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Databases

Querying DynamoDB from Scala

As we’ve seen in this comparison with Apache Cassandra, DynamoDB may be a valuable choice for storing data. It’s fully maintained by AWS you just have to configure your instance and you can start using it straight away.

Being one of the Amazon web services DynamoDB offers a JSON/HTTP interface and Amazon provides a set of client for a variety of language. Unfortunately Scala is not one of them but there is a Java client available. Sadly it suffers from big performance issues due to poor design choices.

So what are the alternatives? This is what we’re going to see in this post: How can we get the most out of the official driver and are there better alternatives both in terms of performance or integration with the Scala ecosystem.

Categories
Databases

Amazon DynamoDB vs Apache Cassandra

Cassandra and DynamoDB both origin from the same paper: Dynamo: Amazon’s Highly Available Key-value store. (By the way – it has been a very influential paper and set the foundations for several NoSQL databases).

Of course it means that DynamoDB and Cassandra have a lot in common! (They have the same DNA). However both AWS DynamoDB and Apache Cassandra have evolved quite a lot since this paper was written back in 2007 and there are now some key differences to be aware of when choosing between the two.

This post aims at comparing these 2 distributed databases so that you can choose the one that best matches your requirements.

Categories
Databases

The Cassandra Java Driver

Cassandra drivers are not just a dumb piece of software that sends CQL strings to a Cassandra node and waits for responses.

They are actually quite smart and are architectured in a way that should make your life easier while still attempting to get the most performance out of Cassandra.

In this post I am going to focus on the Java driver, have a quick look at its architecture and on some of the features it offers.

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Databases

Querying Cassandra from Scala

When it comes to accessing Cassandra from Scala there are 2 possible approaches:

  • The official Java driver
  • A custom DSL like Quill or Phantom

Custom-DSL are nice as they provide all the type-safety you need against your data schema. However in this post I will focus only on the Java driver. Why? Because it’s both a simple and decent solution in my opinion.

The bad thing is that you lose any type-safety as all the queries are just plain strings. On the other hand you don’t have to learn a new DSL because your queries are just CQL. Add a thorough test coverage and you have a viable solution.

Moreover the Java driver provides an async API backed by Guava’s futures and it’s not that difficult to turn these futures into Scala futures – which makes a quite natural API in Scala.

There are still some shortcomings that you’d better be aware of when consuming a result set but overall I think that it’s still a simple solution that is worth considering.

Categories
Databases

Understanding Cassandra tombstones

We recently deployed in production a distributed system that uses Cassandra as its persistent storage.

Not long after we noticed that there were many warnings about tombstones in Cassandra logs.

WARN  [SharedPool-Worker-2] 2017-01-20 16:14:45,153 ReadCommand.java:508 - 
Read 5000 live rows and 4771 tombstone cells for query 
SELECT * FROM warehouse.locations WHERE token(address) >= token(D3-DJ-21-B-02) LIMIT 5000 
(see tombstone_warn_threshold)

We found it quite surprising at first because we’ve only inserted data so far and didn’t expect to see that many tombstones in our database. After asking some people around no one seemed to have a clear explanation on what was going on in Cassandra.

In fact, the main misconception about tombstones is that people associate it with delete operations. While it’s true that tombstones are generated when data is deleted it is not the only case as we shall see.

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Databases

Cassandra lightweight transactions

We all know that Cassandra is a distributed database. However there’re situations where one needs to perform an atomic operation and for such cases a consensus must be reached between all the replicas.

For instance when dealing with payments we might require that we only insert the row once.

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Databases

DSEGraph under the hood

Today we’ll dig deeper inside DSEGraph and see how it uses Cassandra to distribute the graph over the cluster.

We’ll illustrate this post with a classic toy example: a movie graph.

Our model is very simple. I consists in “persons” who “acted in”  “movies” which “belongs to” “genres”.

The graph schema used in the post
The graph schema used in the post
Categories
Databases

DSE Graph vs Neo4J

Last week I took the course on DSEGraph available on Datastax academy so logically this post is about DSEGraph. It turns out I’ve also used Neo4J a couple of months ago. So this post is going to be a comparison between these 2 graph databases.