The most important feature of a relational database is transactional consistency : traditional relational database read and write operations are transactional and acid-based, and this feature enables relational databases to be used in almost all conformance-compliant systems , such as typical banking systems .
However, in the Web application, especially in the SNS application , the consistency is not so important, user a sees the content and User B see the same User C content update inconsistency is tolerable, or, two people see the same friend's data update time difference that a few seconds is tolerable, so , the biggest feature of a relational database is useless here, at least not so important.
On the contrary, the great cost of relational database in order to maintain consistency is its poor read and write performance, and the application of SNS such as Weibo, Facebook, which is very demanding for concurrent reading and writing ability, the relational database has been unable to cope (in terms of reading, traditionally in order to overcome relational database defects, improve performance, is to increase the level of memcache to static Web pages, and in SNS, changes too fast, Memchache is powerless to do so, it is necessary to use a new data structure storage to replace the relational database.
The other characteristic of the relational database is that it has a fixed table structure, so its extensibility is very poor, and in SNS, the upgrading of the system, the increase of the function, often means the huge change of data structure, this relational database is also difficult to cope with, need new structured data storage.
Therefore, the non-relational database comes into being, because it is impossible to meet all the new requirements with a data structure, so the non-relational database is strictly not a database, it should be a collection of data structure storage methods.
It must be emphasized that the persistent storage of data, especially the persistent storage of large amounts of data, or the need for a relational database veteran.
Relational database V.s. Non-relational database
