Replication, Caching, Prefetching and Hoarding for Mobile Computing

1. Replication, Caching, Prefetching and Hoarding for Mobile Computing Mobile Data Access

2. Definitions • Replication: To maintain multiple (consistent) copies of a data item • Static replication: the number and location of copies are statically determined (at compile time, design time). • Dynamic replication: the number and location of copies is determined dynamically (at run-time) • Caching: To maintain a temporary copy of the data in fast (local) memory. The copy is fetched when it is first accessed. • Pre-fetching: To obtain a temporary before it is accessed (to hide access latency). • Hoarding: To preload a copy of a data object so that the mobile client can work while it is disconnected from the network (I.e. prefetching to tolerate disconnections). Mobile Data Access

3. Data Access Model • On-Demand • Broadcast Channel Mobile Data Access

4. Motivation • Caching (Prefetching/Hoarding) at mobile clients is crucial to improve performance of info access and database querying. • Issues: • Read only data: currency guarantees • Read/Write data: consistency in presence of disconnected operations • Server Load/Scalability in presence of numerous clients Mobile Data Access

5. Mobile Database Querying: Requirements • Minimize query delay • Maximize number of queries answered per unit time (system throughput) • Handle client disconnection • Conserve wireless bandwidth and battery power • Minimize server load • Handle mobility Mobile Data Access

6. Advantages of Caching in Mobile Environment • Helps reduce latency caused by narrow bandwidth wireless links • Enable limited functionality in mobile hosts even in disconnected mode • Helps conserve battery power by reducing the number of uplink queries • Conserves bandwidth Mobile Data Access

7. Problems in Maintaining Consistent Cache • Classic solutions do not work • Mobile Clients may be disconnected for long duration => invalidations may be lost • Upon reconnection mobile clients will have to revalidate their cache (wastes energy and bandwidth). • Need new solutions Mobile Data Access

8. Challenges to Efficient Caching Scheme • Efficient caching scheme should take into account: • Data access pattern • Data update rate • Communication/access cost • Mobility pattern of the clients • Connectivity characteristics • Disconnection frequency • Available bandwidth • Data currency requirements • Location-dependence of information Mobile Data Access

9. General Issues in Designing Caching Schemes • Where to cache? How many levels of caching to use? • What to cache (when to cache a data item and how long) ? • How to invalidate cached items? Who is responsible for invalidations? What is the granularity at which the invalidations are done? • What data currency guarantees the system can provide to the user? What are the costs involved? How to charge the user? • What is the effect of the caching scheme on the query delay (response time) and the system throughput (query completion rate)? Mobile Data Access

10. Classification of Cache Invalidation Schemes • Who is in charge of invalidations? • Server or Client (Push or Pull): Callbacks or Validation Checks • Whether or not server maintains per client state information? • Stateless or Stateful Server • How server sends invalidation reports? • Synchronously or Asynchronously • What kind of information is sent in the invalidation report? • State or History based • How information is organized in invalidation reports? • Uncompressed or Compressed Mobile Data Access

11. Cache Maintenance Schemes • Broadcasting Invalidation Reports [Barbara Sigmod 94]. • Disconnected Operation in CODA (Satyanarayanan et. al. ) • Hoarding (Prefetching) • AS (Asynchronous Stateful) Caching Scheme (Kahol et. al. ICDCS 00) Mobile Data Access

12. Broadcasting Invalidation Reports • Uses stateless servers and synchronous broadcasts [Barbara Sigmod 94] • Clients maintain local caches and use the information in invalidation reports to update their cache. • A server broadcasts invalidation reports every L time units which contains ids of all the data items which changed during the past w = kL time units. • A query is satisfies after receiving the next invalidation report. Mobile Data Access

13. Broadcasting IR: Variations • If a client is disconnected from the network and misses k consecutive invalidation reports then it has to discard its cache. • Two variations: • Timestamp Strategy (TS): invalidation reports contain ids of modified data items over a large window (k > 1). • Amnesic Terminal (AT): invalidation reports contain ids of only those data items which changed since the last broadcast (k=1). • TS is better when clients are “sleepers” and AT is better when clients are “workaholics”. Mobile Data Access

14. Disconnected Operation in CODA • Goal: COnstant Data Availability • Mechanisms: server replication and disconnected operations. • Caching scheme (asynchronous, stateful): • Uses callbacks while a client is reachable from a server. • During disconnections permits access to possibly stale data. • Upon reconnection, the client does validity checks on each volume cached. • Uses hoarding to improve data availability Mobile Data Access

15. Drawbacks • Drawbacks of Barbara’s scheme: • Poor delay characteristics due to waiting involved before answering a query. • Poor network utilization characteristics due to answering of queries in bursts. • Does not support arbitrary disconnection pattern. • Drawbacks of CODA caching scheme: • Server has to keep cache state of each client (affects scalability). • A client has to perform volume-by-volume validation check after each reconnection. Mobile Data Access

16. AS Caching Scheme (Kahol et al) • Maintains a Home Location Cache (HLC) at home MSS of a mobile client. • A HLC contains the state of the cache at a MH. • Uses Asynchronous transfer of invalidation reports. • Supports arbitrary disconnection durations by maintaining the timestamp of the last invalidation report destined for an MH at its HLC. Mobile Data Access

17. An Example for AS Scheme • Each cache is associated with a cache timestamp which is the timestamp of the last invalidation report received. • A mobile client sends a probe message to its home MSS when it gets connected to determine whether it missed any invalidation reports while it was disconnected. Mobile Data Access

18. Hoarding • Planned and accidental disconnections are not considered failures. • A technique to reduce the cost of cache misses during disconnection: • load necessary data before disconnect and be ready. • Hoarding techniques: • user-provided information (client-initiated disconnection) • explicitly specify which data (files, tables) to hoard • Implicitly based on the specified application • access structured-based (use past history) E.g., tree-based in file systems, access paths (joins) in databases Mobile Data Access

19. Hoarding versus Prefetching • Both pre-fetch data in anticipation of future use. • Prefetching • Objective is to improve performance (throughput or response time). • Cache miss is not catastrophic. • Hoarding • Objective is to fetch all needed data into MU cache prior to disconnect. Thus the goal is to facilitate disconnected operation. • Cache miss is catastrophic. • OK to overfetch Mobile Data Access

20. Hoarding in Database Systems • Granularity of Hoarding • RDBMS: ranges from tables, set of tables, whole relations • OO DBMS: objects, set of objects or class • Hoard by issuing queries or materialized views • User may explicit issue hoarding queries E.g., Create View with Update-On clause [Lauzac 98] OO query to describe hoarding profiles [Gruber 94] • History of past references both queries and data objects • Hoard Keys - an extended database organization [Badrinath 98] • hoard keys are used to partition a relation in disjoint logical horizontal fragments Mobile Data Access

21. References • D. Barbara and T. Imielinski, Sleepers and Workaholics: Caching Strategies in Mobile Environments, VLDB Journal, 4, 567-602, 1995. • A. Kahol, S. Khurana, S.K. S. Gupta, and P. K. Srimani, A Strategy to Manage Cache Consistency in a Disconnected Distributed Environment, IEEE Transactions on Parallel and Distributed Systems, 12(7), 686-700, July 2001 Mobile Data Access