A-Level信息通讯技术-分布式数据库Distributed Databases。

Distributed Databases
Definition

• A single database that is spread physically across computers in multiple locations that are connected by data communication links (database stored in more than one place)

Why have distributed databases?

• Allows local business units to have control over data
• Allows data in local databases to be used together for decision making based upon the entire dataset

Reduce telecommunications costs by using the local database, rather than a distant option

Reduces the risk of telecommunications failures having a major impact, as less telecommunications hardware used

The database is spread across different sites 
Each remote site has the data that is relevant to itself only 
There are 3 core types:

• Partitioned between sites
• Entire databases duplicated at each site
• Central database with remote local database

Partitioned between sites

• Not every location (node) needs to have all the data. Therefore the partitioned approach is giving each node the data that is relevant to itself.
• If data is required by the node that is not held at the local database then a request for the data can be sent through the central computer (which holds a copy of all data or can link to it).
• The central copy is updated during periods where the load on the database is less – in general this is overnight.
• The data is split between sites either:

……

Advantages

• Data stored close to where it is used leading to increase in efficiency
• Local access optimization leading to better performance
• Only relevant data is available which leads to better security

Disadvantages

• Accessing data across partitions (different sites) leads to inconsistent access speed
• No data replication makes backups essential
• Potential exists for inconsistency in the data stored
• Additional disadvantage for vertical:

……

Entire databases duplicated at each site

• Instead of holding only the data that is relevant at each node, copies of the entire database are held at each node.
• There is a problem with data integrity – Assume that node B updates record 1 locally. Node C also updates record 1 locally but after node B – hence we can assume that the node C data is more up to date and therefore more correct.

Central database with remote local databases

• No data is held at the local node, instead an index is held locally and this is used to find and then access the data is in the central database.
• Indexes are the key data used to search the main database. Re-sorting an index into order when data is changed takes time, but a sorted index allows for fast searching of data
• Very little hardware is required at the nodes, but the indexes need updating. In this method there is a lot of network traffic.
• A ‘light’ alternative is to store the databases relevant to individual sites at that site, with an index being given to all databases
• When data is required, the index gives the location of the data – this is not a central location but the location of the site that holds the required database

Advantages and Disadvantages of Methods 2 and 3
Advantages and disadvantages

• Centralized database is useful for statistical analysis (e.g. sales figures) and backup
• A distributed database may be less secure with more points of access for hackers
• Decentralizing increases complexity but reduces network traffic
• Poor record locking and DBMS causes data reliability/integrity problems

Implementing Distributed Database Systems

Advantages

• Organizational structure
• Breaks the network down with greater control over local access
• Security
• Can readily limit read/write access to different areas of the database
• Local autonomy
• Each local area is responsible for maintenance of its database/access (could be a disadvantage if one site is not up to scratch)
• Errors are simpler to correct at a local level than a national level
• Improved availability (not all or nothing)
• Give access to parts of the database as required (some parts just require longer access time)
• Improved reliability (replication)
• Faster performance when held locally 

……

Disadvantages

• Complexity
• To maintain indexes, locations, updating, etc is complex
• Cost
• Often requires processing and storage at each site

……

• Security
• Many locations and entry points to the system that need to be accounted for
• Integrity control
• Maintenance of data integrity needs to be maintained – it must not be possible to have one record updated in two sites at the same time

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