The Top-Down Classical DDBMs Architecture Global schema Site Independent Fragmentation Schema Schemas Allocation schema Local Mapping Schema Local Mapping Schema Other sites DBMS 1 DBMS 2 Site 1LOCAL LOCAL Site 2 DB 1 DB 2
11 The Top-Down Classical DDBMS Architecture Site 1 Other sites Site Independent Schemas Global Schema Fragmentation Schema Allocation Schema LOCAL DB 1 Local Mapping Schema DBMS 1 LOCAL DB 2 Local Mapping Schema DBMS 2 Site 2
Global relations, Fragments, and Phⅴ sical imas g es R R1 R R (Site 1) R R (Site Global R Relation Fragments (Site3 R3 Physical Imag les
12 Global Relations, Fragments, and Physical Images R Global Relation R3 3 R3 2 R2 2 R2 1 R 1 2 R 1 1 R3 R2 R1 R2 (Site2) R1 (Site 1) R3 (Site3) Physical Images Fragments
DDBMS Schemas Global Schema: a set of global relations as if database were not distributed at all Fragmentation Schema: global relation is split into"non- overlapping (logical) fragments. 1: n mapping from relation R to fragments R Allocation Schema: 1: 1 or 1: n(redundant) mapping from fragments to sites. All fragments corresponding to the same relation R at a site j constitute the physical image R. A copy of a fragment is denoted by RI Local Mapping Schema: a mapping from physical images to physical objects, which are manipulated by local DBMSS
13 DDBMS Schemas Global Schema: a set of global relations as if database were not distributed at all Fragmentation Schema: global relation is split into “nonoverlapping” (logical) fragments. 1:n mapping from relation R to fragments Ri . Allocation Schema: 1:1 or 1:n (redundant) mapping from fragments to sites. All fragments corresponding to the same relation R at a site j constitute the physical image Rj . A copy of a fragment is denoted by Rj i . Local Mapping Schema: a mapping from physical images to physical objects, which are manipulated by local DBMSs
Motivation for this architecture Separating the concept of data fragmentation from the concept of data allocation Fragmentation transparency Location transparency Explicit control of redundancy Independence from local databases allows local mapping transparency
14 Motivation for this Architecture Separating the concept of data fragmentation from the concept of data allocation Fragmentation transparency Location transparency Explicit control of redundancy Independence from local databases allows local mapping transparency
Rules for Data Fragmentation Completeness All the data of the global relation must be mapped into the fragments Reconstruction It must always be possible to reconstruct each global relation from its fragments Disjointedness It is convenient that fragments are disjoint, so that the replication of data can be controlled explicitly at the allocation level
15 Rules for Data Fragmentation Completeness All the data of the global relation must be mapped into the fragments. Reconstruction It must always be possible to reconstruct each global relation from its fragments. Disjointedness It is convenient that fragments are disjoint, so that the replication of data can be controlled explicitly at the allocation level