s-cube lp: proactive sla negotiation

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Proactive SLA Negotiation

Khaled Mahbub and George Spanoudakis

City University London


SLA and SLA Negotiation

Service Level Agreement (SLA)

A Service Level Agreement (SLA) is an explicit formal contract between the

provider and the clients of a service determining guarantees on service quality

properties and penalties in case of defaults

SLA Negotiation

SLA negotiation is the process where service providers and clients reach an

agreement on desired levels of service that satisfy each partner involved in the

negotiation process.

SLA negotiation typically involves the resolution of differences between the

initial positions of the two parties regarding the desired levels of service

provision

Two types of negotiation

• Reactive: after decisions on binding with a service have been made and/or

once SLA violations has occurred

• Proactive: prior to service binding and/or SLA violation


Proactive SLA Negotiation – Triggering Scenarios (SLA Violation Scenarios)

Following forecasts for a higher level of client demand that can put

agreed SLA terms at risk.

Following forecasts that the constituent services of a system will fail or

violate their own SLAs (this applies to both software and infrastructure

services).

If it becomes known that competitors provide or are going to provide

better service offerings


Handling of SLA Violations

Traditional Approaches

– application of penalties depending on the importance of the service

level objectives [1,2]

– runtime negotiation (re-negotiation) of SLA [3,4,5,6,7]:

- To accept a service from the existing provider with reduced service levels

- To terminate the existing SLA and negotiate new SLA with a new provider

At runtime, these approaches have limitations as they

– May affect the quality of the delivered service, or

– fail to guarantee uninterrupted service

Proactive SLA negotiation with alternative service providers is a means

for addressing such limitations

Proactive SLA negotiation needs to be driven and integrated with service

discovery


Two Phase Proactive SLA Negotiation

Identification of potential service providers (aka SLA negotiators): service

discovery, search in published SLA templates (i.e., service levels

advertised by providers)

Pre-agreement: Process of agreeing SLA terms without putting the SLA

into force; it may establish a time frame within which the pre-agreement

can be automatically brought into force without further negotiation

Final agreement: Confirmation that an SLA comes into force

The two phases are required for proactive SLA negotiation but may also

be useful in reactive negotiation.


Architecture for Proactive (& Reactive) SLA Negotiation

The service discovery query may be

specified in order to identify candidate

replacement services that a composite

service (S) uses currently.

The runtime service discovery tool

identifies potential alternative services.

The negotiation broker manages the

negotiation process on behalf of a service

consumer/provider.

– Jess rule engine is used as the negotiation

engine

– Negotiation rules are expressed in XML and

translated into Jess rules by negotiation

rule translator.


Negotiation Rule (1)



if the consumer of a service has made an offer (or counter-offer) where the

response time of the service must be less than 10 milliseconds (ms) and the

price to be paid per service use is 0.5 pounds, the offered values will be

accepted.

<tnsr:NegotiationRule name="rule1">

<tnsr:If>

<tnsr:LogicalExpression>

<slac:Condition relation="LESS-THAN">

<slac:Arg1><slac:QualityAttribute name="RESPONSE_TIME"party="CONSUMER"/></slac:Arg1>

<slac:Arg2><slac:Constant type="NUMERICAL" unit=”ms”>10</slac:Constant></slac:Arg2>

</slac:Condition>

<slac:LogicalOperator>AND</slac:LogicalOperator>

<slac:Condition relation="EQUAL-TO">

<slac:Arg1><slac:QualityAttribute name="PRICE"party="CONSUMER"unit=”GBP”/></slac:Arg1>

<slac:Arg2><slac:Constant type="NUMERICAL">0.5</slac:Constant></slac:Arg2>

</slac:Condition>

</tnsr:LogicalExpression>

</tnsr:If>

<tnsr:Then>

<tnsr:Action>

<tnsr:Accept>

<tnsr:QualityAttribute name="PRICE“ party="CONSUMER" />

<tnsr:QualityAttribute name="RESPONSE_TIME" party="CONSUMER"/>

</tnsr:Accept>

</tnsr:Action>

</tnsr:Then>

</tnsr:NegotiationRule>



A rule in Jess has the form

(defrule rule-name(logical-operator (cond-1 …cond-n))

action)

XML negotiation rules are transformed into Jess rule by applying a set of

translation patterns:

Negotiation Rule Element Jess Representation <slac:Arg><slac:QualityAttribute

name="A” party="P"/></slac:Arg> A-P

<slac:Arg><slac:Constanttype="NUMERICAL">C

</slac:Constant></slac:Arg> C

<slac:Condition relation="REL">

<slac:Arg1>...</slac:Arg1>


</slac:Condition>

{Jess-Rep(Arg1) REL Jess-

Rep(Arg2)}

<tnsr:Action><tnsr:Set>

<tnsr:LogicalExpression>

<slac:Condition relation="EQUAL-TO">



</slac:Condition></tnsr:LogicalExpression>

</tnsr:Set></tnsr:Action>

(modify 0 (Jess-Rep(Arg1)

(fact-slot-value 0 Jess-

Rep(Arg2))))


SLA Negotiation Process (1)

potential candidate services

(RS) are identified.

Services in RS are ranked

based on their distances to the

query.

RS is updated regularly.

An SLA is negotiated and a pre-

agreed SLA is established for

each service in RS.


SLA Negotiation Process (2)

Time validity period is set for each pre-agreed SLA.

pre-agreed SLA is proactively re-negotiated if the service is unused and the validity period is about to expire.

A service in SBA is replaced by the best available service in RS if it is needed.

If a service in RS is selected for dynamic binding, its pre-agreed SLA is automatically enforced.


Proactive SLA Negotiation – Evaluation (1)

Case study and evaluation set up:

– A service based client application, called Route Planner, implemented

by a BPEL service orchestration process.

– Route Planner uses several services, including Global Positioning

Service (GPS) and electronic map service (eMapS).

– A service discovery query is specified in order to identify candidate

replacement services for GPS of Route Planner. The discovery query

expresses the structural discovery criteria as WSDL of GPS service.

– An SLA template with four QoS terms (price, availability, response time

and mean number of service request per hour) for negotiation is used.



Setup

– A set of 15 service consumer negotiation rules (CNR set) is sued.

– 20 different sets of provider negotiation rules sets (PNR set) are used,

where each set contains between 5 and 20 rules. During experiment,

negotiation broker randomly selects one PNR to simulate the different

behaviour that different service provider might have.

– Three different service sets (registries), containing 100, 300 and 500

services, are used. Service sets are populated by geographic location

related services taken from SEEKDA.

– All experiments were carried out using a Pentium 2.33GHz with

3.23GB RAM machine.



Measurements of the time needed to

a) Build the initial candidate service set (RS) by runtime service

discovery (SD)

b) Maintain the RS at runtime due to arrival of new service or change in

the description of an existing service in the registry

c) Select a new service for replacing a service in the service based

system

Times required for (a), (b) and (c) were measured for executions of service

discovery process

• Without SLA negotiation (SD Only)

• With proactive SLA Negotiation (SD with Proactive SLA)

• With reactive SLA negotiation (SD with Reactive SLA)



Results (time measurements)

SD Only SD with Proactive SLA SD with Reactive SLA

100 300 500 100 300 500 100 300 500

Replacement

Service Set (RS)

Building

(Time in Sec.)

160.62 408.05 686.697 157.399 435.985 715.685 159.804 408.03 688.23

Replacement

Service Set (RS)

Maintenance.

avg(trsm-*)

(Time in milli-sec)

690.5 698.3 673.5 837.2 881.2 892 637.4 668.5 677.6

Selection of

Replacement

Service from RS.

avg(trep-*)

(Time in milli-sec)

22 28 21.8 53.2 45.8 50.2 453 459.4 443.8



Results (time measurements)

– service discovery with proactive SLA negotiation is slightly larger than

the time required to identify a replacement without any SLA

negotiation.

– time required to select and bind a replacement service at runtime in

the case of SD with reactive SLA negotiation is significantly larger than

the service selection and binding time in the case of SD with proactive

SLA negotiation



Results (effectiveness)

Service

Registry Agpro = avg(trep-sd-rea-sla)

avg(trep-sd-pro-sla)

RGpro=avg(trep-sd-rea-sla) /

avg(trep-sd-pro-sla)

BEpro= (trsm-sd-pro-sla – trsm-

sd-rea-sla)/(trep-sd-pro-sla – trep-

sd-rea-sla)

100 187 8.51 0.999

300 230.4 10.03 1.028

500 192.8 8.84 1.089



Results (effectiveness)

– possibility of inefficient resource utilization.

– the efficiency of resource utilization with proactive SLA discovery/negotiation over a period of time T can be defined as,

• SRRR: the service replacement request rate;

• SRUS: service registry update rate;

• tmatch : average time required to match a query with a service

• tSLA-Neg : average time required to negotiate an SLA with a service

• tinit-RS : the time needed to build the initial copy of RS

– to have efficient resource utilization, there must be that

SRRR ≥ SRUR + Rinit/T i.e. SRRR ≥ SRUR


Conclusion & Future Works

Although preliminary, our experiments have shown that proactive SLA

negotiation leads to significant reduction of the time required to perform

service replacement at runtime.

This approach has opened new possible lines for future investigation,

– Support for proactive negotiation for hierarchical SLA.

– Heuristics for tuning the triggering of proactive SLA negotiation

process so as to reduce the number of cases where pre-agreed SLAs

never get used

– Support for dynamic adaptation of negotiation rules.


S-Cube Publications

Check the following S-Cube papers for a detail description of Proactive

SLA Negotiation framework.

Khaled Mahbub and George Spanoudakis, "A Framework for Proactive SLA

Negotiation", 5th International Conference on Software and Data Technologies

(ICSOFT 2010), 22 - 24 July, Athens, Greece.

Khaled Mahbub and George Spanoudakis, "Proactive SLA Negotiation for Service

Based Systems", Proceedings of the 2010 IEEE World Congress on Services

(SERVICES 2010), 5-10 July 2010, Miami, Florida, USA.

Khaled Mahbub and George Spanoudakis "Proactive SLA Negotiation for Service

Based Systems: Initial Implementation and Evaluation Experience", 8th IEEE

International Conference on Services Computing, July 4 - 9, 2011, Washington DC,

USA.


References

1. Service Level Agreements”, the Proceedings of the Usage of Service Level Agreements in Grids Workshop, 2007

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3. “SLAM – Service Level Agreement Management”, Whitepaper, Whitestein Technology, August 2005.

4. Dominic Greenwood, Giosue Vitaglione, Lukas Keller and Monique Calisti, “Service Level Agreement Management with Adaptive Coordination”, International Conference on Networking and Services (ICNS’06), July 19-21, 2006, Silicon valley, USA.

5. Di Modica G.,Tomarhio O. and Lorenzo V., "A framework for the management of dynamic SLAs in composite service scenarios", Service-Oriented Computing - ICSOC 2007 Workshops: ICSOC 2007, International Workshops, Vienna, Austria, September 17, 2007

6. Michael Parkin, Peer Hasselmeyer, Bastian Koller, Philipp Wieder: An SLA Re-negotiation Protocol. In proceedings of the 2nd Workshop on Non Functional Properties and Service Level Agreements in Service Oriented Computing at ECOWS 2008. Dublin, Ireland. 12 November 2008.

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