Thursday, September 21, 2017

Creating test data for Apache Cassandra cluster

In many cases you are required to generate test data for Cassandra to do various type of testing. Read the rest of the blogpost if this is something you need and at the end, you should be able to create some test data very quickly.

I use the Ubuntu 16.04.2 LTS and Apache Cassandra 3.0 for this testing. The Cassandra cluster that I'm using here is deployed on AWS but this should not be a consideration factor for test data creation.

Use the steps mentioned below to create the test data.

    1. Download the csv file

curl -O

Note. You can download this file directly on to the EC2 but due to some formatting issue it did not work correctly for me. I downloaded the file to my local machine (Mac) first and then opened the file using TextEditor then copied the contents to the EC2.

    2. Create the csv file on EC2

I just used the vi editor to open a new file called, realstatesdata.csv and then pasted the file contents (including headers) that I copied in step #1. Save the file. (You should be familiar with the VI editor to perform this step.)

After completing this step, you now have "realstatesdata.csv" on EC2.

    3. Connect to the Cassandra cluster using cqlsh

    4. Create a Keyspace

You need to create a Keyspace (in general terms, a database) which is a high level hierarchical object to contain Cassandra tables. The keyspace name I've chosen is "kp_realstate" but feel free to have any name which you want. 

    5. Create a table

You also need to have a table which is the actual object which contains your real data. Unlike MongoDB, you need to have a the table schema created before you insert any data. I already analyzed the data set in csv file we just downloaded and decided the columns based on that. The table name is "realestate_data" again its your choice.

Note that, the order of the columns in CREATE TABLE statement should be the same as the order it appears in the csv file.

    6.  Load the data

You use the COPY command to load data from a file to Cassandra.

COPY kp_realstate.realestate_data (street,city,zip,state,beds,baths,sq__ft,type,sale_date,price,latitude,longitude) FROM 'realstatesdata.csv' WITH HEADER = TRUE;

It has to be executed at CQL prompt.

If the import is successful you will see the messages like below.

Processed: 985 rows; Rate:    1151 rows/s; Avg. rate:    1865 rows/s
985 rows imported from 1 files in 0.528 seconds (0 skipped).

   7. Make sure the data is imported successfully

At CQL prompt, you can execute any of the statements below.

select count(*) from kp_realstate.realestate_data;
select * from kp_realstate.realestate_data limit 30;

At the end, you've full data set for your testing.


Wednesday, September 13, 2017

JEMalloc and Cassandra

Memory management in Cassandra

Cassandra depends on JVM-Java Virtual Machine, to accomplish Cassandra's memory management requirement. The JVM mainly divided into two areas as follows;
  1. Heap - data area which contains the runtime structures. 
  2. Internal data structures - Java methods, thread stack and native methods. 

Cassandra uses its memory in four ways which are mentioned below. This includes OS memory too.
  1. Java heap
  2. Offheap memory (OS memory that is not managed by JVM G.C-Garbage Collector)
  3. OS page cache
  4. OS TCP/IP stack I/O cache

Since Cassandra uses JVM for its memory management, tuning of JVM is necessary to get optimal performance in Cassandra. The tuning of JVM includes the changing the settings in as mentioned below;

What is JEMalloc?

JEMalloc is an enhanced memory allocator in Linux based platforms. With JEMalloc, the memory allocation for multithreaded applications scales well as the no.of processors' increases. The previously used memory allocator, malloc(3) suffered scalability bottleneck for some multithreaded applications that caused JEMalloc to emerged. 

Use of JEMalloc has been introduced in Cassandra after 2.0. 

Ensure JNA-Java Native Access and JEMalloc are installed on Linux AMI. If you're creating an Amazon AMI for Cassandra, then you want to install both of these. 

yum install -y jna
yum install -y jemalloc

Cassandra.yaml configuration requires the change mentioned below in order to use JEMalloc. 

memtable_allocation_type: offheap_objects

Note. The above setting is set to "heap_buffers" by default. 

What is the benefit of using JEMalloc in Cassandra

By enabling JEMalloc in Cassandra, it reduces the amount of Java heap space that Cassandra uses. Data written to Cassandra is first stored in memtables in heap memory. Memtables are then flushed to SStables on disk when they get full. The garbage collection process of JVM is used to clear the heap memory. Sometimes, this garbage collection process causes issues in Cassandra due to garbage collection pause. 

The benefit of JEMalloc is, it reduces the pressure of garbage collection because Cassandra uses off-heap memory allocation with JEMalloc.