• A collection of sites connected by a communications network.
• Each site is a database system in its own right, but the sites have agreed to work together
• A user at any site can access data anywhere as if data were all at the user's own site

• Local autonomy
• No central site
• Continuous operation
• Location independence
• Fragmentation independence
• Replication independence
• Distributed query processing
• Distributed transactions
• Operating system independence
• Network independence
• DBMS independence

• Local autonomy
• Hardware independence
• Operating system independence
• Network independence
• DBMS independence

The sites in a distrubuted database system should be autonomous or independent from each other

Each site should provide its own security, locking, logging, integrity, and recovery. Local operations use and affect only local resources and do not depend on other sites.

A distributed database system should not rely on a central site, which may be a single point of failure or a bottleneck

Each site of a distributed database system provides its own security locking, logging, integrity, and recovery, and handles its own data dictionary. No central site must be involved in every distributed transaction

A distributed database system should never require downtime

A distributed database should provide on-line backup and recovery, and a full and incremental archiving facility. The backup and recovery should be fast enough to be performed online without noticeable detrimental affect on the entire system performance.

Applications should not know, or even be aware of, where the data is physically stored; Applications should behave as if all data were stored locally.

Location independence allows applications and data to be migrated easily fromone site to another without modifications

Relations can be divided into fragments and stored at different sites

Applications should not be aware of the fact that some data may be stored in a fragment of a table of a site different from the site where the table itself is stored

Relations and fragments can be stored as many distinct copies on different sites

Applicatins should not be aware that replicas of the data are maintained and synchronized automatically

A distributed database system should support atomic transactions

Critical database integrity; a distributed database system must be able to handle concurrency, deadlocks and recovery.

A distributed database system should be able to operate and access data spread accross a wide variety of hardware platforms

A truly distributed DBMS should not rely on a particular hardware feature, nor should it be limited to a certain hardware architecture

An ideal distributed database system must be able to support interoperability between DBMS runing on different nodes even if these DBMS are unalike

All sites in a DBMS should use commond standard interfaces in order to interoperate with each other

• Allows localization of acceses of relations by applications
• Allows parallel execution (increases concurrency and throughput)
• There are two types:
• Horizontal - Each fragment contains a subset of tuples of the global relation
• Vertical - Each fragment contains a subset of the attributes of the global relation

Relation R is decomposed into fragments:

F_R = {R1, R2, ..., Rn}

Decomposition (horizontal or vertical) can be expressed in terms of relational algebra expressions

R can be reconstructed if its possible to define a relational operator ▽ such that R = ▽R_i for all R_i ∈ F_R

Note that ▽ will be different for different types of fragmentation

F_R is disjoint if every data item d_i in each R_j is not in any R_k where k≠j

Note that this is only strictly true for horizontal decomposition

For vertical decomposition, primary key attributes are typically repeated in all fragments to allow reconstruction; disjointness is defined on non-primary key attributes.

Each fragment contains a subset of the tuples of the global relation

Two versions:

• Primary horizontal fragmentation performed using a predicate defined on the relation being partitioned
• Derived horizontal fragmentation performed using defined on another relation

• Decomposition
• F_R={R_i: R_i=o_fi(R)} where f_i is the fragmentation predicate for R_i
• Reconstruction
• R = U R_i for all R_i ∈ F_R
• Disjointness
• F_R is disjoint if the simple predicates used in f_i are mutually exclusive

• Decomposition
• F_R={R_i: R_i=R|>S_i}
• where F_s={S_i=o_fi(S)} and fi is the fragmentation predicate for the primary horizontal fragmentation of S
• Reconstruction
• R = U R_i for all R_i ∈ F_R

• Decomposition
• F_R={R_i: R_i=π_ai(R)} where a_i is the subset of attributes of R_i
• Completeness
• F_R is complete if each attribute of R appears in some a_i
• Reconstruction
• R = |><|_K R_i for all R_i ∈ F_R
• Where K is the set of primary key attributes of R
• Disjointness
• F_R is disjoint if each non-primary key attribute R appears in at most one ai.