Mobile Computing and Databases –A Survey Daniel Barbar á

Mobile Computing and Databases –A SurveyDaniel Barbará Pallavi Phene phene@cs.uh.edu November 12th, 2002

“People and their machine should be able to access information and communicate with each other easily and securely, in any medium or combination of media—voice, data, image, video, or multimedia—any time, anywhere, in a timely, cost-effective way” Dr. George H. Heilmeier IEEE Communication Mag. October 1992

MANETsMobile Ad-hoc NETworks Networks formed by Wireless, Mobile hosts without (necessarily) using a pre-existing infrastructure.

Problems a) Routes between nodes may potentially contain multiple hops

Problems b) Hidden Terminal Problem C A B

Problems c) Near Far Problem Near-by terminals over power signals from far-away terminals BS A B C

Problems d) Hand-offs Base Stations (BS) Mobile Host (MH)

Effect of Mobility • Physical Layer • Channel varies with user location and time • Data Link Layer • Reliable communication interrupted by bursts • Network Layer • Re-routing due to movement of hosts • Presentation Layer • Source coding for efficiency • Application Layer • Location Dependent Applications

Mobile Computing “In mobile networking, computing activities are not disrupted when the user changes the computer's point of attachment to the Internet. Instead, all the needed reconnection occurs automatically and non-interactively.” Charles E. PerkinsSun Microsystems

Outline • Characteristics of Wireless Networks • Mobile Computing and Databases • Data Dissemination • Data Consistency • Location Dependent Querying • Interfaces for Browsing • Challenges

A Mobile Environment

Characteristics of Wireless Networks • Asymmetry in Communications • Frequent Disconnections • Power Limitations • Screen Size

Mobile Computing and Databases • Asymmetry & Power Limitations • Data Dissemination • Asymmetry & Frequent Disconnections • Transaction Mgmt and Data Consistency • Frequent Disconnections • Location Dependent Querying • Screen Size • Interface for Browsing

Data Dissemination It is a push-based model for broadcasting data to the client rather than waiting for the client to request the specific data.

Data Dissemination • Advantage • no interruptions by the client requests; • send additional related data; • Disadvantage • figuring out the “relevant” additional data • Depends • On ability of the server to predict the clients’ needs

Types of push-architectures • Periodic Push • delivery is performed on a regular, repeating schedule • client can disconnect and still not lose data • Aperiodic Push • has no such schedule • assumes that clients are either always listening and able to respond to what is being sent • makes more effective use of the downstream communication channel

Methods of Data Dissemination • Broadcast Disks • Interleaved Push and Pull (IPP) • Invalidation Reports (IR)

Broadcast Disks • It is a periodic dissemination architecture • Multiple disks of different sizes are superimposed on the broadcast medium (Server Broadcast Programs) • Exploits the client storage resources for caching data (Client Cache Mgmt)

Server Broadcast Programs • Server takes the union of required items (pages) and broadcasts the resulting set cyclically. • Number of disks (num_disks) determine the number of different frequencies with which pages will be broadcast. • For each disk, the number of pages (num_pages(i)) and the relative frequency of broadcast (rel_freq(i)) are specified.

An Example • num_disks = 3 • Disk 1 • num_pages(1) = 1 • rel_freq(1) = 4 • Disk 2 • num_pages(2) = 2 • rel_freq(2) = 2 • Disk 3 • num_pages(3) = 8 • rel_freq(3) = 1

Client Cache Management • Clients cache the pages for which the local probability of access is higher than the frequency of broadcast.

Interleaved Push and Pull (IPP) • Integration of a pull-based and a push-based Broadcast Disk approach. • Communication Bandwidth shared between 2 Channels • Front-Channel • used by servers for push-based operations i.e. for maintaining Broadcast disks and providing responses to client pull operations • Back-Channel • used by clients for pull-based operations

Improving Scalability of IPP • Adjusting pull bandwidth • Providing a Pull threshold • Increase available pull bandwidth by chopping off the slowest part of the broadcast schedule

Invalidation Report (IR) • The server uses IRs to notify clients about changes in the items being cached by the clients • IRs contain only the changes in the data values; Static/unchanged data is not re-sent • IRs Can cause “false-negatives” • “Quasicopies” and “adaptive IRs” may be used

IRs can be used to check the validity if the items/groups in the clients cache using GCoRe (Group with Cold Update-Set Retention) • Can be combined with clients behavior to broadcast IRs only with a user’s mobility area

Broadcast Disks in Real-Time Environments • Real-time environments are time-critical • Broadcast disks do not accommodate transmission failures • Require a technique to add reliability to the Broadcast Disks technique • Organizations suitable for RT environments • Flat • Rate Monotonic • Slotted Rate Monotonic

Organization of Broadcast Disks to Ensure Fault Tolerance • Achieved using AIDA (Adaptive Information Dispersal Algorithm), which is an elaboration of IDA (Information Dispersal Algorithm) • AIDA uses minimum controlled redundancy to guarantee timeliness and fault tolerance up to any degree of confidence

IDA • Divides a file into N independent pieces, such that combining any m pieces is sufficient to retrieve the file Dispersal and Reconstruction of Information using IDA

AIDA • In AIDA a bandwidth allocation operation is inserted after the dispersal operation but prior to transmission • This allows the system to scale the amount of redundancy used in the transmission • Number of pieces transmitted n is allowed to vary from m (no redundancy) to N (maximum redundancy) • Using bandwidth allocation, the redundancy of unimportant items can be reduced and that of critical items increased

AIDA Dispersal and Reconstruction of Information using AIDA

An Example of a Flat Broadcast Program

Another Consideration: Directories • Indexing on Air • transmitting index of what data items are being broadcast • Advantage • allows clients to be inactive some of the time and hence save power • Distributed Indexing • sending only part of the Index • Methods of Distributed Indexing • Temporal Addresses • Multicast Addresses

Data Consistency “Consistency requires that data bound by a transaction be semantically preserved.”

Session Guarantees • Read Your Writes • Any read operation in the session must reflect the values established by previous writes in that session • Monotonic Reads • Successive reads reflect a non-decreasing set or writes • Writes Follow Reads • Writes are propagated after the reads on which they depend • Monotonic Writes • Writes are propagated after writes that logically precede them

Maintaining Data Consistency • Certification Reports • Isolation Only Transaction (IOT) • Data Replication

Certification Reports • Are used to support transaction management in mobile environments • CRs contain lists of items that are in the read and write set of active transactions • CRs are used by the clients to verify if the transactions being run by them need to be aborted • The server completes the verification only when the client cannot detect any conflict • If the transaction can commit, then the server will put the values in the database and notify the client

Isolation Only Transactions(IOT) • Guarantees only Consistency of Databases • When the transaction completes, it enters either a committed or pending state. • If in committed state, then the results are sent to the servers to be committed • If in pending state, the transactions is in wait state and is validated later

Data Replication • Required so that processing continues smoothly in case of disconnections • Technique requires referee nodes that update and store the core (updateable data items) set descriptions • Other issues • maintaining replication of directories (Primary by Row) • replication in weakly connected systems (Lazy Release Consistency)

Location Dependent Querying • Ad-hoc queries will be addressed to large databases • regarding the local area • e.g.: people, places, routes, services etc • location transparent • e.g.: recent sales figure of a particular product, who stores a particular product on his PDA etc.

Problem • Minimizing communication cost to retrieve the query result • Solutions • integrate GPS into IP to enable creation of location independent services • e.g. Genesis, Advanced Traveler Information Systems (ATIS), Mobisaic (using Dynamic URLs and Active Documents)

Interfaces for Browsing • Query by Icons (QBI) • query processing facility supporting exploration and querying of databases from a mobile computer based on the manipulation of icons • addresses the screen size, memory and battery power, communication bandwidth limitations • Features • Iconic visual language • Semantic Data Model • Meta-query tools

Architecture of QBI

QBI Interface

Query Example

Mobile Computing and Databases –A Survey Daniel Barbar á