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Mobile Databases Mobile computing Portable computing devices and wireless communications Can access data from anywhere, anytime Example: Brokerage services News reporting Traffic/Vehicle services Mobile DB

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mobile computing
Mobile computing
  • Portable computing devices and wireless communications
  • Can access data from anywhere, anytime
  • Example:
    • Brokerage services
    • News reporting
    • Traffic/Vehicle services
mobile db
Mobile DB
  • Mobile database – data management technology enabling use of databases on mobile computing environment
    • Data available anywhere independent of availability of fixed network
      • Can access public data using internet browser
      • Can access private data through distributed DB
    • Data on mobile and fixed hosts sharable in seamless way
      • More complex techniques needed to support this – distributed transaction processing, commit
identifying mobile characteristics
Identifying Mobile characteristics
  • Origins in distributed systems
  • Problems more challenging
    • Asymmetric communication bandwidth
    • Limited and intermittent connectivity
    • Limited life of power supply of mobile units
    • Changing topology of network
  • Mobile database assumes a traditional database requiring ACID properties
mobile databases
Mobile databases
  • How to guarantee ACID properties
  • Environment requires new strategies for:
    • Processing transactions
    • Concurrency control - caching
    • Data dissemination
    • Querying – location dependency
mobile computing architecture
Mobile Computing Architecture
  • Mobile units MU or Mobile Hosts MH
  • Fixed hosts FH on fixed network
  • Base Station BS –serves as gateway to fixed and wireless network
  • Geographic mobility domain divided into cells
  • Mobile host wireless connection to BS of cell
  • Movement of mobile units unrestricted
  • Must maintain info for access contiguity
mobile db7
Mobile DB
  • Mobile DB mix of fixed and wireless network
    • DBS distributed among wired and wireless componen
    • Data management shared among base stations, fixed hosts and mobile units
  • MU – can be data client and/or data server
    • If a server, with DBMS functionality
      • minimally need R, W, C, A
slide8

DB

DB

mobile db9
Mobile DB
  • When Mobile DB mix of fixed and wireless network
    • Fixed – FH location, high capacity, reliability, low connection cost
    • Wireless – support dynamic network topology, low capacity, reliability, high connection cost
transaction
Transaction
  • What is a transaction?
    • A Transaction is not always just an SQL query
    • A transaction is also:
      • From the time you login to SQL Plus until you exit
mobile strategies
Mobile strategies
      • Provide data cache on mobile host
  • Cache replicas of frequently accessed data
  • Work offline
  • Reduce power consumption
  • Client may be unreachable
    • Dozing - energy conserving state
    • Out of reach
    • Proxies – used for unreachable (e.g. update info)
    • What if data cached updated during disconnection?
mobile strategies12
Mobile strategies
  • Resources of MU can be limited
    • Mobile hosts personalized
    • Bring in fraction of data need to access
  • MU has low security
  • Mobile DBs high degree of unavailability
  • Broadcasting accepted way to disseminate data
mobile db conservative
Mobile DB - Conservative
  • Can assume entire DB distributed among wired components
    • Full or partial replication
    • Base station or fixed host has DBMS functionality
    • Must be able to locate mobile units
    • Need query and transaction management features for mobile environment
mobile db conservative14
Mobile DB - Conservative
  • How is this different from distributed non-mobile?
    • Difficult to maintain sustained connection to server
    • Database server typically is stateless, especially under broadcast systems
    • Mobile clients often cannot maintain a sustained network connection
manet extreme db
MANET – Extreme DB
  • Mobile adhoc networks
  • MUs do not need to communicate via a fixed network
  • In MANET, MU responsible for routing own data, acting as BS
  • Must be able to handle changes in network topology
manet extreme db16
MANET – Extreme DB
  • Peer-to-peer
  • No central control
  • Difficult for transaction processing and data consistency
  • Example applications:
    • Multi-user games
    • Shared white-boards
    • Battle information sharing
    • Distributed calendars
mobile dbs best of both
Mobile DBs – Best of both
  • Alternatively assume DB distributed among wired and wireless components
    • What if MU has DBMS functionality?
      • MU can be laptop
    • Data management shared among base stations, fixed hosts and mobile units
  • More interesting problems!! But solvable
data management issues
Data Management Issues
  • Environment requires new strategies for:
    • Querying – location dependency
    • Concurrency control
    • Processing transactions
    • Security
    • Data dissemination
    • Recovery/fault tolerance
query processing
Query processing
  • Must know location of data
  • Query optimization more difficult because of mobility and resource changes of MU
  • MU may be in transit or may cross cell boundaries
location based services
Location-based services
  • Location dependent cache information may become stale
  • Frequently updated location dependent queries
  • Apply spatial queries to refresh cache problem
transaction models
Transaction models
  • Mobile transaction may execute on several BSs
  • Central coordination lacking if data distributed among wireless components
  • Long lived transactions
  • ACID properties difficult to guarantee
    • Can add proxies for unreachable components
      • Proxies keep track of updates to cache
data distribution and replication
Data distribution and replication
  • Data unevenly distributed among BS and MU
  • To compensate for high latency and unreliable connectivity
    • Frequently accessed data is cached
    • Can work offline if necessary
  • Consistency constraints and cache management
recovery and fault tolerance
Recovery and Fault tolerance
  • Site, media, transaction and communication failures
  • Voluntary shutdown not a site failure
  • Transaction failures can occur during handoff
security
Security
  • Mobile data less secure than data at fixed location
  • Data is more volatile
  • Must manage and authorize access to critical data
data dissemination broadcasting
Data Dissemination -Broadcasting

Assumptions:

Requests are read-only (Most are)

  • Because of latency, server can handle fewer clients in same amount of time
  • Broadcasting acceptable solution
    • Scalable – single broadcast of data item can satisfy all outstanding requests for data item
data dissemination broadcasting27
Data Dissemination -Broadcasting

Assumptions:

Requests are read-only (Most are)

  • Because of latency, server can handle fewer clients in same amount of time
  • Broadcasting acceptable solution
    • Scalable – single broadcast of data item can satisfy all outstanding requests for data item
broadcasting
Broadcast-based data dissemination approaches

Push-based data broadcasting

Pull-based data broadcasting

Hybrid data broadcasting

Broadcasting
push based broadcasting
Push-based broadcasting
  • Data contents within a file or database are repeatedly broadcast through the broadcast channel
  • channel becomes a “disk”
  • clients can retrieve data as it goes by
  • expected wait time for a data item is the same
broadcast disks
Broadcast Disks
  • broadcast data in different frequencies according to their relevant importance
  • multi-level memory hierarchy
  • hot data are broadcast more frequently then cold data
  • Data with similar access frequency are grouped into disks
pull based broadcast
Pull-based broadcast
  • also called adaptive approaches
  • data items are broadcast on-demand
  • only requested data will appear as “data on air”
pull based
Pull-based
  • Data broadcasting is prioritized according to some metrics
  • Most common algorithms are:
    • First come First Served (FCFS): broadcasts the pages in the order they are requested.
    • Most Requests First (MRF): broadcasts the page with maximum number of pending requests.
    • Longest Wait First (LWF): selects the page that has the largest total waiting time, i.e., the sum of the time that all pending request for the item have been waiting. (R*W is approximation)
pull based35
Pull-based
  • MRF – best response time at high system loads and page requests uniformly distributed
  • LWF – best response time when page request distributed by Zipf
hybrid data broadcasting
Hybrid data Broadcasting
  • mixes both push and pull
  • clients to send pull requests for misses on the backchannel
  • server supports a Broadcast Disk plus interleaved responses to the pulls on the front channel
  • alleviate the problem of excessively long waiting time for some data
indexing
Indexing
  • Clients can save battery power by turning into active mode only when interested data are broadcast
  • (1, m) index method (Imielinski, et al. )
    • Index is broadcast m times during the broadcast of one version of the file
data consistency
Data Consistency
  • Assumption: Read and Write transactions
  • Challenges in mobile environments
    • Difficult to maintain sustained connection to server
    • Database server typically is stateless, especially under broadcast systems
    • Mobile clients often cannot maintain a sustained network connection
    • How to ensure conflict serializability?
research in data consistency
Research in Data Consistency
  • Assumptions:
    • Read and Write transactions
    • MU has DBMS functionality
  • Mobile unit may often experience voluntary/involuntary disconnections
  • Then, it can only read and update data copied onto their local cache
  • What if data cached updated during disconnection?
concurrency control
Concurrency Control
  • Two-tier replication algorithm (Gray et al. 1996)
  • Tentative and Base transactions
    • Tentative transactions are transactions executed over local copies if disconnected
      • tentative transaction will be submitted to the server and reprocessed before final installation
      • Can be aborted by the server due to conflicts with other transactions
    • Base transactions (transactions work only on master data)
      • transaction becomes durable when the base transaction completes
    • Drawback – deadlocks, system unscalable
concurrency control42
Certification reports - CR (Barbara, ’97)

Consists of the read/write sets of recently committed transactions

Broadcast periodically by the server

Clients execute part of validation work locally

Must submit to server for final validation

Concurrency Control
concurrency control43
Concurrency Control
  • Optimistic Concurrency Control with Update Timestamp (OCC-UTS)
    • Server broadcasts invalidation report (IR), which contains new timestamps of newly updated data items
    • If any accessed data item in a local executing transaction has an older timestamp, the local transactions is aborted
mobile databases44
Mobile Databases
  • Research Issues – 2007 MDM
    • Atomic commit protocol
    • Data dissemination
    • Spatio-temporal range queries
    • Adhoc networks - data integrity, data replication
slide45

Ph. D. student: Weigang Ni

Data Management in Adaptive Broadcast Environments

lazy data request ldr
Lazy Data Request (LDR)
  • Pull-based data broadcasting – data are broadcast on demand (Stathatos, et al. )
  • Scheduling algorithms
    • First Come First Serve (FCFS)
    • Most Requests First (MRF)
    • Longest Wait First (LWF)
    • Requests times Wait (R * W)
    • Other algorithms based broadcast histories, estimation of the probabilities of access for each data item.
slide47
LDR
  • Existing algorithms mainly concern data access time.
  • Whenever a client has a data request, it sends the request to the server – Eager Data Request (EDR).
  • Sending message consumes more battery power than receiving message.
slide48
LDR
  • Motivation: wanted data may have already been requested by other clients. Why not wait instead. Two possible results.
  • Issues need to be addressed
    • Mobile clients do not communicate with each other. Therefore, they cannot decide whether to wait or go ahead and send the request
    • The system load changes dynamically. A predefined waiting time will not work well.
slide49
LDR
  • Features of Lazy Data Request
    • Client do not need to contact the server to get the system load information and waiting time.
    • The waiting time is dynamically changing according to system load.
    • LDR approach can apply to nearly all the existing on-demand broadcast algorithms
server side algorithm of ldr
Server-side algorithm of LDR
  • Step1. Let n be the total number of requested data items
  • Step 2. Choose *n data items to be broadcast next based on some scheduling algorithm (0 <  ≤ 1)
  • Step 3. Clear all existing requests for these *n data times.
  • Step 4. Broadcast the index section consisting of these *n data items.
  • Step 5. Broadcast the data items.
    • Eg. Will broadcast ( *100)% of data items
client side algorithm of ldr
Client-side algorithm of LDR

Wait until wanted data or index section is broadcast

If wanted data items come

download the data

drop the local pending request

else

check the index section

if wanted data ids in index section

wait until data are broadcast

else

send the pending request(s) to the server

discussion
Discussion
  • Algorithm still work without using index. However, index makes the data broadcast more predictable and further saves the data request messages.
  • Adjust the value of ,
    • If  = 1, LDR becomes first come first served (FCFS) algorithm
    • If  is very small, LDR virtually becomes EDR as every time only a couple of data items are chosen
  • Client waiting time is bounded.
system parameters
System Parameters
  • Parameter Description Value
  • Dbsize The number of items in DB 1000
  •  Mean request arrival rate (exp) 10 ~ 100
  •  Skewness of access pattern (zipf) 0.1 ~ 0.9
  •  Selection factor 0.1
impact on hot data
Impact on hot data
  • Requests saved for hot data
  • More likely hot data broadcast before requested
  • MRF has 50% reduction in messages
  • FCFS, < 20% (better for cold data
why is it faster
Why is it faster?
  • Saving the requests for hot data essentially changes the access skewness of data requests sent to the server, i.e., the access pattern appears to be more evenly distributed than it actually is
ldr conclusions
LDR Conclusions
  • LDR decreases number of messages sent
  • Decreases average data access time
    • By up to 50% for both
    • Data access time does not increase
      • Data access time actually decreased by over 50%
      • Due mainly to cold data
  • Works with a variety of scheduling algorithms
overall conclusions
Overall Conclusions
  • Data management in mobile environments:
    • Concurrency control
      • Algorithms proposed produce serializable histories
      • Outperform existing algorithms
    • Adaptive data broadcasting
      • Algorithm proposed shows number of data request messages can be reduced
      • Data access time does not increase
virtual locks
Virtual Locks
  • Lock-based concurrency control approach
  • Using server authorization to eliminate transaction restarts
    • Authorization information is broadcast in the broadcast cycle header
  • Treat read-only and update transactions differently
virtual locks cont
Virtual Locks (cont.)
  • Server schedules the data broadcast in the following way:
    • Read-only transaction’s data requests will be satisfied unconditionally
    • Update transaction’s data requests are satisfied only if they pass the conflict resolution.
virtual locks cont67
Virtual Locks (cont.)
  • Read-only transaction
    • Does not need explicit server authorization
    • May not send data requests to the server as long as the required data is “on the air” within one broadcast cycle.
    • Commit locally
virtual locks cont68
Virtual Locks (cont.)
  • Update transaction
    • Client always sends the data requests to the server
    • Can only proceed when it is authorized to begin, i.e., its transaction id appears on the air
    • Transaction will be submitted to the server for final installment
study
Study
  • Proved conflict serializable
  • Compare performance to existing strategies
    • CR – certification report
      • Receive RW sets from committed transactions
      • Must always request validation from server, even if only read
read only transactions
Read-only transactions

tune into the first index segment in current broadcast cycle;

if (read_set  BC_SET and no data in read_set has been broadcast)

download data in read_set from current

broadcast cycle;

else

send a data request message to the server;

while (true)

wait till the beginning of next broadcast

cycle;

if (read_set  BC_SET)

download the data in read_set;

break;

process the transaction and commit locally;

write transactions
Write Transactions

send a data request message to the server;

while (true)

wait till the next broadcast cycle;

if (tran_id  auth_list)

download the data in read_set and write_set;

process the transaction;

send the old and updated value to the

server for commit;

break;

vl conclusions
VL Conclusions
  • Locking strategy VL better than optimistic CR
  • Optimistic causes large amount of aborted transactions
  • VL better for both uniform and skewed data access pattern
  • VL better scalability
optimistic concurrency control with dynamic adjusting timestamp ordering occ dto
Optimistic Concurrency Control with Dynamic Adjusting Timestamp Ordering (OCC-DTO)
  • Problems with existing algorithms
    • Each transaction must be submitted to the server for validation regardless whether it is read-only or update transaction.
    • During the transaction execution, if it misses a CR or IR, transaction must be aborted.
    • Many read-only transactions can finish even if they find that accessed data have been updated.
existing algorithms
Existing algorithms
  • optimistic concurrency control with updatetimestamp (OCC-UTS)
    • uses timestamps and caching to improve the system performance.
    • Different from the CR approach, only updated data items are broadcast in the invalidation report (IR).
problems with existing algorithms
Problems with existing algorithms
  • CR and OOC-UTS algorithms can cause many transactions to be restarted due to conflicts with updated data items.
  • we propose these transactions be completed without restarting by dynamically adjusting the serialization order among transactions.
wasted abort
Wasted Abort
  • Server commits T1
  • T3 and T4 are aborted due to the conflict on data item y.
  • Only T2 proceeds to finish
  • if we adjust the serialization order between T1 and T3 so that T3T1, we find that transaction T3 also can proceed without aborting.
proposed strategy occ dto
Proposed strategy - OCC-DTO
  • Features of OCC-DTO
  • Clients are allowed to disconnect during the execution of a transaction.
  • Read-only and update transactions are handled differently.
    • Dynamically set transaction threshold timestamp if conflict between read-set and IR
    • Read-only transactions are allowed to commit locally.
    • Read-only transactions do not have to restart if there is a conflict.
  • Proved global serialization is still maintained.
occ dto server side algorithm
OCC-DTO server-side algorithm
  • Validate submitted updated transactions
  • Commit the transactions passing the validation test
  • Broadcast data and invalidation report (IR)
    • Committed transactions
    • Aborted transactions
    • Updated data items (with new timestamps)
occ dto client side algorithm
OCC-DTO client-side algorithm
  • Each transaction has two variables thresh_ts () and adjusted (false)
  • Upon reading data item x:
    • if adjusted is true and ts(x) > tresh_ts, abort; else continue to execute
  • Upon writing data item x:
    • If adjusted is true, abort transaction
extended to include reconnection
Extended to include reconnection
  • Handle mobile clients disconnection
    • A client disconnects from the network for various reasons.
    • It may miss IRs during disconnection.
    • In earlier algorithms, transactions will be aborted if any IR is missed.
  • Upon reconnection, a client will reset its timestamp equal to the maximum ts of its accessed data item if adjusted is false.
performance
Performance

Baseline Restart Ratio Performance of OCC_DTO

occ dto conclusions
OCC-DTO Conclusions
  • Dynamically adjust serialization order using timestamps to allow transactions to complete without restarts
  • Read commit locally, same uplink bandwidth
  • Better performance than CR – fewer restarts