Engine

Features


Cassandra replication

Cassandra replication

  • Replication factor = 3
  • Consistency level = QUORUM
  • Clients talks to any node, the node hashes the partition key and finds the location of the data
  • Data is read from all the replicas waiting for responses until we reach a quorum

Cassandra write

Cassandra write

  • Acknowledged when we write to both the commit log (append only) and the memtable
  • When the memtable becomes full it’s flushed into an SSTable
  • Periodically SSTables are merged

Cassandra read

Cassandra read

  • Check if the key is in the in-memory row cache
  • Query the bloomfilters of the existing SSTables to find the record, if it doesn’t exist then skip the SSTable
  • If the bloomfilter says that there may be data check the in-memory key cache
  • On miss get the data from the SSTable and merge it with the data in the memtable, write the key to the in-memory key-cache and merged result to the in-memory row cache

Data modeling

Goals

  • spread data evenly around the cluster
  • minimize the number of partitions read
  • keep paritions manageable

Process

  • Identify initial entities and relationships
  • Key attributes (map to PK colums)
  • Equality search attributes (map to the beggining of the PK)
  • Inequality search attributes (map to clustering columns)
  • Other attributes
    • Static attributes are shared within a given partition
primary key = partition key + clustering columns

Validation

  • Is data evenly spread?
  • 1 partition per read?
  • Are writes (overwrites) possible?
  • How large are the partitions? $n_{cells} = n_{rows} * (n_{cols} - n_{PK} - n_{static}) + n_{static} < 1M$
  • How much data duplication?

Examples

Store books by ISBN

Attribute Special
isbn K
title
author
genre
publisher
  • Is data evenly spread? Yes
  • 1 partition per read? Yes
  • Are writes (overwrites) possible? Yes
  • How large are the partitions? $1 * (5 - 1 - 0) + 0 < 1M$
  • How much data duplication? 0

Register a user uniquely identified by an email/password, we also want their fullname. They will be accessed by email and password or by UUID

Attribute Special
email K
password C
fullname
uuid

Q1: find users by login info

Q3: find users by email (to guarantee uniqueness)

  • Is data evenly spread? Yes
  • 1 partition per read? Yes
  • Are writes (overwrites) possible? Yes
  • How large are the partitions? $1 * (4 - 1 - 0) + 0 < 1M$
  • How much data duplication? 0
Attribute Special
uuid K
fullname
Q2: get users by UUID

  • Is data evenly spread? Yes
  • 1 partition per read? Yes
  • Are writes (overwrites) possible? Yes
  • How large are the partitions? $1 * (2 - 1 - 0) + 0 < 1M$
  • How much data duplication? 0

Find books a logged in user has read sorted by title and author

Attribute Special
uuid K
title C
author C
fullname S
ISBN
genre
publisher
  • Is data evenly spread? Yes
  • 1 partition per read? Yes
  • Are writes (overwrites) possible? Yes
  • How large are the partitions? (up to 200k book reads per user)
$$ \begin{align*} n_{books} * (7 - 1 - 1) + 1 & < 1M \\\\ n_{books} & < \frac{1M}{5} - 1 \\\\ n_{books} & < 200k \end{align*} $$
  • How much data duplication? 0

Interaction of every user in the website

Attribute Special
uuid K
time C (desc)
element
type
  • Is data evenly spread? Yes
  • 1 partition per read? Yes
  • Are writes (overwrites) possible? Yes
  • How large are the partitions? (up to 333k book reads per user, 333k actions may be low number of actions to store therefore we should store actions by bucket)
$$ \begin{align*} n_{actions} * (4 - 1 - 0) + 0 & < 1M \\\\ n_{actions} & < 333K \end{align*} $$
  • How much data duplication? 0
Attribute Special
uuid K
month K
time C (desc)
element
type
  • Is data evenly spread? Yes
  • 1 partition per read? Yes
  • Are writes (overwrites) possible? Yes
  • How large are the partitions? (up to 333k book reads per user)
1 year  = 333k / 365 / 24 = 38 actions / h
1 month = 333k / 30 / 24  = 462 actions / h (most realistic case)
1 week  = 333k / 7 / 24   = 1984 actions / h
$$ \begin{align*} n_{actions} * (5 - 2 - 0) + 0 & < 1M \\\\ n_{actions} & < 333K \end{align*} $$
  • How much data duplication? 0

Resources