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An IEEE ICDE 2000 Tutorial on. Mobile and Wireless Database Access for Pervasive Computing. Panos K. Chrysanthis University of Pittsburgh & Carnegie Mellon University Evaggelia Pitoura University of Ioannina [email protected] [email protected] Outline. Motivating Example

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Mobile and wireless database access for pervasive computing l.jpg

An IEEE ICDE 2000 Tutorial on

Mobile and Wireless Database Access for Pervasive Computing

Panos K. Chrysanthis

University of Pittsburgh & Carnegie Mellon University

Evaggelia Pitoura

University of Ioannina

[email protected]@cs.uoi.gr


Outline l.jpg
Outline

  • Motivating Example

  • Issues: Mobility, Wireless Communication, Portability

  • Adaptability and Mobile Client-Server Models

  • Location Management

  • Broadcast data dissemination

  • Disconnected database operations

  • Mobile Access to the Web

  • Mobility in Workflow Systems

  • State of Mobile DB Industry and Research Projects

  • Unsolved Problems


Party on friday l.jpg
Party on Friday

  • Update Smart Phone’s calendar with guests names.

  • Make a note to order food from Dinner-on-Wheels.

  • Update shopping list based on the guests drinking preferences.

  • Don’t forget to swipe that last can of beer’s UPS label.

  • The shopping list is always up-to-date.


Party on friday4 l.jpg
Party on Friday

  • AutoPC detects a near Supermarket that advertises sales.

  • It accesses the shopping list and your calendar on the Smart Phone.

  • It informs you the soda and beer are on sale, and reminds you.

    that your next appointment is in 1 hour.

  • There is enough time based on the latest traffic report.


Party on friday5 l.jpg
Party on Friday

  • TGIF…

  • Smart Phone reminds you that you need to order food by noon.

  • It downloads the Dinner-on-Wheels menu from the Web on your PC with the guests’ preferences marked.

  • It sends the shopping list to your

    CO-OP’s PC.

  • Everything will be delivered by the time

    you get home in the evening.


Mobile applications l.jpg
Mobile Applications

  • Expected to create an entire new class of Applications

    • new massive markets in conjunction with the Web

    • Mobile Information Appliances - combining personal computing and consumer electronics

  • Applications:

    • Vertical: vehicle dispatching, tracking, point of sale

    • Horizontal: mail enabled applications, filtered information provision, collaborative computing…


Mobile and wireless computing l.jpg
Mobile and Wireless Computing

  • Goal: Access Information Anywhere, Anytime, and in Any Way.

  • Aliases: Mobile, Nomadic, Wireless, Pervasive, Invisible, Ubiquitous Computing.

  • Distinction:

    • Fixed wired network: Traditional distributed computing.

    • Fixed wireless network: Wireless computing.

    • Wireless network: Mobile Computing.

  • Key Issues: Wireless communication, Mobility, Portability.


Wireless communication l.jpg
Wireless Communication

  • Cellular - GSM (Europe+), TDMA & CDMA (US)

    • FM: 1.2-9.6 Kbps; Digital: 9.6-14.4 Kbps (ISDN-like services)

  • Public Packet Radio - Proprietary

    • 19.2 Kbps (raw), 9.6 Kbps (effective)

  • Private and Share Mobile Radio

  • Wireless LAN - wireless LAN bridge (IEEE 802.11)

    • Radio or Infrared frequencies: 1.2 Kbps-15 Mbps

  • Paging Networks – typically one-way communication

    • low receiving power consumption

  • Satellites – wide-area coverage (GEOS, MEOS, LEOS)

    • LEOS: 2.4 Kbps (uplink), 4.8Kbps (downlink)



  • Wireless characteristics l.jpg
    Wireless characteristics

    • Variant Connectivity

      • Low bandwidth and reliability

    • Frequent disconnections

      • predictable or sudden

    • Asymmetric Communication

      • Broadcast medium

    • Monetarily expensive

      • Charges per connection or per message/packet

    • Connectivity is weak, intermittent and expensive


    Portable information devices l.jpg
    Portable Information Devices

    • PDAs, Personal Communicators

      • Light, small and durable to be easily carried around

      • dumb terminals [InfoPad, ParcTab projects],

        palmtops, wristwatch PC/Phone, walkstations

    • will run on AA+ /Ni-Cd/Li-Ion batteries

    • may be diskless

    • I/O devices: Mouse is out, Pen is in

    • wireless connection to information networks

      • either infrared or cellular phone

    • specialized HW (for compression/encryption)


    Portability characteristics l.jpg
    Portability Characteristics

    • Battery power restrictions

      • transmit/receive, disk spinning, display, CPUs, memory consume power

    • Battery lifetime will see very small increase

      • need energy efficient hardware (CPUs, memory) and system software

      • planned disconnections - doze mode

    • Power consumption vs. resource utilization


    Portability characteristics13 l.jpg
    Portability Characteristics

    • Resource constraints

      • Mobile computers are resource poor

      • Reduce program size – interpret script languages (Mobile Java?)

      • Computation and communication load cannot be distributed equally

    • Small screen sizes

    • Asymmetry between static and mobile computers


    Mobility characteristics l.jpg
    Mobility Characteristics

    • Location changes

      • location management - cost to locate is added to communication

    • Heterogeneity in services

      • bandwidth restrictions and variability

    • Dynamic replication of data

      • data and services follow users

    • Querying data - location-based responses

    • Security and authentication

    • System configuration is no longer static


    What needs to be reexamined l.jpg
    What Needs to be Reexamined?

    • Operating systems

    • File systems

    • Data-based systems

    • Communication architecture and protocols

    • Hardware and architecture

    • Real-Time, multimedia, QoS

    • Security

    • Application requirements and design

    • PDA design: Interfaces, Languages


    Query transaction processing l.jpg
    Query/Transaction Processing

    • Concern moves from CPU time and network delays to battery power and communication costs (including tariffs)

    • Updates may take the form of long-running transactions

      • nodes may continue in disconnected mode

      • need new transaction models [Chrysanthis 93, Satya 94]

    • Move data vs. move query/transaction

    • Context (location) based query responses

    • Consistency, autonomy, recovery

      • Approximate answers

      • Stable storage for logs, data -- stabilize at servers?

    • Providing uniform access in a heterogeneous environment

    • Design of human-computer interfaces (pen-based computing)

    • Updated system info: Location information, user profiles


    Recurrent themes l.jpg
    Recurrent Themes Query/Transaction Processing

    • Handling disconnections (planned failures?)

      • caching strategies

      • managing inconsistencies

    • Delayed write-back and prefetch: use network idle times

      • increases memory requirements

    • Buffering/batching: allows bulk transfers

    • Partitioning and replication

      • triggered by relocation

    • Compression: increase effective BW

      • increases battery power requirements

    • Receiving needs less power than sending


    Outline18 l.jpg
    Outline Query/Transaction Processing

    • Motivating Example

    • Issues: Mobility, Wireless Communication, Portability

    • Adaptability and Mobile Client-Server Models

    • Location Management

    • Broadcast data dissemination

    • Disconnected database operations

    • Mobile Access to the Web

    • Mobility in Workflow Systems

    • State of Mobile DB Industry and Research Projects

    • Unsolved Problems


    Mobility in db applications l.jpg
    Mobility in Db Applications Query/Transaction Processing

    • Need to adapt to constantly changing environment:

      • network connectivity

      • available resources and services

    • By varying and (re)negotiating:

      • the partition of duties between the mobile and static elements

      • the quality of data available at the mobile host

        Example: Fidelity (degree to which a copy of data matches thereference copy at the server)


    Adaptability l.jpg
    Adaptability Query/Transaction Processing

    Where should support for mobility and adaptability be placed?

    Application-Aware

    Laissez-Faire

    Application Transparent

    (-) applications must be re-written which may be very complicated

    (-) no focal point of control to resolve potentially incompatible application demands or to enforce limits on resource usage

    (+) existing applications continue to work unchanged

    (-) too general, cannot take advantage application semantics

    (-) may not be attainable (e.g., during a long disconnection)


    Adaptive applications l.jpg
    Adaptive Applications Query/Transaction Processing

    • Need:

      • Measurement of QoS and communication with application

        • A mechanism to monitor the level and quality of information and inform applications about changes.

      • Programmer Interface for Application-Aware Adaptation

        • Applications must be agile: able to reveive events in an asynchronous manner and react appropriately

      • A central point formanagingresources and authorizing any application-initiated request.


    C sa c server side agent l.jpg

    Wireless Link Query/Transaction Processing

    Fixed Network

    Server

    Client

    Agent

    C-SA-C: Server-side Agent

    • C-SA-C: The Client/Server-side Agent/Server Model

    • Splits the interaction between the mobile client and server: client-agent and agent-server

      • different protocols for each part of the interaction

      • each part may be executed independently of the other


    Responsibilities of the agent l.jpg
    Responsibilities of the Agent Query/Transaction Processing

    • Messaging and queying

    • Manipulate data prior to their transmission to the client:

      • perform data specific compression

      • batch together requests

      • change the transmission order


    Role of the agent l.jpg
    Role of the Agent Query/Transaction Processing

    • Surrogate or proxyof the client

      • Any communication to/from the client goes through the agent

      • Offload functionality from the client to the agent

    • Application (service) specific

      • provides a mobile-aware layer to specifc services or applications (e.g., web-browsing or database access)

      • handles all requests from mobile clients

    • Filters

      • provide agents that operate on protocols

      • E.g., an MPEG-agent or a TCP-agent


    C ca s client side agent l.jpg

    Wireless Link Query/Transaction Processing

    Fixed Network

    Client

    Agent

    Server

    Mobile Host

    C-CA-S: Client-side Agent

    • C-SA-S: The Client/Client-side Agent/Server Model

      • caching

      • background prefetching and hoarding

      • various communication optimizations


    C i s client server agents l.jpg
    C-I-S: Client & Server Agents Query/Transaction Processing

    Wireless Link

    • C-I-S: Client/Intercept/Server Model

      • Caching, prefetching etc

      • various communication optimizations at both ends

        • E.g., asynchronous queued RPC

      • relocate computation between the agents

      • Client interoperability

    Fixed Network

    Agent

    Client

    Agent

    Server

    Mobile Host


    Mobile agents l.jpg
    Mobile Agents Query/Transaction Processing

    • Mobile agents are migrating processes associated with an itinerary

      • dynamic code and state deployment

    • Implement the agents of the previous architectures as mobile agents, E.g.,

      • server-side agents can relocate during handoff

      • client-side agent dynamically move on and off the client

        • Relocatable dynamic objects (RDO) [Rover]

    • Implement the communication using mobile agents:

      • clients submit/receive mobile agents to/from the server

      • E.g., Compacts [Pro-Motion]


    A taxonomy l.jpg
    A Taxonomy Query/Transaction Processing


    Outline29 l.jpg
    Outline Query/Transaction Processing

    • Motivating Example

    • Issues: Mobility, Wireless Communication, Portability

    • Adaptability and Mobile Client-Server Models

    • Location Management

    • Broadcast data dissemination

    • Disconnected database operations

    • Mobile Access to the Web

    • Mobility in Workflow Systems

    • State of Mobile DB Industry and Research Projects

    • Unsolved Problems


    Locating moving objects l.jpg
    Locating Moving Objects Query/Transaction Processing

    • Example of moving objects

      • mobile devices (cars, cellular phones, palmtops, etc)

      • mobile users (locate users independently of the device they are currently using)

      • mobile software (e.g., mobile agents)

    • How to find their location - Two extremes

      • Searcheverywhere

      • Store their current location everywhere

      • Searching vs. Informing


    Locating moving objects31 l.jpg
    Locating Moving Objects Query/Transaction Processing

    • What (granularity), where (availability) and when (currency) to store

    at all sites

    Availability

    At selective sites (e.g., at frequent callers)

    the whole network

    Exact location

    nowhere

    some partition

    Currency

    Granularity

    Never update

    Always update (at each movement)


    Architectures of location dbs l.jpg
    Architectures of Location DBs Query/Transaction Processing

    • Two-tier Schemes (similar to cellular phones)

      • Home Location Register (HLR): store the location of each moving object at a pre-specified location for the object

      • Visitor Location Register (VLR): also store the location of each moving object mo at a register at the current region

    • Hierarchical Schemes

      • Maintain multiple registries


    Two tier location dbs l.jpg
    Two-tier Location DBs Query/Transaction Processing

    • Search

      • Check the VLR at your current location

      • If object not in, contact the object’s HLR

    • Update

      • Update the old and new VLR

      • Update the HLR


    Hierarchical location dbs l.jpg
    Hierarchical Location DBs Query/Transaction Processing

    Maintain a hierarchy of location registers (databases)

    A location database at a higher level contains location information for all objects below it


    Hierarchical location dbs35 l.jpg
    Hierarchical Location DBs Query/Transaction Processing

    Call

    caller


    Hierarchical location dbs36 l.jpg
    Hierarchical Location DBs Query/Transaction Processing

    Move

    new location

    old location


    Hierarchical vs two tier l.jpg
    Hierarchical vs. Two-tier Query/Transaction Processing

    (+)No pre-assigned HLR

    (+)Support Locality

    (-)Increased number of operations (database operations and communication messages)

    (-)Increased load and storage requirements at the higher-levels


    Locating moving objects38 l.jpg
    Locating Moving Objects Query/Transaction Processing

    Partitions

    P3

    P4

    P5

    P1

    P2

    User x

    User x


    Locating moving objects39 l.jpg
    Locating Moving Objects Query/Transaction Processing

    • Caching

      • cache the callee’s location at the caller

        (large Call to Mobility Ratio)

    • Replication

      • replicate the location of a moving object at its frequent callers (large CMR)

    • Forwarding Pointers

      • do not update the VLR and the HLR, leave a forwarding pointer from the old to the new VLR (small CMR)

      • When and how forwarding pointers are purged?

    • Concurrency, coherency and recovery/checkpointing of location DBs


    Querying moving objects l.jpg
    Querying Moving Objects Query/Transaction Processing

    • Besides locating moving objects, answer more advanced queries, e.g.,

      • find the nearest service

      • send a message to all mobile objects in a specific geographical reafion

    • Location queries: spatial, temporal or continuous

      • Issues: representation, evaluation and imprecision

    Most current research assumes a centralized location database


    Querying moving objects41 l.jpg
    Querying Moving Objects Query/Transaction Processing

    How to model the location of moving objects?

    Dynamic attribute (its value change with time without an explicit update) [e.g., in MOST]

    For example, dynamic attribute A with three sub-attributes: A.value, A.updatetime and A.function

    (function of a single variable t that has value 0 at time t=0)

    • The value of A at A.updatetime is A.value

    • at time A.updatetime + t0 isA.value + A.function(t0)


    Querying moving objects42 l.jpg
    Querying Moving Objects Query/Transaction Processing

    How to represent and index moving objects?

    • Spatial indexes do not work well with dynamically changing values

    • Value-time representation

      • An object is mapped to a trajectory [Kollios 99]


    Outline43 l.jpg
    Outline Query/Transaction Processing

    • Motivating Example

    • Issues: Mobility, Wireless Communication, Portability

    • Adaptability and Mobile Client-Server Models

    • Location Management

    • Broadcast data dissemination

    • Disconnected database operations

    • Mobile Access to the Web

    • Mobility in Workflow Systems

    • State of Mobile DB Industry and Research Projects

    • Unsolved Problems


    Information dissemination l.jpg
    Information Dissemination

    Goal : Maximize query capacity of servers, minimize energy per query at the client.

    Focus: Read-only transactions (queries).

    • Clients send update data to server

    • Server resolves update conflicts, commits updates

      1. Pull: PDAs demand, servers respond.

  • backchannel (uplink) is used to request data and provide feedback.

  • poor match for asymmetric communication.


  • Information dissemination45 l.jpg

    F Information Dissemination

    E

    G

    D

    A

    C

    B

    . .

    Clients

    BC

    EG

    AB

    Server

    Information Dissemination…

    2. Push: Network servers broadcast data, PDA's listen.

    • PDA energy saved by needing receive mode only.

    • scales to any number of clients.

    • data are selected based on profiles and registration in each cell.


    Information dissemination46 l.jpg

    F Information Dissemination

    E

    G

    D

    A

    C

    B

    . .

    Clients

    BC

    EG

    AB

    Server

    14.4 Kbps

    Information Dissemination…

    3. Combinations Push and Pull (Sharing the channel).

    • Selective Broadcast: Servers broadcast "hot" information only.

      • "publication group" and "on-demand" group.

    • On-demand Broadcast: Servers choose the next item based on requests.

      • FCFS or page with maximum # of pending requests.


    Broadcast data dissemination l.jpg

    Data Server Information Dissemination

    Broadcast Data Dissemination

    • business data, e.g., Vitria, Tibco

    • election coverage data

    • stock related data

    • traffic information

    • sportscasts, e.g., Praja

      • Datatacycle [Herman]

      • Broadcast disks


    Organization of broadcast data l.jpg

    A B C Information Dissemination

    A A B C

    Organization of Broadcast data

    • Flat: broadcast the union of the requested data cyclic.

    • Skewed (Random):

      • broadcast different items with different frequencies.

      • goal is that the inter-arrival time between two instances of the same item matches the clients' needs.


    Broadcast disks l.jpg

    A Information Dissemination

    Disk1

    B

    C

    Disk2

    Broadcast Disks

    • Multi-Disks Organization[Acharya et. al, SIGMOD95]

      • The frequency of broadcasting each item depends on its access probability.

      • Data broadcast with the same frequency are viewed as belonging to the same disk.

      • Multiple disks of different sizes and speeds are superimposed on the broadcast medium.

      • No variant in the inter-arrival time of each item.

    A B A C


    Selective tuning l.jpg
    Selective Tuning

    • Basic broadcast access is sequential

    • Want to minimize client's access time and tuning time.

      • active mode power is 250mW, in doze mode 50μW

    • What about using database access methods?

    • Hashing: broadcast hashing parameters h(K)

    • Indexing: index needs to be broadcast too

      • "self-addressable cache on the air"

        (+) "listening/tuning time" decreases

        (-) "access time" increases


    Access protocols l.jpg
    Access Protocols Selective Tuning

    • Two important factors affect access time:

      • Size of the broadcast

      • Directory miss factor - you tune in before your data but after your directory to the data!

        Trade-Off:  Size means  Miss factor

        Trade-Off:  Size means  Miss factor


    Indexing l.jpg
    Indexing Selective Tuning

    • (1,M) Indexing:

      • We broadcast the index M times during one version of the data.

      • All buckets have the offset to the beginning of the nextindex segment.

    • Distributed Indexing

      • Cuts down on the replication of index material

      • Divides the index into:

        • replicated topL levels, non-replicated bottom 4-L levels

    • Flexible Indexing

      • Broadcast divided into p data segments with sorted data.

      • Abinary control index is used to determine the data segment

      • A local index to locate the specific item within the segment


    Caching in broadcasting l.jpg
    Caching in Broadcasting

    • Data are cache to improve access time

    • Lessen the dependency on the server's choice of broadcast priority

    • Traditionally, clients cache their "hottest" data to improve hit ratio

    • Cache data based on PIX:

      Probability of access (P)/Broadcast frequency (X).

    • Cost-based data replacement is not practical:

      • requires perfect knowledge of access probabilities

      • comparison of PIX values with all resident pages

    • Alternative: LIX, LRU with broadcast frequency

      • pages are placed on lists based on their frequency (X)

      • lists are ordered based on L, the running avg. of interaccess times

      • page with lowest LIX = L/X is replaced


    Prefetching in broadcasting l.jpg
    Prefetching in Broadcasting Caching in Broadcasting

    • Client prefetch page in anticipation of future accesses

    • No additional load to the server and network

    • Prefetching instead of waiting for page miss can reduce the cost of a miss

    • PT prefetching heuristic [Archarya et al. 96]

      - pt: Access Probability (P) * period (T) before page appears next

      - A broadcast page b replaces the cached page c with lowest pt value

    • Team tag - Teletext approach [Ammar 87]

      • Each page is associated with a set of pages most likely to be requested next

      • When p is requested, D (D:cache size) associated pages are prefetched

      • Prefetching stops when client submit a new request


    Cache invalidation techniques l.jpg
    Cache Invalidation Techniques

    • When?

      • Synchronous: send invalidation reports periodically

      • Asynchronous: send invalidation information for an item as soon as its value changes; E.g., Bit Sequences [Jing 95]

    • To whom?

      • Stateful server: to affected clients

      • Stateless server: broadcast to everyone

    • What?

      • invalidation: only which items were updated

      • propagation: the values of updated items are sent

      • aggregated information/ materialized views


    Synchronous invalidation l.jpg
    Synchronous Invalidation

    • Stateless servers are assumed.

    • Types of client: Workalcholic and sleepers [Barbara Sigmod 94]

    • Strategies:

      • Amnestic Terminals: broadcast only the identifiers of the items that changed since the last invalidation report

        abort T, if x є RS(T) appears in the invalidation report

      • Timestamp Strategy: broadcast the timestamps of the latest updates for items that have occurred in the last w seconds.

        abort T, if ts(x) > tso(T)

      • Signature Strategy: broadcast signatures.

        A signature is a compressed checksum similar to the one used for file comparison.


    Consistency and currency l.jpg
    Consistency and Currency

    • Only committed data are included in the broadcast

    • Does a client read current and consistent data?

    • Currency interval is the fraction of bcycle that updates are reflected

    • Span(T) is the # of currency intervals from which T read data

    • if Span(T) = 1, the T is correct (read consistent data)

      else ?

      ... several consistency models


    Consistency criteria l.jpg
    Consistency Criteria Consistency and Currency

    • Latest value: clients read the most recent value of a data item [Garcia-Molina TODS82, Acharya VLDB96]

    • Serializability: Certification reports [Barbara ICDCS97]

    • Update consistency: clients commit of their reads are not invalidated – read mutually consistent data

      • F-Matrix method [Shanmugasundaram SIGMOD99]

    • 2-level serializability: Each client is serializable with respect to the server

      • SGT method [Pitoura&Chrysanthis ICDS99]

      • Multiversion [Pitoura&Chrysanthis VLDB99]


    Currency in multiversion schemes l.jpg
    Currency in Multiversion Schemes Consistency and Currency

    Versioning

    Multiversioning

    Multiversioning

    with invalidation

    Invalidation

    begin (first read)

    first invalidation

    commit

    T’s lifetime

    10

    VLDB 1999


    Adaptive hybrid broadcast l.jpg
    Adaptive Hybrid Broadcast Consistency and Currency

    • Cycle-based, bidirectional hybrid broadcast server

    • Issues:

      • Dynamic computation of bandwidth allocated to each broadcast mode

      • Dynamic classification of data items (periodic vs. on-demand)

      • Scheduling periodic and on-demand broadcasts


    An approach l.jpg
    An Approach Consistency and Currency

    • After each broadcast cycle, items classified as periodic or on-demand, depending on bandwidth savings expected

    • Periodic broadcast occupies up toBWThreshold

    • Periodic broadcast program is computed to satisfy all deadlines of periodic data

    • On-demand broadcast uses on-line EDF

      (Earliest Deadline First) algorithm + batching


    Outline62 l.jpg
    Outline Consistency and Currency

    • Motivating Example

    • Issues: Mobility, Wireless Communication, Portability

    • Adaptability and Mobile Client-Server Models

    • Location Management

    • Broadcast data dissemination

    • Disconnected database operations

    • Mobile Access to the Web

    • Mobility in Workflow Systems

    • State of Mobile DB Industry and Research Projects

    • Unsolved Problems


    Disconnected operations l.jpg
    Disconnected Operations Consistency and Currency

    • Issues:

      • Cache misses are more expensive in mobile environments.

      • Data availability for disconnected operation

      • Data consistency given that global communication is costly

      • Autonomy vs. Consistency

    • Solutions:

      • Caching

      • Prefetching

      • Hoarding

      • Eventual consistency

        • Assumption: simultaneous sharing other than read is rare.

      • Update conflict detection/resolution


    Caching l.jpg
    Caching Consistency and Currency

    • What to cache?

      • Entire files, directories, tables, objects

      • Portions of files, directories, tables, objects

    • When to cache? Is simple LRU sufficient?

      • LRU captures an aspect of temporal locality

      • Predictive/semantic caching: based on the semantics distance between data/request

        E.g., clustering of queries [Ren 99]


    Prefetching l.jpg
    Prefetching Consistency and Currency

    • Online strategy to improve performance

      • prepaging

      • prefetching of file

      • prefetching of database objects

    • What to fetch?

      • access tree (semantic structure)

      • probabilistic modeling of user behavior

    • Old idea that can be used during network idle times

    • Combine delayed writeback and prefetch


    Hoarding l.jpg
    Hoarding Consistency and Currency

    • Planned and Accidental disconnections are not considered failures.

    • New idea - Hoarding:

      a technique to reduce the cost of cache misses during disconnection.

      That is, load before disconnect and be ready.

    • How to do hoarding?

      • user-provided information (client-initiated disconnection)

        • explicitly specify which data

        • Implicitly based on the specified application

      • access structured-based (use past history)

        E.g., tree-based in file systems, access paths (joins) in DBs


    Hoarding in db systems l.jpg
    Hoarding in DB Systems Consistency and Currency

    • Granularity of Hoarding

      • RDBMS: ranges from tables, set of tables, whole relations

      • OO & OR DBMS: objects, set of objects or class

    • Hoard by issuing queries or materialized views

      • User may explicit issue hoarding queries

        E.g., Create View with Update-On clause [Lauzac 98]

        OO query to describe hoarding profiles [Gruber 94]

      • History of past references both queries and data objects

      • Hoard Keys - an extended database organization [Badrinath 98]

        • hoard keys are used to partition a relation in disjoint logical horizontal fragments


    Processing the log l.jpg
    Processing the Log Consistency and Currency

    • What information to keep in the log for effective reintegration and log optimization?

      • Data values, timestamps, operations

    • Goal: Keep the log size small to

      • Save memory

      • Reduce cost for update propagation and reintegration

    • When to optimize the log

      • Incrementally each time a new operation is added

      • Before propagation or integration

    • Optimizations are system specific

      • E.g., keep last write record, drop records of inverted operations


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    Cache Coherence/Data Consistency Consistency and Currency

    • "Lazy" or weak consistency promises high availability

      • Consider some conflicts (e.g., write-write conflicts)

      • Read-any/Write-any

    • Other schemes are costly:

      • Pessimistic replication schemes/Quorum schemes

      • Server-initiated callbacks for cache invalidation (e.g., Leases).

      • Optimistic replication schemes

    • System support for

      • detection of conflicts: version vector, timestamps

      • automatic resolution or manual resolution (tools)


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    Techniques to Increase Autonomy Consistency and Currency

    • Mobile Database Consistency

      • When a mobile database M shares a data item with another database D, it is involved in a global integrity constraint C.

      • Transactions on both M and D may suffer unbounded and unpredictable delays - No local commitment.

    • What about localizing the constraints – no global constraints?

    • Localization:

      reformulates C so that M accepts a local constraint C’ instead

      • Local transactions remain local.

      • When C’ is violated at a node, it requests the others for re-localization, i.e., a dynamic readjustment of C’.

        • No need for a distributed transaction.

        • No inconsistency from concurrent requests


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    Simple Example Consistency and Currency :

    Let x and y be two data items at two nodes.

    C = J.x + K.y > D is a global constraint.

    Localization yields two local constraints:

    x > L1 and y > L2

    where L1 and L2 are constants chosen such that J.L1 + K.L2 > D

    Re-localization: L1, L2 can be changed: node y increases L2 before node x decreases L1

    Localization of Constraints


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    Localization Methods Consistency and Currency

    • Escrowing:Logically partitions aggregated items

      • Escrow transactions [O’Neil 86]

      • Demarkation protocol [Barbara 91]

    • Geormetric Method [Mazumdar 99]: Enhanced Escrowing

      • It tackles quadratic inequalities

    • Fragmentation [Walborn 95]: Physically partitions item with constraints localized within the individual fragments

      • Fragmentable objects: fragments are merged to the originating position

      • Reorderable Objects: fragments can be re-organized during the merging


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    Two-tier Transaction Models Consistency and Currency

    • Tentatively Committed Transactions

      • Transactions tentatively commit on a mobile unit

      • Make their results locally visible leading to abort dependencies

      • Certification based on application or system defined criteria

      • invalidated trans. are aborted, reconcile, or compensated

    • Isolation-Only Transactions [Lu 94]

      • First-class transactions for connected operations

        • immediately commit at the server, globally serializable

      • Second-class transactions for disconnected operations

        • tentatively commit, locally serializable, no failure atomicity

        • validation criteria: global serializability, global certifiability

        • invalidated trans. are aborted, reexecuted, or compensated.


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    Two-tier transaction Models Consistency and Currency

    • Two-tier Replication [Gray 95]

      • Base transactions and Tentative transactions (disconnected)

      • Upon reconnection, tentative transactions are reprocessed as base transactions on master data version

      • Application semantics are used to increase concurrency and acceptance (e.g., commutative operations)

    • (Mobile) Escrow Transactions

      • Transactions are validated locally by localizing constraints

      • Local commitment ensures global commitment


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    Mobile Transactions Consistency and Currency

    • Distributed transactions involving both mobile and fixed hosts.

    • Traditional approaches are too restrictive.

    • Mobile Open Nested Transactions [Chrysanthis 93]

      Goals: sharing of partial results while in execution,

      maintaining computation state on a fixed host,

      moving transactions on/off a mobile host and across fixed hosts.

      • Components: Atomic transactions, Compensatable transaction, Reporting transactions and Co-transactions.

      • Properties: Component isolation, semantic atomicity Components may commit/abort independently


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    Mobile Transactions Consistency and Currency

    • Kangaroo Transactions [Dunham 97]

      • Transaction relocation is achieved by splitting the transaction during hand-off. One Joey transaction per cell.

    • The Clustering Model [Pitoura 95]

      • A distributed database is divided into weak and strict clusters

      • Data in a cluster are mutually consistent

      • Inconsistency between clusters is bounded and resolved by merging them either

        • during transaction commitments, or

        • when connectivity improves

      • A mobile transaction is decomposed into Strict and Weak transactions based on consistency requirements

      • Only strict transactions ensure durability and currency of reads


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    Failure Recovery Consistency and Currency

    • Emphasis has been on recording global checkpoints

      • Periodically store the state of a distributed application with mobile components.

    • DB Failure Recovery: Logging and checkpointing

    • Failures can be soft or hard

      • Soft failure can be recovered from the locally stored log and checkpoint

      • Hard failure require hard checkpoints stored in the fixed network.

    • Issues:

      • When to propagate the log and create a hard checkpoint?

      • Where to store hard checkpoints to speed up recovery and reduce its cost?


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    Database Interface Consistency and Currency

    • Desirable features:

      • Semantic simplicity: formulation of queries without special knowledge

      • Interaction with a pointing device

      • Disconnected query specification

    • QBI (Query By Icons) [Massari-Chrysanthis 95]

      • Iconic language requiring minimum typing

      • Semantic data model that hides details

      • Metaquery tools for query formulation during disconnections


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    Outline Consistency and Currency

    • Motivating Example

    • Issues: Mobility, Wireless Communication, Portability

    • Adaptability and Mobile Client-Server Models

    • Location Management

    • Broadcast data dissemination

    • Disconnected database operations

    • Mobile Access to the Web

    • Mobility in Workflow Systems

    • State of Mobile DB Industry and Research Projects

    • Unsolved Problems


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    Mobile Access to the Web Consistency and Currency

    • Three-tier Architectures: Client - Web Server - Data Server

    • Web Server can act like a server-side agent

      • Prefetching at its cache can hide some latency

      • Scripts at the Web server can perform user-specified filtering and processing.

    • Most solutions use a Web proxyto avoid any changes to the browsers and servers.

      • Pythia [Fox96]

      • Mobile Browser (MOWSER) [Joshi 96]

        • Distillation: highly lossy, real-time,datatype specific compression that preserves semantic content

      • WebExpress [Housel 97]


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    WebExpress Consistency and Currency

    • Utilizes the C-I-S Model

    • Goals: reduce traffic volume and reduce latency

    • Intercept any http request and perform four optimizations:

      • Caching at both CSA & SSA of graphics and html objects

      • Differencing: only changes are communicated

      • Long-live TCP/IP Connection: CSA & SSA use a single TCP connection

      • Header reduction: SSA includes the required browser capabilities. They are not sent by the CSA.

    • While disconnected (off-line mode) uses CSA cache


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    Advances in Mobile Web Servers Consistency and Currency

    • W4 for Wireless WWW [bartlett 94]: Mosaic on PDA

    • Dynamic Documents: Tcl scripts that execute within the mobile browser to customize the html documents

    • Dynamic URLs [Mobisaic 94]: They support mobile web servers and work with active pages.

    • IPiC [Shrinivasan 99]: A match head sized web server


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    Mobility in Workflows Consistency and Currency

    • Workflows are automated business processes.

      • involve coordinated execution of multiple long-running tasks or activities

    • Workflow system coordinates the workflow execution.

    • Processing entities (clients) are where the activities are executed and can be mobile.

      • disconnections among procesing entities (clients)


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    Workflow Disconnected Operations Consistency and Currency

    A pessimistic approach: Exotica

    • Prior to disconnection, each client:

      • reserves the activities it plans to work by locking

      • hoards the relative to the activities data (requests from the server the input containers of the activities)

    • During disconnection,

      • stores results in its local stable memory

    • Upon reconnection,

      • the results are reported back to the server


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    Mobile Agents in Workflows Consistency and Currency

    • A Mobile Agent Workflow Model: INCAS

    • No centralized workflow server

    • Each workflow process is model as a mobile agent called Information Carrier (INCA). Each INCA

      • encapsulates the private data of the workflow

      • carries a set of rules that control the flow between the activities of the INCA computation

      • maintains the history (log) of its execution

    • Each INCA is initially submitted to a procesisng entity (client) and roams among processing entities to achieve its goal


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    Outline Consistency and Currency

    • Motivating Example

    • Issues: Mobility, Wireless Communication, Portability

    • Adaptability and Mobile Client-Server Models

    • Location Management

    • Broadcast data dissemination

    • Disconnected database operations

    • Mobile Access to the Web

    • Mobility in Workflow Systems

    • State of Mobile DB Industry and Research Projects

    • Unsolved Problems


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    Mobility Middleware in the Market Consistency and Currency

    • Most middleware market are based on TCP/IP and socket-oriented connections

    • Wireless-friendly TCP versions have been proposed but no major products adopted it

    • Microsoft’s Remote Access supports cellular communication by integrating Shiva’s PPP suite

    • Shiva’s PPP (Point-to-Point protocol) suit provide a remote access client to either wired or mobile servers

      • E.g., mobile clients can access Tuxedo transaction services

    • MobileWare Office Server: An agent-based middleware that supports Lotus Notes, Web access, database replication, etc.

      • Connection profiles, checkpointing,compression, security


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    State of Mobile DB Industry Consistency and Currency

    • Sybase SQL Remote (Sybase SQL AnyWhere)

      • MobiLink: Centralized model to control replication

      • Application-specific bi-directional synchronization using scripts

      • UltraLite: in-memory dbms (50KB)

    • ORACLE

      • Oracle Mobile Agents middleware

      • Oracle 8 Lite: supports bi-directional replication between a client and a server (50-750KB)

      • Oracle Replication Manager: supports replication across multiple servers based on the peer-to-peer model

    • MS SQLServer

      • Merge replication and conflict resolution

      • Alternative clients: Outlook and MS ACCESS

    • IBM DB2 Everywhere (100KB)


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    Case Study: Coda Consistency and Currency

    • Client-Server System with two classes of replication w.r.t. consistency

    • Disconnected vs. Weakly connected

      • Hoarding, Caching/Server callback, No Prefetching

    • During connections: Allows AFS clients (Venus) to hoard files.

      • hierarchical, prioritized cache management  equilibrium.

      • track dependencies, bookmarks

    • During disconnections: Venus acts as (emulates) a server

      • generates (temp) fids, services request to hoarded files.

    • On reconnection, Venus integrates locally changed files to servers.

      • Considers only write-write conflicts - no notion of atomicity

      • User conflict resolution/ Application-aware adaptation [Odyssey]

      • Use optimistic replication technique


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    Case Study: Consistency in Bayou Consistency and Currency

    • A bottom-up approach to specific design problems

      • more distributed than coda, more emphasis on "small" clients

    • Key features:

      • read-any/write-any to enhance availability

      • anti-entropy protocol for eventual consistency

      • dependency checks on each write

        • dependency set

        • If queries (run at server) do return the expected results

        • Application-specific resolution of update conflicts

      • Primary server to commit writes and set order

      • Session consistency guarantees

    • How effective is anti-entropy?


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    Anti-entropy Protocol Consistency and Currency

    • Server propagates write among copies.

    • Eventual all copies "converge" towards the same state.

    • Eventual reach identical state if no new updates.

    • Pair-to-peer anti-entropy

      • each server periodically selects another server

      • exchange writes and agree on the performed order

      • reach identical state after performing the same writes in the same order.


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    Case Study: Rover Consistency and Currency

    • Rover [Joseph 97] provides an environment for the development of mobile applications

    • Applications are split into client and server part communicating with Queued RPCs

    • Application code and data are encapsulated within Relocatable Dynamic Objects (RDOs)

    • Access Managers at client and server handle RDOs

    • Client’s operational log is lazily transfer to the server

    • Disconnections are supported by the local cache

    • Some support for primary copy, optimistic consistency


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    Case Study: Pro-Motion Consistency and Currency

    • Pro-Motion [Chrysanthis 97] is designed for the development of mobile database applications.

    • It shares similar architecture as Rover with a multi-tier C-I-S model.

    • Compact is the unit of caching and hoarding

      • It encapsulates cached data, methods, consistency rules and obligations (e.g., deadlines).

    • Supports both tentatively committed transactions

      and two-tier replication.


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    Case Study: Rome Consistency and Currency

    • Rome [Fox 99] goals is the timely and in context delivery of information

    • Information should be received when and where it is needed

    • Its fundamental building block are the triggers:

      • pieces of data bundled with contextual information

      • Condition: (location  R)  (time  t) action

    • It is similar to active databases but with decentralized management

    • It provides an extensible framework and building blocks leveraging on internet service.


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    Unsolved Problems Consistency and Currency

    • Integration and evaluation of algorithms with applications

    • Broadcast disks

      • Information update/consistency and data temporal coherence- meet time constraints of requests

      • Relation between server broadcasting and client caching.

    • Multiple broadcast channels and multiple database access

    • Efficient, scalable, adaptive mechanisms

      • Location handling

      • Trigger management

    • Programmer Interface for Application-aware adaptation

      • Data fidelity vs. consistency

      • Semantic consistency needs metadata/requirements

    • Multimedia and QoS


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    To Recap Consistency and Currency

    Mobile and wireless computing attempts to deliver today’s and tomorrow’s applications on yesterday’s hardware and communication infrastructure!


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