Information Dissemination Services – Usage patterns discussion document
Author: Vijay Dialani (vkd00r@ecs.soton.ac.uk)
Optimizing Interaction between IDSs
An information dissemination service (IDS) consists of four distinct artefacts – the
publication, the subscription, the propagation and the consumption. IDS uses
publish/subscribe notification mechanism to facilitate information sharing between a set
of publishers and consumers. A publication is described by means of a publication rule,
which essentially consists of data access statement and the data access schedule. It is
assumed that a DS (DAIS Service) is used to provide the data resource access for the
IDS. An access statement (usually a query statement) is evaluated by the associated data
access service. As specified in DS specification, a synchronous or an asynchronous
access mechanism can be used to access a DS. This document discusses probable usage
scenarios of the IDS and elaborates its probable interactions with the DS.
It should be noted that there exist multiple possibilities for optimizing the performance of
the information dissemination service. Examples of such optimization include, but are not
limited to, optimizing data access by identifying similarity between query expressions,
optimizing publication process by sharing access across multiple subscriptions,
optimizing data propagation by intelligently managing the staging issues, etc. However,
this document focuses on the service level semantics and/or interface modifications that
may allow incorporation of known optimization techniques.
This document suggests modifications to operational semantics of IDS. Following
general principles underline the modifications suggested in this document:
 Maximize the resource sharing capabilities between multiple subscriptions.
 Centralize the maintenance of the state to within the publication service and
facilitate recovery of state for the consumer and the analyst services.
 Optimize resource management by automating object lifetime management by the
use of policies.
Modification to usage patterns
Four usage scenarios described in the GDD specification are:
 Asynchronous SQL Result Set delivery to 3rd party consumers.
 Publication on Request: Asynchronous SQL Result Set Delivery.
 Workflow.
 Continuous replication.
Above scenarios were analyzed to abstract the usage patterns for the operations provided
by the IDS. Modifications have been suggested to introduce the brokerage and discovery
pattern for out of bounds communication between the analyst and prospective subscribers
of the information. In addition, the assumption of direct co-relation between the
publication and the subscription has been relaxed to introduce the notion of a publication
shared for a set of subscriptions.
Suggested Modification-1
Explicitly creating and managing a publication (with brokerage pattern),
involves
 An analyst creates an explicit publication and controls the rights for
subscription to the data.
 An analyst acts as a broker to create a “placeholder” subscription object
with the IDS and distributes the subscription identifier to potential
subscribers. Use of brokerage pattern prevents unauthorized propagation
of rights to subscription.
 The analyst determines the lifetime management policy of the publication.
 Destruction of the publication results in explicit destruction of all the
subscription objects.
Suggested Modification-2
Implicitly creating and managing a publication (with discovery pattern), involves
 A subscription request results in the creation of the publication object and
the primary subscription.
 The implicit publication object is published in a discovery service
(provided the policy permits).
 Subsequent subscription objects share the publication object.
 Dropping of the primary subscription does not necessarily result in
dropping of the publication. The publication object is automatically
dropped when there are no more subscription objects for the specified
publication.
 The dropping of the publication object is automatically preceded by unpublishing of the object from the discovery service.
Suggested Modification-3
Selective association of the state with IDS objects, involves
 An analyst creates a publication for continual data. The analyst controls
the publication process by explicit calls to publishData( ). In accordance to
the publication rule, a query is evaluated by the data access and a data set
is generated. The data sets are either immediately transported for
consumption or are cached for retrieval by explicit calls to the getData( )
method. Either the subscription object or the consumption object ends up
caching the dataset. The scenario represents a staging problem, one similar
to that encountered in data access mechanism. The state maintenance
needs to take into account the staging policy, to facilitate efficient usage of
resources. It is suggested that the subscription rule be modified to include
the policy for resolving the staging issues.
Suggested Modification-4
Providing reflection for publication and subscription objects and means to
interrelate their lifetime management.
 The IDS service maintains the state of the valid publications and
subscriptions, an associated object identifier (for example publication ID,
subscription ID) are associated with the messages to maintain the context.
In certain cases, for example a terminal fault in the analyst, it is not
possible for the client to retain the state identifiers to recreate the state at
the client side. Providing enumeration on publication and subscription
objects allows state recovery for the analyst and consumers.
 Explicit creation and maintenance of objects in an SOA introduces
requirements for autonomous destruction of the objects. By default the
lifetime of the objects are associated with the temporal constraints, for
example, timeout. However, the lifetimes of the various resources are
inter-related. For example, in case of managing an explicit publication, the
lifetime of all the subscription objects is associated to the lifetime of the
publication. Providing added features to associate lifetime of the objects
within the service seems to be a probable way of efficiently utilizing the
resources. A detailed scenario is explained in the following section.
A probable usage pattern
An IDS publication is activated by either an explicit request for a publication or an
implicit request for a subscription. In either case the invocation results in the creation of a
publication object, a subscription object and a data access object. In case of an explicit
creation of the publication object all the subsequent subscription objects lifetimes are
controlled by the lifetime of the publication object, where as if the subscription request is
used to create an implicit publication the subscription object controls the lifetime of the
publication. The lifetime of this combined entity controls the lifetime of the data access
session.
In its current form the IDS specification necessitates explicit management of the service
objects. For example, createPublication() and dropPublication() control the lifetime of an
specific publication object. However, use of policy based control for the publication
process facilitates resource sharing. For example, consider that an analyst has requested
for a publication of the “customer data” to be accessed and propagated at sometime in the
future. If the publication is advertised in a discovery service or is available through
reflection provided by the IDS -a number of probable subscribers can subscribe for the
publication and as a result optimize the data access process. In addition, the publication
lifetime policy can determine the semantics for the dropping of the publication.
Probable candidate policies for dropping of the publication include- first, consider that
the analyst holds exclusive authorizations for extracting the data from the data resource.
In this if the analyst has exclusive rights to drop the publication. However, if any of the
subsequent subscriptions also have access rights to the underlying data resource the
dropPublication() by the analyst will only drop the publication and subscription requests
associated with the analyst, while retaining the remaining subscriptions.
Successful evaluation of the doPublish() may result in service staging the data for
multiple subscribers or each subscriber initiating its own propagation. The creation of the
propagation object proceeds in accordance to the propagation rules. A probable
propagation policy may allow automatic propagation of the data on the completion of the
publication process.
The above example uses policy based object management for maximizing resource
sharing between multiple subscriptions.
DS Access Patterns
An IDS publication object creates a DS based access to the underlying data resource. An
IDS may utilize the DS in either of the following three patterns:
1. A synchronous access mechanism
2. An asynchronous access mechanism.
3. A continual data access mechanism – In this pattern, the IDS receives multiple
datasets from the DS. Each of these datasets may be asynchronously delivered at
different temporal points. For example, online replication mechanism with
periodic updates.
The first two access patterns have been described in the existing DS specification, while
the continual access pattern is not yet supported. Additionally, there are some other
unresolved issues both in IDS and DS that have been highlighted below:
1. If a IDS accepts a publication request, does it attempts to verify the publication
rule at the time of acceptance, does it verify the schema and access rights with the
underlying DS?
2. Does the DS notify the IDS of any schema changes that may effect its publication
requests?
3. Does the DS provide an IDS with metadata information on the authorization
model?