Cassandra Storage Engine Overview

Installing

If using the YUM repositories on Fedora, Red Hat, or CentOS, first install the Cassandra storage engine package with:

If using the Debian or Ubuntu repositories, the Cassandra plugin is in the main MariaDB server package.

To install/activate the storage engine into MariaDB, issue the following command:

You can also activate the storage engine by using the --plugin-load command on server startup.

Introduction

The Cassandra Storage Engine allows access to data in a Cassandra cluster from MariaDB. The overall architecture is shown in the picture below and is similar to that of the NDB cluster storage engine.

You can access the same Cassandra cluster from multiple MariaDB instances, provided each of them runs the Cassandra Storage Engine:

The primary goal of Cassandra SE (Storage Engine) is data integration between the SQL and NoSQL worlds. Have you ever needed to:

• grab some of Cassandra's data from your web frontend, or SQL query?

• insert a few records into Cassandra from some part of your app?

Now, this is easily possible. Cassandra SE makes Cassandra's column family appear as a table in MariaDB that you can insert to, update, and select from. You can write joins against this table; it is possible to join data that's stored in MariaDB with data that's stored in Cassandra.

Versions in MariaDB

What about CQL?

The Cassandra Query Language (CQL) is the best way to work with Cassandra. It resembles SQL on first glance; however, the resemblance is very shallow. CQL queries are tightly bound to the way Cassandra accesses its data internally. For example, you can't have even the smallest join. In fact, adding a mere... AND non_indexed_column=1 into a WHERE clause is already invalid CQL.

Our goal is to let one work in SQL instead of having to move between CQL and SQL all the time.

Does this make Cassandra an SQL database?

No. Cassandra SE is not suitable for running analytics-type queries that sift through huge amounts of data in a Cassandra cluster. That task is better handled by Hadoop-based tools like Apache Pig or Apache Hive. Cassandra SE is rather a "window" from an SQL environment into NoSQL.

Data mapping

Let's get specific. In order to access Cassandra's data from MariaDB, one needs to create a table with engine=cassandra. The table will represent a view of a Column Family in Cassandra and its definition will look like so:

The name of the table can be arbitrary. However, primary key, column names, and types must "match" those of Cassandra.

Cassandra's rowkey

The table must define a column that corresponds to the Column Family's rowkey.

• If Cassandra's rowkey has an alias (or name), then MariaDB's column must have the same name.

  • Otherwise, it must be named "rowkey".

• The type of MariaDB's column must match the validation_class of Cassandra's rowkey (datatype matching is covered in more detail below).

Note: Multi-column primary keys are currently not supported. Support may be added in a future version, depending on whether there is a demand for it.

Cassandra's static columns

Cassandra allows one to define a "static column family", where column metadata is defined in the Column Family header and is obeyed by all records.

These "static" columns can be mapped to regular columns in MariaDB. A static column named 'foo' in Cassandra should have a counterpart named 'foo' in MariaDB. The types must also match; they are covered below.

Cassandra's dynamic columns

Cassandra also allows individual rows to have their own sets of columns. In other words, each row can have its own unique columns.

These columns can be accessed through MariaDB's feature. To do so, one must define a column:

• with an arbitrary name

• of type blob

• with the DYNAMIC_COLUMN_STORAGE=yes attribute

Here is an example:

Once define, one can access individual columns with the new variant of the Dynamic Column functions, which now support string names (they used to support integers only).

Super columns

Cassandra's SuperColumns are not supported, there are currently no plans to support them.

Datatypes

There is no direct 1-to-1 mapping between Cassandra's datatypes and MySQL/MariaDB datatypes. Also, Cassandra's size limitations are often more relaxed than MySQL/MariaDB's. For example, Cassandra's limit on rowkey length is about 2G, while MySQL limits unique key length to about 1.5Kb.

The types must be mapped as follows:

For types like "VARBINARY(n)", n should be chosen sufficiently large to accommodate all the data that is encountered in the table.

Command mapping

INSERT

Cassandra doesn't provide any practical way to make INSERT different from UPDATE. Therefore, INSERT works as INSERT-or-UPDATE, it will overwrite the data, if necessary.

INSERT ... SELECT and multi-line INSERT will try to write data in batches. Batch size is controlled by the system variable, which specifies the max. batch size in columns.

The status variables and allow one to see whether inserts are actually batched.

UPDATE

UPDATE works like one would expect SQL's UPDATE command to work (i.e. changing a primary key value will result in the old record being deleted and a new record being inserted)

DELETE

• DELETE FROM cassandra_table maps to the truncate(column_family) call.

• The DELETE with WHERE clause will do per-row deletions.

SELECT

Generally, all SELECT statements work like one expects SQL to work. Conditions in the form primary_key=... allow the server to construct query plans which access Cassandra's rows with key lookups.

Full table scan

Full table scans are performed in a memory-efficient way. Cassandra SE performs a full table scan as a series of batches, each of which reads not more than records.

Batched Key Access support

Cassandra supports Batched Key Access in no-association mode. This means that it requires the SQL layer to do hashing, which means the following settings are required:

• optimizer_switch='join_cache_hashed=on'

• join_cache_level=7|8

Cassandra SE is currently unable to make use of space in the join buffer (the one whose size is controlled by ). Instead, it will limit read batches to reading not more than at a time, and memory are allocated on the heap.

Note that the buffer is still needed by the SQL layer, so its value should still be increased if you want to read in big batches.

It is possible to track the number of read batches, how many keys were looked-up, and how many results were produced with these status variables:

System and status variables

The following are available:

The following are available:

A note about Cassandra 1.2

Cassandra 1.2 has slightly changed its data model, as described at . This has caused some of Thrift-based clients to no longer work (for example, here's a problem experienced by Pig:).

Currently, Cassandra SE is only able to access Cassandra 1.2's column families that were defined WITH COMPACT STORAGE attribute.

See also

• Slides from talk at Percona Live 2013 -

• - JIRA task for Cassandra SE work

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