Data replication is common in distributed databases. Which site to query? Which site to update first and then the rest (with cascades)? If a site (or a link) fails then, on re-connecting the isolated sites, the relevant data commits done during the interruption must be updated. Transaction scheduling over multiple sites is much harder than [...]
- Data replication is common in distributed databases. Which site to query? Which site to update first and then the rest (with cascades)?
- If a site (or a link) fails then, on re-connecting the isolated sites, the relevant data commits done during the interruption must be updated.
- Transaction scheduling over multiple sites is much harder than the centralised case.
- Lack of experience or complete knowledge.
- Complexity.
- Costs.
- Control.
- Security.
- Upgrading a database to a distributed database is not self evident.
- Performance.
- Reliability.
- Upsizing.
- Shareability.
- Local autonomy.
A distributed database is a collection of a number of logically related databases partitioned over a computer communications network, but whose data is available to all of its end users, irrespective of their physical location. A distributed DBMS maintains distributed databases and attempts to achieve a high level of distribution transparancy.
- A distributed database is a collection of a number of logically related databases partitioned over a computer communications network, but whose data is available to all of its end users, irrespective of their physical location.
- A distributed DBMS maintains distributed databases and attempts to achieve a high level of distribution transparancy.
Strange combination.
- Databases are a logical repository of structured data. With most implementations having a centralised framework.
- High level networks promote sharing.
Could a database be made available to a number of users without specifying a "single" computer? – Yes.
How?
Distributing the data. Distributing the DBMS processing. Distributing the database control etc.
What do we need?
More refined database techniques. Better and new DBMS technology. A networking infrastructure that connects all of [...]
Could a database be made available to a number of users without specifying a "single" computer? – Yes.
How?
- Distributing the data.
- Distributing the DBMS processing.
- Distributing the database control etc.
What do we need?
- More refined database techniques.
- Better and new DBMS technology.
- A networking infrastructure that connects all of the databases' end users.
If there are two fragments F and S, allocation problem involves finding optimal distribution of F to S.
Optimality can be defined;
- Minimal cost.
- Performance.
Major information requirement is related to applications. The major requirement for applications is access frequencies.
Algorithms to calculate vertical partitioning include:
- Clustering algorithm.
- Partitioning algorithm.
To obtain optimal solutions in vertical fragmentation, heuristics is used.
Two types of heuristics approach exists:
Grouping: assigns each attribute to one fragment, and at each step joins some of the fragments until some criteria is satisfied. Splitting: starts with a relation and decides on beneficial partitionings based on the access behaviour applications [...]
To obtain optimal solutions in vertical fragmentation, heuristics is used.
Two types of heuristics approach exists:
- Grouping: assigns each attribute to one fragment, and at each step joins some of the fragments until some criteria is satisfied.
- Splitting: starts with a relation and decides on beneficial partitionings based on the access behaviour applications to the attributes.
Splitting fits more naturally in the top-down methodology and the optimal solution is more likely found in a full relation.
Horizontal fragmentation partitions a relation along its tuples. There are two versions of horizontal partitioning:
Primary horizontal fragmentation. Derived horizontal fragmentation.
Primary horizontal fragmentation: is defined by a selection operation on the owner relations of a database schema.
Derived horizontal fragmentation: is defined on a member relation of a link according to [...]
Horizontal fragmentation partitions a relation along its tuples. There are two versions of horizontal partitioning:
- Primary horizontal fragmentation.
- Derived horizontal fragmentation.
Primary horizontal fragmentation: is defined by a selection operation on the owner relations of a database schema.
Derived horizontal fragmentation: is defined on a member relation of a link according to a selection operation specified by its owner.
