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Database

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Concepts

ACID SQL NOSQL Transaction Consistency

Types

Scaling

Replication

  • A master database generally only supports write operations.
  • A slave database gets copies of the data from the master database and only supports read operations.
  • All the data-modifying commands like insert, delete, or update must be sent to the master database.
  • Most applications require a much higher ratio of reads to writes; thus, the number of slave databases in a system is usually larger than the number of master databases.

Benefits

  • Better performance
    • In the master-slave model, all writes and updates happen in master nodes; whereas, read operations are distributed across slave nodes.
    • This model improves performance because it allows more queries to be processed in parallel.
  • Reliability
    • If one of your database servers is destroyed by a natural disaster, such as a typhoon or an earthquake, data is still preserved.
  • High availability
    • By replicating data across different locations, your website remains in operation even if a database is offline as you can access data stored in another database server.

Fail-over

  • If only one slave database is available and it goes offlin
    • Read operations will be directed to the master database temporarily.
    • As soon as the issue is found, a new slave database will replace the old one.
    • In case multiple slave databases are available, read operations are redirected to other healthy slave databases.
    • A new database server will replace the old one.
  • If the master database goes offline
    • A slave database will be promoted to be the new master.
    • All the database operations will be temporarily executed on the new master database.
    • A new slave database will replace the old one for data replication immediately.
    • In production systems, promoting a new master is more complicated as the data in a slave database might not be up to date.
      • The missing data needs to be updated by running data recovery scripts.
      • Some other replication methods like multi-masters and circular replication could help, those setups are more complicated

Clustered Database

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