M obile Transactions and Synchronization M obile Transactions and - PowerPoint PPT Presentation

M obile Transactions and Synchronization M obile Transactions and Synchronization Can Türker ETH Zurich S eminar "Ubiquitous Information", 28.1 1 .2001

... towards Pervasive Computing ... towards Pervasive Computing • Gartner Group: "by 2003 more than 1 37 million business users will be involved in some form of remote work" • Accenture: "by 2005 over 500 million mobile devices will offer Internet access" • Conclusions – mobile hosts (M H; laptops, palmtops, smart phones, etc.) outstrips fixed hosts (FH; personal computers, desktops, etc.) – the way information is created and processed will change within this increasingly ubiquitous network • Need – infrastructure to coordinate concurrent information access and processing in the presence of mobile hosts and users 2

Characteristics of M obility Characteristics of M obility • M obile information sources and consumers – physical access point to the network may change: sources as well as consumers may move – sources as well as consumers may be disconnected • User and Context Awareness – tracking/ monitoring information sources and consumers – consumer's information needs may shift with location change • Data management techniques have to be revisited Resource Limitations (Bandwidth, Resource Limitations (Bandwidth, Optimization + Careful Optimization + Careful M emory, Computing Power, ... ) M emory, Computing Power, ... ) Resource Sharing Resource Sharing Scalability Scalability Correctness Concerns Correctness Concerns Transactional Guarantees Transactional Guarantees Combining M any Sources Data Integration Combining M any Sources Data Integration 3

M ovements and Disconnections M ovements and Disconnections MH m ovem ent MH m ovem ent Laptop Laptop Laptop Disconnected Palmtop Data MH Data MH Disconnected base base Laptop disconnection disconnection M obile Data Phone base Data (WAP) base MH MH Data reconnection reconnection base CELL CELL M obile M obile Support Support Station Station Terminals Fixed Network Disconnected Fixed Laptop Fixed Disconnected Host Database Host Fixed Cluster Host Data Data base Data FH FH base base User/ User/ FH FH disconnection disconnection W orkspace reconnection W orkspace reconnection m ovem ent 4 m ovem ent

Abstractions Abstractions Abstraction of Data Storage (Relational) DBMS Abstraction of Concurrency DBMS & TP-Monitors with Concurrency & System Failures Control & Recovery Abstraction of Object-relational DBMS with Object Method I mplementations Methods, Triggers & Stored Procedures Abstraction of Distribution, Distributed & Federated DBMS, Data Heterogeneity & Autonomy Integration, Conflict Resolution Abstraction of Movements Mobile DBMS, Context Maintenance, & Disconnections Replication & Synchronization, Profiling 5

Transparency of M obility Transparency of M obility • Challenge: information access and processing everywhere and at anytime while supporting – transparent disconnections and – transparent movements of users and information components � How much transparency is indeed needed and reasonable? How much transparency is indeed needed and reasonable? � 6

Characteristics of Transactions Characteristics of Transactions • Logically related set of operations executed under certain guarantees, e.g., – atomicity – consistency – isolation – durability • Advanced transaction models rely on multi-tier transactions – increase parallelism by exploiting application semantics – parent-child relationships • M obile Transactions involve execution/ initiation on M H – ACID cannot be supported generally – nevertheless: certain transactional guarantees shall be ensured always and everywhere 7

Transaction Tiers Transaction Tiers Laptop Laptop Mobile Mobile Transactions Transactions Data Data base base Global Global CELL CELL M obile M obile Support Support Subtransactions Subtransactions Station Station Terminals Fixed Network Local Local Fixed Laptop Fixed Disconnected Host Database Transactions Host Transactions Fixed Cluster Host Data Data base Data base base 8

Disconnection- -resistant Transactions resistant Transactions Disconnection Laptop Data Start Transaction Start Transaction Disconnect Disconnect Execute Execute Execute Commit Commit Execute Reconnect Reconnect base & Execute & Execute Operation 3 Operation 3 & Sync & Sync Operation 2 Operation 2 Operation 1 Operation 1 Operation 1 Operation 1 CELL CELL M obile M obile Operation 2 Operation 2 Support Support Operation 3 Operation 3 Station Station Terminals Fixed Network Fixed Laptop Fixed Disconnected Host Database Host Fixed Cluster Host Data Data base Data base base 9

Disconnections Disconnections • "The flight worker The flight worker": Working in the "intended" disconnected mode requires some lazy replication techniques – updates are precommitted locally transparent to the user? – precommitted updates are propagated asynchronously when reconnected to the network • conflicts may occur • conflict resolution when a conflict arises • Conflict detection via timestamps, version vectors, etc. • Conflict handling – optimistic (resolution): function-based, manual – pessimistic (avoidance): primary copy, ROWA, quorum 1 0

M ovement- -resistant Transactions resistant Transactions M ovement MH m ovem ent MH m ovem ent Laptop Laptop join leave join leave Data Data Execute Execute Commit Commit Start Transaction Start Transaction Execute Execute base base Operation 3 Operation 3 & Execute & Execute Operation 2 Operation 2 Operation 1 Operation 1 Operation 3 Operation 3 Operation 1 Operation 1 CELL CELL M obile M obile Operation 2 Operation 2 Support Support Station Station Terminals hand- - off off hand Fixed Network Fixed Laptop Fixed Disconnected Host Database Host Fixed Cluster Host Data Data base Data base base 1 1

M ovements M ovements • "The train/ tram/ bus worker The train/ tram/ bus worker": Working while physically moving requires transparent support of cell migration and "unintended" disconnections – create subtransactions on several mobile support stations – coordinate these substranactions correctly transparent to – wireless communication and cost issues the user? • "The home worker The home worker": Resume and/ or continue work at another host (mobile or fixed) – continue transactions – create new subtransactions (within the existing workspace / transaction sphere) 1 2

Issues of M obile Transaction M odels Issues of M obile Transaction M odels • M H may have transaction processing capabilities – M H may run transactions locally – M H may only initiate transactions on FH • M H may change location and network connection while transactions are being executed – split computations • M H may disconnect while transactions are being executed – long-lived processes – data replication 1 3

Characteristics of Advanced Transaction M odels Characteristics of Advanced Transaction M odels • closed vs. open – child's results are visible to the parent only or to all? • vital vs. non-vital – parent's commitment depends on the child's commitment? • dependent vs. independent – child's commitment depends on the parent's commitment? • substitutable vs. non-substitutable – does there exists an alternative transaction? • compensatable vs. non-compensatable – are the results semantically undoable? 1 4

An Example of a M obile Transaction M odel: An Example of a M obile Transaction M odel: Kangaroo Transactions Kangaroo Transactions Kangaroo Transaction M H Level Hop Hop J oey J oey J oey M SS T1 T2 T3 Level FH T1 1 T1 2 T1 3 T21 T22 T31 Level • On disconnect: J Ts and Ts can complete but no more Ts are created • On reconnect: the T resumes • Hand-offs: new J T runs on the new M SS (transaction split) 1 5

An Example of a M obile Transaction M odel: An Example of a M obile Transaction M odel: Kangaroo Transactions Kangaroo Transactions • Builds on open nested and split transaction models • Supports mobility and disconnections • M H starts a Kangaroo Transaction (KT) – A J oey Transaction (J T) is started at the connected M SS – T run on the FH (as open nested transaction) – If the M H changes location, the previous J T is split and a new J T runs on the new location M SS – J T1 can commit independently from J T2 • If J T1 fails: – Compensating mode: undoes the entire KT – Split mode: • Previously committed J Ts are not compensated • No new J Ts are initiated • Current child transactions are committed or aborted upon decision of the local DBM S 1 6