Group Title: Department of Computer and Information Science and Engineering Technical Reports
Title: Architectures and monitoring techniques for active databases : an evaluation
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Title: Architectures and monitoring techniques for active databases : an evaluation
Series Title: Department of Computer and Information Science and Engineering Technical Reports
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Language: English
Creator: Chakravarthy, Sharma
Publisher: Department of Computer and Information Sciences, University of Florida
Place of Publication: Gainesville, Fla.
Copyright Date: 1992
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University of Florida
Computer and Information Sciences

SDepartment of Computer and Information Sciences
"' Computer Science Engineering Building
SUniversity of Florida
Gainesville, Florida 32611

Architectures and Monitoring
Techniques for Active Databases : An

Sharma Chakravarthy


(Submited for publication)

(This work was (in part) supported by the Office of Naval Technology and the Navy
Command, Control and Ocean Surveillance Center RDT&E Division and in part by
the NSF grant IRI-9011216. Also, part of this work was carried out when the author
was at Xerox Advanced Information technology, Cambridge, MA and was supported
by the DARPA and Rome Air Development Center under contract No.


The need for active capability for non-traditional applications and its concomitant
benefits are well-established. Although the event-based technique for monitoring condi-
tions (leading to the integrated architecture) is the most versatile of all the techniques,
from a practical viewpoint there is a need for enhancing pre-existing non-active DBMSs
to support active capability. The set of techniques that can be used for providing this
add-on active capability (leading to the layered architecture) imposes limitations on the
active capability that can be supported. Insights into the details of techniques as well as
their impact on the architecture entails a better design that meets the active database

This paper identifies a repertoire of techniques for condition monitoring and dis-
cusses their suitability to different architectures. This paper argues that from a prag-
matic viewpoint, both layered and i;itclratd approaches to support active capability
need to be pursued. Then it compares polling and event-based or asynchronous mon-
itoring techniques using an implementation of flavors on Symbolics. The focus of this
comparison is on: techniques, performance, influence of implementation strategies on
performance, and identification of opportunities for optimization.

Index Terms: Active databases, Polling, Performance evaluation, Implementation
strategies, Layered approach, Object-oriented design

1 Introduction

The concept of monitoring conditions, which forms the basis for supporting active capability, is
not entirely new. ON conditions in programming languages and early DBMSs [01178], and signals
in operating systems [B... '-, Geh84, Hug79, LMKQ89] have been used early on. Later, Artificial
Intelligence (AI) systems have used daemons for asynchronous execution of procedures attached to
frame slots [CRM80, WB79]. Furthermore, multi-paradigm systems, such as LOOPS [BS83] and
KEE III, ".] have incorporated active values as a new technique that generalizes asynchronous rule
processing. However, most of these systems do not support typical database functionality, such as
data sharing, consistency, and multi-user execution.

A large class of non-traditional applications, such as process control, threat assessment and analysis,
air traffic control, Computer Integrated Manufacturing, and cooperative problem solving, need to
react (often subject to timing-constraints) to a variety of conditions that are defined over the
database state and events that change the state of the database. Hence, there is a critical need for
providing a generic capability which: is at a higher level of abstraction (than ON statements and
daemons), has well-defined semantics, is efficient, and is tailored to the specific needs of Database
Management Systems (DBMSs).

In contrast, traditional DBMSs are passive. They only respond to external requests in the form of
transactions/applications to change their state. Hence, the current trend is to augment a conven-
tional DBMS with active capability. The effect of condition monitoring can also be achieved either
by encoding condition evaluation as part of the application program (equivalent to posing external
queries) or by polling the database (periodically) to detect whether any of the conditions have

become true. Encoding condition evaluation within the application program not only transfers the
burden -of determining the conditions, formulating queries for these conditions, and the timing of
their evaluation -to the application programmer but also interferes with application development.
Furthermore, optimization of such conditions is extremely difficult. Polling seems to waste resources
and transfers the onus of determining the frequency of polling to the user/application developer.
Polling frequency is likely to be dependent on a variety of parameters such as the frequency of
update, timeliness (the time window within which the condition needs to be detected) etc.

Active databases typically accept a declarative (non-imperative is perhaps more accurate) specifica-
tion of situation-action rules (or event-condition-action or ECA rules) and manage their execution.
Each rule consists of an event, a condition, and an action; when an event occurs, the condition is
checked and if it evaluates to true the action is executed. Support for these rules enable a database
system to react to changes in its state and perform specified actions (e.g., alerting application pro-
grams, notifying users, restoring the consistency of the database, or denying access) asynchronously
and without user/application intervention.

Most of the work on active database systems [C+89, DBAB+>", RCBB89, SRi, SHP87, DB87,
DKM86, KDM",- WCB91, WF90, GJ91, GJS92, Han89, Int90a, Int90b, Ati 'i', C:.I'l, CHS93,
GrD93, DPG91] is aimed at supporting some form of rule processing capability (e.g., alerters,
triggers, situation-action rules) and techniques for their management and optimization (e.g., lazy,
eager, overlapped execution). Active capability is viewed as a unifying mechanism for supporting a
number of DBMS functionality, such as integrity/security enforcement, maintenance of materialized
(e.g., view) data, constraint management, and rule-based inferencing.
Although a number of active database issues such as, expressive event specification language [C':.I'll,
GJS92, HJ91], integration of ECA rules into an object-oriented database [A,-- *'i, GJ91, GrD93]
and others [Tan92, C+93, CN90] are being pursued vigorously, to the best of our knowledge, there
is no work on the discussion of architectural alternatives, techniques for condition monitoring,
and evaluation and comparison of the suitability of these techniques. In this paper, we identify
a suite of techniques for condition monitoring and analyze their appropriateness for the proposed
architectures. We also compare two of the techniques identified from the viewpoint of their impact
on the architecture. Towards this end, we designed and implemented active capability for an object-
oriented data model. Specifically, the thrust of this work is towards answering basic questions, such
as the ones listed below, rather than an in-depth performance evaluation.

Is asynchronous condition monitoring always better than polling?,
Should polling be retained as a viable alternative to be used by the system?,
What issues need to be considered when an existing DBMS is made active?, and
What techniques are meaningful for a given architecture?

The prototype object-oriented DBMS1 was designed and implemented on a Symbolics machine
using Symbolics Common Lisp and flavors. This prototype was modularly extended to include
active objects for supporting automatic condition monitoring in addition to other useful DBMS
functions. Needless to say that transaction processing and concurrency control were not included
in the prototype.
1The prototype has the functionality of a subset of PROBE -a passive, extensible DBMS developed at CCA
[D+85, DMB+87].

The remainder of this paper is structured as follows. In section 2 we briefly overview a subset of the
active database issues that are relevant to this paper. Section 3 identifies several techniques useful
for supporting active capability along with an analysis and proposes alternative active database
architectures. Section 4 describes the implementation of an object-oriented DBMS using Common
Lisp. Section 5 discusses the design and implementation of active objects and the functionality
of the resulting prototype. Section 6 describes the evaluation of the approaches and contrasts the
expected behavior with the observed behavior and provides an analysis of the observed results.
Section 7 contains conclusions and future work.

2 Active Database Issues

There seems to be consensus among the researchers in the database community that support for
ECA (event-condition-action) rules is at the core of active capability. However, the semantics of
rule execution in the context of databases requires that several additional issues be considered
[WF90, Cha89, C+89]. Here we include only those issues that are relevant to the rest of the
discussion in the paper:

Rule execution points: In HiPAC [HLM"], three coupling modes for rule execution were in-
troduced to support application needs. Their semantics with respect to triggering transactions
is defined as follows: in the immediate coupling mode a rule is executed at the point where
the event occurs, in the deferred coupling mode a rule is executed at the end of the transac-
tion prior to its commit, and in detached coupling mode a rule is executed as an independent
transaction. A causally dependent variation of the detached mode was introduced in which
the independent rule transaction is not committed unless the triggering transaction commits.
These modes can be specified on a finer granularity (i.e., independently between event and
condition as well as between condition and action).
Rule scheduling: When several rules are tib---. .1 at the same time, there has to be a policy
for their order of execution. Either a conflict resolution strategy can be used to totally order
the rules or traditional serializability theory can be applied to execute rules concurrently.
Starburst [WF90] uses the former approach whereas HiPAC [HLM''] uses the latter.
Nested rules: When rule actions can trigger other rules, there is potential for nested (or even
cyclic) rule execution. Again scheduling strategies (depth-first, breadth-first etc.) for these
rules need to be outlined.
Rule management: If rules are to be shared by applications (like any other shared data), then
modification of rules should be possible. This entails subjecting rules to the same concurrency
control mechanism used for any other shared data. Otherwise, rules have to be treated as
meta-data whose manipulation is deemed different from shared data.

3 Active Database Architectures

In this section, we identify and describe various techniques for condition monitoring which lead to
different architectures for supporting active capability. We further analyze and highlight the need,
advantages, and limitations of architectures as well as condition monitoring techniques.

3.1 Condition Monitoring Techniques

A number of techniques can be employed for accomplishing the (re)active capability in a database.
Below, we identify some of them and discuss their advantages and disadvantages with respect to
the issues outlined in the previous section.

Busy waiting: Typically used in single user or single CPU systems without multi-programming
or multi-tasking. In these systems, it is not possible to use the CPU for any other purpose.
This technique is used only in custom-crafted process control applications where the time
taken to execute the loop in which readings are collected is carefully .,.1i1-I. .1 to the frequency
with which data has to be gathered. Also, in some of the process control applications, the
busy-waiting period is utilized for performing computations (e.g., refresh display).
Encoding or embedding situation monitoring in applications: In this approach, the
application developer includes condition monitoring code at appropriate places in the appli-
cation code. The main advantage of this technique is that it does not require any changes
to the underlying system as all the condition monitoring is done by the application developer
as part of the application. A secondary advantage of this approach is that various coupling
modes proposed in HiPAC [HLMX-] (except the causally dependent mode) can be easily im-
plemented. The main problems with this approach are: i) the application developer has to
be aware of the conditions to be monitored and code it as part of the application, ii) the
condition monitoring code is not maintained or managed by the system and hence cannot
be shared among applications, iii) software maintenance of application programs is extremely
difficult as there is no modularity and no reusability of code. Although a library approach
for condition monitoring is possible in principal, at least in the host language context it is
extremely difficult to accomplish. Data Base Programming Language (DBPL) environments
will be more amenable to this approach.
Polling: In this technique, each condition is checked with a pre-determined polling frequency
and if the condition evaluates to true then appropriate actions are executed. However, the
frequency of polling (or the polling interval) needs to be determined for each rule or classes
of rules. The polling interval needs to take into account the size of the relation (as one may
have to check all the tuples for a condition), the complexity of the condition to be evaluated,
and the frequency of update. In general, it would be a burden on the part of the user (or
even the Data Base Administrator or the Data Base Customizer) to explicitly indicate the
polling frequency. This technique does not lend itself to coupling mode enforcement and nested
execution of rules. Also timeliness of checking is tied to the polling interval and hence this
technique may not be suitable for monitoring rules with time-constraints. In this paper, we
present some conclusions on the feasibility of this approach as an alternative that the system
can use for implementing situation monitoring.
Aperiodic checking of situations: This is a variation of polling in which the system or a
component of the system checks whether the conditions are true periodically by evaluating
queries. The event which triggers this evaluation has to be specified explicitly or determined
by the system. This approach will allow conditions to be specified once but the checking
has to be explicitly invoked. Again, timeliness of checking is not guaranteed and hence this
technique is not suitable for monitoring rules with time-constraints.
Asynchronous monitoring: This is similar to interrupt-driven processing where the system
is responsible for the detection of events, evaluation of conditions, and execution of actions.

Basic Techniques Architecture Advantages Disadvantages
busy waiting, application managed
encode applications Application-based condition monitoring,
no changes to the rules cannot be shared,
underlying DBMS no modularity
busy waiting, no nested rules,
polling, Layered limited coupling modes
periodic checking developed as add-on
polling, versatile, enhancement needed
asynchronous monitoring Integrated enhanced DBMS at the kernel level

Table 1: Techniques-Architecture summary

This approach has the potential for timely detection of events and execution of actions as well
as opportunities for optimizing conditions and actions that are specified to the system declar-
atively. This is the most versatile of the techniques outlined here. However, this technique
requires extensions to the core of the DBMS and a re-examination of the DBMS architecture
to accommodate active capability. This technique lends itself very well for addressing various
active database issues outlined in the previous section.

3.2 Architectures

Table 1 depicts three distinct architectures that result from using one or more of the techniques
listed above.

In the application-based architecture the burden of monitoring is on the user and can use either
busy waiting or encode condition checking as part of applications. No generality is lost as the user
can control when to check the conditions (either at the point of state change or later). Certain
coupling modes (for example, causally dependent) cannot be supported in this approach. The
main drawbacks are that the same condition checking code is distributed among application pro-
grams violating modularity and software engineering principles, leading to difficulties in software

Figure 1 shows the layered architecture. A number of condition monitoring techniques described
earlier are applicable here. The underlying DBMS is augmented with a layer that is responsible for
providing active capability. The architecture shown permits access to the augmented system either
through a user interface tool that transforms user active database design to underlying system
constructs [Tan92] or through a stand-alone interface. Although full active capability cannot be
obtained in this approach, a number of techniques can be used and some optimizations can be
performed by the situation monitor layer. For instance, the layer can decide whether to rewrite a
transaction to include the condition monitoring code (similar to the application-based architecture
but the rewrite is done by the situation monitor layer) or use either the polling or periodic checking
approach depending upon the meta-data used by the system. All applications that require active
capability have to interact with the system through this interface; otherwise, active capability will


Application Layer 1
User Interface

Storage, Analysis, Translation

Monitor/DBMS Interface

Aperiodic Checking
Situation Monitor Layer Transaction rewrite
Creating Meta Data
for Rule Processing



Figure 1: Layered Active Database Architecture





, _________________________i ______



Object Oriented / Relational (Passive)
Extended Transaction Management
Rule (ECA) Management
Event detection & processing
Condition Monitoring
Inter Application Communication Support

Figure 2: Integrated Active Database Architecture

not be available. Alert [S+91] proposed a layered architecture.

Layered Architecture requires that a layer be built (on top of a passive DBMS such as ORACLE
or INGRES) through which all the ECA rule specifications are routed. The layer is responsible for
monitoring the situations and executing appropriate rules which also means that all transactions
are routed through the layer (although eventually processed by the underlying system's transaction
manager). There may be some limitations on the class of ECA rules that can be supported using
this approach. For example, immediate mode coupling may not be possible as the layer may not be
able to suspend a transaction that is being executed by the underlying DBMS. Also, explicit and
other temporal events cannot be supported in this approach without resorting to polling. A number
of optimizations can be incorporated when the cardinality and frequency of database operations
are known. It may even be possible to cache some data in the situation monitor layer for condition
monitoring purposes.

Not surprisingly, most of the research developmental efforts on active databases have opted for
the integrated architecture shown in Figure 2. In this architecture, the kernel functionality of
a passive DBMS is enhanced to include event detection, condition monitoring, and an extended
transaction management to support concurrent rule processing. An integrated approach does not
necessarily have to support full active capability as evidenced by commercial systems such as Sybase
[DB87], Interbase [Int90a, Int90b] and others. All the active database issues outlined earlier are
being addressed in various research efforts in the context of an integrated architecture. Currently,
Ode [GJS92, GJ91], Sentinel [CHS93, A-' *-'1], SAMOS [GrD93] are some of the efforts that are
integrating active capability into an object-oriented database.

3.3 Analysis

Of all the architectures discussed above, the integrated architecture is the most versatile and flexible
which does not have any of the problems associated with the other two architectures. However,
there are compelling reasons for pursuing the layered architecture:

1. To provide a migration path between currently used non-active DBMSs that are not likely to
be replaced in the near future with systems that have some active capability, and
2. To support the integration of a number of pre-existing DBMSs with differing (or no) active

From a practical viewpoint, both 1) and 2) are extremely important. It is unlikely that currently
used non-active systems will be replaced by active systems in the near future. However, the need
for active functionality is clear and the layered architecture can play a significant role in providing
a useful migration path.

Currently, there are no tools or interfaces available that support either the design of active databases
or even rule specification at the application level and translate them to ECA rules. Irrespective
of the architecture used, there is a need for an interface that will accept trigger/rule specifications
from the user and translate them into a form accepted by the layered architecture (shown in Figure
1) or by the integrated active system (shown in Figure 2). In the case of the layered approach, it is
possible to combine both the situation monitor layer and the application level trigger specification
interface into a single one. The application shown in Figure 1 is a tool that supports design
interface at one end (extended ER diagram, for example) and generates ECA rules either for the
layered or for the integrated architecture. Such a system is being implemented in [Tan92, NTC93].

Figure 3 shows the utility of the architectures proposed in this paper. In order to support feder-
ated/heterogeneous active DBMSs, it is critical that the underlying systems support some active
capability 2. The approach proposed in this paper will help address some of the problems faced
by a global transaction manager [SRK92] in a multidatabase environment. Also, federated active
databases is likely to be useful for enforcing interdatabase dependencies [SLE91] as the situation
monitor layer (or the active capability of the integrated architecture) can be used to reveal some
of the information in the underlying system to help make global decisions. For example, if each
system can inform the global transaction manager before the commit of a transaction, then the
global transaction manager can use that information for global serializability. Currently, we are
researching the issues related to multidatabase constraints using this framework.

4 Design of a Prototype Active OODBMS

For the purposes of this effort, a subset of PROBE3, excluding spatial/temporal objects and re-
cursion, was designed and implemented on Symbolics using Common Lisp and its object-oriented
otherwise this has to be built as the part of the mediator layer; however, as the layer is likely to have differences
depending upon the underlying system, it is best done as part of the system rather than as part of the mediator layer.

3PROBE [MD86, RHDM86, D+87], supports a rich data model incorporating spatial/temporal objects as well as
recursion at the kernel level. The Probe Data Model (PDM) provides objects and functions as basic constructs. An
object is used to represent a real world object. Functions can be applied to objects to obtain properties of objects,
invoke operations on objects, and describe relationships among objects. Objects and functions are manipulated

Figure 3: Heterogeneous Active Database Architecture


Figure 4: Object hierarchy used in the prototype

extension, flavors. In this version, stored and virtual relations are supported. A generalization of
relational algebra is used as the query language.

The algebra consists of relational operators (such as select, project, natural join etc.) with suitable
extensions (apply-and-append, for example) to support the PROBE object classes. The system is
composed of a hierarchy of objects and operations on objects. For example, relation is an object
consisting of file objects, attribute objects and other instance variables. These objects are in turn
built using other objects. Figure 4 shows the object hierarchy (including the objects introduced
for active condition monitoring) used for supporting the relational model. Basic object of Figure
4 is inherited by all other objects (not shown to keep the diagram comprehensible).

A simple user interface that embeds algebra operators in LISP has been implemented. It allows
the use of lists, strings, numbers, and conditional expressions using LISP syntax instead of objects
and functions.

4.1 Active Objects

The conceptual ingredients of condition monitoring are: events, conditions, and actions. An event is
an indicator of a happening (recognized by the system or signaled by an application program/user).
For example, events such as an arithmetic overflow or a timer underflow are recognized by the
hardware whereas events corresponding to updates (e.g., insert, delete, and modify) are detected

through an algebra that is a generalization of the relational algebra.

by the DBMS software. A condition is a predicate over the database state that evaluates to a
boolean value. (In the presence of free variables, the predicate is false if there are no bindings to
the free variables.) Actions are operations to be performed when an event occurs and the condition
associated with the event evaluates to true. Actions can be arbitrary programs. Actions, conditions
and events can be packaged in various ways.

Situation-action rules (henceforth rules) have been used to group a condition and its associated
action. One or more rules can be associated with a specific event. When an event occurs, one or
more conditions are evaluated and corresponding actions) invoked.

Our design assumes that a set of primitive events (such as read, write, execution of a function, clock
event) on objects can be detected by the underlying system (either by the operating system or the
kernel of a DBMS). These events are independent of the model and the object classes in the system.
Model and object specific events can be built from these events as we will demonstrate later. A
condition and its associated action is packaged as a rule in our prototype and is implemented as
instances of an object class (active-object).

We have introduced two new object classes active-object and activelist-object which provide the
basis for supporting condition monitoring in the prototype. The object class active-object en-
compasses the functionality of a rule including scope and context information. The object class
activelist-object has been introduced to support multiple rules for a specific event. These object
classes, along with the methods defined on them and the handlers for read and write events, provide
all the necessary mechanisms for making the prototype DBMS active.

The active-object is a Common Lisp Flavor consisting of the following components:

local to hold the value of an object,
readfn a function to be executed on a read event,
writefn a function to be executed on a write event,
relation-id, tuple-id context variables (for the relational model4), and
hist-vars history variables.

The readfn stores the rule (in the form of a LISP function that may include calls to database
operations) to be executed on a read event. When an object with which an active-object is associated
is read, the readfn of the active-object is executed passing the current value of local, the context,
and history variables as parameters. The read function, usually, returns the value of the object as
its final action. As part of the readfn execution, complex computations (including side-effects) can
be performed. A NIL value for readfn is interpreted as a no-op and the value of localis immediately
returned. Similarly, the writefn is a function to be executed when the object (with which the
active-object is associated) is assigned a new value. The final action of a write function usually
changes the value of local. Again, a NIL writefn value immediately assigns new value to local.

The local component of an active-object is a place holder for storing the value of the data object
with which an active-object instance is associated. The value of local is used by the functions
readfn and writefn.

The relation- and tuple- ids are essentially variables providing the context in the form of the
relation and the tuple identification, respectively. Storing references to the relation and the tuple
provides context for the active-object's read and write functions. The context information has
4The discussion is cast in the relational framework for comparison purposes, although the design is object-oriented.

several potential uses. First, the name, type, and the details of the object can be extracted from
the tuple or the relation. Such accesses are often part of a ti -'-. I. I. action. Second, the information
in the tuple or relation may be used in computation performed by the read and/or write functions.
For example, a function which computes the amount of time it would take an enemy ship object
to reach a monitoring ship object must have access to the X and Y coordinates, speed, direction
of the enemy ship object, and possibly other information about the object or other objects of the
same type. Some of this information may be contained in the active object itself. The rest can be
obtained from the tuple or relation containing the active object. Finally, it is useful to know the
relation instance or the tuple instance containing the active object in order to pass it to a function
called from within the rule. In our applications, we use the context information in all these ways.

The history variables can be used for a variety of purposes. Specifically, they can be used to store
information (such as a previous value, the number of times the object is accessed or modified, an
.--:'i.-..i.. value computed using other objects or even a complex derived value) to be used for
optimization and for optimizing rule evaluation.

The activelist-object is also a Common Lisp Flavor consisting of a list of active-object instances.
For the sake of uniformity, an activelist-object is always used to make an object active.

4.2 Semantics of Active Objects

The semantics of an active-object can be easily inferred from the way readfn and writefn are invoked.
The semantics of nested active objects is as follows. For read events, the readfn's are executed inside
out. In other words, the first operation is a read of local which may result in executing a nested
active-object. The value of local generated for the innermost local is passed to the outer level readfn
and so forth. Analogously, for write events, the writefn's are executed outside in. Functions for
both read and write events can be nested to arbitrary levels.

In the case of multiple rules, if rules are to be executed one at a time, then a 'conflict resolution'
strategy is required to choose the appropriate rule. If multiple rules can be executed when an event
occurs, they can be executed concurrently. In either case, ordering of the rules based on some
criterion (e.g., priority) is useful. In the prototype, the read and write functions of an active object
are executed in the order in which they appear in the activelist-object. The implementation of read
function assumes that at least one of them will return the value of the object and the first non-NIL
value returned is used as the value of the object. For multiple write functions, each of them is
executed in the order specified. In this case, it is assumed that values are kept consistent by the
functions. Other strategies for evaluating multiple rules are possible.

In addition to the active- and ,, 'I 1/'- l-,'1;,, I a set of methods are defined over the object classes
introduced above. They include methods for: creating active-object instances, associating them
with attributes and tuples of a relation, activating and deactivating them, and executing rules for
read and write events.

4.3 Implementation of Event Handlers

In order to perform condition monitoring effectively and efficiently, events of interest (read and
write on object instances in our case) need to be recognized. A variety of techniques can be used
for recognizing the events of interest and an important criterion is to keep the overhead of the

detection of events to a minimum. New types of locks were introduced into the lock table for
recognizing various types of events in the initial design of POSTGRES [S+86]. As it is necessary to
have the locks for condition monitoring survive system failures, it seems appropriate to associate
the mechanism for recognizing events with the data items (which survive system failures). This
approach is also beneficial when the number of active objects are large as it distributes the locks
among object instances rather than storing them in a single data structure.

The approach taken for the prototype already treats events as functions (read and write) or methods
defined for a component of an object class. Hence, a component of an object class needs to be made
active only at the object class level and will be inherited by all instances of the object class. It is
clear that the access functions need to be intercepted in order to evaluate conditions and execute
actions associated with the events.

Choice of the strategy for implementing event handlers was based upon:

1. Simplicity of the technique (without sacrificing the functionality of the resulting system),
2. Ease of implementation (one that would not require extensive modifications to existing code),
3. Ease of maintenance (a single version of code which can be made active incrementally).

The following alternatives were considered:

1. Define our own access functions for components of object classes of interest,
2. Use error signaling mechanism to get control of the occurrence of an event,
3. Use before- and after-daemons to get control of the occurrence of an event, and
4. Use v Il,.p. I combined methods to intercept the invocation of methods.

The first three options (listed above) were rejected as they did not satisfy our criteria. The first
alternative involves extensive changes to the existing code in terms of introducing a user-defined
access function for each component of an object we wanted to make active. Also, this option
entails an implementation at the application level overriding the system level access functions.
This approach would also have resulted in maintaining two versions of the system. Hence, this
option violated the ease of implementation and ease of maintenance criteria.

The second option is mostly used for debugging and hence lacked the generality and simplicity
required for our purpose. The third alternative permits one to extend the primary method of an
object class using the before- and after- daemons. Briefly, before- and after- daemons permit adding
code before and after the execution of a function or a method associated with a flavor (object class).
This mechanism is very useful for combining independent methods of component flavors, but does
not permit the binding of variables during the execution of a method. Also, this mechanism does
not provide the flexibility of skipping the method for which before- and after-daemons are defined.

5There is another mechanism called wrappers which also permits adding code around the execution of any method.
However, there are some fundamental differences between them. A wrapper is similar to a macro whereas a whopper
is similar to a function; if a wrapper is modified, all combined methods using it must be recompiled whereas if a
whopper is modified only the whopper needs to be recompiled; the body of a wrapper is expanded (by duplicating
code) in all the combined methods in which it is involved whereas the body of a whopper is not expanded in multiple
places. Whoppers, on account of the above differences, are slightly slower than wrappers.

;;; definition of stringobj object class (flavor)

(defflavor stringobj (string)

;;; a whopper around the read function of string component of stringobj

(defwhopper (stringobj-string stringobj) ()
(if (typep string 'activelistobj)
(p_triggerreadfns string)

;;; a whopper around write function of string component of stringobj

(defwhopper ((setf stringobj-string) stringobj) (new-string)
(if (typep string 'activelistobj)
(p_triggerwritefns string new-string)
(continue-whopper new-string)))

Figure 5: A Sample whopper

The last alternative provided the type of control and at the level that satisfied our objectives.
Whoppers provide a means for wrapping code around any method including system generated
methods for an object class. Furthermore, it is a general mechanism that permits one to gain
control prior to the execution of any method (both system defined as well user defined) and permits
passing of parameters to the actual method itself. This mechanism is superior to other alternatives
in providing a lexical scope within which the actual method itself can be invoked (optionally, of
course) and local variables can be created and passed as parameters. In addition, whoppers delay
the detection of triggers as far as possible thereby reducing the overhead incurred for condition

Whoppers need to be defined as a separate function for each flavor whose components are active.
One has to only load the whopper definitions in order to make objects active in this scheme and
hence does not require any change to existing code. Furthermore, object instances can be made
active incrementally. This incremental way of making objects active keeps the two versions of code
used for comparison identical, except for the whopper code execution at run time. This makes our
results and hence conclusions very reliable.

Figure 5 shows the definition of string-object and whoppers for read and write methods. Figure
6 shows the definition of the active object and how it is created. Note that the readfn and writefn
are passed as parameters.

;;; -*- Mode: LISP; Package: USER; Base: 10; Syntax: Common-lisp -*-

;;; a flavor for active objects consisting of
;;; a value, history variables, context variables (tupid and relid),
;;; a readfn and a writefn.
;;; Currently the data members readfn and writefn (which are functions)
;;; are NOT assumed to be active objects, although theoretically,
;;; nothing prevents us from making them active.

(defflavor activeobj (value locals tupid relid readfn writefn)
(:writable-instance-variables value locals tupid relid)
(:locatable-instance-variables value))

(defvar *null-activeobj*
(make-instance 'activeobj :value nil
:locals *null-obj*
:tupid *null-obj*
:relid *null-obj*
:readfn nil
:writefn nil
:hist-vars nil))

;;;function for creating an active object

(defun createactive (val readfn writefn
&key (locals *null-obj*) (tupid *null-obj*) (relid *null-obj*))
(if (and
(null readfn) (p_typecheck *null-activeobj* readfn 'function 'createactive
readfn writefn locals tupid relid))
(null writefn) (p_typecheck *null-activeobj* writefn 'function 'createactive
readfn writefn locals tupid relid)))
(make-instance 'activeobj :value val
:locals locals
:tupid tupid
:relid relid
:readfn readfn
:writefn writefn
:hist-vars nil)

Figure 6: Active object class and its create method

4.4 Functionality of the Resulting System

Incorporation of active objects into a passive DBMS dramatically enhances the capabilities of the
DBMS. We indicate below how active objects of the prototype support the following.

Triggers/alerters: The prototype supports triggers and alerters. For a relational DBMS, trig-
gers/alerters can be associated with: an attribute instance (by associating an active object with
an attribute instance), a tuple instance (by associating an active object with a tuple instance), an
attribute name (by associating an active object with an attribute name to be associated with every
instance of that attribute), and a relation instance (by associating an active object with a relation
name to be associated with every tuple of that relation).

In order to support the last two, an activelist-object is maintained for each attribute of a relation
which stores the active objects to be associated when a tuple is inserted into the relation. Similarly,
active objects to be associated with the tuples of a relation are stored as part of the relation. This
information is used when a tuple is inserted (deleted) into (from) a relation.

Multiple and nested rules: Multiple rules and nesting of rules (one active object invoking methods
of other active objects including recursive invocation) are readily supported in our prototype. The
mechanism used for nested rules is the same as the one used for supporting a rule in the first place
and hence no special treatment is needed for handling nested rules.

Forward/Backward C(i/ ,. ,-i The prototype supports both backward and forward chaining. The
function associated with a read event can be used to support backward chaining (read on a virtual
field or a virtual tuple) using the history variable to indicate whether the local value needs to
be computed or not. The function associated with the write event simulates forward chaining and
alerters. Embedded reads in rules cause deeper backward chaining. It is assumed that the functions
defined as part of active objects are well-behaved (i.e., there are no self references generating infinite

Optimization: The prototype can also be used to implement materialization, lazy, eager, and one-
shot rule evaluation techniques. History variable mentioned earlier plays a key role in this regard.
The computations encoded in readfn and writefn can be distributed differently and intermediate
results can be stored in history variables. Currently, the functions are not analyzed/processed by
the system and hence lazy and eager evaluations have to be encoded by the implementor of these
functions judiciously. However, with a higher-level user interface these functions can be generated
to exploit lazy, eager, and one-shot evaluation.

Active Relational DBMS: Data manipulation operators are built in terms of the basic events sup-
ported by the system. That is, relational level operations cause the corresponding basic events
which in turn execute the functions associated with the events. For example, a modify operation
on an active field value (or field values) of a relation will activate the write functions associated
with the corresponding fields. Similarly, a retrieve on an active field (or field values) will activate
the read function associated with the corresponding field (or field values), and a delete operation
will activate the write functions associated with the corresponding field (or field values).

5 Evaluation

In this section we compare monitoring of conditions using active objects and polling. We first
describe the scenario used. We then analytically compare polling and active objects and predict
their behavior. Finally, we conduct several simulations and analyze the observed behavior resulting
in identifying potential optimization opportunities.

5.1 Scenario and Measurements

The scenario used for the purposes of comparison is a simulation of a simple command and control
application. In this application, the database is populated with various kinds of platforms (such
as airborne, naval, and submarine) which are converging towards a stationary platform. The
stationary platform is monitoring the movement of all the platforms around it to determine the
threat posed by other platforms based on the distance of a platform from itself. The application
simulates updates to the position of the platforms in a way that moves the platforms towards the
stationary platform at specified rates. When, during the simulation, the distance becomes less than
a certain threshold value (which is distinct for each class of platforms and there are three thresholds
for each class) the commander is alerted by displaying the appropriate alert code on the console.

The database consists of three relations friendll-i,1.,f'rrms for the stationary platform (having
its name, X, and Y coordinates), hostile-l,'1lf,'rrms for the rest of the platforms (having their
name, class, X and Y coordinates) and thresholds for the thresholds (having the platform class
and the threshold values three for each class). Each tuple of hostile-Jlll'.Ifrm relation is made
active. Functions corresponding to write events obtain the appropriate threshold value based on the
class of the platform and compare the distance computed with the threshold values to determine
whether there is a change in the alert code. Graphic updates are performed by triggers for active
values using the same functions used for updating graphics for polling.

For comparing the performance of polling and active objects, the following were measured at run
time using the system clock on Symbolics.

computations -the number of times a rule is executed,
comp-time -the sum of times spent in executing rules, for a given number of updates,
polling-time -the amount of time spent in polling (only for polling), and
total-time -total time taken to run a fixed number of simulated updates.

The parameters input for each experiment are: strategy (polling or active), number of hostile
platforms, number of updates, and polling interval. The hostile-l'1'ilf'rms relation is populated with
X and Y coordinate values generated randomly making sure that none of the platforms is within
the threshold to start with. After setting up the initial graphics, specified number of updates are
performed in a loop measuring the values indicated above. At the end of the experiment the values
are displayed. A running average of the above values is also computed and displayed.

All measurements used for plotting graphs in this paper are the average of at least two simulations.
We also eliminated the effect of paging objects from secondary storage by discarding initial sim-
ulations until the measurements stabilized. Hence most of the readings should reflect simulations

with the database resident in the main memory.6

5.2 Expected Behavior

Of the values measured, computations is a measure which is independent of the way in which tuples
are accessed in a relation. For active objects the number of computations is the same as the number
of updates (assuming that all tuples in the relation are active) and is independent of the size of the
relation. For polling, it is directly proportional to size of the relation and inversely proportional to
the time interval of polling. The crossover point of these curves (when number of tuples and the
computations are plotted on the X- and Y- axis respectively) is a function of the size of the relation
and the polling frequency.

Analytically, suppose N is the number of tuples in a relation, u is the update interval (of tuples)
for the relation (average), and p is the polling interval. The polling interval is defined as the time
between the beginning of one polling cycle to the beginning of the next polling cycle (a polling cycle
corresponds to scanning the relation and evaluating the conditions associated with each tuple). The
number of computations over a time period T for active objects is f. The number of computations
for polling is T *N. They are equal when p = N*u. For a fixed u, p increases directly with
respect to N and hence the timeliness of detection (which is inversely proportional to p) is directly
dependent on the size of the relation in the case of polling. If the same timeliness of detection is
to be maintained as N increases, the amount of computation increases in every polling cycle which
may give rise to thrashing (i.e., losing updates). The system will thrash if p is less than the time
it takes to perform N computations, i.e., if p < N*a where a is the time it takes to perform an
action (ignoring the access time of the tuple for the time being). The system can also thrash when
conditions are monitored actively, if u < a. But the size of the relation does not have any impact in
this case. Ideally, p should be large enough not only to perform all the computations in one polling
cycle, but should also allow updates to occur after the polling cycle. If in each polling interval p,
x units of time is left after performing N1 operations (N1 is the size of the relation) and if the
size of the relation were to increase to N2, the new and old relation sizes are related (assuming
p is same) by the equation N2 = N1 + O( ). Note that the two sizes are related additively
(and not multiplicatively) and the computation performed for each tuple (a) is a significant factor.
Preferably, the ratio x/a should be chosen to accommodate the expected size of the relation. As a
increases, x needs to be increased, losing timeliness of detection as a result.

A similar analysis indicates that, for active objects, the total-time (or even the comp-time) should
not be influenced by the size of a relation. However, our initial measurement of total-time (shown
in Figure 7) clearly indicated the dependence of the operation on the size of the relation. It was
not difficult to trace this anomaly to the implementation of our update operation which retrieved
tuples sequentially instead of using a direct access.

The above observation implies the significance of physical database design in the presence of active
values. The physical database design is influenced by the rules and their association with data
6Readings of time reported in this paper need to be qualified further. First, time measurements reported here
are not comparable to DBMS benchmarks (in terms of number of transactions per second) as the implementation
is a prototype on Symbolics. Furthermore, the conclusions drawn in this paper do not depend upon on absolute
values of execution time, but instead depend upon relative values (for polling and active objects) and hence are
valid in our opinion. The prototype primarily used for demonstrating the active capability visually uses graphics and
manipulation of graphic icons (using expensive operations such as drawing and erasing line segments) extensively. In
addition, time is measured with a granularity of 1/60th of a second and not in microseconds.

Figure 7: Total Time Vs. Number of Tuples


Figure 8: Total time vs. number of tuples (revised)

objects. A relation which may not need indexing when the database is passive may have to be
indexed (on one or more attributes) when the same relation (or parts of it) is made active.

In response to the above observed behavior, we implemented a set of operators that were based on
object identifiers (or cursors). Cursors can be used to access tuples directly from relations without
having to scan the entire relation. Results obtained using the revised implementation are discussed

5.3 Comparison with Polling

Simulation 1: 200 database updates were executed. For polling, intervals of 2, 5, and 10 seconds
were considered. In this experiment, we measured the total-time to run the simulation as the number
of platforms in the database increases. In agreement with our analysis, the total-time for active
values is flat (as shown in Figure 8) and does not seem to be influenced by the size of the relation.
As expected, the total-time increases with the decrease of polling intervals as can be inferred from
the graph. The curves for polling seem to diverge and the total-time spent on polling and active
monitoring are more pronounced as the number of tuples increase and the polling intervals decrease.
Note that the total-time used in this simulation includes the overhead incurred for the execution of
active objects (whoppers in our case). The effect of the overhead does not appear to be significant
for the number of updates and the size of the relation used in this simulation.

The measurements obtained from this simulation also indicate that active condition monitoring

Figure 9: Percent of total time vs. polling interval (revised)

is better than polling beyond the crossover point. In general, the crossover point shifts towards
the Y-axis (non-linearly) as the polling interval decreases. This strengthened our initial conjecture
that although active condition monitoring is often better than polling, the decision for choosing a
strategy (polling or active objects) has to be based upon the various parameters of the application
at hand.

Simulation 2: 200 database updates on 30 platforms were conducted. In the polling case, the
polling interval is varied from 1 to 10 seconds. In this simulation, we measured the fraction of the
total-time used for condition monitoring as the polling interval varied. Figure 9 shows the polling
interval (in seconds) on the X-axis and the fraction (as a percentage) of the total-time spent on
condition monitoring on the Y-axis.

In this experiment, the overhead for active condition monitoring does not come into the picture
enabling us to compare the waste of resources in the case of polling as compared to active condition
monitoring. It can be easily observed from the graph that as the polling interval decreases, the
fraction of time spent on condition monitoring increases almost exponentially.

The results of the above simulation clearly demonstrate two limiting aspects of polling, namely,
waste of resources and the timeliness of detection. These are conflicting requirements and hence
have to be balanced against application needs. In real-time (time critical) applications, the lack
of timeliness may not be acceptable. On the other hand, the increased overhead of polling at very
short time intervals may not be tolerable.

Polling, by its nature, involves excessive computation to support timely notification. However, even

Figure 10: Percent of total time vs. number of tuples (revised)

with a very small polling interval, the notification of a condition being met in the database is not
ideal. On the other hand, the timeliness of notification approaches the ideal for active objects.
There is no need to wait for the next polling cycle, however soon that might be.

Simulation 3: In this simulation, we measured the fraction of time spent on condition monitoring
as the number of platforms in the database increases.

For active objects, the fraction of time spent on condition monitoring does not change significantly
as the number of platforms in the database increases. For polling, on the other hand, the fraction
of time spent on condition monitoring does increase as the number of platforms increases. This
result reflects the fact that more work is done in polling a large number of objects, all other factors
being equal.

This simulation strongly indicates a subtle aspect of polling. Note that the crossover points in
Figure 10 occur much earlier than those in Figure 8 While the same fraction of the total time is
spent on condition monitoring in active objects (as indicated by a flat line), for polling, the fraction
increases with the increase in the number of tuples indicating that the condition is evaluated for
all the tuples whether they were changed or not. This strongly suggests that there is potential for
optimizing polling to reduce the overhead.

5.4 Analysis of Simulation Results

Below, we analyze the results of the above simulations and identify opportunities for improving the
performance of both active objects and polling. The following analysis assumes that polling and
active condition monitoring are being carried out on a relation where all the tuples are assumed to
be active.

For a relation of size N, the amount of time spent during the time interval T using the polling
technique is T *(access time for (N,, + Nch) tuples + (N, + Ncm) *a), where T represents the
P p
number of polling cycles and the expression in the outermost parenthesis indicates the computing
time during each polling cycle. N,, and Nch represent the number of tuples that were not changed
and the number of tuples that were actually changed, respectively, after the previous polling cycle.
Clearly, N = N , + Nch. For active condition monitoring the above can be expressed as (access
time for T tuples + *a). Assuming that T *a and Tp *(N,*a) are the same (i.e., the same
amount of work is done for tuples that were really changed in both cases, though it is potentially
less for polling as several changes may get grouped for each polling cycle), any optimization that
can be incorporated for evaluating the conditions can be done for both techniques.

Simulations with revised implementation eliminated the dependency of the access time for f tuples.
However, for polling there are still additional overheads in the form of access time for (Nu, + Nch)
tuples and N,, *a and this accounts for the observed results in Figures 8 and 10. When access
time for N tuples was measured as polling-time (though not shown in any of the Figures), the access
time increased with the number of tuples as a multiplicative factor of T

The above analysis indicates that the polling technique used for condition monitoring can be im-
proved in at least two ways: by reducing the access time in each polling cycle and by reducing or
eliminating the condition evaluation on tuples that have not changed in a way to affect the outcome
of the condition. These two improvements are further elaborated in the next section.

6 Summary and Future Directions

In this paper, we have proposed two distinct active database architectures based on available con-
dition monitoring techniques. We have argued for using the layered architecture as an intermediary
step that is beneficial in the short term, facilitates heterogeneous active DBMSs, and provides a
better migration path for the long term. In order to facilitate the design of the layered architec-
ture, we have compared polling and asynchronous monitoring techniques leading to the following

The results presented in this paper confirm our initial hypothesis that active monitoring is, in
general, better than polling when the relation being monitored is large or when timely response
is important. However, our simulations strongly indicate that a naive implementation of active
objects (or making a passive DBMS active without taking the physical redesign into consideration
or adding active capability without analyzing the implementation of operations) will not provide
the performance advantage that one would expect of active condition monitoring. In addition, there
is clear indication that polling is better than active condition monitoring below the crossover point
substantiating retention of polling as one of the strategies. Pragmatically, an active DBMS should
intelligently choose from among a set of strategies to reduce the waste of resources and increase
the timeliness of condition detection. A DBMS should also be capable of dynamically restructuring

the physical access to support active objects.

Typically, polling is implemented by scanning the entire relation during each polling cycle. The
analysis of simulated results .i-.- -I opportunities for reducing the amount of work done during
a polling cycle. First, the access time can be reduced by employing direct access techniques such as
indexing for those tuples that have been modified from the previous polling cycle. Second, the time
spent on condition evaluation can be reduced by storing some information as part of the tuples
which can be used for deciding whether the condition should be evaluated at all. Though this
optimization can be easily made part of condition evaluation, it will add an additional burden, of
incorporating this into the code, on the implementor.

We are currently designing ECA rule support for object-oriented database management systems.
We are developing a layered architecture for providing limited active capability. Further, we plan on
comparing all the three approaches for condition monitoring (user-encoded, polling with and with-
out optimizations, and active objects) to ascertain conditions under which each of the approaches
offer maximum performance advantages. We also propose to evaluate the above approaches for large
databases involving secondary storage access (which is not addressed in the present discussion).

We have already identified a set of techniques for optimizing the evaluating situation-action rules
for the Sentinel project [CHS93] and are investigating them in more detail. Our long term goal is
to develop and consolidate a variety of techniques that are useful for condition monitoring and to
develop a set of comprehensive criteria using which appropriate techniques can be chosen.

7 Acknowledgments

I want to thank Ms. Susan Nesson for her contribution (both design and implementation) to the
comparison of polling and active condition monitoring technique. I want to thank Michael Brodie
and Umeshwar Dayal for many fruitful discussions during the course of this work at CCA/XAIT.
The implementation reported in this paper was carried out on a Symbolics machine given to CCA
on loan by Symbolics Inc. I gratefully acknowledge the support Symbolics Inc. provided during
the course of the work reported in this paper. I want to thank Ms. Eman Anwar for her useful
comments and suggestions.


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