I'll second Ed's comment.

The documentation should be more careful when using phrases "like relational databases". When we look at the history of relational databases, people expect certain things like ACID transactions, primary/foriegn key constraints, query planners, joins and relational algebra. Clearly Cassandra's storage engine does not follow most of those principals for a good reason.

The term row oriented storage would be more descriptive and appropriate. It avoids conflating Cassandra storage engine with "traditional" relational storage engines. Those of us that have spent over a decade using IBM DB2, Oracle, Sql Server and Sybase tend to think of relational databases in a certain way. If we go back to 1998, most RDBMS storage engine had a max row size limit. Databases like Sybase before version 9 preferred RAW disk for optimal performance. I can go on and on, but there's no point really.

Cassandra's storage engine is "row oriented", but it's not relational in RDBMS sense. We do everyone a huge disservice by using confusing terminology and then making fun of those who get confused. No one wins when that happens. At the end of the day, what differentiates cassandra's storage engine is it support static and dynamic columns, which traditional RDBMS don't support today. Calling Cassandra storage "distributed tables" doesn't really help in my bias opinion.

For example, if you tell a SqlServer or Oracle RAC admin "cassandra uses distributed tables" they might answer "so what, sql server and oracle can do that too." The difference is with RDBMS the partitioning is optional and requires more work to configure. Whereas with Cassandra you can have everything in 1 node, which means there is only 1 partition and no different to 1 instance of sql server. Where you win is when you need to add 2 more nodes, Cassandra makes this easier whereas with SqlServer and Oracle you have to do a little bit more work. I've lost count of how many times I've to explained noSql databases to RDBMS admins and had to explain the official docs are stupid.



On Sat, Oct 1, 2016 at 11:31 AM, Edward Capriolo <edlinuxguru@gmail.com> wrote:
https://github.com/apache/cassandra

Row store means that like relational databases, Cassandra organizes data by rows and columns. The Cassandra Query Language (CQL) is a close relative of SQL.

I generally do not know what to say about these high level "oversimplifications" like "firewalls block hackers". Are there "firewalls" or do they mean IP routers with layer 4 packet inspections and layer 3 Access Control Lists?

We say (and I catch myself doing it all the time) "like relational databases" often as if all relational databases work alike. A columnar store like HP Vertica is a relational database.MySql has different storage engines does MyIsam work like InnoDB?

Google docs organizes data by rows and columns as well. You can wrap any storage system into an API that makes them look like rows and columns. Microsoft LINQ can enumerate your network cars and query them https://msdn.microsoft.com/en-us/library/bb308959.aspx , that really does not make your network cards a "row store"

"Theoretically a row can have 2 billion columns, but in practice it shouldn't have more than 100 million columns."
In practice (In my experience) the number is much lower than 100 million, and if the data actually is deleted and readded frequently the number of live columns(rows, whatever) you can use happily is even lower


I believe on twitter (I am unable to find the tweet) someone was trying to convince me Cassandra was a "columnar analytic database".  ROFL

I believe telling someone it "row store" "like a database", is not a good idea. They might away content with that explanation. You are setting them up to walk into an anti-pattern. Like a case where the user is attempting to write and deleting 1 row and 1 column 6 billion times a day. Then you end up explaining to them http://stackoverflow.com/questions/21755286/what-exactly-happens-when-tombstone-limit-is-reached 

and how the cassandra storage model is not "like a relational database". 

On Fri, Sep 30, 2016 at 9:22 PM, Edward Capriolo <edlinuxguru@gmail.com> wrote:
I can iterate over JSON data stored in mongo and present it as a table with rows and columns. It does not make mongo a rowstore. 

On Fri, Sep 30, 2016 at 9:16 PM, Edward Capriolo <edlinuxguru@gmail.com> wrote:
The problem with calling it a row store:

https://en.wikipedia.org/wiki/Row_(database)

In the context of a relational database, a row—also called a record or tuple—represents a single, implicitly structured data item in a table. In simple terms, a database table can be thought of as consisting of rows andcolumns or fields.[1] Each row in a table represents a set of related data, and every row in the table has the same structure.

When you have static columns and rows with maps, and lists, it is hard to argue that every row has the same structure. Physically at the storage layer they do not have the same structure and logically when accessing the data they barely have the same structure, as the static column is just appearing inside each row it is actually not contained in.

On Fri, Sep 30, 2016 at 4:47 PM, Jonathan Haddad <jon@jonhaddad.com> wrote:
+1000 to what Benedict says. I usually call it a "partitioned row store" which usually needs some extra explanation but is more accurate than "column family" or whatever other thrift era terminology people still use.
On Fri, Sep 30, 2016 at 1:53 PM DuyHai Doan <doanduyhai@gmail.com> wrote:
I used to present Cassandra as a NoSQL datastore with "distributed" table. This definition is closer to CQL and has some academic background (distributed hash table).


On Fri, Sep 30, 2016 at 7:43 PM, Benedict Elliott Smith <benedict@apache.org> wrote:
Cassandra is not a "wide column store" anymore.  It has a schema.  Only thrift users no longer think they have a schema (though they do), and thrift is being deprecated.

I really wish everyone would kill the term "wide column store" with fire.  It seems to have never meant anything beyond "schema-less, row-oriented", and a "column store" means literally the opposite of this.

Not only that, but people don't even seem to realise the term "column store" existed long before "wide column store" and the latter is often abbreviated to the former, as here: http://www.planetcassandra.org/what-is-nosql/ 

Since it no longer applies, let's all agree as a community to forget this awful nomenclature ever existed.



On 30 September 2016 at 18:09, Joaquin Casares <joaquin@thelastpickle.com> wrote:
Hi Mehdi,

I can help clarify a few things.

As Carlos said, Cassandra is a Wide Column Store. Theoretically a row can have 2 billion columns, but in practice it shouldn't have more than 100 million columns.

Cassandra partitions data to certain nodes based on the partition key(s), but does provide the option of setting zero or more clustering keys. Together, the partition key(s) and clustering key(s) form the primary key.

When writing to Cassandra, you will need to provide the full primary key, however, when reading from Cassandra, you only need to provide the full partition key.

When you only provide the partition key for a read operation, you're able to return all columns that exist on that partition with low latency. These columns are displayed as "CQL rows" to make it easier to reason about.

Consider the schema:

CREATE TABLE foo (
  bar uuid,
  boz uuid,
  baz timeuuid,
  data1 text,
  data2 text,
  PRIMARY KEY ((bar, boz), baz)
);

When you write to Cassandra you will need to send bar, boz, and baz and optionally data*, if it's relevant for that CQL row. If you chose not to define a data* field for a particular CQL row, then nothing is stored nor allocated on disk. But I wouldn't consider that caveat to be "schema-less".

However, all writes to the same bar/boz will end up on the same Cassandra replica set (a configurable number of nodes) and be stored on the same place(s) on disk within the SSTable(s). And on disk, each field that's not a partition key is stored as a column, including clustering keys (this is optimized in Cassandra 3+, but now we're getting deep into internals).

In this way you can get fast responses for all activity for bar/boz either over time, or for a specific time, with roughly the same number of disk seeks, with varying lengths on the disk scans.

Hope that helps!

Joaquin Casares
Consultant
Austin, TX

Apache Cassandra Consulting

On Fri, Sep 30, 2016 at 11:40 AM, Carlos Alonso <info@mrcalonso.com> wrote:
Cassandra is a Wide Column Store http://db-engines.com/en/system/Cassandra

Carlos Alonso | Software Engineer | @calonso

On 30 September 2016 at 18:24, Mehdi Bada <mehdi.bada@dbi-services.com> wrote:
Hi all,

I have a theoritical question:
- Is Apache Cassandra really a column store?
Column store mean storing the data as column rather than as a rows.

In fact C* store the data as row, and data is partionned with row key.

Finally, for me, Cassandra is a row oriented schema less DBMS.... Is it true for you also???

Many thanks in advance for your reply

Best Regards
Mehdi Bada
----

Mehdi Bada | Consultant
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