Important metrics and alerts

Information about various tools for monitoring clusters and which metrics are most useful.


The location of the cassandra.yaml file depends on the type of installation:
Package installations /etc/dse/cassandra/cassandra.yaml
Tarball installations installation_location/resources/cassandra/conf/cassandra.yaml

Monitoring Apache Cassandra® and DataStax Enterprise (DSE) clusters is a very important activity that allows you to identify problems in your clusters and react faster to mitigate them.

Both Apache Cassandra and DSE expose metrics for observation and analysis. Cassandra uses Java Management Extensions (JMX) to expose various metrics; allow temporary configuration changes, such as changing the compaction throughput; and provides the ability to execute actions, such as triggering compaction. JMX is also used by nodetool and other Cassandra tools. The different types of the exposed metrics are described in the Cassandra documentation.
Note: JMX is a technology within Java that provides tools for managing and monitoring applications.

You can use the following tools for collection of metrics for analysis:

  • Tools for one-off analysis, including JConsole, jmxterm, and nodetool sjk, use JMX and are described below.
  • DSE OpsCenter collects metrics using JMX, stores them in DSE, and uses them for visualization and alerts. Metrics collection requires that the DataStax Agent is running on your DSE nodes.
  • DSE Metrics Collector collects metrics from DSE and other entities, such as CPU and disks, using collectd.

    The DSE Metrics Collector also enables integration with different monitoring systems using collectd plugins. For example, you can expose data to Prometheus with visualization via Grafana using predefined dashboards. Because metrics are exposed directly, you don’t need the DataStax Agent running on your nodes.

  • The Metrics Collector for Apache Cassandra together with Prometheus and Grafana (also with predefined dashboards) provides the same functionality as DSE Metrics Collector.
  • External tools for integration with monitoring systems like Prometheus (via JMX Exporter for Prometheus) and other monitoring tools may require additional tuning and dashboard creation.

When using any of these methods, you get a lot of information. There are approximately 40 metrics per keyspace, 60 to 70 metrics per individual table, and even more metrics for different subsystems. The remainder of this topic provides guidance for understanding the most important metrics.

What do you need to monitor?

The important metrics that require monitoring can split into several groups:

  • Metrics related to client request:

    How the system performs from the point of view of the client application.

    • Coordinator level latency for read and write operations, especially for 95/99th percentiles.
    • Number of client connections.
  • Metrics related to threadpools that process data and execute different tasks:

    Examples include compaction, and flushing of data.

    • How many threads are in the blocked state. For example, memtable flush writer, memtable pool allocations, and so on.
    • How many threads are in the aborted state, like aborted compactions.
    • How many threads are in the pending state, such as pending compactions and pending flushes.
  • Metrics related to Thread-per-Core (TPC).

    Applies only to DSE 6.0 and later.

  • Metrics related to individual tables:

    It’s useful to track such metrics for your most important tables to make sure that SLAs are met and avoid problems.

    • Partition size.
    • Number of SSTables overall.
    • Number of SSTables read per request.
    • Number of tombstones scanned during read request.
    • Coordinator-level read and write latencies.
  • Metrics related to inter-cluster communication:

    These metrics provide information on how data exchange happens in the cluster: replication, hinted handoff, and so on:

    • Number of dropped mutations, and other messages.
    • Total number of timeouts and timeouts per host.
    • Cross-data center latency.
    • Number of hints on disk.
    • Hint replay (number of failed and timed out hint messages).
  • Metrics related to the Java Virtual Machine (JVM):
    • Amount of memory used.
    • Duration of garbage collection pauses.
  • Metrics related to operating system and hardware:
    • CPU usage on the node.
    • Amount of disk space available.

Important metrics exposed via JMX

The following list of metrics are recommended by DataStax for monitoring and generating alerts that cross their threshold. Note that some values, such as latency, are general recommendations and could be lower or higher depending on your requirements.

Read and write latencies (at the coordinator level)
Total and per keyspace/table.
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: 99pt greater than 200ms for more than 1 minute
Overall internode latency
JMX in MBean org.apache.cassandra.metrics:
Internode latency for datacenter with name DC-Name
JMX in MBean org.apache.cassandra.metrics:
Number of pending compaction tasks
Total per node, and/or for tables in specific keyspace.
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: more than 30 for more than 15 minutes.
Number of dropped mutations
Ttotal and/or per table in given keyspace.
JMX (in MBean org.apache.cassandra.metrics):
Alerting condition: Non-zero value.
Number of timeouts occurring on a specific node
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: Heavy number increase during the last 5 to 15 minutes.
Required reaction: Possible sign of network problems and similar problems.
Maximum partition size in bytes
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: Partitions greater than 100 Mb.
Required reaction: Alert development team, as this indicates problems with the data model.
Total number of SSTables in system and per table
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: More than 200 per individual table (depends on the used compaction strategy).
Required reaction: Too many big tables, which will lead to performance degradation.
Number of hints stored on individual node
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: Value greater than zero indicates that some nodes aren't reachable.
Hint Replay Success/Failure/Timeout Rate
JMX in MBean org.apache.cassandra.metrics:
Number threads are blocked by memtable allocation
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: Non-zero value
Number of blocked memtable flush writer tasks.
Note: This condition could lead to heavy write performance degradation.
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: Non-zero value.
Required reaction: Investigate. This condition caused by failing disks, excessive disk operations, and so on.
Number of blocked compaction tasks
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: Non-zero value.
Number of aborted compaction tasks
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: Non-zero value.
Information about Java's garbage collection
Caused by Mac GC Elapsed and similar.
JMX in MBean org.apache.cassandra.metrics:
Number of segments waiting on commit
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: High count during last minute.
99th percentile of time spent waiting on commit
JMX in MBean org.apache.cassandra.metrics:
Number of pending flushes
JMX in MBean org.apache.cassandra.metrics:
Hit ratio for key cache
Only Cassandra and DSE prior to version 6.0.
JMX in MBean org.apache.cassandra.metrics:
Alerting condition: Hit ratio is lower than 0.9.
Required reaction: If the cache is full (capacity is equal to size), increase the size of the key cache.

Visualizing important metrics in OpsCenter

OpsCenter collects metrics (6.8 | 6.7 | 6.5 | 6.1) from all nodes in a cluster and stores original data, together with rollups in the DSE cluster. This data is then used to create graphs and alerts. When using OpsCenter for monitoring, the following list of the metrics/graphs is useful for setting up an effective monitoring of the cluster:

  • Active Alerts
  • Cluster Health
  • Storage Capacity
  • Read and Write Request Latency
  • Read and Write Requests
  • Data Size
  • Compactions Pending
  • Dropped Messages: Mutations
  • Dropped Messages: Reads
  • Native Clients
  • For specific tables (setup for most important tables):
    • TBL: SSTables per read (percentiles)
    • TBL: Tombstones per read (percentiles)
    • TBL: Partition size (percentiles)
  • Related to hinted handoff:
    • Hints on Disk
    • TP: Hint Dispatcher Active
    • TP: Hint Dispatcher Completed
    • Dropped Messages: Hinted Handoff
  • Related to operating system:
    • OS: Disk Latency
    • OS: Load
    • OS: CPU Iowait
    • OS: Memory Free
  • Related to Java Virtual machine:
    • Heap Used
    • JVM G1 Old Collection Count and Time
    • JVM G1 Young Collection Count and Time
  • When DSE Search is enabled:
    • Search: Core Size
    • Search: Read Latency
    • Search: Timeouts
  • When NodeSync is enabled, NodeSync related:
    • TP: Read Range NodeSync Active
    • NodeSync: Uncompleted Pages, Failed Pages

Alerts in OpsCenter

OpsCenter alerts (6.8 | 6.7 | 6.5 | 6.1the operator when certain conditions are met. Examples include when a node is down and latency is too high for a long period of time. OpsCenter can deliver alerts by email, SNMP, and HTTP requests.

Configure the following alerts to operatively react to problems in DSE clusters.

Node Down
When a node is marked as down by OpsCenter.
Condition: “<event>” for more than X <minutes|hours|days>
Recommendation: X: “immediately” or “1 minute” (depending on whether some level of tolerance of the event is possible)
Criticality/Notification frequency: Urgent
Agent Issue
When a DataStax Agent is monitored having some issues.
Condition: “<event>” for more than X <minutes|hours|days>
Recommendation: X: “30 minutes”
Criticality/Notification frequency: Low
CPU Usage
The percentage of CPU was busy.
Condition: “<event>” is above X for more than Y <minutes|hours|days>
Recommendation: X: 100, Y: 1 hour
Criticality/Notification frequency: Low
The overall amount of work that a computer system performs.
Condition: “<event>” is above X for more than Y <minutes|hours|days>
Recommendation: X: 0.7 x total number of CPU cores, Y: 1 hour
Criticality/Notification frequency: High
Write Request Latency (percentiles)
The response time (in milliseconds) for successful write operations.
Condition: “<event>” is above X ms/op for more than Y <minutes|hours|days> for selected Z percentile
Recommendation: X: <depending on application SLA>, Y: 4 hours, Z: 99
Criticality/Notification frequency: Medium
Read Request Latency (percentiles)
The response time (in milliseconds) for successful read operations.
Condition: “<event>” is above X ms/op for more than Y <minutes|hours|days> for selected Z percentile
Recommendation: X: <depending on application SLA>, Y: 4 hours, Z: 99
Criticality/Notification frequency: Medium
Advanced -> System -> Disk Usage(%)
The percentage of disk being used for a particular disk partition.
Condition: “<event>” is above X % for more than Y <minutes|hours|days>
Recommendation: X: 50, Y: 4 hours
Criticality/Notification frequency: Medium
Advanced -> Tables -> TBL: SSTables per Read (percentiles)
For a specified percentile, how many SSTables are accessed during a read
Condition: “<event>” is above X SSTables for more than Y <minutes|hours|days> for W table at Z percentile.
Recommendation: X: 10, Y: 1 day, W: table of interest, Z: 99
Criticality/Notification frequency: Low
Advanced -> Tables -> TBL: Tombstones per Read (percentiles)
For a specified percentile, how many Tombstones are accessed during a read.
Condition: “<event>” is above X tombstones for more than Y <minutes|hours|days> for W table at Z percentile.
Recommendation: X: tombstone_warn_threshold in cassandra.yaml, Y: 1 day, W: table of interest, Z: 99
Criticality/Notification frequency: Low
Advanced -> Tables -> TBL: Partition Size (percentiles)
For a specified percentile, what is the size (in bytes) of partitions of this table.
Condition: “<event>” is above X for more than Y <minutes|hours|days> for W table at Z percentile.
Recommendation: X: 200 MB (in bytes), Y: 1 day, W: table of interest, Z: 99
Criticality/Notification frequency: Low

Tools for work with JMX

A number of tools exist for one-off analysis of specific metrics. Usually you only use these tools for debugging as they aren’t designed to replace monitoring solutions. These tools primarily provide access to individual metrics at the moment on a specific node. They do not generate a view of what happens over time or provide multiple metrics on multiple nodes.


Jconsole is a GUI tool included in a Java distribution, such as, OpenJDK. It allows easy browsing of the metrics and inspection of their values. It also provides the possibility to graph them over the time.
Note: Many more tools exist that are listed here.
Figure 1: Java Monitoring & Management Console

Java Monitoring and Management Console

To access metrics, JMX either needs to be running on the server (usually requiring installation of the GUI libraries), or network access, which exposes JMX to the externally and may have a security impact.


This is a very popular JMX command-line tool. After downloading, it’s easy to run and connect to a local node, or to other nodes when JMX is exposed externally:
$>open localhost:7199
#Connection to localhost:7199 is opened
You can access to specific metrics with commands like this:
$>get -b org.apache.cassandra.metrics:type=Cache,scope=KeyCache,name=Size Value
#mbean = org.apache.cassandra.metrics:type=Cache,scope=KeyCache,name=Size:
Value = 0;

To get a full list of supported commands, run help inside the interactive console. Besides getting the values for attributes, you can also set values for attributes (if they are settable), or call the function, which you can use to temporarily modify the behavior of Cassandra (the same way as nodetool commands).

nodetool sjk (DSE and Cassandra 4.0)

DSE provides nodetool sjk (6.8 | 6.7 | 6.0 | 5.1), which is a wrapper for the well-known library called Swiss Java Knife1 (SJK). This subcommand is really handy, as you don’t need to specify the DSE process PID or other parameters; you just provide necessary flags. For example, to get a hit rate of key cache, use following command, where -b flag specifies the name of the bean and -f specifies the field:
nodetool sjk mx -b "org.apache.cassandra.metrics:type=Cache,scope=KeyCache,name=Size" -mg -f Value

Similarly to jmxterm, you can use this command to set values (when settable) or call functions.

1 The scope of the SJK library is not limited to JMX. You can use it to get the thread dump, information about threads, and other functionality.