DataStax Academy FAQ

DataStax Academy migrated to a new learning management system (LMS) in July 2020. We are also moving to a new Cassandra Certification process so there are changes to exam bookings, voucher system and issuing of certificates.

Check out the Academy FAQ pages for answers to your questions:


question

chandrasekar.b03_190734 avatar image
chandrasekar.b03_190734 asked ·

In the DS201 quiz, how many vnodes does each node have?

In DS 201, the video lecture tells that by default, each node has 256 vnodes. But when i tried to attend the quiz on vnodes, it tells that there is 128 vnodes per node. Here i have attached a screenshot of the quiz i attended on DS201. Kindly provide me the valid answer !

academyds201virtual nodes
vnodes.jpg (240.4 KiB)
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Erick Ramirez avatar image
Erick Ramirez answered ·

I completely agree with you -- the question and the course content hasn't been clear about this.

The problem is that DS201 course question is referring to the num_tokens value configured in the DSE 6.0 cassandra.yaml. As Bettina already pointed out, I've previously explained it in question #4773. Allow me to explain.

Apache Cassandra

In open-source Cassandra, num_tokens is explicitly set to 256 in cassandra.yaml. This is true for both the latest C* 3.11.6 release and the current alpha release C* 4.0-alpha4:

num_tokens: 256

However if num_tokens is not set in cassandra.yaml, the number of tokens defaults to just 1 (see Config.java):

    public int num_tokens = 1;

DataStax Enterprise

The default cassandra.yaml that ships with DSE (for example, DSE 6.0 used in the DS201 course) has num_tokens set to 128 but it is commented out so it has no effect during bootstrap:

# num_tokens: 128

Unless num_tokens is explicitly set, a new node will bootstrap with just 1 token.

DS201 course

Specifically on your query about the Academy exercise on vnodes, the default value hasn't changed.

The quiz was asking about the default in the DSE 6.0.0 cassandra.yaml -- the version included in the VM image you downloaded for the course. And it was commented out with 128:

# num_tokens: 128

For the purposes of the DS201 course, 128 tokens is the answer. But I understand the confusion since if you install DSE without explicitly setting num_tokens, the node will bootstrap with just 1 token since the default configuration in DSE's Config.java is also 1 token:

    public int num_tokens = 1;

I've requested that the course be revised to avoid this confusion since it isn't really useful to someone who will eventually be a Cassandra administrator. Cheers!

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bettina.swynnerton avatar image
bettina.swynnerton answered ·

Hi @chandrasekar.b03_190734,

this has been answered in detail in this post, let me redirect you:

https://community.datastax.com/questions/4773/default-vnodes-on-caasandra.html

Cheers!


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