The document provides an overview of the publish-subscribe model from the perspective of a database. It discusses key aspects of the publish-subscribe model including decoupling of publishers and subscribers, subscription models, and quality measures. It also examines applying publish-subscribe concepts in databases through expressions, continuous queries, and using XML with XFilters and SQL queries.

1. Publish-Subscribe
Model Overview
Presented by: Ishraq Fatafta

2. Agenda
O Introduction.
O Publish-Subscribe model overview.
O Publish-Subscribe a database perspective:
O Expressions.
O Continuous query.
O XML.
O Conclusion

3. Introduction
O Traditional system-centric approach
O Request/Response query of data.
O Data volume and response time.
O Data-centric approach
O Publishers
O Subscribers
O Notification system

4. Introduction Cont.

5. Introduction Cont.
O Publish-Subscribe model advantages:
O Enhanced response time.
O Enhanced results.
O Database resources utilization and
increased capacity.
O Loosely coupled relationship between
publishers and subscribers.
O Scalability.

6. Publish-Subscribe Model
Overview
O Described as events or pattern of events
produced by publishers that subscribers
interested in and notified when they are
available.
O Information has been referred to as
Notifications in this paradigm.
O Subscribers can continue their tasks until
the notification service delivers
notifications.

7. Publish-Subscribe Basic Model
Overview

8. Publish-Subscribe Basic Model
Overview Cont.
O Decoupling types between publishers and
subscribers:
O Space decoupling: in which publishers and
subscribers do not need to know each other.
O Time decoupling: in which publishers and
subscribers do not need to be running at the
same time.
O Synchronization decoupling: in which publishers
and subscribers operations and tasks are not
halted during publishing and receiving
notifications.
O Scalable system that fits well in distributed
systems.

9. Publish-Subscribe Model
Overview Cont.
O Other communication models existed aside from
publish-subscribe model:
O Message passing:
O Relies on messages for establishing communication
between the sender and the receiver.
O Message production done Asynchronously.
O Message consumption done Synchronously.
O Both need to be available in the same time.
O Not decoupled in terms of time and space

10. Publish-Subscribe Model
Overview Cont.
O Other communication models existed aside from
publish-subscribe model:
O Remote call procedure (RPC):
O Intends to make remote interactions looks the same as
local interactions.
O Coupled in time, space and synchronization.
O Notifications:
O Notifications sent by client to the server including
callback arguments.
O Notifications sent by server to the client including the
result.
O Coupled in time and space.

11. Publish-Subscribe Model
Overview Cont.
O Other communication models existed aside from
publish-subscribe model:
O Shared space:
O Based on tuple-space: ordered collection of tuples
accessed by all parties.
O Adding and deleting tuples from tuple-space
Synchronously.
O Decouple time and space.
O Message queuing:
O Uses tuple-space, queues are provided with messages
from producers and additional transactional, ordering
and timing functionalities are provided by the message
queue.
O Same as Shared space.

12. Subscription models
O Topic-based subscription model
O Also referred to as Subject-based models.
O subscriber shows interest in a particular topic and
receives notifications filtered based on that.
O Similar to joining to a group but more dynamic.
O Hierarchy based.
O Limited amount of expressions provided for
subscribers to filter and limit their interested criteria.
O Subscribe to more than one topic in a single
subscription.

13. Subscription models Cont.
O Content -based subscription model
O Bound to the content of events themselves
rather than external criteria.
O Subscription language is used for filtering
O CarBrand = „Mercedes‟ and Price <= 20,000
O StockName = „T*‟ and change > 3
O Needs more expressive criteria to determine
which will generate a lot of traffic on the network.
O More advanced and complex notification system
to be able to filter each event and extract
subscriptions

14. Subscription models Cont.
O Type-based subscription model
O Built using concepts from Object-Oriented.
O Events are objects that can hold attributes and
methods and notifications are objects of specific
type.
O Subscribers of specific object types will only
receive instances of that type or its sub-types.
O Performance issues when a large amount of
events that need to be processed all at runtime.

15. Subscription related
characteristics
O Push and Pull
O Time driven and data driven
O Full update and incremental update
O Broadcast and unicast data delivery

16. Quality measures of publish-
subscribe services
O Quality measures and metrics when
designing any publish-subscribe model:
O Reliability.
O Security.
O Priority.
O Latency.

17. Publish-Subscribe model:
Database Perspective
O Publish-subscribe with expressions.
O Continuous Query.
O XML.

18. Publish-subscribe with
expressions
O Boolean expression used to specify subscribers‟
interest in an event by filtering their criteria using
name-value, comparison operators
(=, >, >=, <, <=) and regular expressions.
O We will use SQL and relational database.

19. Publish-subscribe with
expressions Cont.
O Example: Interested in cars for sale
O Brand Cadillac and price less than 35000
O Rules :
ON Car4Sale
IF (Model = ‘Cadillac’ and Price < 35000)
THEN notify(‘abc@yahoo.com’)

20. Publish-subscribe with
expressions Cont.
SubscriberI Address … Interest
D …
100 Amman … Model = „Cadillac‟ and
…. Price < 35000
101 Irbid … Model = „Mercedes‟
…. and Year > =2007
SELECT * FROM [SUBSCRIBERS]
WHERE
EVALUATE(SUBSCRIBERS.Interest,
<DATA ITEM>) = 1

21. Publish-subscribe with
expressions Cont.
O Queries can be simple, complex, with any type of
join.
O Publishers can put limitations on predicates.
SELECT * FROM SUBSCRIBERS
WHERE
EVALUATE(SUBSCRIBERS. Interest,
<CAR DETAILS>) = 1
AND SUBSCRIBERS.Address = „Amman‟
ORDER BY SUBSCRIBERS.SubscriberID
DESC

22. Publish-subscribe with
expressions Cont.
O Storing expressions as Table data
O Store these conditions as data in special type
columns.
O Metadata is needed
O To store information about values stored in the
condition predicates.
O A list of built-in and user-defined functions
referenced by the condition.
O Validate values stored when new or existing
columns are modified.
O Indexes can be added.

23. Publish-subscribe with
expressions Cont.
O Evaluating expressions
O Evaluate operator is new to SQL.
O Conditional expression is translated into a
WHERE condition in SQL.
O Expression Metadata used to determine
the structure of the FROM clause.
O The result returned is 1 (true) when the
condition is satisfied.

24. Publish-subscribe with
expressions Cont.
SELECT DISTINCT SUBSCRIBERS.SubscriberID,
(CASE WHEN SUBSCRIBERS.annual_income > 100000
THEN notify_salesperson (SUBSCRIBERS.PhoneNumber)
ELSE
create_email_msg (SUBSCRIBERS.EmailAddress
)
FROM SUBSCRIBERS, INVENTORY
WHERE
EVALUATE(SUBSCRIBERS.Interest, <car details FROM
INVENTORY>) = 1
AND
Sub_DISTANCE(SUBSCRIBERS.Address,:DealerLoc,‟distance=50‟)
= ‟TRUE‟
Group BY SUBSCRIBERS.SubscriberID

25. Continuous Query
O Queries constructed only once and stored
for continuous use over the database.
O Used over Append-Only databases.
O First used to support Tapestry systems.
O Uses time-based approach rather than
triggers.
O Uses a language called TQL (Tapes-try
query language).

26. Continuous Query Cont.
FOREVER DO
Execute Query Q
Return results to user
Sleep for some period of time.
ENDLOOP

27. Continuous Query Cont.
O Continuous query suffers some dis-efficiency:
O Non-deterministic results.
O Duplicate.
O Inefficiency of the system
O To overcome this:
O Incremental queries which run periodically.
O Has two timestamps: last execution time (t) and
current time (T).
O Only results in the period (T-t) are returned.

28. Continuous Query Cont.
O Incremental queries:
Set T. –∞
FOREVER DO
set t:= current time
Execute query Q (z, t)
Return result to user
set T:= t
Sleep for some period of time
ENDLOOP

29. Continuous Query Cont.
O To overcome duplicates, queries are transformed into Monotone
queries.
O Queries whom results are not increased as new tuples added to
the database
SELECT * FROM tbl
WHERE tbl.field = “test”
AND tbl.ts < t
SELECT m.msgid
FROM m
WHERE NOT EXISTS(
SELECT * FROM m ml
WHERE ml.inreplyto = m.msgid
AND t< ml.ts + 2 weeks
)

30. XML
O Importance of XML as a standard
information exchange mechanism.
O Its capabilities of encoding structural
information in documents.
O Using XML in creating user profiles.
O Using XFilters, which is a mechanism that
matches XML documents to user profiles
and relational databases, matched
documents are returned using XPath to
interested users .

31. XML Cont.

32. XML Cont.
O XPath query is decomposed into a set of
path nodes using XPath parser.
O Tags are extracted from these nodes and
stored in a TagPath table.
O linear path is extracted from user
subscription XPath profile and stored in
the LinearPath table.
O TagPath table is used to match linear
paths in users‟ subscriptions with TagPath
from XML documents.

33. XML Cont.
O SQL query can perform any DML
operation on them.
O SQL query is ran recursively to match
XML messages with subscriptions.
O Values of stored path tags are used as
predicates in the join

34. Conclusion
O Publish-subscribe system consists of:
O publishers, who wish to disseminate
messages in a form of events to interested
users,
O Subscribers, who wish to be notified with
these events by subscribing to them.
O Notification management system that
maintains a database with all publishers
and subscribers.

35. Conclusion Cont.
O Database is used to match events and
subscriptions by evaluating events based on:
O Expressions provided by subscribers in a form
of queries stored in the database.
O Continuous queries that target Append-only
databases using a time-based approach.
O XML that uses XFilters to match XML
documents against user profiles, filter and
return them using SQL queries in relational
databases and XPath queries.