Objective
1. Relational database and non-relational database
A, what is a database?
A database is a warehouse of data. The main difference between a database and a common file system is that the database can quickly find the corresponding data.
B, what is a relational database?
A relational database is a database that is created from a relational model. The so-called relationship model is "a pair of one or one-to-many, many-to-many" relational model, the relationship model refers to a two-dimensional tabular model, and thus a relational database is a two-dimensional table and the relationship between a data organization. The relational model includes the data structure (the problem of the datastore, the two-dimensional table), the set of Operations instructions (SQL statement), the integrity constraint (the data constraint in the table, the constraint between the table and the table).
A common relational database:
Oracle, DB2, PostgreSQL, Microsoft SQL Server, Microsoft Access, MySQL
The characteristics of a relational database:
- Safe (because it is stored on disk, it does not say that suddenly the power loss data is gone);
- Easy to understand (built on relational models);
- But not space-saving (because it is based on a relational model, you have to follow certain rules, like the data in which a field value is allocated space even if it is empty)
C, what is a non-relational database?
Non-relational database is mainly based on the "non-relational model" of the database (because the relationship is too large, so the general "non-relational" to represent other types of databases).
Non-relational models such as:
- Column model: The stored data is a column of columns. The relational database takes one row as a record, and the column model database is listed as a single record. (This model, data is index, IO is very fast, mainly some distributed database)
- Key-value pair model: The stored data is a "key-value pair", k/v mode.
- Document Class Model: stores data in a document, somewhat like a "key-value pair."
Common non-relational model databases:
- Column model: Hbase
- Key-value pair model: Redis,memcachedb
- Document Class Model: MongoDB
Features of non-relational databases:
- High efficiency (because it is stored in memory).
- But not safe (power loss data, but where redis can synchronize data to disk), many non-relational databases are now starting to support dump to disk.
2. Introduction to NoSQL Cache database
NoSQL (NoSQL = not-only sql), meaning "not just SQL", refers to non-relational databases.
Four categories of NoSQL databases:
- Key value (Key-value) stores the database: The database is primarily used to a hash table with a specific key and a pointer to the specific data. The advantage of the Key/value model for IT systems is simplicity and ease of deployment. such as: Tokyo cabinet/tyrant, Redis, Voldemort, Oracle BDB.
- Columnstore database: A database is often a massive amount of data used to address distributed storage. Keys still exist, but they are characterized by pointing to multiple columns. such as: Cassandra, HBase, Riak.
- Document database: A document Database model is a versioned document, and semi-structured documents are stored in a specific format, such as JSON. A document database can be considered an upgraded version of a key-value database, allowing for the nesting of key values. and the document database is more efficient than the key-value database query. such as: CouchDB, MongoDb.
- Graph database: The graphical structure of the database is different from the other columns and the rigid structure of the SQL database, it is using a flexible graphical model, and can be extended to multiple servers. NoSQL databases do not have a standard query language (SQL), so database queries require a data model. Many NoSQL databases have rest-type data interfaces or query APIs. such as: neo4j, Infogrid, Infinite Graph.
NoSQL databases are more appropriate in these cases:
- The data model is relatively simple;
- Need for more flexible IT systems;
- The database performance requirements are high;
- No need for high data consistency;
- For a given key, it is easier to map the environment for complex values.
Four classified tabular analysis of NoSQL databases:
3. Mongodb\redis\memcached Difference
Redis and Memcache differences:
- Both Redis and Memcache store data in memory, both in-memory databases. But memcache can also be used to cache other things, examples, videos, and so on.
- Data type--memcache Specify the byte length of the data when adding data; Redis does not need to.
- Virtual Memory--redis When the physical memory is exhausted, you can swap some long-unused value to disk. LRU algorithm.
- Expiration policy--memcache is specified when set, such as set Key1 0 0 8, which never expires. Redis can be set by example expire, for example expire name 10
- Distributed-set memcache cluster, use Magent to do a master multi-slave, Redis can do a master multi-slave. Can be a master one from
- Storage data security--memcache after hanging, the data is gone; Redis can be saved to disk periodically (persisted).
- After the disaster recovery--memcache hangs, the data cannot be recovered; Redis data can be recovered by aof after it is lost.
The three differences are as follows:
2, Eighth week-network programming advanced-Understanding of database type
