Cassandra Compaction Strategies

SSTables are immutable, so every update writes a new version and reads must merge across SSTables. Compaction merges them back together. The strategy decides how, and it is a per-table setting.

The strategies

Size-tiered (default)

Merges SSTables of similar size into larger ones. Cheap in write terms, but a partition's data can be spread across many SSTables, so reads may touch several.

Leveled

Organises SSTables into levels where each level is ten times the previous, and guarantees a partition appears in at most one SSTable per level. Reads typically touch far fewer SSTables.

The cost is write amplification: a row may be rewritten several times as it moves through the levels. Use it for tables that are read often and updated in place, and avoid it for pure write-heavy ingest.

ALTER TABLE shop.user_profiles
WITH compaction = {'class': 'LeveledCompactionStrategy', 'sstable_size_in_mb': 160};

Time-window

Groups SSTables by time window and compacts only within a window. Once a window is closed, its SSTables are never rewritten.

This is the right strategy for time-series data with TTL: an entire window expires and its SSTables are dropped whole, without the rewriting that other strategies do.

CREATE TABLE metrics (
    device_id uuid, bucket date, ts timestamp, value double,
    PRIMARY KEY ((device_id, bucket), ts)
) WITH compaction = {
    'class': 'TimeWindowCompactionStrategy',
    'compaction_window_unit': 'DAYS',
    'compaction_window_size': 1
  }
  AND default_time_to_live = 7776000
  AND gc_grace_seconds = 3600;

Monitoring compaction

nodetool compactionstats            # what is running and how much remains
nodetool tablestats shop.events     # SSTable count per table
nodetool tablehistograms shop events  # SSTables touched per read

A rising SSTable count and a growing pending-compaction backlog mean compaction cannot keep up with the write rate. Options: raise compaction_throughput_mb_per_sec, add concurrent_compactors, use faster storage, or reduce the write rate.

nodetool setcompactionthroughput 128     # MB/s, takes effect immediately