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Recovery in database system new
1. Recovery in
Mobile
Database
System
Presented by:
PRAVEEN PALLAV
(1DS09IS061)
ISE - DSCE - 2013 1
2. RECOVERY IN MOBILE DATABASE SYSTEM
Outline
Introduction
Previouswork
System model
Recovery and its issue
Proposed recovery scheme
Advantages
Disadvantages
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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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Problem Specifications
Location is not fixed
Batteries have limited power
Different types of failures
Hard failure
Soft failure
Weak wireless link
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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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Recovering the MU state on its failure
at the time of handoff
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Key terms
Region: BSs under the same MSC.
Designated BS: First BS for a MU registration
inside a region.
MSC
BS1 BS3
BS2
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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Inter-region
MSC MSC
BS1 BS3 BS1 BS3
DBS
BS2 DBS
BS2
LOG
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Intra-region
Log transfer from MU cache to BS:
When?
EitherMU 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.
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Case 1 log management
MSC
DBS
BS1 BS3
MHID is with me
BS2
Connect(MHID, PBSID, DBSID)
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Case 2 log management
Going 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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Case 2 log management
MSC
MHID is with me
DBS
BS1 BS3
Since previous BS is DBS, hence no
BS2
delete message is send Connect(MHID, PBSID, DBSID)
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Case 3 log management
In 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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Case 3 log management
MSC
MHID is with me
BS4
DBS
Acknowledgement
Delete Connect(MHID, PBSID, DBSID)
BS3
BS2
Delete LOGS entry
the log
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Case 4 log management
When 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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Case 4 log management
MSC
PBSID==
BS4
DBS LOGS?
CBSID
MHID is with me
Append with previous
BS3
Log & delete the PBSID.
Connect(MHID, PBSID, DBSID)
BS2
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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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