Guide to Cassandra Thread Pools

Posted in: Cassandra, Technical Track

This guide provides a description of the different thread pools and how to monitor them. Includes what to alert on, common issues and solutions.


Cassandra is based off of a Staged Event Driven Architecture (SEDA).  This separates different tasks in stages that are connected by a messaging service.  Each like task is grouped into a stage having a queue and thread pool (ScheduledThreadPoolExecutor more specifically for the Java folks).  Some stages skip the messaging service and queue tasks immediately on a different stage if it exists on the same node.  Each of these queues can be backed up if execution at a stage is being over run.  This is a common indication of an issue or performance bottleneck.To demonstrate take for example a read request:


Note: For the sake of this example the data exists on a different node then the initial request, but in some scenarios this could all be on a single node.  There are also multiple paths a read may take.  This is just for demonstration purposes.
  1. NODE1> The request is queued from node1 on the ReadStage of node2 with the task holding a reference to a callback
  2. NODE2> The ReadStage will pull the task off the queue once one of its 32 threads become available
  3. NODE2> The task will execute to and compute the resulting Row, which will be sent off to the RequestResponse stage of node1
  4. NODE1> One of the four threads in the RequestResponseStage will process the task, and pass the resulting Row to the callback referenced from step 1 which will return to the client.
  5. NODE1> It will also possibly kick off an async read repair task on node1 to be processed in the ReadRepairStage once one of its four threads become available

The ReadRepairStage and how long it takes to process its work is not in the feedback loop from the client.  This means if the rate of reads occurs at a rate higher than the rate of read repairs the queue will grow until it is full without the client application ever noticing.  At the point of the queue being full (varies between stages, more below) the act of enqueueing a task will block.

In some stages the processing of these tasks are not critical.  These tasks are marked as “DROPPABLE” meaning the first thing the stage will do when executing it is to check if its exceeded past a timeout from when it was created.  If the timeout has passed it will throw it away instead of processing.

Accessing metrics


For manual debugging this is the output given by

nodetool tpstats

Pool Name                    Active   Pending      Completed   Blocked  All time blocked
ReadStage                         0         0         113702         0                 0
RequestResponseStage              0         0              0         0                 0
MutationStage                     0         0         164503         0                 0
ReadRepairStage                   0         0              0         0                 0
ReplicateOnWriteStage             0         0              0         0                 0
GossipStage                       0         0              0         0                 0
AntiEntropyStage                  0         0              0         0                 0
MigrationStage                    0         0              0         0                 0
MemoryMeter                       0         0             35         0                 0
MemtablePostFlusher               0         0           1427         0                 0
FlushWriter                       0         0             44         0                 0
MiscStage                         0         0              0         0                 0
PendingRangeCalculator            0         0              1         0                 0
commitlog_archiver                0         0              0         0                 0
InternalResponseStage             0         0              0         0                 0
HintedHandoff                     0         0              0         0                 0

Message type           Dropped
RANGE_SLICE                  0
READ_REPAIR                  0
PAGED_RANGE                  0
BINARY                       0
READ                         0
MUTATION                     0
_TRACE                       0
REQUEST_RESPONSE             0
COUNTER_MUTATION             0

The description of the pool values (Active, Pending, etc) is defined in the table below. The second table lists dropped tasks by message type, for more information on that read below.


These are all available via JMX as well in:




with the attributes (relative to tpstats) being:

MBean Attribute tpstats name Description
ActiveCount Active Number of tasks pulled off the queue with a Thread currently processing.
PendingTasks Pending Number of tasks in queue waiting for a thread
CompletedTasks Completed Number of tasks completed
CurrentlyBlockedTasks Blocked When a pool reaches its max thread count (configurable or set per stage, more below) it will begin queuing until the max size is reached.  When this is reached it will block until there is room in the queue.
TotalBlockedTasks All time blocked Total number of tasks that have been blocked

 Metrics Reporters

Cassandra has pluggable metrics reporting capabilities you can read more about here

Droppable Messages

The second table in nodetool tpstats displays a list of messages that were DROPPABLE. These ran after a given timeout set per message type so was thrown away. In JMX these are accessible via or org.apache.cassandra.metrics:DroppedMessage. The following table will provide a little information on each type of message.

Message Type Stage Notes
BINARY n/a This is deprecated and no longer has any use
_TRACE n/a (special) Used for recording traces (nodetool settraceprobability) Has a special executor (1 thread, 1000 queue depth) that throws away messages on insertion instead of within the execute
MUTATION MutationStage If a write message is processed after its timeout (write_request_timeout_in_ms) it either sent a failure to the client or it met its requested consistency level and will relay on hinted handoff and read repairs to do the mutation if it succeeded.
COUNTER_MUTATION MutationStage If a write message is processed after its timeout (write_request_timeout_in_ms) it either sent a failure to the client or it met its requested consistency level and will relay on hinted handoff and read repairs to do the mutation if it succeeded.
READ_REPAIR MutationStage Times out after write_request_timeout_in_ms
READ ReadStage Times out after read_request_timeout_in_ms. No point in servicing reads after that point since it would of returned error to client
RANGE_SLICE ReadStage Times out after range_request_timeout_in_ms.
PAGED_RANGE ReadStage Times out after request_timeout_in_ms.
REQUEST_RESPONSE RequestResponseStage Times out after request_timeout_in_ms. Response was completed and sent back but not before the timeout


Stage Details

Below lists a majority of exposed stages as of 2.0.7, but they tend to be a little volatile so if they do not exist don’t be surprised.  The heading of each section is correlates with the name of the stage from nodetool tpstats. The notes on the stages mostly refer to the 2.x data but some references to things in 1.2. 1.1 and previous versions are not considered at all in making of this document.

Alerts; given as a boolean expression, are intended as a starting point and should be tailored to specific use cases and environments in the case of many false positives.

When listed as “number of processors” its value is retrieved first by the system property “cassandra.available_processors” which can be overridden by adding a


to JVM_OPTS. Falls back to default of Runtime.availableProcessors



Performing a local read. Also includes deserializing data from row cache.  If there are pending values this can cause increased read latency.  This can spike due to disk problems, poor tuning, or over loading your cluster.  In many cases (not disk failure) this is resolved by adding nodes or tuning the system.

JMX beans: org.apache.cassandra.request.ReadStage
Number of threads: concurrent_reads (default: 32)
Max pending tasks: 231-1
Alerts: pending > 15 || blocked > 0


When a response to a request is received this is the stage used to execute any callbacks that were created with the original request

JMX beans: org.apache.cassandra.request.RequestResponseStage
Number of threads: number of processors
Max pending tasks: 231-1
Alerts: pending > 15 || blocked > 0


Performing a local including:

  • insert/updates
  • Schema merges
  • commit log replays
  • hints in progress

Similar to ReadStage, an increase in pending tasks here can be caused by disk issues, over loading a system, or poor tuning. If messages are backed up in this stage, you can add nodes, tune hardware and configuration, or update the data model and use case.

JMX beans: org.apache.cassandra.request.MutationStage
Number of threads: concurrent_writers (default: 32)
Max pending tasks: 231-1
Alerts: pending > 15 || blocked > 0


Performing read repairs. Chance of them occurring is configurable per column family with read_repair_chance. More likely to back up if using CL.ONE (and to lesser possibly other non-CL.ALL queries) for reads and using multiple data centers. It will then be kicked off asynchronously outside of the queries feedback loop, demonstrated in the diagram above. Note that this is not very likely to be a problem since does not happen on all queries and is fast providing good connectivity between replicas. The repair being droppable also means that after write_request_timeout_in_ms it will be thrown away which further mitigates this. If pending grows attempt to lower the rate for high read CFs:

ALTER TABLE column_family WITH read_repair_chance = 0.01;
JMX beans: org.apache.cassandra.request.ReadRepairStage
Number of threads: number of processors
Max pending tasks: 231-1
Alerts: pending > 15 || blocked > 0


CounterMutation in 2.1, also counters changing dramatically so post 2.0 should consider this obsolete. Performs counter writes on non-coordinator nodes and replicates after a local write. This includes a read so can be pretty expensive. Will back up if the rate of writes exceed the rate that the mutations can occur. Particularly possible with CL.ONE and high counter increment workloads.

JMX beans: org.apache.cassandra.request.ReplicateOnWriteStage
Number of threads: concurrent_replicates (default: 32)
Max pending tasks: 1024 x number of processors
Alerts: pending > 15 || blocked > 0


Post 2.0.3 there should no longer be issue with pending tasks. Instead monitor logs for a message:

Gossip stage has {} pending tasks; skipping status check ...

Before that change, in particular older versions of 1.2, with a lot of nodes (100+) while using vnodes can cause a lot of cpu intensive work that caused the stage to get behind.
Been known to of been caused with out of sync schemas. Check NTP working correctly and attempt nodetool resetlocalschema or the more drastic deleting of system column family folder.

JMX beans: org.apache.cassandra.internal.GossipStage
Number of threads: 1
Max pending tasks: 231-1
Alerts: pending > 15 || blocked > 0


Repairing consistency. Handle repair messages like merkle tree transfer (from Validation compaction) and streaming.

JMX beans: org.apache.cassandra.internal.AntiEntropyStage
Number of threads: 1
Max pending tasks: 231-1
Alerts: pending > 15 || blocked > 0


Making schema changes

JMX beans: org.apache.cassandra.internal.MigrationStage
Number of threads: 1
Max pending tasks: 231-1
Alerts: pending > 15 || blocked > 0


Operations after flushing the memtable. Discard commit log files that have had all data in them in sstables. Flushing non-cf backed secondary indexes.

JMX beans: org.apache.cassandra.internal.MemtablePostFlusher
Number of threads: 1
Max pending tasks: 231-1
Alerts: pending > 15 || blocked > 0


Sort and write memtables to disk. A vast majority of time this backing up is from over running disk capability. The sorting can cause issues as well however. In the case of sorting being a problem, it is usually accompanied with high load but a small amount of actual flushes (seen in cfstats). Can be from huge rows with large column names. i.e. something inserting many large values into a cql collection. If overrunning disk capabilities, it is recommended to add nodes or tune the configuration.

JMX beans: org.apache.cassandra.internal.FlushWriter
Number of threads: memtable_flush_writers (1 per data directory)
Max pending tasks: memtable_flush_queue_size (default: 4)
Alerts: pending > 15 || blocked > 0


Snapshotting, replicating data after node remove completed.

JMX beans: org.apache.cassandra.internal.MiscStage
Number of threads: 1
Max pending tasks: 231-1
Alerts: pending > 15 || blocked > 0


Responding to non-client initiated messages, including bootstrapping and schema checking

JMX beans: org.apache.cassandra.internal.InternalResponseStage
Number of threads: number of processors
Max pending tasks: 231-1
Alerts: pending > 15 || blocked > 0


Sending missed mutations to other nodes. Usually a symptom of a problem elsewhere so dont treat as root issue. Can use nodetool disablehandoff to prevent further saving. Can also use JMX (nodetool truncatehints) to clear the handoffs for specific endpoints or all of them with org.apache.cassandra.db:HintedHandoffManager operations. This must be followed up with repairs!

JMX beans: org.apache.cassandra.internal.HintedHandoff
Number of threads: max_hints_delivery_threads (default: 1)
Max pending tasks: 231-1
Alerts: pending > 15 || blocked > 0


Measures memory usage and live ratio of a memtable. The pending should not grow beyond size of number of memtables (front ended by set to prevent duplicates). This can take minutes for large memtables.

JMX beans: org.apache.cassandra.internal.MemoryMeter
Number of threads: 1
Max pending tasks: 231-1
Alerts: pending > column_family_count || blocked > 0


Calculates the token ranges based on bootstrapping and leaving nodes. Instead of blocking this stage discards tasks when one already in progress so no value in monitoring

JMX beans: org.apache.cassandra.internal.PendingRangeCalculator
Number of threads: 1
Max pending tasks: 1


(Renamed CommitLogArchiver in 2.1) Executes a command to copy (or user defined command) commit log files for recovery. Read more at DataStax Dev Blog

Alerts:pending > 15 || blocked > 0

JMX beans: org.apache.cassandra.internal.commitlog_archiver
Number of threads: 1
Max pending tasks: 231-1


Number of active repairs that are in progress. Will not show up until after a repair has been run so any monitoring utilities should be able to handle it not existing.

Alerts:pending > 1 || blocked > 0

JMX beans: org.apache.cassandra.internal.AntiEntropySessions
Number of threads: 4
Max pending tasks: 231-1

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