (BSTR OldSymbol, BSTR NewSymbol ); HRESULT DowJonesUpdate ( double Current ); }; • Each method may have 0 - n subscriptions • Subscribers expose this interface as well You subscribe to individual methods on the firing interface IStockEvents StockEvents.1 { 877E7998-E8EC-11D1-B994-00C04F990088 }
- 12. option explicit Dim EventSystem Dim EventClass set EventSystem = CreateObject("EventSystem.EventSystem") ' ---------------------------------------------- ' Subscription for IStockEvents.StockPriceChange ' ---------------------------------------------- set EventClass = CreateObject("EventSystem.EventClass") EventClass.EventClassID = "{58A7E1A1-5A4A-11d2-BD11-0080C72B9899}" EventClass.EventClassName = "StockEvents" EventClass.FiringInterfaceID = "{58A7E1A2-5A4A-11d2-BD11-0080C72B9899}" EventSystem.Store "EventSystem.EventClass", EventClass Set EventClass = Nothing CREATING AN EVENTCLASS
- 14. HOW DO I SUBSCRIBE? • Traditionally: • Subscriber gives pointer to self to a publisher • This is problematic! • Lifecycles must be shared • Assumes one publisher of the event • Adds complexity to subscriber • Subscriber now “knows about” the publisher • Subscriber includes subscription building logic
- 15. Subscriber (Start-up) Publisher (Start-up) Time Publisher (Shutdown) Subscriber (Shutdown) Subscribe Un-Subscribe TRADITIONAL LIFE-CYCLES (A LA CONNECTION-POINTS)
- 16. TIGHTLY-COUPLED SUBSCRIPTIONS • This model cannot be used if: • Subscribers do not want to be active until the event is fired • Subscribers want to subscribe prior to publisher start-up or installation • Administrative management of subscriptions is required • Publisher of event changes over time
- 17. ◆ A subscription is a relation composed of: ◆ EventClass (IID inferred) ◆ A specific method within the EventClass interface ◆ A subscriber ◆ May be a CLSID, moniker or an interface pointer ◆ The use of CLSID solves the activation problems, but not the life-cycle issue Publisher Event class Subscriber StockEvents StockPriceChange StockSubscriber StockSymbol = “MSFT” EventClass EventMethod Subscriber PublisherProperties SUBSCRIPTION AS A FORMAL OBJECT
- 18. SUBSCRIPTION DETAILS • MachineName • Used to indicate that the subscribing component must be called remotely • Subscriber • May be a CLSID, moniker or interface pointer • Security • May be Secured • Publisher Properties • May be used by the publisher for filtering, etc. • Subscriber Properties • May be used by the subscriber for filtering, forwarding, etc. • Roaming • Subscription follows the user (owner)
- 19. Publisher Event class Subscriber COM+ Events Subscription DECOUPLING THE LIFE-CYCLES • Achieving independent life-cycles requires that a third-party become involved: COM+ events • Subscribers add subscriptions to COM+ events • Publisher’s EventClass retrieves subscriptions from COM+ events
- 20. Subscriptions SURVIVING SYSTEM FAILURES • Ensuring subscriptions survive software failures requires persistence • COM+ Events stores subscriptions in a database Publisher Event class Subscriber COM+ Events Subscription
- 21. FORMALIZING THE ROLE OF “SUBSCRIPTION BUILDER” • Most publish/subscribe models do not delineate between: • Subscribers • Subscription builders • Subscriber = Component which receives an event call • Subscription Builder = Entity that registers the subscriber to receive the event
- 22. Admins WHO BUILDS SUBSCRIPTIONS? • Answer: Everyone! Subscriptions Publisher Subscriber COM+ events Subscription
- 23. set EventSub = CreateObject("EventSubscription") EventSub.EventClassID = “...” EventSub.MethodName = “...” EventSub.SubscriberCLSID = “...” EventSystem.Store(EventSub) MMC Snap-in Programmatic Subscriptions StockEvents StockPriceChange StockTicker COM EVENTS - WIRING COM+ Events Subscription
- 24. Dim EventSystem Dim EventSubscription set EventSystem = CreateObject("EventSystem.EventSystem") ' ---------------------------------------------- ' Subscription for IStockEvents.StockPriceChange ' ---------------------------------------------- set EventSubscription = CreateObject("EventSystem.EventSubscription") EventSubscription.SubscriptionID = "{0019B161-69D9-11D1-88D1-0080C7D771BF}" EventSubscription.SubscriptionName= "ESSample.StockPriceChange" EventSubscription.EventClassID = "{F89859D1-6565-11D1-88C8-0080C7D771BF}" EventSubscription.MethodName = "StockPriceChange" EventSubscription.SubscriberCLSID = "{C658CAB0-89A2-11D1-891C-0080C7D771BF}" 'EventSubscription.PutPublisherProperty "StockSymbol", "MSFT" EventSystem.Store "EventSystem.EventSubscription", EventSubscription Set EventSubscription = Nothing CREATING A SUBSCRIPTION
- 25. SUBSCRIPTION SUMMARY • Subscriptions as objects are more expressive than mere “object pointers” • Subscriptions are added to COM+ events • Subscriptions are managed by COM+ events • Subscriptions are persisted by COM+ events • Subscriptions may be queried • Subscriptions may be secured • Subscriptions may be shared by multiple publishers (instances) • Publishers and subscribers • May be uncoupled • Need not share life-cycles
- 26. NT Service Firing Subscribers IStockEvents IStockEvents EventClass Wiring COM EVENTS OVERALL ARCHITECTURE Subscriptions StockEvents StockPriceChange StockTicker Subscription
- 27. Event service Subscriptions Event store COM EVENTS FIRING Subscribers IStockEvents IStockEvents EventClass Publisher
- 28. COM EVENTS FIRING Subscribers IStockEvents IStockEvents EventClass Publishers Subscription cache Event service Subscriptions Event store ◆ Publishers cache subscriptions ⚫ Cache is kept coherent with Event Store automatically ⚫ Cache may contain private subscriptions
- 29. Publisher Event class Subscribers “MSFT” “CPQ” “INTC” interface IStockEvents { HRESULT StockPriceChange ( BSTR StockSymbol, double Price ); }; EVENT FILTERING • Most Subscribers will want to be called only when a specific stock price changes • This requires filtering
- 30. Publisher Subscribers IStockEvents IStockEvents EventClass Event service Event store EventFilter Subscription cache interface IStockEvents { HRESULT StockPriceChange ( BSTR StockSymbol, double Price ); }; Subscription EventClass Method Subscriber StockSymbol = “MSFT” COM EVENTS FILTERING IN PUBLISHER
- 31. Publisher Subscribers IStockEvents IStockEvents EventClass Event service Subscriptions Event store EventFilter Subscription Cache COM EVENTS FIRING (CUSTOM CACHE)
- 32. Publisher process Stock events IStockEvents Subscriber Subscriber Subscriber Machine 1 Machine 2 IStockEvents IStockEvents Subscriber process Subscriber process REMOTEABLE EVENTS
- 33. COM+ EVENTS STORE • Component-based • Load balancing, security, transactions • Secure • Accessible via Windows NT® service (COM+ event system) • Publishes events • EventObject added, modified, deleted • Not required for firing events
- 34. COM+ EVENTS SECURITY • Subscriber • Subscriber components use standard COM/COM+ security to protect against “fake” publishers • EventClass • COM+ role based security prevents unauthorized subscriptions from being registered to receive secure events • All objects in Event Store may be Secured • Event Store • Only accessible by Windows NT Service running as “Local system”
- 35. COM+ EVENTS COMPOSITION • COM/COM+ features: • Interfaces • Transactions • Object pooling • Just-in-time activation • Queued components • Deployment • Load balancing • Role-based security • Windows NT features: • MMC-based admin • Windows NT roaming
- 36. Publisher Subscribers Logger Screen saver (Per-user, roaming)) Emailer IStockEvents IStockEvents StockEvents Event Service Subscriptions Event store MAPI, SMTP MSMQ Audit log Debug/trace (Transient subscriptions) Performance monitor (Transient subscriptions) STOCK TICKER SCENARIO
- 37. MS PRODUCTS IMPLEMENTING COM+ EVENTS • SENS - System Event Notification Service • Publishes login/logout, network connect/disconnect, Plug and Play, shell start/end, etc. • SyncMgr • Subscribes to SENS and coordinates application synchronization • COM+ • All COM+ components fire debug/trace events using COM+ events • Tools • Support currently being added across both our languages and tools