Triggers and Active Databases Alexandra Klenova Meghan Russ Josh Sunshine Abe Weinograd

Timeline  1:40-3:05 Trigger History and Application Josh and Abe  3:05-3:15 Break  3:15-4:00 Scalable Trigger Processing Sasha and Meghan  4:00-4:30 Discussion

Practical Applications of Triggers and Constraints: Successes and Lingering Issues Stefano Ceri Roberta J. Cochrane Politecnico di Milano IBM Almaden Research Center Jennifer Widom Stanford University

References  S. Ceri, R.J. Cochrane, and J. Widom. Practical Applications of the Twenty-Sixth International Conference on Very Large Data Bases, pages , Cairo, Egypt, September Invited Paper  U. Dayal, E. Hanson, and J.Widom. “Active Database Systems. In: Modern Database Systems: The Object Model, Iteroperability and Beyone, Addison-Wesley, Reading, Massachusetts, Sep 1994  N. Paton, O. Diaz “Active Database Systems”  S. Ceri, J. Widom. “Deriving Production Rules for Constraint Maintenance” In Proceedings of the Six-teenth International Conference on Very Large Data Bases, pages , Brisbane, Australia, August 1990

What are Triggers and where do they come from?  Idea of triggers were developed from data constraints  Triggers make a passive database active and allows it to react to situation Example: Relation of baseball players and salary. If new player is added/old player is updated, trigger will check to see if team is over salary cap and will impose a penalty to team.  Event Condition Action rule On event insert/update/delete, if condition C is true then do action

DEMO

Outline  How does this relate to Streaming Databases?  Issues and Advantages  Generated Triggers  Handcrafted Triggers  Further Classification

So How Does This Relate to Streams?  Triggers are the foundation for continuous queries New data, or updated data can cause a reaction  Transforms old passive database, into an active one that waits on data in order to react to it.

STREAM and Triggers Similarities  Based on traditional DBMS  Uses continuous queries  Extending DBMS to become Active  Triggers are like continuous queries

STREAM and Trigger Differences  Triggers are simple but have no sense of order of evaluation  Triggers aren’t scalable  Many triggers cause confusion Trigger cycle

Issues  Expressive Events Events that can be triggered are limited to insert, delete, and update No support for time actions Example: At 4am copy essential tables and send alert to DBA No sophisticated trigger processor to handle complex triggers

Issues cont’d  Limitations Limit to prioritization and optimization when multiple triggers are opened Language is low-level and more difficult to deal with  Uniformity Each DBMS has subtle differences in language  Subtle Behavior Triggers only execute after data is finished modifying External actions do not know if the trigger that executed it committed causing possible inconsistency.

Issues cont’d  Development Support No support for handcrafted triggers, which result more generated triggers and trigger “wizards”  Performance Increasing problems when there are many triggers on one table

Advantages  To move application logic and business rules into database This allow much more functionality for DBAs to establish vital constraints and rules that apply to their applications

Types of Triggers  Two types of Triggers Generated: Handcrafted: usually specific to application  Three types of uses Kernel DBMS: hard coded into kernel DBMS services: enhances database functionality External applications: creating triggers specific to app.

Generated Triggers  DBMS Kernel Referential integrity If foreign key in a table is deleted or updated, it causes an action usually specified by user: set null/cascade Materialized View Set of triggers that keep data consistent  Either re-computes view or changes view each time base data is changed

Generated Triggers cont’d  DBMS Services Alerter When data changes, message can be sent to user  Example: A sensor will notice that only one milk carton is left on the shelf in a market, and a message could be send to manager. Replication If a table is copied, a trigger will notice updates to table and will change copied table.

Handcrafted Triggers  External Applications Straightforward use of triggers Application Specific May compute derived columns (remember that this was a shortcoming of the relational model) Use trigger wizard to let developer specify trigger in higher level language

Further Classifications  Generated Constraint preserving Constraint restoring Invalidating Materializing Metadata Replication Extenders Alerters  Handcrafted Ad hoc

What to Take Away  Triggers are foundation to Sreams  Triggers are limited in expression and performance  Two Types of Triggers:Generate and Handcrafted