db:dynamodb

About

Amazon implementation inspired by dynamo

Amazon DynamoDB is a nosql database that store its data as a key/value.

DynamoDB was introduced to address the limitations of SimpleDB

DynamoDb combine the best parts of:

the original Dynamo design (incremental scalability, predictable high performance)
SimpleDB

Table

DynamoDB tables do not have a fixed schema but instead allow each data item to have any number of attributes, including multi-valued attributes.

The key (primary and sort) can not be changed.

Installation

Local

https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/DynamoDBLocal.html

Management

Create table

AWS Management Console > Services > DynamoDB
Choose Create table.
Enter Table name. This field is case sensitive.
Enter the id for the Partition key and select the key type. This field is case sensitive.
Check the Use default settings box and choose Create.
Scroll to the bottom of the Overview section of your new table where you will see the ARN.

Key features

Service Level Agreement (SLN) at the 99th percentile, and not on mean/median/variance (the heavy users are penalized) “Respond within 300ms for 99.9% of its requests”
DHT with replication:
- Store value at k, k+1, …, k+N-1
- Eventual consistency through vector clocks
Reconciliation at read time:
- Writes never fail (“poor customer experience”)
- Conflict resolution: “last write wins” or application specific

Consistency

A feature of Dynamo is eventual consistency through vector clocks.

vector clock

Each data item associated with a list of (server, timestamp) pairs indicating its version history.

Which pairs of vector clocks do not conflict? Select all that apply.

Data 1: ([SX, 10], [SY, 20], [SZ, 3]); Data 2: ([SX, 10], [SY, 10], [SZ, 60])
Data 1: ([SX, 5], [SY, 10]); Data 2: ([SX, 10], [SZ,30])
Data 1: ([SY, 10]); Data 2: ([SY, 1])
Data 1: ([SX, 5], [SY, 10]); Data 2: ([SX, 10], [SY, 20], [SZ,30])

Answer C and D

Vector clocks conflict when all values in one clock are not later than or equal to all values in the other clock. This means the vector clock [(SX, A), (SY, B), (SZ, C)] does not conflict with the vector clock [(SX, D), (SY, E), (SZ, F)] if and only if A ≥ D, B ≥ E, and C ≥ F OR D ≥ A, E ≥ B, and F ≥ C. If a server has no entry in a specific clock, you can treat its time stamp as 0.

Configurable Consistency

R = Minumum number of nodes that participate in a successful read
W = Minumum number of nodes that participate in a successful write
N = Replication factor

If R + W > N, you can claim consistency but R + W < N means lower latency.

Table of Contents

Aws - DynamoDB