Distributed and mobile DBMSs Transparencies. ©Pearson Education 2009 Chapter 16 - Objectives Main...
-
Upload
barnard-hodge -
Category
Documents
-
view
219 -
download
1
Transcript of Distributed and mobile DBMSs Transparencies. ©Pearson Education 2009 Chapter 16 - Objectives Main...
Distributed and mobile DBMSsTransparencies
©Pearson Education 2009
Chapter 16 - ObjectivesMain concepts of distributed DBMSs
(DDBMSs)Differences between DDBMSs and distributed
processing.Advantages and disadvantages of distributed
databases.Main issues associated with distributed
database design.How fragmentation should be carried out.The importance of allocation and replication.
2
©Pearson Education 2009
Chapter 16 - ObjectivesLevels of transparency that should be
provided by DDBMS.Main concepts associated with database
replication.Main concepts associated with mobile
DBMSs.
3
©Pearson Education 2009
DDBMS concepts Distributed Database
A logically interrelated collection of shared data (and a description of this data), physically distributed over a computer network.
Distributed DBMSSoftware system that permits the management of the distributed database and makes the distribution transparent to users.
4
©Pearson Education 2009
DDBMS conceptsCollection of logically-related shared data.Data split into fragments.Fragments may be replicated.Fragments/replicas allocated to sites.Sites linked by a communications network.Data at each site is under control of a DBMS.DBMSs handle local applications
autonomously.Each DBMS participates in at least one global
application.
5
©Pearson Education 2009
Distributed DBMS
6
©Pearson Education 2009
Distributed processingA centralized database that can be accessed
over a computer network.
7
©Pearson Education 2009
Advantages of DDBMSs
Reflects organizational structure
Improved shareability and local autonomy
Improved availability
Improved reliability
Improved performance
Economics
Modular growth.
8
©Pearson Education 2009
Disadvantages of DDBMSs
Complexity
Cost
Security
Integrity control more difficult
Lack of standards
Lack of experience
Database design more complex.
9
©Pearson Education 2009
Types of DDBMSHomogeneous DDBMS
All sites use same DBMS product. Much easier to design and manage. Approach provides incremental growth and
allows increased performance.Heterogeneous DDBMS
Sites may run different DBMS products, with possibly different underlying data models.
Occurs when sites have implemented their own databases and integration is considered later.
Translations required. Typical solution: gateways.
10
©Pearson Education 2009
Multidatabase system (MDBS)
DDBMS in which each site maintains complete autonomy.
DBMS that resides transparently on top of existing database and file systems and presents a single database to its users.
Allows users to access and share data without requiring physical database integration.
Unfederated MDBS (no local users) and federated MDBS.
11
©Pearson Education 2009
Taxonomy of DBMSs
12
©Pearson Education 2009
Distributed database designThree key issues:
Fragmentation,Allocation,Replication.
13
©Pearson Education 2009 14
Distributed database design
FragmentationRelation may be divided into a number of sub-relations, which are then distributed.
AllocationEach fragment is stored at site with “optimal” distribution.
ReplicationCopy of fragment may be maintained at several sites.
©Pearson Education 2009 15
FragmentationDefinition and allocation of fragments
carried out strategically to achieve:Locality of Reference.Improved Reliability and Availability.Improved Performance.Balanced Storage Capacities and Costs.Minimal Communication Costs.
Involves analyzing most important applications, based on quantitative/qualitative information.
©Pearson Education 2009 16
FragmentationQuantitative information may include:
frequency with which an application is run;site from which an application is run;performance criteria for transactions and
applications.
Qualitative information may include transactions that are executed by application, type of access (read or write), and predicates of read operations.
©Pearson Education 2009 17
Why fragment?Usage: Applications work with views
rather than entire relations. Efficiency: Data is stored close to where
it is most frequently used. Data that is not needed by local applications is
not stored.Parallelism: With fragments as unit of
distribution, transaction can be divided into several subqueries that operate on fragments.
Security: Data not required by local applications is not stored and so not available to unauthorized users.
©Pearson Education 2009 18
Why fragment? - DisadvantagesPerformance,Integrity.
©Pearson Education 2009 19
Correctness rules for fragmentationThree correctness rules:
Completeness,Reconstruction,Disjointness.
©Pearson Education 2009 20
Correctness rules for fragmentationCompleteness
If relation R is decomposed into fragments R1, R2, ... Rn, each data item that can be found in R must appear in at least one fragment.
ReconstructionMust be possible to define a relational
operation that will reconstruct R from the fragments.
Reconstruction for horizontal fragmentation is Union operation and Join for vertical.
©Pearson Education 2009 21
Correctness rules for fragmentationDisjointnessIf data item di appears in fragment Ri, then
it should not appear in any other fragment. Exception: vertical fragmentation, where
primary key attributes must be repeated to allow reconstruction.
For horizontal fragmentation, data item is a tuple.
For vertical fragmentation, data item is an attribute.
©Pearson Education 2009 22
Types of fragmentation Four types of fragmentation:
Horizontal,Vertical,Mixed,Derived.
Other possibility is no fragmentation:
If relation is small and not updated frequently, may be better not to fragment relation.
©Pearson Education 2009 23
Horizontal and vertical fragmentation
©Pearson Education 2009 24
Mixed fragmentation
©Pearson Education 2009 25
Horizontal fragmentationConsists of a subset of the records of a table.
Defined using WHERE clause in SQL SELECT.
For example:
DC1: SELECT * FROM DVDCopy WHERE dCenterNo=‘D001’;
DC2: SELECT * FROM DVDCopy WHERE dCenterNo=‘D002’;
DC3: SELECT * FROM DVDCopy WHERE dCenterNo=‘D003’;
DC4: SELECT * FROM DVDCopy WHERE dCenterNo=‘D004’;
©Pearson Education 2009 26
Vertical fragmentationConsists of a subset of the columns of a table.
Defined using SELECT clause of SQL SELECT.
For example:
S1: SELECT staffNo, position, salary FROM Staff;
S2: SELECT staffNo, name, eMail, dCenterNo
FROM Staff;
©Pearson Education 2009 27
Mixed fragmentationConsists of a horizontal fragment that is
vertically fragmented, or a vertical fragment that is horizontally fragmented.
For example, from above example:
SELECT * FROM S2 WHERE dCenterNo = ‘D001’;
SELECT * FROM S2 WHERE dCenterNo = ‘D002’;
SELECT * FROM S2 WHERE dCenterNo = ‘D003’;
SELECT * FROM S2 WHERE dCenterNo = ‘D004’;
©Pearson Education 2009 28
Derived horizontal fragmentation A horizontal fragment that is based on horizontal
fragmentation of a parent relation.Ensures that fragments that are frequently joined
together are at same site.For example:
DR1: SELECT dr.* FROM DVDRental dr, DC1
WHERE dr.DVDNo = DC1.DVDNo;
DR2: SELECT dr.* FROM DVDRental dr, DC2
WHERE dr.DVDNo = DC2.DVDNo;
DR3: SELECT dr.* FROM DVDRental dr, DC3
WHERE dr.DVDNo = DC3.DVDNo;
DR4: SELECT dr.* FROM DVDRental dr, DC4
WHERE dr.DVDNo = DC4.DVDNo;
©Pearson Education 2009 29
Data allocation
Centralized: Consists of single database and DBMS stored at one site with users distributed across the network.
Partitioned: Database partitioned into disjoint fragments, each fragment assigned to one site.
Complete Replication: Consists of maintaining complete copy of database at each site.
Selective Replication: Combination of partitioning, replication, and centralization.
©Pearson Education 2009 30
Comparison of data allocation strategies
©Pearson Education 2009 31
Distributed database design methodology1. Use normal methodology to produce a design for
the global relations.2. Examine topology of system to determine where
databases will be located.3. Analyze most important transactions and identify
appropriateness of horizontal/vertical fragmentation.
4. Decide which tables are not to be fragmented.5. Examine tables on 1 side of relationships and
determine a suitable fragmentation schema. Tables on many side may be suitable for derived fragmentation.
©Pearson Education 2009 32
Transparencies in a DDBMSDistribution Transparency
Fragmentation TransparencyLocation TransparencyReplication TransparencyLocal Mapping TransparencyNaming Transparency
©Pearson Education 2009 33
Transparencies in a DDBMSTransaction Transparency
Concurrency TransparencyFailure Transparency
Performance Transparency
DBMS Transparency
©Pearson Education 2009 34
Distribution transparencyDistribution transparency allows user to
perceive database as single, logical entity. If DDBMS exhibits distribution
transparency, user does not need to know:data is fragmented (fragmentation
transparency), location of data items (location transparency),otherwise call this local mapping
transparency. With replication transparency, user is
unaware of replication of fragments.
©Pearson Education 2009 35
Transaction transparencyEnsures that all distributed
transactions maintain distributed database’s integrity and consistency.
In IBM’s Distributed Relational Database Architecture (DRDA), four types of transactions:Remote requestRemote unit of workDistributed unit of workDistributed request.
©Pearson Education 2009 36
IBM’s DRBA
©Pearson Education 2009 37
Concurrency transparencyAll transactions must execute independently
and be logically consistent with results obtained if transactions executed one at a time, in some arbitrary serial order.
Same fundamental principles as for centralized DBMS.
DDBMS must ensure both global and local transactions do not interfere with each other.
Similarly, DDBMS must ensure consistency of all subtransactions of global transaction.
©Pearson Education 2009 38
Concurrency transparencyReplication makes concurrency more complex. If a copy of a replicated data item is updated,
update must be propagated to all copies. Could propagate changes as part of original
transaction, making it an atomic operation. However, if one site holding copy is not reachable, then transaction is delayed until site is reachable.
Could limit update propagation to only those sites currently available. Remaining sites updated when they become available again.
Could allow updates to copies to happen asynchronously, sometime after the original update. Delay in regaining consistency may range from a few seconds to several hours.
©Pearson Education 2009 39
Failure transparencyDDBMS must ensure atomicity and
durability of global transaction.Means ensuring that subtransactions of
global transaction either all commit or all abort.
Thus, DDBMS must synchronize global transaction to ensure that all subtransactions have completed successfully before recording a final COMMIT for global transaction.
Must do this in presence of site and network failures.
©Pearson Education 2009 40
Failure transparency – Two-phase CommitTwo phases: a voting phase and a decision phase. Coordinator asks all participants whether they are
prepared to commit transaction. If one participant votes abort, or fails to respond within a
timeout period, coordinator instructs all participants to abort transaction.
If all vote commit, coordinator instructs all participants to commit.
All participants must adopt global decision.If participant votes abort, free to abort transaction
immediatelyIf participant votes commit, must wait for
coordinator to broadcast global-commit or global-abort message.
©Pearson Education 2009 41
Failure transparency – Two-phase Commit
©Pearson Education 2009 42
Performance transparencyDDBMS must perform as if it were a centralized
DBMS. DDBMS should not suffer any performance degradation
due to distributed architecture. DDBMS should determine most cost-effective strategy to
execute a request.
Must consider fragmentation, replication, and allocation schemas.
Distributed QP maps data request into ordered sequence of operations on local databases. DQP has to decide: which fragment to access; which copy of a fragment to use; which location to use.
©Pearson Education 2009 43
Performance transparencyDQP produces execution strategy optimized
with respect to some cost function. Typically, costs associated with a distributed
request include:
I/O cost;CPU cost;communication cost.
©Pearson Education 2009 44
Performance transparency - ExampleMember(memberNo, mCity) 100000 records in
Seattle
DVDCopy(DVDNo, catalogNo) 1000000 records in NY
Rental(memberNo, DVDNo) 1000000 records in Seattle
SELECT dc.DVDNo
FROM DVDCopy dc, Rental r, Member m
WHERE dc.DVDNo=r.DVDNo AND r.memberNo=m.memberNo AND dc.catalogNo = ‘634817’ AND m.mCity = ‘Chicago’;
Each record is 20 characters long.1000 members in Chicago; 200 000 rentals of ‘War of the
Worlds’ (634817).Computation time negligible compared to communication
time.
©Pearson Education 2009 45
Performance transparency - ExampleStrategy Time
(1) Move DVDCopy table to Seattle and process query there. 33.3 mins
(2) Move Member and Rental tables to New York and process query there. 1.17hours
(3) Join Member & Rental at Seattle, select records for Chicago members, and for
each of these in turn, check at New York for an associated catalogNo = ‘634817’. 33.3mins
(4) Select DVD copies with catalogNo = ‘634817’ at New York and for each
one found, check at Seattle for a rental involving a member living in Chicago. 11.12hours
(5) Join Member and Rental tables at Seattle, select Chicago members, project
result over DVDNo, and move this result to New York for matching with
catalogNo = ‘634817’. 2 seconds
(6) Select DVD copies with catalogNo = ‘634817’ at New York and move the
result to Seattle for matching with rentals associated with Chicago members. 41 seconds
©Pearson Education 2009 46
Dates 12 rules for a DDBMS0. Fundamental Principle
To the user, a distributed system should look exactly like a nondistributed system.
1. Local Autonomy2. No Reliance on a Central Site3. Continuous Operation4. Location Independence5. Fragmentation Independence6. Replication Independence
©Pearson Education 2009 47
Dates 12 rules for a DDBMS7. Distributed Query Processing8. Distributed Transaction
Processing9. Hardware Independence10. Operating System Independence11. Network Independence12. Database Independence
Last four rules are ideals.
©Pearson Education 2009 48
Replication serversReplication
Process of generating and reproducing multiple copies of data at one or more sites.
Provides users with access to current data where and when they need it.
Provides number of benefits, including improved performance when centralized resources get overloaded, increased reliability and data availability, and support for mobile computing and data warehousing.
©Pearson Education 2009 49
Replication serversSynchronous – updates to replicated data are part of enclosing transaction. If one or more sites that hold replicas are
unavailable transaction cannot complete. Large number of messages required to
coordinate synchronization.Asynchronous - target database updated after source database modified. Delay in regaining consistency may range
from few seconds to several hours or even days.
©Pearson Education 2009 50
Replication - functionalityAt basic level, has to be able to copy data
from one database to another (synch or asynchronous).
Other functions include:Scalability.Mapping and Transformation.Object Replication.Specification of Replication Schema.Subscription mechanism. Initialization mechanism.
©Pearson Education 2009 51
Replication – data ownershipOwnership relates to which site has
privilege to update the data. Main types of ownership are:
Master/slave (or asymmetric replication), Workflow, Update-anywhere (or peer-to-peer or
symmetric replication).
©Pearson Education 2009 52
Replication – Master/Slave OwnershipAsynchronously replicated data is owned by
one (master) site, and can be updated by only that site.
Using ‘publish-and-subscribe’ metaphor, master site makes data available.
Other sites ‘subscribe’ to data owned by master site, receiving read-only copies.
Potentially, each site can be master site for non-overlapping data sets, but update conflicts cannot occur.
©Pearson Education 2009 53
Replication – Workflow OwnershipAvoids update conflicts, while providing
more dynamic ownership model. Allows right to update replicated data to
move from site to site. However, at any one moment, only ever one
site that may update that particular data. Example is order processing system, which
follows steps, such as order entry, credit approval, invoicing, shipping, and so on.
©Pearson Education 2009 54
Replication – Update Anywhere OwnershipCreates peer-to-peer environment where
multiple sites have equal rights to update replicated data.
Allows local sites to function autonomously, even when other sites are not available.
Shared ownership can lead to conflict scenarios and have to detect conflict and resolve it.
©Pearson Education 2009 55
Mobile databasesWitnessing increasing demands on mobile computing to
provide support for growing number of mobile workers. Such a workforce require to work as if in the office but
in reality they are working from remote location places. With rapid expansion of cellular, wireless, and satellite
comms, soon be possible for mobile users to access any data, anywhere, at any time.
However, , business etiquette, practicalities, security, and costs may still limit communication (eg. can’t get online connection for as long as want, whenever you want).
Mobile databases offer a solution for some of these restrictions.
©Pearson Education 2009 56
Mobile databasesMobile database
A database that is portable and physically separate from a centralized database server but is capable of communicating with that server from remote sites allowing the sharing of corporate data.
Components of a mobile database environment include:corporate database server and DBMS that manages and
stores the corporate data and provides corporate applications;
remote database and DBMS that manages and stores the mobile data and provides mobile applications;
mobile database platform that includes laptop, PDA, or other Internet access devices;
two-way communication links between the corporate and mobile DBMS.
©Pearson Education 2009 57
Mobile database environment
©Pearson Education 2009 58
Mobile DBMSsAdditional functionality required of mobile
DBMSs includes ability to:communicate with centralized database server
through modes such as wireless or Internet access;replicate data on centralized database server and
mobile device;synchronize data on centralized database server
and mobile device;capture data from various sources such as the
Internet; manage data on the mobile device; analyze data on a mobile device; create customized mobile applications.