report on recovery in mobile database system

RECOVERY IN MOBILE DATABASE SYSTEM

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TABLE OF CONTENT

ABSTRACT

INTRODUCTION

PREVIOUS WORK

SYSTEM MODEL

PROBLEM SPECIFICATION

METHODOLOGY

PROPOSED RECOVERY SCHEME

MERITS AND DEMERITS

FUTURE ASPECTS

CONCLUSION

REFERENCES


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ABSTRACT

Mobile computing is a rapidly emerging trend in distributed computing. This

new mobile computing environment poses challenging problems in designing

fault-tolerant systems because of the dynamics of mobility, and limited

bandwidth available on wireless links. Traditional fault-tolerance schemes,

therefore, cannot be directly applied to these systems. Mobile systems are often

subject to environmental conditions which can cause loss of communication or

data. Presented here is a scheme for recovery upon failure of a mobile host.

This report portrays the limitations of the mobile wireless environment and

improvement over existing recovery scheme is presented which suit the mobile

environment.


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INTRODUCTION

The portable computers are fast catching up with the desktop workstations in

terms of computing power, memory, display and disk storage. This gives users

the information accessing capability regardless of the location.

Telecommunication companies are continuously improving the quality,

security, availability, and reliability of cellular services and trying to enhance

its scope by providing new services like data management capabilities, which is

highly desirable. Motivated by such growing demand, an information processing

system is referred as the Mobile Database system (MDS). Mobile Database

System (MDS), in essence is a distributed client/server database system where

the entire processing environment is mobile. The actual database may be static

and stored at multiple sites but the data processing nodes, such as laptop,

PDA, cell phones, etc., may be mobile and can access desired data to process

transactions from anywhere and at any time. In this report the main aim i s to

provide a scheme which will help a Mobile Unit (MU) recover efficiently,

taking reduced amount of time to recover from its last secure state. In the

process a strategy for efficiently managing a unified log for a MU is provided.


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PREVIOUS WORK

LAZY SCHEMES:- Lazy and Pessimistic schemes (asynchronous

schemes) are reported in [8]. In a lazy scheme, logs are stored in the

base station (BS) and, if the MU moves to a new BS, a pointer to the old

BS is stored in the new BS. Pointers can be used during failure to

recover the log distributed over several BS. This scheme has the

advantage that it incurs relatively less network overhead during handoff

as no log information needs to be transferred. Unfortunately, this

scheme has a large recovery time.

PESSIMISTIC SCHEME:- In the pessimistic scheme, the entire log

and checkpoint record, if any, are transferred at each handoff. Hence,

the recovery is fast but each handoff requires large volume of data

transfer.

SCHEMES BASED ON THE MU’S MOVEMENT:- In these

schemes, the list of BSs where the log is distributed is transferred during

a handoff. This schemes fail when a MU recovers in a BS different than

the one in which it crashed.

Distance based:- In the distance-based scheme, logunification is done when the distance covered by MUincreases above a predefined value.

Frequency based :- In the frequency-based scheme, logunification is performed when the number of handoffs seenby the MU increases above a predefined value.


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SYSTEM MODEL

Mobile networks have two components, a fixed backbone and a wireless

network. There are two distinct sets of entities, namely, mobile hosts and fixed

hosts. A host that can move while retaining its network connection is a mobile

host. The static network comprises of the fixed hosts and the communication

links between them. Some of the fixed hosts, called base stations (BS) are

augmented with a wireless interface, and, they provide a gateway for

communication between the wireless network and the static network.

Due to the limited range of the wireless transceivers, a MU can communicate

with a BS only within a limited geographical region around it. This region is

referred to as a Cell. The geographical area covered by a cell is a function of

the medium used for wireless communication. Due to mobility, the MU may

cross the boundary between two cells while being active. Thus, the task of

forwarding data between the static network and the mobile host must be

transferred to the new cell's BS. This process is known as Handoff.


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PROBLEM SPECIFICATIONMDS recovery process is significantly more complex than conventional

systems mainly because of the following unique aspects of Mobile Environment

LOCATION IS NOT FIXED:- As the user moves from one place to

another, the location of the MU in the network changes.

DISCONNECTION:- A MU can get disconnected. While disconnected,

the mobile host is not able to send or receive any message. Protocols

that need to exchange messages will not work correctly in this situation.

During disconnection, the checkpoint protocol should provide a local

recovery mechanism that allows the MU to recover from its own failures.

BATTERIES HAVE LIMITED POWER:- The MU is often powered by

batteries. Network transmissions and disk accesses are the two major

power consuming operations. To minimize power consumption, the

checkpoint protocol should reduce the amount of information that it

adds to the application messages, and it should avoid sending extra

messages.

DIFFERENT TYPES OF FAILURES:- MU failure can be separated

into two different categories.

Hard Failure:- It includes all failures that can not be

repaired; for example, the MU falls and breaks, or is lost

or stolen.

Soft Failure:- It does not permanently damage the MU; for

example, the battery is discharged and the memory

contents are lost, or the operating system crashes.


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RANDOM HANDOFF :- MU’s may be subjected to handoff randomly.

A handoff may affect recovery mainly because the location of the desired

MU may not be immediately available for communication.

WEAK WIRELESS LINK :- The BS and the MU are connected with an

unreliable wireless link which is usually slow.


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METHODOLOGY

Recovery is a mechanism to restore an intermediate consistent state of the

system which may be useful to avoid loosing all the work performed. Recovery

is made possible by maintaining log. Thus, an efficient recovery scheme

requires that the log management should be very efficient and must consume

minimum system resources and recreate the execution environment as soon

as possible after MU reboots.

The entire log must be carefully managed to help MU recover from every

possible failure in minimum time. Now Depending on the system specifications

and requirements, the appropriate recovery and handoff strategy needs to be

chosen. There are several factors which affect the recovery:-

FAILURE RATE OF THE HOST:- System failures are caused by

defects introduced in manufacturing or by transient or permanent faults

occurring during operation.

COMMUNICATION/MOBILITY RATIO:- Communication refers to

the number of messages sent/received by the mobile host and mobility

refers to the number of moves the MU makes in a given period of time.

MESSAGE SIZE:- Transmitting and receiving data consumes

additional power. In general, transmitting a given amount of data

consumes twice as much power as receiving the same amount of data.

MEMORY CONSTRAINTS:- A BS generally has many MUs in its cell. It

is necessary to evaluate average memory requirements based on statistical

data and the recovery schemes used.


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RECOVERY TIME: This is essentially the time required to recover a

process upon failure. If the process has hard real-time deadlines, or

requires high availability, the recovery upon a failure should be quick.


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PROPOSED RECOVERY SCHEME

In our approach we concentrate on recovering the MU state on its failure at the

time of handoff. A region in our approach is a set of all BS controlled by a

Mobile Switching Center. Designated Base Station(DBS) is the station where a

MU registers itself for first time within a Region. It has following

characteristics:-

1. All log activity performed by a MU will be ultimately collected at this DBS

2. In every Region MU will have a different DBS.

Concept of Designated Base Station

INTER-REGION STRATEGY:- Amongst the region the approach works in

a way so as to avoid extra communication steps when MU moves from one

region to a new region. In this case MU has some log maintained at DBS in

Region1, now when the MU moves to Region 2, the first BS with which MH

registers becomes its DBS for new Region say Region 2. So Log will be

transferred from Region1 to Region2.


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Region (R)1 Region (R)2

Inter Region Strategy

INTRA-REGION STRATEGY

LOG TRANSFER FROM MOBILE CACHE TO BS:- When should

MU transfer the entire gathered log to the BS. There are

two cases, first of all if suppose MU’s cache is exhausted,

immediately entire Log will be copied to current BS. In

other case whenever MU moves away from the current BS

and system detects Handoff, the MU will copy the entire log

to BS and will move to new cell and the log would be

appended to the previous Log file.

Log Transfer from Mobile Cache to Base Station

Log appended


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LOG MANAGEMENT:- MU carries following information for the

purpose of registration-

1. Previous BS identity.

2. DBS identity.

3. Own identity in the form of Connect (MHid, PBSid, DBSid).When any MU registers with a new BS which is not the DBS then the

new BS say BS1 informs DBS about its reachability, saying ‘MU is with

me’. Now since this message is received by DBS so it won’t do anything,

just discard that message. The sequence of messages is shown in the

figure below

Case 1 of Log ManagementNow when MU moves from BS1 to BS2, then BS2 informs BS1 about

its reachability. After receiving this info BS1 won’t pass delete

message to DBS as BS1 knows that previous BS for the MU is DBS

where all Log is collected. Now when MU moves further to another BS

after handoff, say BS3 then again the same procedure is repeated. BS3

will inform BS2 about MU’s reachability. Then BS2 will send delete

message to BS1.After receiving the Delete message BS1 copies the

entire log for MU which exists locally over to DBS and after it gets

acknowledgement for data transfer it deletes that log entry from local

space.


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Now if MU moves back to BS2 from BS3 then:-

1. BS2 informs BS3 about MU’s reachability.

2. BS3 checks if MU’s previous BS i.e. BS2 and its current BS are same.

If both the BS are same then:

- BS3 won’t send delete message to BS2.

- BS3 will append Log to BS2 log.

- BS3 will delete its copy of log.

- BS2 clears its previous BS identity.

- From BS2 MU will continue with original chain of activities.

Case 2 of Log Management


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Depending on where the MU recovers, there exist two cases for recovery:

Case 1: If MU recovers in same BS where it crashed, then, that MU can

use Log stored at that BS to rollback to previous state. If that log is not

sufficient, then MU will need Log from its Previous BS. If even that is

also not enough, then BS will contact MU’s DBS to restore MU with all

its previous state.

Case 2: If MU fails in one BS, goes to another BS, and recovers there,

then Handoff does not happen and also the new BS does not know

about Previous BS identity. In this case the BS in which MU fails

notifies DBS about the same and whenever MU tries to recover, the new

BS fires query to DBS asking about the log for MU.



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MERITS AND OF PROPOSED SCHEME

Preceding BS log maintenance gives latest recovery option.

Unification at DBS avoids redundancy of logs.

Loop is tackled.

Auto unification of logs at one site helps:

Quick recovery.

Achieve real time system deadlines.

Didn’t require to traverse a link list

DEMERITS OF PROPOSED SCHEME

DBS with many MUs gets overburdened.

Single point of failure.


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FUTURE ASPECTS

For more efficient management of recovery information and faster recovery,

some other schemes can be employed which uses checkpoints and message

logs together in order to guarantee recovery and overcome the cost of storing

entire log at one place. The movement- based scheme can also be used which

considers Mobile Switching Center as a stable storage which considers both

of the failure-free operation cost and the recovery cost. Maintaining entire log

at one place in long run may make BS to run out of space so strategies should

be developed for Garbage Collection to flush the log at regular intervals.


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CONCLUSION

In this paper, an efficient recovery scheme based on message logging for mobile

computing systems is presented. The mobile host carrying its recovery

information to its current BS can recover instantly in case of a failure. In the

proposed approach, the recovery information is not dispersed over a wide range

of cells so the recovery cost is not too high. As described in the scheme, while

the mobile host moves within a region, recovery information of the mobile host

is transferred among the BS of that region. However, if the mobile host moves

out of the region, then DBS transfers the recovery information to another

region’s DBS. The scheme controls the transfer cost as well as the recovery

cost.


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REFERENCES

[1] Nuno Neves, W. Kent Fuchs, “Adaptive Recovery for Mobile Environments”,

ACM Press, Vol.40, Issue 1, pp. 68 – 74, 1997.

[2] Bin Yao, W. Kent Fuchs, Kuo-Feng Ssu. "Message Logging in Mobile

Computing", IEEE Twenty-Ninth Annual International Symposium on Fault-

Tolerant Computing, 1999.

[3] Cris Pedregal Martin, Krithi Ramamritham, “Recovery Guarantees in

Mobile Systems”, Proc. of the 1st ACM international workshop on Data

Engineering for Wireless and Mobile Access, pp. 22-28, 1999.

[4] Cris Pedregal-Martin, Krithi Ramamritham, “Support for Recovery in

Mobile Systems”, IEEE Transactions on Computers, v.51 no.10, pp. 1219-

1224, October 2002.

[5] Dhiraj K, Pradhan, P. Krishna, Nitin H. Vaidya, “Recoverable Mobile

Environments: Design and Trade-off Analysis”, ACM Press, 2001.

[6] Taesoon Park, Namyoon Woo, Heon Y. Yeom, “An Efficient Recovery

Scheme for Mobile Computing Environment”, ICPADS.

[7] Taesoon Park, Namyoon Woo, Heon Y. Yeom,“An EfficientOptimistic

Message Logging Scheme for the Recoverable Mobile Computing Systems”,

ISAS-SCI (1), 2001.

[8] P. Krishna, N. H. Vaidya and D. K. Pradhan, "Recovery in Distributed Mobile

Environments", IEEE Workshop on Advances in Parallel and Distributed

Systems, 1993.

[9] Sashidhar Gadiraju, Vijay Kumar, “Recovery in the Mobile Wireless

Environment Using Mobile Agents”, IEEE Transaction on Mobile Computing,

Vol. 3, No. 2, pp. 180-191.

[10] R. Koo, and S. Toueg,“Checkpointing and Rollback-Recovery for

Distributed Systems”,IEEE Trans. Software Eng.,Vol.13,No.1, pp. 23-31, 1987.


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[11] L. Alvisi, and K. Marzullo, “Message Logging: Pessimistic, Optimistic,

Causal, and Optimal”, IEEE Trans. Software Eng., Vol. 24, No. 2, pp. 149-159,

1998.

[12] H. Higaki, M. Takizawa, “Checkpoint Recovery Protocol for Reliable Mobile

Systems”, the 17th IEEE Symposium on Reliable Distributed Systems.

[13]CHENG-MIN LIN AND CHYI-REN DOW, “Efficient Checkpoint-based Failure

Recovery Techniques in Mobile Computing Systems”, Journal of information

science and engineering 17, pp. 549-573, 2001.

[14] T. Park, N. Woo, H. Yeom, “Efficient Recovery Information Management

Schemes for the Fault Tolerant Mobile Computing Systems”, 20th IEEE

Symposium on Reliable Distributed Systems (SRDS'01).


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not even use the network's internet connection for Web surfing or other online

activities, this is referred to as piggybacking. [2] The idea behind this is to just

find the network and see that it exist, but don't connect to it in any way. This

will prevent you from breaking the law, because connecting to it is stealing and

is illegal by today's laws. Just because as a wardriving you don't mean to

connect doesn't mean that it is illegal. This can be a problem with some client

adapters that auto connect to the a network whenever it comes into range. This

is still illegal to connect to any of there non-public access points without

permission. The keys on this part is a wardriver is responsible for the hardware

that they choose, so make sure that it is running as expected and don't get

caught in a situation where something illegal could be taking place. The main

thing with the legal issues here is that wardriving is legal and can be beneficial,

but this does not mean that everything a wardriver does can be ethical.

Wardrivers should take the knowledge they gain from doing such task and help

notify people of their vulnerabilities. Having all this information on networks in

a given area brings into some ethical issues that a wardriver must make sure

they follow, because a wardriver has a database of all these vulnerable

networks, it makes it easy for them to get caught into ethical dilemmas. This

information can be both used for good and for bad. The good notifying people of

their vulnerabilities and the bad taking advantage of these networks or selling

information about them to others who might take advantage of this

information. This is each users responsibility to ask themselves, is what I'm

doing ethical. If the answer is no or even shady it is time to stop doing it and


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think about the purpose of wardriving and the effects each wardriver has on

the technology community.

WARDRIVING METHODOLOGY

THE RIG

A “rig” refers to all of the hardware and software components used while

wardriving. The forum is a valuable resource, especially to new wardrivers,

because they may learn what components are effective and not effective for

wardriving. This forum also allows security professionals to observe how the

activity i s progressing over time and may even assist in recognizing a

wardriver on the road.

COMPUTER SYSTEM

The foundation of every wardriving rig is a computer system. Most wardrivers

choose to use either a laptop or PDA since these devices are relatively light and

portable and they may function on batteries if necessary. A simple power

inverter may be used with the cigarette lighter adaptor found in most vehicles

to provide power for the entire rig.

The selection of a laptop or PDA will affect your available scanning software

options. An x86-based laptop may utilize software on several Linux and Wi

ndows operating systems. A PowerPC-based laptop will be able to uti lize

software on Mac OS X or Yellow Dog Linux. W ith a Pocket PC PDA, one would

be restricted to software that runs on Familiar Linux or Microsoft Pocket PC,

while Palm OS devices do not have any relevant software availabl e for

wardriving at the moment.


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report on recovery in mobile database system

Technology

mobile hosts

mobile systems

mobile unit mu

mobile database system

existing recovery scheme

mobile databasesystem

wireless network

new bs