Distributed Databases for Challenged Networks Vinoth Chandar, Ashwini Athalye Most delay tolerant networking research has focussed for query processing in a distributed Database for DTNon developing routing protocols that optimize data delivery environments. The beneﬁts that an application can enjoy fromof a single data unit from a source to potentially multiple such a system are location transparency and data decoupling.destinations. However, distributed applications built on top of Achieving location transparency in a DTN setting obviouslysuch intermittently connected networks, typically involve more results in more ﬂexible applications.complex data access mechanisms. Distributed databases arewidely used to capture such interactions. Thus, exploring the To our knowledge, no previous work has examined thisfeasibility of building a distributed database system for DTNs problem of distributed query processing in a completelyis an intriguing research problem. Applications can beneﬁt decentralized DTN environment. This work tries to put intoimmensely from the resulting location transparency and data perspective the issues that are to be addressed in order todecoupling. build a practical system, over which multitude of interesting The paper proposes a distributed database system for such applications can be written. The problem requires additionalnetworks. The key contributions of this work are 1) practical mechanisms to improve latency involved in answering queries,heuristics for query scheduling 2) a prereplication scheme due to the inherently time varying nature of the links. Such athat reduces the cost of on-demand retrieval by actively pre- scheme is vital to ensure the practical usefulness of the system.caching data. To our knowledge, this is the ﬁrst work toexamine these issues in an uniﬁed framework. We describe The paper is organized as follows. Section 2 details thethe results of several simulations conducted on a wide variety relevant background and the other related research in thisof artiﬁcial and real world traces. area. Section 3 presents the motivations and challenges to solving this problem.Section 4 discusses the problem of query I. I NTRODUCTION scheduling in DTNs. Section 5 presents the prereplication DTN is a category of networks that is characterized by scheme proposed. Section 6 and 7 contain details about meta-intermittent connectivity, irregular connectivity patterns, data exchanges involved and the packet scheduling criteriaunpredictable communication opportuntity and diverse involved. Section 8 presents detailed results from simulationsdevices. As explained in , these types of networks are of the scheme under various scenarios. This is followed by abecoming important with the pervasiveness of wireless short observations section. Section 10 concludes after outliningtechnology. For example, the campus wide deployment some of the future directions that we intend to pursue.of wireless technology coupled with the large number ofusers having laptops/cell phones with wireless capability, II. BACKGROUND AND R ELATED WORKprovides enormous opportunities for exchange of ’good-to- Traditional query processing schemes in distributedknow’ information amongst students. Such good-to-know databases construct an execution plan for an input query basedinformation can substantially augment the quality of decision on the sites participating in the query and the size of themaking in daily life. In the past, there have also been many data set that needs to be moved from one site to anotherreal world DTN deployments like Zebranet, DakNet for the execution of the query. Algorithms that optimize forand DieselNet. Zebranet tries to study the movement response time typically do so by reducing the amount of datapatterns and the behaviour of zebras, by attaching sensor shipped between sites, to execute the query. These schemesnodes to the Zebras. Daknet is one of the earliest practical  however assume a ﬁxed network topology which meansDTN systems, that provides Internet connectivity to rural that there are statically deﬁned paths between nodes and thatvillages in India. DieselNet is a DTN infrastructure deployed they are always up. This is contrary to what DTNs possess.over the bus transport system in Amherst. These practical In a DTN, simple optimization for communication cost maysystems have clearly proved the utility of delay tolerant not minimize the response time, since there can be schedulesnetworking. which involve earlier contacts between sites, leading to lower delays, even with larger data shipping. Hence query scheduling Thus, the wide applicability of DTNs call for a deeper look is consequently more complex for such an environment. Thisinto the needs of the applications that can be supported over is the focus of the research conducted in this paper.it.  stresses on the need to develop robust mechanisms ICEDB  proposes a system, where in users query a webfor DTN based collaborative applications to improve data portal, connected to devices on the road. Devices are sensorpropagation rates in realistic deployments. Using this nodes that build databases with the information of interest. Theinformation as a guiding light, we propose mechanisms portal is responsible for sending the user queries and getting
the results back from the devices. The queries are annotated track the nodes that carry interesting information, increasingwith priorities and the devices schedule results to be moved the complexity of the application.to the portal based on the priorities. This work illustrates Hence, if applications query using an indirection of databasethe utility of using database queries to retrieve content from tables, which are then mapped to the nodes that containintermittently connected nodes, since the application can work fragments of the global information, it can lead to reducedwith a simpler abstraction of a database table.  builds a overhead for applications and seamless integration of differentfederated database over Bluetooth. However, it assumes that applications to work with each other. Moreover multiplethere is no mobility and hence, does not apply to most practical applications can be built on top of this layer of abstractionscenarios.  is a mobile surveillance system, where buses since they no longer need to have node speciﬁc information.upload spatially and temporally tagged images to a base station In essence, building a distributed database system over DTNsplaced in the bus depot. These data stores are then queried would foster the development of myriad of interesting appli-from a central cloud server. cations, that in turn enrich the practical beneﬁts of conducting A survey of the related work gives insight into some delay tolerant networking research.practical aspects of DTNs.  formally analyzes the problemof DTN routing at various degrees of knowledge of the future IV. Q UERY P ROCESSINGand the network. MaxProp is a DTN routing protocol thatbuilds relative meeting probabilities and uses it to forward In conventional distributed databases, selectivity factors arepackets to a destination. RAPID  is a DTN routing protocol used to perform reductions on JOIN operations and optimizethat can be optimized speciﬁcally for a routing metric such as for the response time of the query. Computing the selectivityworst-case delay, average delivery delay. In this paper DTN factor is a linear time operation that can be performed instan-routing has been formulated as a resource allocation problem taeneously. However, the traditional schemes break in a DTNunder a realistic scenario where both storage and bandwidth since there is no way of maintaining selectivity factors in aare limited. The selected routing metric is used to prioritize DTN. Most traditional schemes reduce the response time bya packet at each hop and locally optimize the delivery of reducing the amount of data shipped across the network. Sincepackets in the order of the change in their utility function. the data size shipped, directly contributed to the delay involvedAnother interesting work on DTN routing , DPSP, is tuned in processsing the query , in a DTN setting, our approachfor applications that use a publish scheme to advertise content, largely pursues this path.on topics and the nodes subscribe to different topics. This We work with approximations of contact times/probabilitiesscenario does not require either the content provider or the through metadata exchanges between the nodes. Deadlines arecontent subscriber to have knowledge about each other. Also, useful in identifying the time until the data is valid. Thisthe knowledge given out by the publisher and the subscriber is correlates closely with the type of ’good-to-know’ informa-made use of to effectively allocate the resources and to prevent tion that the system intends to work with. Queries can beﬂooding of the network with replicated packets. tagged with deadlines to indicate the amount of time that the To summarize, the approach to query processing in DTNs results will be useful for. We also intend to reduce the queryhas been largely centralized i.e one central server holds the processing delay, by sending results as and when intermediatenode-to-data mapping and uses it to perform query scheduling. results are available, without waiting for the completion of theMost practical DTN routing mechanisms work with heuristics entire query, that potentially provides additional results. Theand rely on empirical results to prove their usefullness, in an duplicate results can be ﬁltered out through a delta detectionattempt to keep the problem tractable. mechanism at either the query initiator or the intermediate nodes, using standard techniques. III. M OTIVATION AND C ONTEXT We assume that applications that talk to each other have knowledge about the database tables. The knowledge about DTNs offer opportunities to develop some exciting dis- the horizontal fragmentation, including cardinalities, can betributed applications. For example, a newcomer to Austin built based on metadata exchanges. The packets that aremight want to know about good places to dine, live music sent through the network can be either those carrying theevents etc. A distributed peer to peer application can help query or packets that carry the results. We divide the sethim to query the people around for this information, even of queries that are issued by applications into Unions andwithout Internet connectivity. Typically, this good-to-know Joins. Unions consist of queries on single tables that areinformation is not very time sensitive and hence, can be horizontally fragmented across various nodes. Unions aredelivered over DTNs. Existing routing protocols in DTNs simply sent to all the nodes that the query initiator knowstypically operate with a known source and destination. The of, which contain the target query. Joins consist of queries onapplication developer needs to schedule packets to different multiple tables that can be horizontally fragmented and havedestinations, according to the needs of the application. The at least one common attribute. Joins require a much moreproblem becomes worse in case of open networks, where not complicated query scheduling mechanism. Joins, which areevery node knows each other, even though unknown nodes costly operations even in a conventional distributed databasemay carry relevant information. Thus, the application needs to system, are far more expensive in DTN environments.
For distributed databases, a number of factors contribute For each t in T:to heuristics that determine how the query gets processed. B[q] += cardinality(t)These are typically the cardinality of a database at a node, Processing node = i , such thatthe size of a tuple in the database, and together the two B[i] = max(B[q]), for all q in Q.numbers give the size of the data that needs to be movedbetween sites for complex queries. Query processing in DTN C. Nearest sitewould require heuristics to be designed such that they take This heuristic picks the site which all of the relevantinto account, packet scheduling criteria for DTN as well as sites (sites that house the relevant data) are more likely todatabase query optimization parameters. In this paper, we meet. Note that this processing site may not be amongst thediscuss two heuristics for scheduling Joins 1)Heaviest site relevant sites. The relevant sites may accumulate data at some2)Nearest site. intermediate node, that every node is most likely to meet. This heuristic tries to directly optimize for the time spent inA. Optimal scheme getting the required data to the processing site. The meeting Assuming the knowledge of contact times and contact probabilities used in MaxProp are used directly to computedurations of the nodes helps in improving the performance the nearest site.of routing mechanisms . In many real life scenarios, suchaccurate contact information can be obtained, such as schedule N : set of nodes known to current nodeof students in a campus. But, when this information is not Q : set of all nodes that are relevantavailable,statistical approximations can be used in their place, to this query.based on contact histories. Even with this complete informa- C[n] : cummulative meeting probabilitytion, computing an optimal path is NP Hard .Given that the of nodes in Q to each node n in NDTN routing problem, even with a perfect Oracle is NP Hard, Pseudo-code:the problem of scheduling the queries, which involves moving For node n in N:relevant data to the processing site in the best possible manner For each q in Q:also becomes NP hard, since the query processing problem is C[n] += meetingprobability(q,n)composed of independent subproblems of the routing problem. Processing site = i such that The non availability of selectivity factors, precludes any C[i] = max(C[n] ), for all n in Nchances of reducing joins at intermediate sites. Also, with V. P REREPLICATIONthe prereplication scheme discussed in section 6, even ﬂoatingapproximate and possibly stale values of the selectivity factors As mentioned earlier, the cost of on-demand retrieval isin the network might not perform well, since the prereplication much higher in a delay tolerant network, since the links maywould invalidate the selectivity factors very quickly due to not become available again for a long time. In such cases,addition of new content at the node. Moreover, selectivity actively moving content to nodes that are likely to be involvedfactors are very speciﬁc to a particular query. For a selectivity in relevant queries, would help reduce the latency involved infactor to be useful, the new query needs to match very closely answering the queries. Each node builds a popularity indexto the original query. Hence, the relevant data must be brought for table combinations that are involved in the queries that itto a single common processing site, for query evaluation. In sees. Nodes also build the popularity index through exchangethis paper, we primarily focus on choosing this processing site. of this information amongst themselves. Thus, a global pop-Hence, we try to approach the problem using heuristics, local ularity metric is built for all the table combinations. Over aoptimizations, and techniques from existing work on DTN longer duration of system operation, this metric captures therouting and distributed database query optimization. probability that a future query will be generated for each of the table combinations. Let us deﬁne the servability, of a nodeB. Heaviest site as the amount of popular content possessed by the node. When This heuristic tries to optimize for lower delays, by moving two nodes meet, they exchange more popular content amongstthe data to the site which possesses the largest amount of themselves.data relevant to the query. The heaviest site is chosen as the When a node meets another,processing site. Note that the processing site is one amongthe sites that have the relevant data. The intuition is that if the * Updates popularities from other nodeheaviest site is prevented from shipping its data elsewhere, * Computes a Integer Knapsack to deter -mine the transfers that are to bethe lesser bandwidth consumption would result in signiﬁcant made to increase servabilitysavings in the response time. * Utility of transfering a table fragQ : set of all nodes that are relevant to -ment to another node is the amount this query. of increase in servability due toT : set of tables that are required. that transferPseudo-code: * Cost of transfer is the bandwidthFor each node q in Q: consumed
* The maximum cost that can be incurr -ed by deadline -ed is total bandwidth of the conta Exchange prereplication content. -ct Schedules the transfers that it Note that the prereplication content is sent at the last, in needs to make to the other node effect , using only any amount of spare bandwidth that mayA. Returning results be available. Thus, prereplication is used as an add-on and it does not affect the data trafﬁc. As more content is prereplicated, the more popular contentis widely spread across the nodes in the network. As a result, VIII. E VALUATIONthe number of relevant sites in a join query increases with We have implemented our scheme on a DTN simulatorthe amount of prereplication. Hence, the latency involved in called  The ONE [see ﬁgure 1]. This Simulator providesprocessing the join also increases prohibitively. To keep the the following desirable features which led us to eventuallylatency low, two mechanisms can be used to return the results. choosing it for our project: • Best : The processing site waits for all table fragments • Various Routing mechanisms like Epidemic Routing, from relevant sites to arrive, to begin processing the query MaxProp Routing, Spray and Wait etc. are supported. • Fast: The processing site processes the query and sends • Different mobility models for simuating movement of the result out, as soon as atleast one portion of all the nodes. tables involved in the join are available. • A visualization tool to view node movement as well as The two mechanisms are a tradeoff between accuracy of data exchanges.results (best) and response time for the query (fast). Weconsidered two mechanisms at the two extremes for simplicityof analysis. Obviously, an intermediate scheme that balancesbetween the two can also be used. VI. M ETADATA EXCHANGES The unstructured nature of DTNs requires that a mechanismbe in place for each node to either have a global view of thenodes in the system or have some way of updating its localinformation based on interactions with other nodes. The latterinvolves learning state of neighbors by exchanging relevantmetadata between nodes in contact. Gradually as more andmore node exchanges happen, the metadata grows and getsreplaced with newer state. The following meta data are exchanged • Meeting probability, as described earlier • List of tables possessed by each node, and their cardinal- ities • Popularity indexes Fig. 1. Screenshot of ONE Simulator VII. PACKET SCHEDULING We have implemented our scheme on top of the MaxProp The packet scheduling scheme used must reﬂect the goals Router, provided by the simulator. The router maintains meet-of the database layer above. It needs to decide on speciﬁc ing probabilities of each node with other nodes it meets, whichper-packet and per-packet-per-hop metrics to replicate packets is useful for nearest site selection.in the network. We chose replication over forwarding since As mentioned earlier, there is no existing work in thisreplication has better chances of delivering the packets quickly, space to compare our scheme against. Hence we have tried toeven though it involves more resources. The following sum- evaluate our scheme by testing it against different movementmarizes the packet scheduling scheme used in the paper. models. We have also varied the number of nodes in theWhen node A meets node B network to see the effect of node size on the performanceExchange metadata of the proposed scheme. Through our evaluation, we hope toExchange directly deliverable content get an insight into the goodput of the scheme as well as its ordered by deadline applicability to different DTN scenarios.For all packets p in buffer: We have analysed our scheme against 5 movement models: if B has higher meeting probability • Random Way Point: This movement model captures to dest[p] than A: completely randomized node movement. Pick p for transfer • Shortest Path Based: This model simulates the movementSend all picked packets in buffer order of people. The nodes move along ﬁxed paths, however
their movement along these ﬁxed paths is random and real world events are roughly deterministic, it serves as a good based on certain points of interest. starting point for our analysis. The graphs in ﬁgure 2,3 convey • Map Based: This movement model simulates DTN where results generated for 20 and 40 nodes in a Random Way Point nodes have periodicity in their movement. We have model. utilized the movement of trams in Helsinki as part of this model. • DieselNet: This is a bus system that has been deployed by UMass consisting of 34 buses. • ZebraNet: This is a trace of a network of 5 zebras which have sensors deployed on them. The ﬁrst 3 movement models are provided by the simulator.Due to simulation time constraints, we have performed anal-ysis on these models with 20 and 40 nodes. The remaining2 movement models - DieselNet and ZebraNet are real traceswhich were converted into a format that was acceptable to thesimulator. Hence we have been able to run our scheme onsimulation models as well as real traces.A. Metrics To evaluate the performace of our scheme, we consider thefollowing metrics. Fig. 2. Query processing throughput for Randomly moving nodes • Percentage of answered queries: This metric was choosen to get the throughput of the system for the total of forward(path along which query is sent) and reverse paths(path along which query result is sent). • Percentage of Unions: This metric gives the peformance for single table queries. • Percentage of Joins: This metric gives the peformance for multi table queries. This metric is especially useful to verify our scheme since we expect performance to improve for joins, especially with our processing site selection scheme and the prereplicaton scheme. • Average query latency: This metric is again important to understand the behavior of our scheme. We expect our scheme to reduce the query latency i.e. decrease the delay between the time when the query was issued by a node and the time that it received the results. The average query latency also provides a useful insight into the kind of deadlines that each DTN scenario is typcially able to handle. Fig. 3. Latency for Randomly moving nodesB. Scenarios We present the results of the metrics introduced in the D. Map based Tram movementsprevious section, for three scenarios. This model simulates the movement of trams in Helsinki, • No repl - This represents the behavior of the system, with using WKT ﬁles that were generated from a real GIS system. no prereplication, ’best’ return of results and heaviest It simulates periodicity in meeting patterns. Figures 4,5 show site query processing. This serves as the vanilla scenario the performance of the scheme for 20 and 40 people in the against which we compare the next two scenarios. system. • Repl(Heavy,Fast) - This involves prereplication, ’fast’ E. Shortest path People movement return of results and heviest site query processing. • Repl(Near,Fast) - This is same as Repl(Heavy,Fast) ex- With the popularity of portable devices like hand held PDA’s cept that nearest site query processing scheme is used. and cell phones, DTNs with such devices as nodes is another potential scenario. Hence the movement patterns for peopleC. Random Way point give an idea of connectivity in such an environment. The The random way point model simulates randomly moving scheme was evaluated for 20 and 40 nodes in the system [seenodes. Although, it is a unrealistic mobility model since most ﬁgure 6,7].
Fig. 4. Query processing throughput for Tram system Fig. 7. Latency for People interactions were generated by contacts between metro buses, in Amherst. The number of nodes in the DieselNet traces is 34. Figures 8,9 present the results of the simulation run for about a week. Fig. 5. Latency for Tram system Fig. 8. Query processing throughput for DieselNet G. ZebraNet Monitoring wildlife and natural phenomena is another in- teresting area for DTNs. It also presents a good case, where a query processing scheme could be invaluable. ZebraNet is a real world deployment of sensors on 5 zebras. See ﬁgure 10,11 for results. IX. O BSERVATIONS In general, Pre-replication provides improvements in latency Fig. 6. Query processing throughput for People interactions as well as the success ratio of the queries.However, the extent of beneﬁt varies from one movement model to another.F. DieselNet • The pre-replication beneﬁts are around 30-40% for suc- DieselNet is a realtime trace which exempliﬁes the appli- cess rate.cability of our scheme for a real world scenario. The traces • Beneﬁts for latency are around 30-50%.
latency. X. C ONCLUSION AND F UTURE W ORK This is the ﬁrst work to address the problem of query scheduling in a completely decentralized and adhoc , delay tolerant network. This is a very interesting research area with enormous scope for improvement. We intend to pursue further research along these lines. • Building a DB Aware packet replication scheme with multicast capability would help improve performance since the query issuals are inherently multicast trafﬁc. • If the intermediate nodes cache previous results, queries could be answered from these caches, improving latency. However, this would involve complex tradeoffs in buffer management. Also, the applications should be willing to accept some degree of staleness in the results. Fig. 9. Latency for DieselNet • Given that many real world DTN environments have near periodic events, a local prereplication scheme could be used wherein the nodes perform the prereplication based on the queries that they answer and route. This is different from the current scheme where the global popularities are used. • A very important contribution would be to improve the heuristics used for query scheduling. Ways to reduce multi table joins, parallelly in this environment, is a open problem. • In our scheme we were not able to use bandwidth between nodes as a metric since the simulator doesnot provide a way to extract contact durations between nodes. Looking into this aspect can yield better more practical heuristics. • With better simulation results, the system needs to be im- plemented on real hardware to exemplify its real beneﬁts. The work identiﬁes an area that has not received due Fig. 10. Query processing throughput for ZebraNet attention before. We have proposed query scheduling and prereplication mechanisms to enable applications to manage data in a ﬂexible manner, in a DTN setting. The simulation results prove the validity of the approaches, followed in the paper. We strongly believe that such a distributed solution, will really foster the growth of killer applications that can popularize the notion of DTNs. XI. R EFERENCES  Kevin Fall, Delay Tolerant Network Architecture for Challenged Internets, Sigcomm 03, August 25-29  Xuwen Yu and Surendar Chandra, Delay tolerant collaborations among campus-wide wireless users, IEEE Infocom 2008 Aruna Balasubramanian, Brian Neil Levine and Arun Venkataramani, DTN Routing as a Resource Allocation Problem, Sigcomm 07, August 27-31  Janico Greifenberg and Dirk Kutscher, Efﬁcient Fig. 11. Latency for ZebraNet Publish/Subscribe-based Multicast for Opportunistic Networking with Self Organized Resource Utilization, IEEE 2008• In most cases the Nearest site heuristic performs better  Donald Kossmann,The State of the Art in Distributed than Heaviest Site in terms of sucess rate as well as Query Processing, ACM 2001
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