Transaction Processing in Mobile Distributed Databases Sherida Jacob CSC 536 5/2/2005

Distributed Database Distributed Database Client/Server type controlled by a central DBMS Client/Server type controlled by a central DBMS Portions of the DB stored on multiple computers Portions of the DB stored on multiple computers Users can access data without interfering with each other Users can access data without interfering with each other DBMS operates like there is only one computer DBMS operates like there is only one computer

Mobile Distributed Database DDB used by wireless devices DDB used by wireless devices Focus is on location awareness as opposed to transparency Focus is on location awareness as opposed to transparency Ex. Requesting a bus timetable from a laptop and having the result sent to your mobile phone via text message Ex. Requesting a bus timetable from a laptop and having the result sent to your mobile phone via text message

Mobile Computing Environment Wired Network

Data Transmission Problems in a MC Environment Battery power Battery power Low bandwidth Low bandwidth Frequent disconnects – topology changes Frequent disconnects – topology changes Limited storage space Limited storage space Location dependency – result changes as you move Location dependency – result changes as you move

Transaction Characteristics in a MC Environment An interaction that can access or change the DB An interaction that can access or change the DB Longer processing times Longer processing times Execute on heterogeneous networks Execute on heterogeneous networks Can execute when not connected to network Can execute when not connected to network

Do not strictly adhere to ACID rules Do not strictly adhere to ACID rules Atomicity – transactions are split up Atomicity – transactions are split up Consistency – questionable atomicity and durability Consistency – questionable atomicity and durability Isolation – application specific Isolation – application specific Durability – too many disconnections Durability – too many disconnections Transaction Characteristics in a MC environment cont’d

Transaction Processing Problems Deadlock detection – impedes, costly Deadlock detection – impedes, costly Disconnections – identify fault tolerance levels Disconnections – identify fault tolerance levels Recovery – isolation rule is relaxed, recovery may not be possible Recovery – isolation rule is relaxed, recovery may not be possible Replication – costly, method needed Replication – costly, method needed Validation – impedes, costly Validation – impedes, costly

Concurrency – ability to handle multiple transactions Concurrency – ability to handle multiple transactions Security – use encrypting algorithms and data logs Security – use encrypting algorithms and data logs Fragmentation – how and where Fragmentation – how and where Transaction Processing Problems cont’d

Transaction Management Models No consensus on how to execute a mobile transaction No consensus on how to execute a mobile transaction Many models to resolve some of the issues Many models to resolve some of the issues

Kangaroo Transaction Model A kangaroo transaction is a global transaction A kangaroo transaction is a global transaction KT’s are split into Joey transactions KT’s are split into Joey transactions JT’s are executed independently at MSS’s JT’s are executed independently at MSS’s JT’s split as the MH moves to new cells JT’s split as the MH moves to new cells

Kangaroo Model cont’d Execution status and recovery info for the uncommitted transactions are forwarded to the MH as it moves Execution status and recovery info for the uncommitted transactions are forwarded to the MH as it moves A KT is successful if the last order of execution of the JT commits A KT is successful if the last order of execution of the JT commits Ex. Airplanes communicating with air traffic control Ex. Airplanes communicating with air traffic control

Cluster Model DB’s are grouped into clusters depending on their network links or mutually consistent data DB’s are grouped into clusters depending on their network links or mutually consistent data Mobile transactions sent from the MH for processing are broken into 2 types – weak and strict Mobile transactions sent from the MH for processing are broken into 2 types – weak and strict Strict trans can read/write data on any cluster while the MH is connected Strict trans can read/write data on any cluster while the MH is connected Weak trans can only access data within their cluster. Weak trans can only access data within their cluster. Fixed Network

Cluster Model Proxies are used to mirror the trans. on MSS’s as the MH moves Proxies are used to mirror the trans. on MSS’s as the MH moves Inconsistency is allowed between clusters but not within them Inconsistency is allowed between clusters but not within them When the clusters are merged the weak transactions are committed across the clusters as long as they do not conflict with any strict transactions When the clusters are merged the weak transactions are committed across the clusters as long as they do not conflict with any strict transactions Ex wireless subscribers, sports, word of the day Ex wireless subscribers, sports, word of the day Fixed Network

PRO-Motion (Pro-active Management) Supports disconnected transaction processing Supports disconnected transaction processing Can execute if it has the needed data and methods Can execute if it has the needed data and methods Compacts enable local executions on the MH Compacts enable local executions on the MH Compacts provide support for dynamic replication for escrowable items and improved caching techniques for non-escrowable items Compacts provide support for dynamic replication for escrowable items and improved caching techniques for non-escrowable items

PRO-Motion (Pro-active Management) While connected the MH identifies a group of compacts that are updated by locally committed sub-transactions While connected the MH identifies a group of compacts that are updated by locally committed sub-transactions It caches the compacts it needs then disconnects to process the trans, it can be committed locally It caches the compacts it needs then disconnects to process the trans, it can be committed locally When the MH reconnects to the network it resynchronizes with the FH When the MH reconnects to the network it resynchronizes with the FH The compacts are sent back to the DBS The compacts are sent back to the DBS A commit/abort message is sent back to the MH A commit/abort message is sent back to the MH Ex. UPS deliveries Ex. UPS deliveries

PRE-Write Model Increases data availability at MH’s, good for MH’s with little processing power Increases data availability at MH’s, good for MH’s with little processing power Allows an MH to submit a pre-committed state of the trans to be executed at the FH or other MH’s at a later time Allows an MH to submit a pre-committed state of the trans to be executed at the FH or other MH’s at a later time A pre-write op. announces the value or future state that the data object will have after the commit of the write op. A pre-write op. announces the value or future state that the data object will have after the commit of the write op.

Pre-Write Model Pre-committing makes all the data items that will be updated at the commit phase visible to other transactions Pre-committing makes all the data items that will be updated at the commit phase visible to other transactions Permanent updates on the DB are performed later by the write operation at commitment time Permanent updates on the DB are performed later by the write operation at commitment time Ex. Newspaper reporter Ex. Newspaper reporter MH Fixed Network MSS FH Start transaction Perform Reads/Pre-writes Pre-commit WriteCommit

TCOT (Time out based commitment) PROTOCOL Supports weakly connected less powerful MH’s Supports weakly connected less powerful MH’s Assumes MH has cache transaction processing power Assumes MH has cache transaction processing power Guarantees the complete execution of fragments of a mobile transaction within in a predefined timeframe Guarantees the complete execution of fragments of a mobile transaction within in a predefined timeframe

TCOT Protocol Fixed Network MSS FH MSS t6 t5 t3 t4 T T t1 T T T MSS T t2 MH extracts a sub-transaction from the MT, estimates a timeout period, and sends the rest of the MT to the MSS MH extracts a sub-transaction from the MT, estimates a timeout period, and sends the rest of the MT to the MSS The MSS distributes it to a FH and other MH’s for further fragmentation The MSS distributes it to a FH and other MH’s for further fragmentation At the end of each timeout period the fragments can be committed independently At the end of each timeout period the fragments can be committed independently If the FH does not receive a message from the MH or other executing nodes it aborts the trans and sends an abort message to all the nodes If the FH does not receive a message from the MH or other executing nodes it aborts the trans and sends an abort message to all the nodes

MANET (Mobile Ad Hoc Network) Model Self-organizing, infra-structureless, multi-hop network, uses spread spectrum techniques for security Self-organizing, infra-structureless, multi-hop network, uses spread spectrum techniques for security Every MH acts like a router, has a radius of influence Every MH acts like a router, has a radius of influence If a route is broken a route error message is sent to the MH that uses that route If a route is broken a route error message is sent to the MH that uses that route Uses three types of routing algorithms Uses three types of routing algorithms Proactive – routes already known Proactive – routes already known Reactive – routes found only when needed Reactive – routes found only when needed Hybrid – uses proactive and reactive techniques Hybrid – uses proactive and reactive techniques

MANET Model Ex. Battle field surveillance Ex. Battle field surveillance

Reporting and Co-transactions Model Assumes the MH never disconnects only moves from cell to cell, a GDBS exists at each MSS Assumes the MH never disconnects only moves from cell to cell, a GDBS exists at each MSS Reporting trans are sub-transactions that are able to share their partial results at any time with the parent transaction Reporting trans are sub-transactions that are able to share their partial results at any time with the parent transaction Co-trans are reporting trans that behave like co-routines Co-trans are reporting trans that behave like co-routines Reporting trans and Co- trans can exchange info with each other – parallel or step-wise Reporting trans and Co- trans can exchange info with each other – parallel or step-wise Able to relocate their executions to new MSS’s as the MH moves Able to relocate their executions to new MSS’s as the MH moves MSS’s remain in the computation state to minimize costs MSS’s remain in the computation state to minimize costs

Reporting and Co-Transactions Update commitments are dependent on the transaction receiver (MH) Update commitments are dependent on the transaction receiver (MH) If the MH commits or aborts the reporting action will either have its results permanently committed or aborted If the MH commits or aborts the reporting action will either have its results permanently committed or aborted Ex. Online insurance policies Ex. Online insurance policies

Semantic Based Model Assumes MT’s are long lived tasks plagued by frequent disconnections, network delays Assumes MT’s are long lived tasks plagued by frequent disconnections, network delays Reduces demand on bandwidth, utilizes cache Reduces demand on bandwidth, utilizes cache Design Phase - at MH, defines applications and transaction model Design Phase - at MH, defines applications and transaction model Execution Phase - at FH, constructs the transaction model Execution Phase - at FH, constructs the transaction model

Semantic Based Model Queues and stacks are split into fragments Queues and stacks are split into fragments Split is based on selection criteria and consistency conditions Split is based on selection criteria and consistency conditions MH requests fragments from the server MH requests fragments from the server Processed transaction is returned and then merged Processed transaction is returned and then merged Ex. Booking a vacation online Ex. Booking a vacation online