Investigating Distributed Investigating Distributed Database SystemsDatabase Systems

Challenges and Technology

Kishore Puppala Rao

DefinitionsDefinitions

A database is a logically related collection of data, stored in one or many files

A distributed database is a collection of multiple, logically interrelated databases distributed over a computer network

ArchitectureArchitecture

Client/server architecturesMultiple clients, single server – this is the

most common and straightforward implementation

Multiple clients, multiple servers – more flexible. DB distributed over multiple servers. Each client directs requests to a “home” server.

Architecture (cont’d)Architecture (cont’d)

DB is physically distributed by fragmenting and replicating data (discussed later)

Regardless of architecture, implementation details of queries, transactions and DB operations should be transparent to users.

Architecture (Peer-to-peer)Architecture (Peer-to-peer)

No distinction between client and serverEach site has functionality of both client

and serverE.g. File-sharing apps such as BearShare,

LiveWireSophisticated protocols needed to manage

data distributed across multiple sites

FragmentationFragmentation

Partitions the dataSubdivides each relation either vertically

(by project operation) or horizontally (by selection operation)

Facilitates the placement of data close to its place of use, reducing transmission costs

ReplicationReplication

Refers to duplication of data for access and/or security purposes

Fragments or whole database may be replicated

Replication involves keeping physical separate copies of data at different sites

Distributed vs. ParallelDistributed vs. Parallel

Distributed DBMS are not parallel DBMS, although distinction may be unclear

Distributed DBMS assume loose connection between processors operating independently, perhaps under different operating systems

Parallel DBMSParallel DBMS

Multiple processors under same operating system.

Architecture: Shared-none, shared-disk, or shared memory

Shared-Nothing: Each processor has exclusive access to its main memory and disk. Each processing element (PE) is a local site.

Parallel DBMS (cont’d)Parallel DBMS (cont’d)

Shared-memory: Each PE has access to any memory module or disk through some fast connection (e.g. LAN or cross-bar switch)

Shared-disk: Each PE has exclusive access to its own memory, but shared access to any disk via a fast connection. PE accesses DB pages on shared disk and copy to local cache

TransparencyTransparency

Distributed (and Parallel) DBMS must provide same functionality and consistency of centralized DBMS.

Transparency implies presenting a consistent view that shields the user from implementation details such as fragmentation, replication, and distribution.

Introduces major challenges

ChallengesChallenges

Query processing and optimizationConcurrency controlReliability protocolsReplication protocols

Query Processing and Query Processing and OptimizationOptimization

Techniques needed to address difficulties arising from data distribution and fragmentation. Localization techniques employed.

Algebraic queries on global relations are transformed to operate on fragments

Opportunities for parallel processing are identified (fragments are stored at different sites), unnecessary work is eliminated (not all fragments may be involved in the query)

Query optimizationQuery optimization

Determining the execution sites for distributed operations

Identifying the best distributed algorithm for distributed operations

Changing the order of operations in a query

Concurrency ControlConcurrency Control

Challenge in synchronizing user transactions is to extend serializability and concurrency to the distributed execution environment

Serializability: The ability to perform a set of operations in parallel with the same effect as if they were performed in a certain sequence, requires:

(a) execution of the set of transactions at each site must be serializable

(b) the serialization orders of these transactions at all these sites must be identical

Concurrency (cont’d)Concurrency (cont’d)

If locking-based algorithms used, lock management may be centralized or distributed

Deadlocks must be avoidedDeadlock detection and management in a

distributed database can be difficult

Reliability protocolsReliability protocols

Several types of failures: System, media, transaction, communication

May be difficult to differentiate type of failure

Distributed reliability protocols enforce transaction atomicity (commit all or commit nothing)

Reliability (cont’d)Reliability (cont’d)

E.g. of Atomic commitment protocol: Two-phase commit

All sites involved in the execution of a distributed transaction must agree to commit the transaction before it is made permanent.

Replication protocolsReplication protocols

Each logical data item has a number of physical instances

Challenge is to maintain (or approximate) consistency among physical copies as user updates logical data

Example criterion: One-copy equivalence – All physical copies of logical data should be equivalent after being updated by a transaction

Read-One/Write All (ROWA) protocol – enforces one-copy equivalence. Disadvantage: failure of one site may block entire transaction

Replication (cont’d)Replication (cont’d)

Alternative algorithms relax ROWA by mapping each write to a subset of the physical copies

Quorum-based voting: Copies are assigned votes; read and (especially) write operations have to collect votes and reach a quorum to commit data. (see class notes)

Research and TrendsResearch and Trends

Workflow models (advanced transaction models)

Network scaling problemsMulti-database systems and interoperabilityDistributed object management

Trends (cont’d)Trends (cont’d)

Primitive objects are not simple-structured data. Can consist of programs, voice, images, etc.

Distributed DBMS must handle increasingly larger data objects. E.g. 1MB storage needed for 1 digital X-Ray image (1024x1024) @ 8 bits/pixel

Most commercial DBMS (e.g. MS SQL Server 2000, Oracle 8i) provide some sort of distribution

Emergence of broadband networks eliminates the network as a bottleneck

Trends (cont’d)Trends (cont’d)

Mobile computing is escalating in interest and prevalence

Mobile stations may download data as needed Alternatively, more powerful mobile stations may

store native data for sharing with others Mobility raises issues of address migration,

maintenance of directories, and determining the location of stations

Object-oriented DBMS e.g. CORBA (platform independent), COM/OLE (MS-specific)

CORBACORBA

Common Object Request Broker Architecture

Facilitates the maintenance and DB access of data from a number of autonomous and heterogeneous sources (e.g. file systems, spreadsheets) via a multidatabase approach

Provides a generic platform for distributed computing

CORBA (cont’d)CORBA (cont’d)

In multidatabase systems, the main problem is the heterogeneity extant at four levels: platform, communication, database system, and semantic.

CORBA facilitates implementation transparency by providing client access via interfaces defined in a special Interface Definition Language (IDL), independent of the databases actual software and hardware environment.

Provides location transparency, allowing clients to access DB objects independent of location and communication protocols

CORBA (cont’d)CORBA (cont’d)

Provides a common interface to mask heterogeneity among native database system implementations based on different data models (e.g. flat-file, relational, spreadsheet) and query languages

Common interface overcomes semantic conflicts such as schema and data conflicts

ReferencesReferences

M.T. Ozsu and P. Valduriez, "Distributed and Parallel Database Systems – Technology and Current State-of-the-Art", ACM Computing Surveys, 28(1): 125 - 128, March 1996.

A. Dogac, C. Dengi and M.T. Ozsu, "Distributed Object Computing Platforms", Communications of ACM, 41(9): 95-103, September 1998.

J. N. Gray, “Notes on Data Base Operating Systems.” Operating Systems: An Advanced Course. R. Bayer, R.M. Graham (eds.) New York: Springer-Verlag, 1979, pp. 393-481.

References (cont’d)References (cont’d)

M.T. Ozsu, "The Push/Pull Effect - Can Distributed Database Technology Meet The Challenges of New Applications?", Database Programming & Design, April 1997.

Thank youThank you