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NoSQL Databases

COMP207 Lectures

These are non-relational databases that generally cater to web applications:

  • Very fast access, will millions of users in parallel.
  • Vault Tolerance
  • Semi-structured
    • Flexibility in the type of data stored.
  • Full ACID compliance can sometimes be relaxed.

NoSQL Example

Consider that you have the following setup:

flowchart
ws[Web Service] <--> nsql[NoSQL Database] & rd[Relational Database]
nsql --- n([User Profiles, Shopping Cart])
rd --- r([Inventory, Transactions])

We can store data that must be compliant on the relational database and high volume, non-compliant data can be stored on the noSQL database.

In the NoSQL database we can use tables like so:

Key Value
User1 {“name”:”John Smith”,
“items”:{
“1”:{“name”:”product 1”,”quantity”:3}
…}
}

They have the following properties:

  • Simple Queries:
    • Return teh value for key $k$.
  • Fast reads/writes via index on keys.
  • ACID can sometimes be relaxed.

NoSQL Characteristics

NoSQL databases are often distributed:

  • Nodes run on commmidity hardware.
  • Standard network.

They are designed to guarantee:

  • Availability - Every non-failing node always executes queries.

  • Consistency - Every read receives the most recent write or an error.

    This is not the same as in ACID

  • Scalability - More capacity by adding new nodes.
  • High Performance - Supports few, fast, interfaces.
  • Partition-Tolerance - Even if nodes, or messages, fail - the remaining sub-network can continue their work.

They often give up ACID to improve performance.

The CAP Theorem

This theorem says we can only choose two of:

  • Consistency
  • Availability
  • Partition-Tolerance

Single node DBMS can have:

  • C and A

NoSQL can have:

  • C and P
  • A and P.

BASE

NoSQL databases drop ACID and often provide the weaker guarantee of:

  • Basically
  • Available
  • Soft State
  • Eventually Consistent

Soft state & eventual consistency mean that the database might occasionally be inconsistent but will eventually be consistent after propagation occurs.

NoSQL Classification

  • Key-Value Stores
    • Efficient lookups and insertions of key-value pairs.
  • Document Stores
    • Similar to key-value stores, where the value is a semi-structured datatype.
    • Lookups can involve values from the documents.
  • Column Stores
  • Graph Databases