Managing Adaptive Presentation Executions inDistributed Multimedia Database SystemsH. Thimm, W. KlasGMD-Integrated Publication andInformation Systems Institute�Darmstadt, Germanyfthimm, klasg@darmstadt.gmd.de J. Walpole, C. Pu, C. CowanOregon Graduate Instituteof Science and TechnologyPortland, OR, USAfwalpole, calton, crisping@cse.ogi.eduAbstractThe paper describes recent progress in research on database systemsupport for executing multimedia presentations. It proposes the generalconcept of Reactive Playout Management to adjust multimedia presenta-tions. This adjustment can be required due to performance uctuations asin the absence of end-to-end quality of service guarantees, cost constraintsspeci�ed by the user, and other reasons. A concrete concept to facilitatePerformance-Based Reactive Playout Management in distributed Multi-media Database Systems is proposed. This covers the identi�cation ofrespective components in distributed database system architectures alongwith corresponding algorithms.1 IntroductionAn adaptive execution of multimedia presentations can be required for severalreasons. For example, one reason is that although there exists promissing workon resource reservation for multimedia presentations, e.g. [2, 7, 13, 15, 10, 35,24], it is likely that within the next decade most general purpose computingenvironments will not fully support end-to-end quality of service (QoS) guaran-tees. In order to also allow in those environments the execution of multimediapresentations1 performance uctuations must be handled. This should go be-yond a trivial pausing and resume. It is much more adequate to adapt thepresentation execution to the performance available. A further reason is thatusers might want to constrain the costs they are willing to pay for a multi-media presentation. Given such cost constraints, an adaptive execution of a�GMD is the German National Research Center for Information Technology1In this context, we mean preorchestrated multimedia presentations also regarded as storedor scripted multimedia presentations. 1

multimedia presentation means that the presentation is still entirely performed.However, depending on his cost constraints, the presentation that the user getsdeviates from its original speci�cation but is still useful for the viewer.It is our assumption that many multimedia applications, especially presenta-tion intensive ones (e.g. multimedia systems for education, training, advertising,amusement) can bene�t if the multimedia database system (MM-DBS) supportsthe adaptive execution of multimedia presentations. Our research presented inthis paper is especially concerned with adaptive presentation executions to over-come performance uctuations. We present the general concept of Performance-Based Reactive Playout Management (PB-RPM) that provides adaptive presen-tation executions in the absence of end-to-end guaranteed QoS. Then, we pro-pose a comprehensive general approach to facilitate PB-RPM especially withindistributed MM-DBS. Our approach provides a distributed control algorithmwith local PB-RPM at the presentation sites and global PB-RPM at the centraldatabase environment. We identify concrete components and describe how theymust work together to achieve PB-RPM. This proposal is an extention of ourgeneral concept of Playout Management in distributed MM-DBS [29] that wehave realized within the context of IPSI's AMOS system. The AMOS systemis a research prototype of a distributed MM-DBMS [20, 16, 25, 1, 17]. Thedevelopment of AMOS is driven by experiences gathered by the realization ofseveral concrete multimedia applications [26, 27, 28, 33].The remainder of this paper is organized as follows. Section 2 introducesPB-RPM independent of a concrete type of system. Section 3 discusses thespeci�c requirements for PB-RPM in distributed MM-DBS. Section 4 presentsan approach for facilitating PB-RPM in distributed MM-DBS by a respectiveadaptive presentation service. Related work is discussed in section 5 and section6 concludes the paper.2 PB-RPM: Performance-Based ReactivePlayout ManagementIn general, RPM [30, 32, 31] is targeted at the adaptation of executing multi-media presentations. We call it performance-based if its adaptations are drivenby the system's performance such that they handle performance uctuations.With performance, we mean the performance to make the data available at thepresentation site, called data delivery performance. It covers all componentsalong the data delivery path from external storage devices to the receiving pre-sentation site.2.1 Presentation Quality and AdaptationsRPM in general must provide the notion of presentation quality as one mea-surement criterion for adaptations. However, a general and objective de�nition2

for presentation quality cannot be given. [22] proposes an interesting modelfor measuring presentation quality as perceived by the viewer. Alternatively,presentation quality can also be de�ned from the viewpoint of the presentationauthors. In the remainder, we call this an author-centered de�nition of presenta-tion quality. For example, the author's speci�cation can be taken as a referencequality which speci�es a quality of 100%. When the executing presentation doesnot deviate from the speci�cation then the current presentation quality is 100%.Otherwise, the presentation quality is correspondingly lower than 100%. In thispaper, we exactly deploy this de�nition of presentation quality.In general, RPM should strive at gaining advantage from the naturally largerange of adaptability which multimedia presentations provide. Hence, adapta-tions should not be limited to manipulating just one media presentation. In-stead, they should potentially tackle several media presentations at the sametime. This is advantageous since for many presentations, e.g., it is superiorto have a necessary adaptation uniformly distributed over all component mediapresentations instead of only adapting one of them. In [30, 31], we have proposedso-called adaptation schedules to achieve complex adaptations. Such adapta-tion schedules consist of a sequence of adaptation actions. Considering theauthor-centered de�nition of presentation quality introduced above, we call anadaptation schedule which actions decrease the current presentation quality ofan executing multimedia presentation, a presentation quality decreasing or justd-adaptation schedule. Vice versa, adaptation schedules which increase the pre-sentation quality are called presentation quality increasing or just i-adaptationschedule.2.2 A Model for PB-RPMPB-RPM must support two variants of adaptations:� In case of lacking performance, PB-RPM decreases the demanded de-livery performance by reducing the presentation quality (see section 2.1)of the executing presentation. For example, this can be done by a d-adaptation schedule.� In case of left-over performance and a presentation quality below 100%,PB-RPM increases the demanded performance by improving the presen-tation quality, e.g., via an i-adaptation schedule.A very prominent approach for executing multimedia presentations in dis-tributed environments in the absence of end-to-end QoS guarantees is the use ofbu�er-level playout combined with a proper prefetching scheme for the multime-dia data, e.g. [14, 5, 4]. We have re ected this in our proposal for PB-RPM byassuming a model (referred to as a presentation quality-based control model)which provides a producer/consumer concept with the following components(Figure 1): 3

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Figure 1: A model for PB-RPM with presentation quality-based control� A best-e�ort prefetch process which loads the multimedia data fromexternal storage devices over a network into a bounded bu�er at the pre-sentation site. The current throughput of this process is denoted by T . Itis the objective of this process to maintain a so-called bu�er safety level[17] denoted by S. S is that amount of bu�ered data necessary for the nnext presentation seconds relative to the current presentation time. Thecurrent presentation quality that the prefetch process supports is denotedby Qb. The current bu�er level is denoted by B.� An asynchronously running playout process which handles the playoutof the multimedia data [29] such that the executing multimedia presen-tation provides a current playout presentation quality denoted by Qp. Inessence, this process manages the output devices according to Qp and userinteractions.� A third process called PB-RPM enforcement process which:{ keeps track of the current bu�er level (B) and the current throughput(T ) of the prefetch process,{ identi�es situations that require d- or i-adaptations (see section 2.1)depending on B and the behavior of T within a certain time period ,{ determines in these situations proper d- or i-adaptation schedules(see section 2.1), and{ realizes the adaptation schedules which has the following e�ects:� after the data that were already in the bu�er prior to the adapta-tion are completely presented, the playout process provides the4

new presentation quality Q0p implied by the adaptation schedule,i.e., Qp becomes Q0p,� the prefetch process immediately changes its presentation qualityQb to the new one Q0b implied by the adaptation schedule. Forexample, a d-adaptation schedule can achieve this by leaving outevery third frame instead of loading every frame of a video clipfrom the data source.2.3 A Scenario for PB-RPMWe now describe a sample PB-RPM scenario. We assume a multimedia presen-tation which, e.g., could be a multimedia product show of a car manufacturer.We suppose an author-centered approach for measuring presentation qualityand the use of adaptation schedules as described in section 2.1. Furthermore,we assume a system which is based on the model described in section 2.2. Hence,in the description of our scenario (see Figure 2), we reuse some of the modelparameters introduced in section 2.2.

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1. Four snapshots of the multimedia presentation taken at points in time �2,�4, �6, and �8 together with their current presentation quality Qp.2. The temporal composition of the presentation which consists of one imagepresentation (I1), two audio presentations (A1 and A2), and four videopresentations (V1 � V4).3. The curve Qp that shows the presentation quality during the presentationexecution.4. The curve P that shows �ctive phases with lacking data delivery perfor-mance within the presentation execution. This lacking performance hasto be understood relative to achieving a presentation quality Qp of 100%.5. The curve F that shows non-deterministic uctuations of the data deliveryperformance (lacking as well as left-over performance) relative to the cur-rent presentation quality Qp. As this sub�gure illustrates, performance islacking or left-over relative to the current presentation quality Qp only fora short moment. This comes from the fact that PB-RPM reacts againstthese uctuations by d- and i-adaptations. The lacking or left-over perfor-mance is nulli�ed when the entire adaptation schedule becomes e�ective.In more detail, during the execution of the presentation, the following happens:� [�1, �3[: At �1, the multimedia presentation is started and executed pre-cisely as speci�ed during the rest of this time interval. A snapshot of thisprecise execution is taken at �2.� [�3, �5[: At �3, the data delivery performance uctuates which yields a lackof performance (F and also P decline). This would lead at time �3 + d1to a problem for the presentation if an adaptation is not performed. Thedelay d1 results from the future presentation seconds already bu�ered at�3 which are consumed at time �3 + d1. PB-RPM in response to the lack-ing performance at �3 completes a d-adaptation which has the followingconsequences:{ The lacking performance relative to the current presentation quality,after a short moment, is nulli�ed (F declines down to the abszisse).{ The lacking performance relative to a presentation quality of 100%remains unchanged (P remains at the level for the rest of this inter-val).{ The presentation quality Qp of the multimedia presentation falls be-low 100% due to the deviations from the given presentation spec-i�cation introduced by the d-adaptation. It remains on this levelfor the rest of this interval. Note that the presentation quality Qpfalls at point in time �3 + d1). This delay results from the fact that6

at time �3, there is already data for a presentation quality of 100%su�cient for d1 futur presentation seconds in the bu�er2. The snap-shot at �4 indicates possible deviations from the precise executionshown in the snapshot at �2. For example, starting at the time atwhich the adaptation becomes e�ective, video presentations V2 andV4 could continuously leave out some frames (e.g. every 5th frame).Or, instead of presenting audio A2 in 16-bit stereo quality, it couldbe presented in 8-bit quality.� [�5, �7[: At �5, the data delivery performance uctuates which resultsinto left-over performance relative to the current presentation quality (Fgoes up). Compared to the uctuation at time �3, the uctuation at time�5 is less intensive. This is why, in the remainder of this time interval,relative to a presentation quality of 100%, performance is still lacking.However, the lack is lowered at �5 + d1. PB-RPM in response to theleft-over performance at �5 performs an i-adaptation with the followingresults:{ The left-over performance is nulli�ed (F meets abszisse again).{ P goes up, since the total performance has increased such that lessperformance is lacking in order to achieve a presentation quality of100%.{ Q goes up but not fully to the level of 100% since (as P shows) thereis still not enough performance to achieve a presentation quality Qpof 100%. Note that Qp goes up at time �5 + d2. The delay comesfrom the fact that the data in the bu�er at time �5 (i.e. currentbu�er level B with respect to the model of section 2.2) was loadedfor the lower presentation quality Qp valid prior to �5. The snapshotat time �6 shows that there are still deviations (hence, a presentationquality Qp lower than 100%), but with respect to �4 the deviationshave decreased (hence, the current presentation quality Qp is greaterthan the one at �4).� [�7, �9[: At �7, again there is left-over performance such that PB-RPMperforms a second i-adaptation. This i-adaptation leads to a presentationquality Qp of 100%.2Given the model described in section 2.2, d1 can be derived from the current bu�er levelB at point in time �3.7

3 Requirements for PB-RPM in DistributedMM-DBSIn the previous section, we introduced the general concept of PB-RPM indepen-dent from a speci�c type of computing system. We now discuss from di�erentviewpoints the requirements for realizing this general concept for distributedMM-DBS. Note that some of these requirements can con ict. In the givendiscussion, a so-called presentation instance is an individual instance which isinstantiated by a granted presentation request and terminated when the presen-tation is �nished. Whereas, with the notion of presentation, we rather mean thespatio-temporal speci�cation together with the corresponding raw multimediadata which are stored in the multimedia database.3.1 Users' Point of ViewFrom the user's point of view, the following requirements must be considered:(a) Service availability:� availability of each of the o�ered multimedia presentations at any pointin time� presentation instances should start with minimal delay(b) Maximization of presentation quality. With respect to the author-centered de�nition of presentation quality given in section 2.1, this means thatpresentations must be performed with minimal deviations from their speci�ca-tions (regarding, e.g., data quality, synchronization quality). This also coversthe requirement that the presentation must be performed as smoothly as pos-sible. Other approaches towards a measurement of presentation quality can bedeployed as well. For example, [22] suggests a de�nition of presentation qualityas perceived by the individual user who can specify his own error model. Basedon this user-centered de�nition, a maximization of presentation quality meansto minimize presentation errors according to the user's individual error model.(c) Individualizeable adaptation pro�le for presentation instances.Users should be allowed to in uence adaptations of presentation instances tore ect their individual needs and preferences3. For example, this can be doneby allowing them to specify their own adaptation pro�les (general pro�le for allinstances or instance-speci�c pro�le). Such pro�les can, e.g., include among alot more things the following:3Note that such a pro�le could also be part of a user-speci�c QoS speci�cation [22]8

� preferences with respect to adaptation alternatives (e.g., adaptation of thedata quality vs. presentation interruptions),� preferences with respect to the occurence of adaptations within certaintime intervals,� preferences with respect to adaptation intensity (e.g., progressive vs. de-gressive adaptation of presentation parameters).3.2 Presentation Providers' Point of ViewIn the center of interest of the presentation providers are the users who are thecustomers of their multimedia presentations. The providers' overall goal is toachieve a high-level customer satisfaction. The customer satisfaction, on the oneside, depends on the quality of the composition of the multimedia presentations(responsibility of presentation authors). On the other side, it also depends onhow presentations are executed and adapted. Hence, from this point of view,the following requirements must be considered:(a) Minimization of rejected users. Obviously, each provider wants toreach as many users as possible such that the number of rejected users (e.g., dueto an overload situation) must be minimized. However, this can con ict with thesimultaneously requested maximization of the global presentation quality (seenext requirement) if new presentation requests are admitted on the expense ofthe global presentation quality.(b) Maximization of total presentation quality. This parallels the dis-cussion of maximizing presentation quality with respect to the users. However,providers want to maximize the presentation quality of their entire set of presen-tations. This is di�erent to the user`s view who want to have the presentationquality of their particular presentation instance to be maximized by the system.Hence, the notion of total presentation quality.(c) Individualizeable presentation-speci�c adaptation pro�les. Provi-ders should be allowed to in uence how the system adapts instances of theirpresentations. For example, this can be done by allowing them to specify, to-gether with their multimedia presentation, corresponding adaptation pro�les.Such pro�les, e.g., can cover among many more things the following:� general adaptation rules (e.g., similar to those mentioned in 3.1 (c)) and� user-speci�c adaptation rules that make it dependent on the concrete usershow individual presentation instances are adapted. For example, considera multimedia product catalogue application which provides a collection of9

multimedia product presentations. The provider might want to catego-rize the users into \good", \average", and \new" customers consideringinformation from the customer records.� Relative presentation importance which might depend, e.g., on the topic ofthe presentation, or how long it has already been o�ered by the MM-DBS.3.3 MM-DBS's Point of ViewThe adaptive presentation service is realized by the MM-DBS which is the cen-tral management instance for the users as well as providers. Hence, from thispoint of view, the following requirements are to be considered:(a) Fairness with respect to service provision to users and providers.This primarily means that the resources have to be used for users and customersin a way such that the global system acceptance is maximized.(b) Minimization of PB-RPM-overhead. Obviously, the performance over-head imposed by the adaptive presentation service should be minimal.4 Facilitating PB-RPM in a DistributedMM-DBSFor distributed MM-DBS, we introduced a novel DBS service called playoutmanagement service [29, 32]. This service is targeted at executing arbitrarilycomposed multimedia presentations stored in a database. Our proposal forfacilitating PB-RPM in distributed MM-DBS is based on this service. Theproposal consists of a set of dedicated additional system components whichjointly perform PB-RPM. As a basis for our proposal and for RPM in MM-DBMS in general, we suggest to split PB-RPM into global and local PB-RPMexplained next. This is similar to approaches for identifying components forload control in distributed environments as outlined, e.g., in [3].� Global PB-RPM manages multimedia presentation instances from aglobal system point of view. Thus, it deals with all currently executingmultimedia presentation instances at all presentation sites. It decides howmuch each presentation must be adapted in order to achieve a certainimpact on the current total system load. For this reason, global PB-RPMmaintains knowledge about all multimedia presentation instances that areexecuting within the entire system.� Local PB-RPM manages multimedia presentations from the point ofview of a particular presentation instance. Thus, it deals with one par-ticular multimedia presentation instance executing at a particular site. It10

determines and conducts a concrete adaptation of the given multimediapresentation, e.g., by generating and executing an adaptation schedule[30, 31]. This adaptation must lead to a change in the presentation`s datademand which impacts the demanded data delivery performance as re-quested by global PB-RPM. Among other aspects, the determination ofan adaptation must consider the presentation's potential synchronizationconstraints.With respect to a distributed architecture, global PB-RPM is performed at thecentral database environment. Whereas, local PB-RPM which is guided by theneeds of the global system is performed at each site at which a multimediapresentation instance must be executed. That is, similar to [11], we proposea distributed control method for a set of system instances which consists ofa dynamically changing set of presentation sites and one central agent whichbelongs to the central database environment.In the following, we �rst describe the proposed components to facilitate PB-RPM in a distributed MM-DBS. Then, it is explained how the service keepstrack of data delivery performance to recognize performance uctuations whichrequire a system reaction. Furthermore, we also provide an abstract descriptionof the algorithms for the proposed components.4.1 ComponentsIn the following, we describe the components to achieve PB-RPM (Figure 3).First, we introduce those components which perform local PB-RPM and thusbelong to the environment of the presentation site. Second, we indentify thecomponents for global PB-RPM which belong to the central database systemenvironment.4.1.1 Components at Presentation SitePresentation Set-up Manager (PSM). The PSM is responsible for sub-mitting the users' presentation requests to the central database system envi-ronment and the further handling necessary to set-up a presentation instance.This must also cover a respective handling of requests rejected by the centraldatabase environment, e.g., due to the fact that there is not su�cient data de-livery performance available for the requested presentation. Depending on thepolicy supported by the central database environment (see also section 4.1.2),the PSM must support respective functionality. One sample required func-tionality could be that the PSM negotiates with the central database systemenvironment for starting the presentation with a lowered performance demand.Multimedia Playout Manager (MPM). The MPM is responsible for theplayout of the multimedia presentation to the user according to a requested11

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Figure 3: Components of a database system service for PB-RPM12

presentation quality. For example, using the author-centered de�nition of pre-sentation quality as proposed in section 2.1, if 100% presentation quality isrequested then the playout is precisely done as de�ned in the spatio-temporalde�nition. Otherwise, the playout is done with deviations from the speci�ca-tion. Playout management mainly consists of the management of multimediapresenters and the handling of user interactions [29, 32].Feasibility Predictor (FP). The FP tracks the best-e�ort delivery of thedata to the presentation site. If it anticipates that in the near futur the run-ning multimedia presentation cannot be continued with the current presentationquality, it informs the central database environment. Vice versa, if it anticipatesthat a current presentation quality lower than 100% can be improved it also in-forms the central database environment. More details about this are given insection 4.2.Adaptation Manager (AM). When an adaptation of the presentation in-stance is necessary then the AM determines and conducts an optimized adap-tation that adjusts the presentation to a new admissible performance demand.The optimization method proposed in [31] is one promissing approach whichcan be exploited in the context of the AM. Based on this approach, the AMgenerates and performs optimized adaptation schedules which adjust the opera-tion of the MPM to a new presentation quality. Furthermore, the AM maintainsa history of the presentation execution, especially it maintains a record of allperformed adaptations.4.1.2 Components of Central Database EnvironmentFor global PB-RPM, the components of the central database environment main-tain a set of transient objects. Each of these objects models a multimedia pre-sentation instance. These objects provide execution speci�c data (e.g., dataabout the individual user, the execution history such as allocated demand ad-justments, the current presentation quality) as required by global PB-RPM.This knowledge is acquired throughout the execution of the corresponding mul-timedia presentation instance. These objects themselves do not belong to thepersistent contents of the multimedia database. However, the database in addi-tion to the spatio-temporal composition of the multimedia presentation togetherwith its raw data provides meta data about each multimedia presentation (e.g.general information about the presentation author/provider, average data de-mand of the presentation).The operation of some of the components of the central database environ-ment must be based on adequate policies. In this paper, we only show examples,while a comprehensive discussion of respective policies is given in [34]. As one ofour further goals, we will investigate what information a multimedia database13

must provide in order to drive \policy" decisions for PB-RPM in distributedMM-DBS.Presentation Admission Control Component (PAC). The PAC handlespresentation requests issued from the presentation sites according to a givenpolicy. One sample policy is that a requested presentation is admitted onlywhen the performance demand of this presentation can be satis�ed (i.e. whenthe system is not overloaded). When the demand cannot be satis�ed then therequest is rejected. Another, less rigid policy is that in overload situationsthe PAC and the PSM of the presentation site negotiate for a correspondinglydownscaled delivery demand.Performance Demand Adjustment Allocator (PDA). The PDA keepstrack of the global system load and reacts to performance uctuations of a cer-tain intensity according to a given policy. The load tracking which is explainedin more details in section 4.2 is based on anticipated lacking and left-over perfor-mance reported by the presentation sites' FPs. When the system is overloadedor when there is left-over performance, for all presentation instances, a properdemand adjustment is allocated. That is, a new admissible performance de-mand is calculated for each presentation and issued to the presentation sites'AM. More details about this are given in section 4.2.4.2 Tracking the Data Delivery Performance andAdjusting the System LoadThe performance of the best-e�ort data delivery to the presentation sites mustbe tracked to identify and in turn handle relevant performance uctuations.Figure 4 illustrates the system's processing and, in particular, identi�es therespective components of our approach dedicated to this. Given our concretePB-RPM concept introduced in section 2 and these components, we now de-scribe further details of this processing. Note that an even more detailed de-scription is given in section 4.3 which presents corresponding algorithms and themessages which are interchanged between the presentation sites and the centraldatabase environment and vice versa.4.2.1 Processing at Presentation SitesThe FP continuously performs a so-called I/F-Test with a certain frequency4.Taking the current throughput of the data delivery to the presentation site, thebehavior of the delivery throughput within a certain past time frame, and thecurrent bu�er level, the test anticipates that either no reaction is necessary orone of the following two alternatives:4This frequency is dynamically changing and depends on the test results.14

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Figure 4: Load tracking and adjustment for PB-RPM� CONTINUATION INFEASIBILITY means, that it is anticipated that the cur-rent presentation quality Qp of the playout is too high, such that a contin-uation of the presentation with this quality will become infeasible. Thisis anticipated when the actual bu�er level B is lower than the safety levelS and at the same time the throughput of the data delivery T has beentoo low for a certain period of time [30].� DEMAND INCREASE FEASIBILITY means, that it is anticipated that an im-provement of a current presentation quality Qp below 100% is feasible.This is anticipated when B is beyond S and at the same time T has notbeen too low for a certain period of time [30].In case of CONTINUATION INFEASIBILITY, the FP issues the parameterized mes-sage CI(quantified infeas, current pres quality) to the central databaseenvironment's PDA. In case of DEMAND INCREASE FEASIBILITY, the FP issuesthe parameterized message DIF(quantified feas, current pres quality) tothe central database environment's PDA. Both, the quanti�ed infeasibility de-noted by ij which is the �rst message's �rst parameter and the quanti�ed feasi-bility denoted by fj which is the second message's �rst parameter are derived byproper heuristic approximation methods [34]. The second parameters of bothmessages provide the current presentation quality denoted by Qp.15

When the currently demanded data delivery performance must be changed, thenthe parameterized message (ADMISSIBLE PERF DEMAND) is received by the AM.Given such a message, the AM determines �rst an adequate adaptation, e.g.,based on the method described in [31]. The resulting well selected adaptationis carried out next which establishes the requested new performance demand.That is, the AM brings the current presentation quality Qp of the playout inline with the performance demand admitted by the PDA.4.2.2 Processing at Central Database EnvironmentAs a component of the central database environment, the PDA performs threeprocessing steps:1. Global Feasibility Check. Using the potential set of CI(...) andDIF(...) messages received from the presentation sites within a certainpast time frame, a so-called Global I/F-Check is performed. This checkinspects if a global system reaction is necessary and �lters out exceptionsto be handled in another way5. When a global system reaction is requiredthen depending on the concrete type of required reaction (either a reactionto GLOBAL INFEASIBILITY or GLOBAL FEASIBILITY) the global infeasibil-ity denoted by I or feasibility denoted by F is quanti�ed. This is done bya heuristic approximation method [34] that takes into account the valuesof the reported local infeasibility/feasibility and the arrival pattern of thefeasibility messages.2. Global Adjustment Calculation. The calculated global infeasibilityI/feasibility F is taken as input and mapped into a corresponding requiredglobal data demand adjustment denoted by A.3. Local Adjustment Allocation. The global demand adjustment Ais taken as input and for each presentation, a new admissible data de-mand denoted by dj is allocated. Then, to each presentation site's AM,the corresponding allocated dj is issued via a parameterized messageADMISSIBLE PER DEMAND(...).4.3 AlgorithmsIn the following, we present an abstract description of algorithms for our pro-posed components together with messages interchanged between the client siteand the central database environment. We di�er between the set-up and theexecution phase. The set-up phase manages presentation requests. Whereas,the execution phase deals with adaptations to potentially lacking or left-overperformance.5The handling of these exceptions is described in [34].16

4.3.1 Set-Up PhaseThe PSM and the PAC handle the set-up phase. Here are the respective algo-rithms:/*=============================================================*//* Algorithm for PSM *//*-------------------------------------------------------------*/Loop Forever;if (PRES_REQ issued by MPM) thensend PRES_REQ to PAC;wait for response from PAC;if (UNCONST_PRES_PERMIS received from PAC) then/* There is enough performance available. */grant REGULAR_PRES_START to MPM;send NEW_PRES_INVOKED to PAC;if (CONST_PRES_PERMIS received from PAC) then/* There is not enough performance available. Pres. start *//* is possible with some preadaptations. */if (PREADAPTATION_ACCEPTED) then/* application/user acceptance is checked */adapt presentation to admissible demand;/* by using the AM */grant PREADAPT_PRES_START to MPM;send NEW_PRES_INVOKED to PAC;else reject PRES_REQ of MPM;send REJECTION_ACKNOWLEDGE to PAC;if (PRES_REJECTED) then/* There is not enough performance available. */reject PRES_REQ of MPM;send REJECTION_ACKNOWLEDGE to PAC;End;/*=============================================================*/17

/*=============================================================*//* Algorithm for PAC *//*-------------------------------------------------------------*/Loop Forever;if (PRES_REQ received from PSM) thenif (ADMISSION_TEST fails) then/* There is not enough performance available. */send PRES_REJECTED to PSM;else if (PREADAPTATION_NECESSARY) then/* Req. pres. can be started if some adaptations *//* are performed. */send CONST_PRES_PERMIS to PSM;wait for response from PSM;if (NEW_PRES_INVOKED) thenupdate PRES_MONITORING_SET;else;else/* Re. pres. can be started without any adapt. */send UNCONST_PRES_PERMIS to PSM;wait for response from PSM;if (NEW_PRES_INVOKED) thenupdate PRES_MONITORING_SET;End;/*=============================================================*/

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4.3.2 Execution PhaseThe FP and the PDA handle the execution phase. Here are the respectivealgorithms:/*=============================================================*//* Algorithm for FM *//*-------------------------------------------------------------*/Loop Forever;perform I_F_TEST;if (CONTINUATION_INFEASIBILITY) thensend CI(quantified_infeas, current_pres_quality) to PDA;if (DEMAND_INCREASE_FEASIBILITY) thensend DIF(quantified_feas, current_pres_quality) to PDA;End;/*=============================================================*//*=============================================================*//* Algorithm for PDA *//*-------------------------------------------------------------*/Loop Forever;perform GLOBAL_I_F_CHECK;if (GLOBAL_INFEASIBILITY OR GLOBAL_FEASIBILITY) thenperform GLOBAL_ADJUSTMENT_CALCULATION;perform LOCAL_ADJUSTMENT_ALLOCATION;send ADMISSIBLE_PERF_DEMAND(adm_perf_demand) to AMs;End;/*=============================================================*/5 Related WorkIt is the global research goal of the work presented in this paper to providedatabase system support for executing multimedia presentations. So far, mostof the database literature on multimedia presentations focuses on the modellingand not on the execution aspect underneath. For example, [18] proposes toexpress semantic coherency of a multimedia presentation in terms of presenta-tion inclusion and exclusion constraints that are incorporated into the multime-dia data model. Synchronization models for multimedia are discussed, e.g., in[12, 19]. We ourselve [1] suggested an extension of our data modelling languageby a new primitive called SCHEDULE to support the temporal operations inherentto multimedia presentations. 19

In this paper, we proposed a database system service which is even able toadaptively execute the set of executing multimedia presentations according tothe needs of the global system. This is a new idea which has not been proposedfor database systems before. Several research e�orts not speci�cally addressingmultimedia database systems are related to this proposed service. They can becategorized as either concerning QoS speci�cation for multimedia presentationsor the management of adaptive presentation executions.A comprehensive proposal of a QoS speci�cation for multimedia presenta-tions is given in [22]. The proposal provides a data model for continuous mediathat preserves device and physical data independence [23] and allows to de�neQoS constraints from a speci�cation of ideal presentation outputs. Moreover, aset of QoS parameters is proposed that constitute a complete model for presen-tation error and the notion of presentation quality as perceived by the viewer.As part of our further joint research work, we will especially investigate howwe can exploit this much richer model of presentation quality for the purposeof PB-RPM in distributed MM-DBS. The notion of presentation quality is alsoconsidered in [4] in which the quality is dependent on the response time of thepresentation and the quality of the objects. Further work on the QoS speci�ca-tion issue can be found, e.g., in [9, 36].Adaptive techniques for distributed multimedia presentations have beenstudied by several other groups. [21] proposes the use of lightweight messages,called feedback units which are transmitted periodically back to media servers todetect impending discontinuities as well as asynchronies at mediaphones. In con-trast, in our approach the central database environment receives only feedbackmessages from the presentations sites when a system reaction is necessary toresolve lacking or left-over system performance. Practical results of the usage ofadaptive techniques for multimedia presentation executions have been reported,e.g., in [11] and [6]. More elaborated ideas based on the latter are presentedin [8]. Also in [4], work on the adaptive execution of multimedia presentationsis described. In particular, it distinguishes between Presentation Schedules andRetrieval Schedules. In case of a resource shortage, three di�erent modi�cationsare considered, (1) modi�cation of the bu�er resource distribution, (2) modi-�cation of the presentation or retrieval schedule, and (3) modi�cation of thequality of the presentation. The �rst alternative is the most desireable, whilethe last one is the least desireable one. In comparison to our approach, we donot have explicit retrieval schedules. However, as described in [30], our reactivecontrol mechanism manipulates nominal bu�er loading parameters according tothe needs of the presentation speci�cation. This manipulation is driven by animaginary retrieval schedule and in the future we might extend our approach byexplicit retrieval schedules. We also do not have explicit presentation schedules.However, as with respect to retrieval schedules, our reactive control mechanismmanipulates the nominal presentation parameters of the playout process accord-ing to an imaginary presentation schedule. Since, our work is more focusing ondeveloping an optimization method for adapting a set of executing multimedia20

presentations [31], we introduced the notion of adaptation schedule which canbe seen as a highly complementory to the other schedule types.6 ConclusionIn this paper, we proposed the general concept of PB-RPM which adapts thepresentation quality of executing multimedia presentations to the available de-livery performance. Furthermore, we presented an approach to facilitate thisconcept in distributed MM-DBS. This approach consists of an adaptive presen-tation service which performs local PB-RPM at all presentation sites and globalPB-RPM at the central database environment. The distinction between localand global PB-RPM re ects the fact that in database system environments,many concurrently executing multimedia presentations must be adapted at thesame time. In particular, it allows for more intelligent adaptation of the setof executing multimedia presentations to be adapted. For example, among theset of multimedia presentations, there are always presentations which can easilygive up a certain degree of presentation quality and there are also always otherswhich cannot. To the bene�ts of the users, using this knowledge about the setof presentations allows to achieve appropriate presentation adaptations. Thisknowledge can also be exploited for \policy" driven decisions for the optimiza-tion of adaptations of the set of executing multimedia presentations.There is a need for more research on speci�c issues within this framework.Among others, this includes respective methods to calculate local and globalinfeasibility/feasibility. These methods must take into account, e.g., knowledgeabout the past performance behavior of the system. Another issue is that aproper method is necessary to deduce the globally required performance de-mand adjustment. This method must especially be aware of the fact that asystem might begin to icker between decreasing and increasing adaptationswhen the global load is not properly adapted. The development of a properallocation method that splits the globally required demand adjustment amongthe set of executing multimedia presentations is a further issue to be investi-gated. In particular, this method must consider the requirements described inthis paper. So far, we take linear optimization techniques for this problem intoaccount. Furthermore, we will investigate the bene�ts of taking a more elabo-rated approach for presentation quality. In particular, we intend to deploy theapproach proposed in [22]. We will evaluate our proposed concept within theframework of the AMOS research prototype.Our more long term oriented goals in this framework include, among othertopics, the extention towards cost-based RPM. This extention re ects the re-cently started transmission of the internet towards an increasingly commercial-ized market place with information providers on the one side and paying userson the other. 21

References[1] Aberer, K., Klas, W.: \Supporting Temporal Multimedia Operations in ObjectOriented Database Systems'", Proc. of IEEE Multimedia Computing and Sys-tems, Boston, USA, May 1994, pp. 352{361[2] Anderson, D.P.: \Meta Scheduling for Distributed Continuous Media", ReportNo. UCB/CSD 90/599, Computer Science Division, University of CaliforniaBerkeley, October 1990[3] Breitbart, Y., Vingralek, R., Weikum, G.: \Load Control in Scalable DistributedFile Structures", Technical Report No. 254-95, Department of Computer Science,University of Kentucky, Lexington, January 1995[4] Candan, S.K., Prabhakaran, B., Subrahmanian, V.S.: \Retrieval Schedules Basedon Resource Availability and Flexible Presentation Speci�cations", Departmentof Computer Science, University of Maryland, College Park, Technical Report,CSTR 3616, UMIACS TR96-21, 1996[5] Cen, S., Pu, C., Staehli, R., Cowan, C., Walpole, J.: \A Distributed Real-TimeMPEG Video Audio Player", Proc. of the Fifth International Workshop on Net-work and Operating System Support of Digital Audio and Video (NOSSDAV'95),April 1995, Durham, NH, USA[6] Cen, S., Pu, C., Staehli, R., Cowan C.: \Demonstrating the E�ect of SoftwareFeedback on a Distributed Real-Time MPEG Video Audio Player", Proc. of theACM 1995 Multimedia Conference, San Francisco, Ca. November 1995[7] Chou, S. T.C., Tokuda, H.:\System Support for Dynamic QOS Control of Con-tinuous Media Communication", Proc. of the Third Int. Workshop on Networkand Operating System Support for Digital Audio and Video, La Jolla, Ca., USA,November 1992, Springer LNCS 712, pp. 363{368[8] Cowan, C., Cen, S., Pu, C., Walpole, J.: \Adaptive Methods for DistributedVideo Presentation", ACM Computing Surveys, Special Issue on Multimedia Sys-tems - Symposium on Multimedia, Vol. 27, No. 4, December 1995, pp. 580{583[9] Kalkbrenner, G., et al.: \Quality of Service (QoS) in Distributed Hypermedia-Systems", Proc. of RPODP'94, Second Int. WS on Principles of Document Pro-cessing, Darmstadt, April 1994[10] Huang, J., Du, D.: \Resource Management for Continuous Multimedia DatabaseApplications", Honeywell Technical Report SST-P94-011, Honeywell TechnologyCenter, Minneapolis, Minnesota, April 1994[11] Katse�, H.P., Robinson, B.S.: \Predictive Prefetch in the Nemesis MultimediaInformation Service\, Proc. of the ACM Multimedia 94, San Francisco, Ca., USA,October 1994, pp. 201{209[12] Little, T.D.C., Ghafoor, A.: \Synchronization and Storage Models for MultimediaObjects", IEEE J. on Selected Areas in Comm., Vol. 8, No. 3, April 1990, pp.413{427[13] Little, T.D.C., Ghafoor, A.:\Scheduling of bandwidth constrained multimediatra�c", in Computer Communications, Vol. 15 No. 6, July/August 1992, Butter-worth Heinemann Ltd., pp. 381{387 22

[14] Little, T.D.C., Kao, F.: \An Intermedia Skew Control System for MultimediaData Presentation", 3rd Int. WS on Network and Operating System Support forDigital Audio and Video, La Jolla, CA, Nov. 92, Springer LNCS 712, pp. 130{141[15] Little, T.D.C.:\A Framework for Synchronous Delivery of Time Dependent Mul-timedia Data", in Multimedia Systems, Vol. 1, No. 2, 1993, pp. 87{94[16] L�ohr, M., Rakow, T.: \Audio Support for an Object-Oriented Database Manage-ment System", in Multimedia Systems Journal, Vol. 3, Special Issue on Multime-dia Database Systems (2), Springer, October/November 1995[17] Moser, F., Kraiss, A., Klas, W.: \L/MRP: A Bu�er Management Strategy forInteractive Continuous Data Flows in a Multimedia Database Management Sys-tem", Proc. of the VLDB '95, Z�urich, Switzerland, September 1995[18] �Ozsoyoglu, G., Hakkoymaz, V., and Kraft, J.D.: \Automating the Assembly ofPresentations from Multimedia Databases", Proc. 12th IEEE Int. Conf. on DataEngineering, New Orleans, Lo, March 1996, pp. 593{601[19] Qazi, N., Woo M., Ghafoor, A.:\A Synchronization and Communication Modelfor Distributed Multimedia Objects", ACM Multimedia, August 1993, Anaheim,CA, USA, pp. 147{155[20] Rakow, T., L�ohr, M., Neuhold, E.J.: \Multimedia Databases - The Notionsand the Issues", GI-Fachtagung Datenbanksysteme in B�uro, Technik und Wis-senschaft BTW 95, Dresden, March 1995, Springer, pp. 1{29[21] Rangan, P.V., Ramanathan, S., Sampathkumar, S.: \Feedback Techniques forContinuity and Synchronization in Multimedia Information Retrieval", ACMTransactions on Information Systems, Vol. 13, No. 2, April 1995, Pages 145{176[22] Staehli, R., Walpole, J., Maier, D.: \A Quality of Service Speci�cation for Mul-timedia Presentations", ACM Multimedia Systems Journal, Vol. 3, Number 5/6,1995, ACM Press, Springer, pp. 251{263[23] Staehli, R., Maier, D., Walpole, J.: \Device and Data Independence for Mul-timedia Presentations", ACM Computing Surveys, Special Issue on MultimediaSystems - Symposium on Multimedia, Vol. 27, No. 4, December 1995, pp. 640{642[24] Steinmetz, R.: \Inside Operating System for Processing of Multimedia Data",IEEE Multimedia Magazine, February 1995[25] Thimm, H., Rakow, T.C.: \Upgrading Multimedia Data Handling Services of aDatabase Management System by an Interaction Manager", Arbeitspapiere derGMD No. 762, St. Augustin, July 1993[26] Thimm, H., Rakow, T.C.: \A DBMS Based Multimedia Archiving TeleserviceIncorporating Mail", Proc. of the 1st Int. Conf. on Applications of Databases,Vadstena, Sweden, June 1994, Springer LNCS 819, pp. 281{298[27] Thimm, H.: \A Multimedia Enhanced CSCW Teleservice for Wide Area Cooper-ative Authoring of Multimedia Documents", ACM CSCW'94 WS on DistributedSystems, Multimedia, and Infrastructure Support in CSCW, Chapel Hill, NC,USA, October 1994, ACM SIGOIS Bulletin, December 1994, Vol. 15, No.2, pp.49{57 23

[28] Thimm, H., Roehr, K., Rakow, T.C.: \A Mail Based Teleservice Architecturefor Archiving and Retrieving Dynamically Composable Multimedia Documents",Proc. of the Int. COST 237 Workshop, Multimedia Transport and Teleservices,Vienna, Austria, November 1994, Springer LNCS 882, pp. 14{34[29] Thimm, H., Klas, W.: \Playout Management - An Integrated Service of a Mul-timedia Database Management System", Proc. IEEE Int. WS on MultimediaDatabase Management Systems, Blue Mountain Lake, NY, August 1995, IEEEComputer Society Press, pp. 38{47[30] Thimm, H., Klas, W.: \Reactive Playout Management - Adapting Multime-dia Presentations to Contradictory Constraints", Arbeitspapiere der GMD/GMDTechnical Report No. 916, St. Augustin, May 1995[31] Thimm, H., Klas, W.: \�-Sets for Optimized Reactive Adaptive Playout Manage-ment in Distributed Multimedia Database Systems", Proc. 12th IEEE Int. Conf.on Data Engineering, New Orleans, Lo, March 1996, pp. 584{592[32] Thimm, H., Klas, W.: \Playout Management in Multimedia Database Systems",60-pages invited book chapter, in Nwosu, K., Berra, B., Thuraisingham, B. (Ed-itors): "Design and Implementation of Multimedia Database Management Sys-tems", to be published in spring 1996, Kluwer Academic Publishers[33] Thimm, H., Rakow, T., Neuhold, E.J.: \Using Multimedia Archives for Hyper-mnedia Applications in Open Networks", in J. Ebersp�acher (Ed.): New Marketswith Multimedia (Neue M�arkte durch Multimedia), Series Telecommunications,Vol. 21, pp. 153{176, Springer, Berlin 1995[34] Thimm, H.: \Integrated Database System Support for Multimedia Presentationsin the Absence of End-to-End QoS Guarantees", PhD dissertation, TechnicalUniversity Darmstadt, Germany, in progress[35] Tokuda, H., Kitayama, T.: \Dynamic QOS Control Based on Real TimeThreads", Proc. of the 4th Int. Workshop on Network and Operating SystemSupport for Digital Audio and Video, Lancaster, U.K., November 1993, SpringerLNCS 846, pp. 114{123[36] Vogel, A., et al.: \Distributed Multimedia and QOS: A Survey", IEEE Multime-dia Systems Journal, Summer 1995, pp. 10{19

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