sparkey-java

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Java implementation of the Sparkey key value store

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This is the java version of sparkey. It's not a binding, but a full reimplementation.
See [Sparkey](http://github.com/spotify/sparkey) for more documentation on how it works.

### Travis
Continuous integration with [travis](https://travis-ci.org/spotify/sparkey-java).

[![Build Status](https://travis-ci.org/spotify/sparkey-java.svg?branch=master)](https://travis-ci.org/spotify/sparkey-java)

### Dependencies

* Java 6 or higher
* Maven

### Building

mvn package

### Changelog
See [changelog](CHANGELOG.md).

### Usage

Sparkey is meant to be used as a library embedded in other software.

To import it with maven, use this for Java 8 or higher:

<dependency>
      <groupId>com.spotify.sparkey</groupId>
      <artifactId>sparkey</artifactId>
      <version>3.7.0</version>
    </dependency>

Use this for Java 6 or 7:

<dependency>
      <groupId>com.spotify.sparkey</groupId>
      <artifactId>sparkey</artifactId>
      <version>2.3.2</version>
    </dependency>

To help get started, take a look at the
[API documentation](http://spotify.github.io/sparkey-java/apidocs/2.0.0-SNAPSHOT/)
or an example usage: [SparkeyExample](src/test/java/com/spotify/sparkey/system/SparkeyExample.java)

### License

Apache License, Version 2.0

### Performance

#### Java 22+ Optimizations (Recommended)

Sparkey-java achieves best performance on **Java 22+** with **uncompressed** data using MemorySegment-based
zero-copy I/O. The optimizations are automatically enabled when running on Java 22 or higher.

**Benchmark results** (Intel Xeon @ 2.2GHz, 100K entries, Java 25):

| Configuration | Lookup Time | vs Java 8 | Notes |
|--------------|-------------|-----------|-------|
| **Uncompressed, 8 threads (optimal)** | **400 ns/op** | **38% faster** | Best performance |
| Uncompressed, single-threaded | 469 ns/op | 5% faster | |
| Uncompressed, 16 threads | 937 ns/op | 10% faster | |
| Uncompressed, 32 threads | 1,919 ns/op | 15% faster | |
| Compressed SNAPPY, 8 threads | 3,367 ns/op | Same | 8.4x slower than uncompressed |
| Compressed ZSTD, 8 threads | 9,987 ns/op | Same | 25x slower than uncompressed |

**Performance guidance:**
- **Best**: Java 22+, uncompressed data, 8-16 threads → **~400-900 ns/op**
- **Good**: Java 8, uncompressed data → **~500-650 ns/op** per lookup
- **Acceptable**: Compressed SNAPPY → **~3,000 ns/op** (useful when disk space is limited)
- **Slower**: Compressed ZSTD → **~10,000 ns/op** (best compression ratio, CPU intensive)

For maximum performance:
1. Use Java 22+ (or newer)
2. Use uncompressed data (CompressionType.NONE)
3. Use 8-16 threads for concurrent workloads
4. Ensure dataset fits in RAM (or use mlock for predictable performance)

Older Java versions (8-21) continue to work with identical functionality, just without the zero-copy optimizations.

#### Historical Random Lookup Benchmarks

This data is from earlier versions, using the JMH plugin on `Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz`:

Benchmark             (numElements)  (type)   Mode  Cnt        Score         Error  Units
    LookupBenchmark.test           1000    NONE  thrpt    4  4768851.493 ±  934818.190  ops/s
    LookupBenchmark.test          10000    NONE  thrpt    4  4404571.185 ±  952648.631  ops/s
    LookupBenchmark.test         100000    NONE  thrpt    4  3892021.755 ±  560657.509  ops/s
    LookupBenchmark.test        1000000    NONE  thrpt    4  2748648.598 ± 1345168.410  ops/s
    LookupBenchmark.test       10000000    NONE  thrpt    4  1921539.397 ±   64678.755  ops/s
    LookupBenchmark.test      100000000    NONE  thrpt    4     8576.763 ±   10666.193  ops/s
    LookupBenchmark.test           1000  SNAPPY  thrpt    4   776222.884 ±   46536.257  ops/s
    LookupBenchmark.test          10000  SNAPPY  thrpt    4   707242.387 ±  460934.026  ops/s
    LookupBenchmark.test         100000  SNAPPY  thrpt    4   651857.795 ±  975531.050  ops/s
    LookupBenchmark.test        1000000  SNAPPY  thrpt    4   791848.718 ±   19363.131  ops/s
    LookupBenchmark.test       10000000  SNAPPY  thrpt    4   700438.201 ±   27910.579  ops/s
    LookupBenchmark.test      100000000  SNAPPY  thrpt    4   681790.103 ±   45388.918  ops/s

Performance goes down slightly as more elements are added due to:
* More frequent CPU cache misses
* More page faults

If you can mlock the full dataset, performance should be more predictable.

#### Appending data

Benchmark                   (type)   Mode  Cnt         Score         Error  Units
    AppendBenchmark.testMedium    NONE  thrpt    4   1595668.598 ±  850581.241  ops/s
    AppendBenchmark.testMedium  SNAPPY  thrpt    4    919539.081 ±  205638.008  ops/s
    AppendBenchmark.testSmall     NONE  thrpt    4   9800098.075 ±  707288.665  ops/s
    AppendBenchmark.testSmall   SNAPPY  thrpt    4  20227222.636 ± 7567353.506  ops/s

This is mostly disk bound, CPU is not fully utilized. So even though Snappy uses more CPU, it's still faster
because there's less data to write to disk, especially when the keys and values are small.
(The score is number of appends, not amount of data, so this may be slightly misleading)
    
#### Writing hash file

Benchmark                (constructionMethod)  (numElements)  Mode  Cnt   Score   Error  Units
    WriteHashBenchmark.test             IN_MEMORY           1000    ss    4   0.003 ± 0.001   s/op
    WriteHashBenchmark.test             IN_MEMORY          10000    ss    4   0.009 ± 0.016   s/op
    WriteHashBenchmark.test             IN_MEMORY         100000    ss    4   0.035 ± 0.088   s/op
    WriteHashBenchmark.test             IN_MEMORY        1000000    ss    4   0.273 ± 0.120   s/op
    WriteHashBenchmark.test             IN_MEMORY       10000000    ss    4   4.862 ± 0.970   s/op
    WriteHashBenchmark.test               SORTING           1000    ss    4   0.005 ± 0.033   s/op
    WriteHashBenchmark.test               SORTING          10000    ss    4   0.014 ± 0.008   s/op
    WriteHashBenchmark.test               SORTING         100000    ss    4   0.070 ± 0.111   s/op
    WriteHashBenchmark.test               SORTING        1000000    ss    4   0.835 ± 0.310   s/op
    WriteHashBenchmark.test               SORTING       10000000    ss    4  13.919 ± 1.908   s/op

Writing using the in-memory method is faster, but only works if you can keep the full hash file in memory while
building it. Sorting is about 3x slower, but is more reliably for very large data sets.

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