nodetool invalidatekeycache

Invalidates the key cache on the node.

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Synopsis

nodetool [connection_options] invalidatekeycache

See connection options for connection options.

Description

nodetool invalidatekeycache clears all entries from the key cache on the node. The key cache stores partition index entries, allowing Cassandra to skip disk seeks when locating partitions within SSTables.

After invalidation, the cache is empty and will be repopulated as subsequent read operations occur. This causes a temporary performance impact until the cache is warm again.

Performance Impact

Invalidating the key cache causes temporary read latency increases as cached partition locations must be re-read from disk. Use during maintenance windows when possible.

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Examples

Basic Usage

nodetool invalidatekeycache

Invalidate and Monitor Refill

nodetool invalidatekeycache
watch -n 5 'nodetool info | grep "Key Cache"'

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Key Cache Overview

What the Key Cache Stores

The key cache stores partition key to SSTable position mappings:

Cached Data	Purpose
Partition key hash	Identify the partition
SSTable file reference	Which SSTable contains the data
Position offset	Byte offset within the SSTable

How It Improves Performance

Without Key Cache:
  Read Request → Bloom Filter → Partition Index (disk) → Data (disk)

With Key Cache Hit:
  Read Request → Key Cache → Data (disk)
  (Skips partition index read)

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When to Use

After Schema Changes

Clear stale cache entries after significant schema modifications:

# After ALTER TABLE operations that affect partition structure
nodetool invalidatekeycache

Troubleshooting Stale Data

If cached data may be causing issues:

# Clear potentially stale cache
nodetool invalidatekeycache

# Verify issue resolution

Memory Pressure

During memory emergencies (though reducing cache size may be better):

# Immediate memory release
nodetool invalidatekeycache

Before Performance Testing

Ensure cold-cache baseline measurements:

# Clear all caches for baseline test
nodetool invalidatekeycache
nodetool invalidaterowcache
nodetool invalidatecountercache

# Run performance tests

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Impact Assessment

Immediate Effects

Aspect	Impact
Key cache entries	All cleared
Memory usage	Temporarily reduced
Disk I/O	Increases (partition index reads)
Read latency	Temporarily increases

Recovery Timeline

Phase	Duration	Cache State
Immediately after	0	Empty
Warming up	Minutes to hours	Partially filled
Fully warm	Hours (workload dependent)	Optimally filled

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Monitoring Cache Status

Before and After

# Check cache status before
echo "Before invalidation:"
nodetool info | grep -A 5 "Key Cache"

# Invalidate
nodetool invalidatekeycache

# Check immediately after
echo "After invalidation:"
nodetool info | grep -A 5 "Key Cache"

Monitor Refill Progress

monitor_keycache_refill.sh

#!/bin/bash
echo "=== Key Cache Refill Monitor ==="
echo "Press Ctrl+C to stop"
echo ""

while true; do
    info=$(nodetool info 2>/dev/null | grep -A 1 "Key Cache")
    entries=$(echo "$info" | grep "entries" | awk '{print $4}')
    hit_rate=$(echo "$info" | grep "hit rate" | awk '{print $4}')
    size=$(echo "$info" | grep "size" | awk '{print $4}')

    echo "$(date '+%H:%M:%S') - Entries: $entries, Hit Rate: $hit_rate, Size: $size"
    sleep 10
done

Cache Statistics

# Detailed cache statistics
nodetool info | grep -E "Key Cache|Row Cache|Counter Cache"

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Workflow: Controlled Cache Clear

controlled_cache_clear.sh

#!/bin/bash
echo "=== Controlled Key Cache Clear ==="

# 1. Check current state
echo "1. Current key cache status:"
nodetool info | grep -A 2 "Key Cache"

# 2. Record baseline metrics
echo ""
echo "2. Baseline read latencies:"
nodetool proxyhistograms | head -20

# 3. Confirm
echo ""
read -p "Proceed with key cache invalidation? (y/n): " confirm
if [ "$confirm" != "y" ]; then
    echo "Aborted."
    exit 1
fi

# 4. Invalidate
echo ""
echo "4. Invalidating key cache..."
nodetool invalidatekeycache

# 5. Verify invalidation
echo ""
echo "5. Cache status after invalidation:"
nodetool info | grep -A 2 "Key Cache"

# 6. Monitor recovery
echo ""
echo "6. Monitoring cache refill (30 seconds)..."
for i in {1..6}; do
    sleep 5
    entries=$(nodetool info | grep -A 1 "Key Cache" | grep "entries" | awk '{print $4}')
    echo "  $(date '+%H:%M:%S') - Entries: $entries"
done

echo ""
echo "=== Complete ==="
echo "Continue monitoring with: watch 'nodetool info | grep -A 2 \"Key Cache\"'"

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Cluster-Wide Operations

Invalidate on All Nodes

invalidate_keycache_cluster.sh

#!/bin/bash
echo "WARNING: Invalidating key cache cluster-wide!"
echo "This will cause temporary read latency increases."
echo ""

# Get list of node IPs from local nodetool status
nodes=$(nodetool status | grep "^UN" | awk '{print $2}')

for node in $nodes; do
    echo -n "$node: "
    ssh "$node" "nodetool invalidatekeycache" 2>/dev/null && echo "invalidated" || echo "FAILED"
done

echo ""
echo "Monitor recovery with: nodetool info | grep 'Key Cache'"

Rolling Invalidation

For minimal impact, invalidate one node at a time:

rolling_keycache_invalidate.sh

#!/bin/bash
# Get list of node IPs from local nodetool status
nodes=$(nodetool status | grep "^UN" | awk '{print $2}')
WAIT_TIME=300  # 5 minutes between nodes

for node in $nodes; do
    echo "$(date): Invalidating key cache on $node..."
    ssh "$node" "nodetool invalidatekeycache"

    echo "Waiting ${WAIT_TIME}s for cache to warm..."
    sleep $WAIT_TIME

    # Check hit rate before proceeding
    hit_rate=$(ssh "$node" "nodetool info" 2>/dev/null | grep -A 1 "Key Cache" | grep "hit rate" | awk '{print $4}')
    echo "  Hit rate: $hit_rate"
done

echo "Rolling invalidation complete."

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Configuration

Key Cache Size

cassandra.yaml

key_cache_size_in_mb:  # Leave empty for auto (5% of heap)
key_cache_save_period: 14400  # Save interval in seconds
key_cache_keys_to_save:  # Number of keys to save (empty = all)

Adjust Cache Size

# Check current size
nodetool info | grep "Key Cache"

# If cache is too small/large, adjust in cassandra.yaml and restart
# Or consider per-table caching settings

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Troubleshooting

Low Hit Rate After Warmup

# Check cache size vs working set
nodetool info | grep "Key Cache"

# If entries near capacity with low hit rate,
# consider increasing cache size

Cache Not Refilling

# Check if caching is enabled for tables
nodetool describecluster

# Check table-level caching
cqlsh -e "SELECT table_name, caching FROM system_schema.tables WHERE keyspace_name = 'my_keyspace';"

Memory Issues

# If key cache is using too much memory
# Consider reducing size in cassandra.yaml

# Check current usage
nodetool info | grep "Key Cache" | grep "size"

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Best Practices

Key Cache Guidelines

1. Avoid frequent invalidation - Let cache warm naturally
2. Use during maintenance windows - Minimize user impact
3. Monitor after invalidation - Track cache refill and hit rates
4. Consider rolling approach - Invalidate one node at a time
5. Check cache sizing - Ensure cache is appropriately sized
6. Document the reason - Record why cache was invalidated

When NOT to Invalidate

• During peak traffic hours
• Without understanding the performance impact
• Routinely (let cache manage itself)
• On all nodes simultaneously in production

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Related Commands

Command	Relationship
invalidaterowcache	Invalidate row cache
invalidatecountercache	Invalidate counter cache
info	View cache statistics
setcachecapacity	Adjust cache sizes