recovery in database system new

Recovery in Mobile Database System

Presented by:PRAVEEN PALLAV

(1DS09IS061)

ISE - DSCE - 2013 1

Outline Introduction Previous work System model Recovery and its issue Proposed recovery scheme Advantages Disadvantages

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RECOVERY IN MOBILE DATABASE SYSTEM

Introduction Aim to provide a scheme which will help

a MU recover efficiently, taking reduced amount of time to recover from its last secure state

This new mobile computing environment poses challenging problems in designing fault-tolerant systems

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Motivation Telecommunication services need to improve the quality,

availability, and reliability of cellular services.

information accessing capability regardless of the location

Motivated by such growing demand, we envision an information processing system

Need of fault-tolerant systems which suit the mobile environment

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Previous work Lazy schemes

Pessimistic scheme

Schemes based on the MU’s movement

Distance based

Frequency based

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System model

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Problem Specifications

Location is not fixed

Batteries have limited power

Different types of failures

Hard failure

Soft failure

Weak wireless linkISE - DSCE – 2013

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Recovery

A mechanism to restore an intermediate consistent state of the system.

made possible by maintaining log

log management should be very efficient

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……. Appropriate recovery strategy needs to

be chosen

Failure rate

Memory constraints

Recovery time

Communication / Mobility Ratio

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PROPOSED RECOVERY SCHEME

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Recovering the MU state on its failure at the time of handoff

Key terms Region: BSs under the same MSC. Designated BS: First BS for a MU

registration inside a region.MSC

BS1

BS2

BS3

DBS

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……… Characteristics of DBS:

Store all the log entry done by respective MU.

Different DBS for different region.

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Inter-region Step 1: MU registers with a BS in a Region, changes

state of that BS as DBS for that MU. Step 2: MU does some transactions, stores the log at

respective DBS. Step 3: MU goes out of that region and comes under

another region and repeats the same registration procedure, making first registered BS as DBS for that region.

Step 4: Current DBS calls for the logs from the previous region DBS.

Step 5: Previous DBS transfers all the respective MU log to the new DBS.

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MSC

BS1

BS2

BS3

MSC

BS1

BS2

BS3

DBS DBSLOG

Inter-region

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Intra-region Log transfer from MU cache to BS:

When? Either MU memory is exhausted. Or a handoff is detected.

How? Log is updated to the current BS. It is appended to the DBS log.

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Log management Registration:

Connect(MHID, PBSID, DBSID) MHID: ID of the MU. PBSID: Previous BS’s ID. DBSID: ID of respective region’s DBS.

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Case 1 log management Step 1: MU registers with a BS in a Region, changes

state of that BS as DBS for that MU. Step 2: MU does some transactions, stores the log at

respective DBS. Step 3: MU goes out of DBS vicinity and comes under

BS2 cell. Step 4: MU registers itself to new BS with

connect(MHID, PBSID, DBSID). Step 5: BS2 sends a message to PBSID ‘MHID is with

me’. Step 6: Since the PBS is also the DBS for that MU,

hence no action taken and the message is discarded.ISE - DSCE – 2013

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MSC

BS1

BS2

BS3DBS

Connect(MHID, PBSID, DBSID)

MHID is with me

Case 1 log management

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Case 2 log managementGoing on in addition with previous scenario: Step 1: Now MU has moved from BS2 to

BS3, repeats the same registration process as in previous case and the PBS (BS2) is notified by the current BS (BS3) about that MU existence.

Step 2: Now BS2 checks for the PBS for that MU in its memory, since the PBS is the DBS hence no delete message or action required.

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MSC

BS1

BS2

BS3DBS


MHID is with me

Since previous BS is DBS, hence no delete message is send


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Case 3 log managementIn addition with previous scenario: Step 1: MU goes from BS3 to BS4 now and performs the

same registration process with BS4, BS4 notifies BS3 about MU.

Step 2: The with respect to that MU, PBSID stored at BS3 is not the DBS hence a ‘delete’ message from BS3 is sent to BS2.

Step 3: BS2 on receiving of ‘delete’ message transfers all log for that MU to the respective DBS.

Step 4: DBS acknowledges BS2 about log successful transfer.

Step 5: BS2 deletes respective MU’s log entry from its memory.

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MSC

BS2

BS3

DBSBS4


MHID is with me

Delete

LOGS

Acknowledgement

Delete the log entry


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Case 4 log managementWhen we further move on in this scenario: Step 1: MU comes back to the recent visited BS

forming a loop i.e. from BS4 to BS3, registration process is done in the same previous manner.

Step 2: BS4 checks whether the PBS and current BS of that MU in its memory are same? If yes, send the respective MU’s log to current BS i.e. logs are transferred from BS4 to BS3.

Step 3: BS3 appends the BS4 log with its current available log and BS3 deletes the PBSID for that MU from its memory.

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MSC

BS2

BS3

DBSBS4


MHID is with me

PBSID==CBSID

?LOGS

Append with previous Log & delete the

PBSID.

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Recovery case 1 Recovery in same BS where it crashed:

Uses log saved in current BS. If not sufficient go for previous BS. If that is also insufficient, go for MU’s DBS. By using DBS log MU is restored with

required previous state.

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Recovery case 2 Recovery in BS other than in which

failed: Current BS unaware of handoff. PBSID is unavailable. Old BS in which MU failed notifies

respective DBS about MU failure. When MU tries to recover, current BS

queries DBS for log. Using DBS log MU is recovered.

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Advantages 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.

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Disadvantage DBS with many MUs gets overburdened. Single point of failure.

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THANK YOUISE - DSCE – 2013

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