Choice of Tool

New Relic

http://newrelic.com/plugins/3legs/113
Monitor Cassandra statistics using the 3legs plugin. Metrics include Read and Write latency (global and per host), Cache statistics, Pending compactions, flushes and more.

Datastax - OpsCenter

http://www.datastax.com/documentation/opscenter/4.1/pdf/opscuserguide41.pdf
DataStax OpsCenter is a visual management and monitoring solution for Apache Cassandra and DataStax Enterprise.
The DataStax agents are installed on the Real-time (Cassandra), Analytics (Hadoop), and Search (Solr) nodes. They use Java Management Extensions (JMX) to monitor and manage each node. Cassandra exposes a number of statistics and management operations through JMX. Using JMX, OpsCenter obtains metrics from a cluster and issues various node administration commands, such as flushing SSTables or doing a repair. It monitors a variety of metrics at the system and cassandra cluster level including performance and column family metrics.

JConsole

JConsole is a graphical monitoring tool to monitor Java Virtual Machine (JVM) and Java applications both on a local or remote machine.
JConsole uses underlying features of Java Virtual Machine to provide information on performance and resource consumption of applications running on the Java platform using Java Management Extensions (JMX) technology.
JConsole comes as part of Java Development Kit (JDK) and the graphical console can be started using “jconsole” command.

Nagios

http://exchange.nagios.org/components/com_mtree/attachment.php?link_id=3819&cf_id=24
For Cassandra, foll. metrics are monitored:
Storage Load - The amount of data stored on each Cassandra node
Pending Tasks - The number of basic task stages waiting to run
Active Tasks - The number of basic task stages currently running
Blocked Tasks - The number of basic task stages blocked from running
Pending Internal Tasks - The number of internal task stages waiting to run
Active Internal Tasks - The number of internal task stages currently running
And foll. JVM Metrics
Active JVM Threads
JVM Heap memory usage
JVM Non-Heap memory usage

Monitoring through Datastax OpsCenter

Installing OpsCenter Console 4.1

Configure /etc/yum.repos.d/datastax.repo

sudo yum install opscenter  
service opscenterd start  

Connect to the console using http://:8888 Add an existing Cassandra cluster from the Console, by clicking on “Add Cluster” and adding the IP of one of the Cassandra Cluster Nodes (JMX Port:7199, Thrift Port:9160)
All the nodes in the cluster are now identified.
/etc/opscenter/opscenterd.conf is the configuration file

Installing OpsCenter Agent

Add the DataStax Yum repository in the /etc/yum.repos.d/datastax.repo file

yum install datastax-agent

In address.yaml set stomp_interface to the IP address that OpsCenter is using.

echo "stomp_interface: 10.132.56.93"|tee -a /var/lib/datastax-agent/conf/address.yaml

If SSL communication is enabled in /etc/opscenter/opscenterd.conf , use SSL in address.yaml.

echo "use_ssl: 1" | sudo tee -a /var/lib/datastax-agent/conf/address.yaml

Start the DataStax agent.

service datastax-agent start

Perform above step on each node in the cluster

Configuring Access

python /usr/share/opscenter/bin/set_passwd.py <username> admin 
service opscenterd restart  

Enabling HTTPS

vi /etc/opscenter/opscenterd.conf and uncomment the ssl lines
Restart Opscenter daemon
Access console with https://:8443/opscenter/index.html

HA Setup for OpsCenter

Configure VIP and attach it to Primary Opscenter
On Standy Opscenter, copy all the necessary congfis.
On agent side, configure communication to VIP
#Metrics and Alerts Configuration
All configured alerts in the console are stored in OpsCenter Keyspace, in column family Settings, with rowkey global-cluster-alert-rules and each column will represent a metric alert
We need to identify the metrics and thresholds we want to monitor.
We are not using the mail alerting feature, instead we are using the Web URL alerting feature to send mails. This is because the mail alerting feature is not configurable to send mails to different DLs based on the threshold and also customize the mail subject. (The subject will always be static, whereas we want it to have format like alert_level:hostname - alert name

Configure Apache Web Server

yum install 
httpd /sbin/service httpd start  

Configuring PHP for Apache

yum install php --> to install the libphp5 module? 
vi /etc/httpd/conf/httpd.conf --> and add below line
LoadModule php5_module modules/libphp5.so  

Configuring Sendmail

yum install sendmail 
sendmail.cf and submit.cf

Configuring the Alerting Script

cat /etc/opscenter/event-plugins/posturl.conf  
[posturl] 
enabled=1 
url=http://<hostname>/postOPSCevents.php 
# levels can be comma delimited list of any of the following:  
# DEBUG,INFO,WARN,ERROR,CRITICAL,ALERT 
# If left empty, will listen for all levels 
levels=WARN,ERROR,CRITICAL,ALERT  

This script will send warning to Warning mail_id and critical alerts to Critical mail_id.

Monitoring through New Relic

https://github.com/threelegs/newrelic-plugins
Download the plugin from above link onto one of the nodes in the cassandra cluster. Else you can download from New Relic Console, under Plugins Section.

ls /tmp/newrelic_3legs_plugin-0.0.2-cassandra.tar.gz 
/tmp/newrelic_3legs_plugin-0.0.2-cassandra.tar.gz 
cd /root/ gunzip newrelic_3legs_plugin-0.0.2-cassandra.tar.gz 
cd config  

Configure activation.conf, application.conf, newrelic.properties, /etc/init.d/newrelic_3legs

/sbin/service newrelic_3legs start

Now on New Relic Console, you could see Cassandra on the left hand side clicking which you could see the cluster PCS-Cassandra we configured.
Even with one host agent running, it was showing metrics for all hosts
Click on the Settings icon and configure thresholds for Read Latency, Write Latency and Down Hosts.