LMDB JNI provide a Java API to LMDB which is an ultra-fast, ultra-compact key-value embedded data store developed by Symas for the OpenLDAP Project. It uses memory-mapped files, so it has the read performance of a pure in-memory database while still offering the persistence of standard disk-based databases. Transactional with full ACID semantics and crash-proof by design. No corruption. No startup time. No dependencies.
LMDB JNI is available for 64 bit Linux, OSX, Windows and Android.
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[In-Memory Microbenchmark] (http://symas.com/mdb/inmem), June 2014
Multithreaded read performance for a purely in-memory database.
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On-Disk Microbenchmark, November 2014
Multithreaded read performance for a database that is over 5 times larger than the size of RAM.
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LMDB JNI Microbenchmark, February 2015 (source)
Row scanning speed per second compared with RocksDB, LevelDB and MapDB. Mongodb is difficult to setup in JMH but de.flapdoodle.embed.mongo indicate that it is around 50x slower than lmdb_zero_copy.
Benchmark Mode Cnt Score Error Units Iteration.leveldb thrpt 10 6965637.351 ± 784589.894 ops/s Iteration.lmdb_buffer_copy thrpt 10 3157796.643 ± 265830.424 ops/s Iteration.lmdb_zero_copy thrpt 10 16372428.882 ± 1812316.504 ops/s Iteration.mapdb thrpt 10 1358748.670 ± 87502.413 ops/s Iteration.rocksdb thrpt 10 1311441.804 ± 176129.883 ops/s
<!-- required java classes -->
<dependency>
<groupId>org.deephacks.lmdbjni</groupId>
<artifactId>lmdbjni</artifactId>
<version>${lmdbjni.version}</version>
</dependency>
<!-- prebuilt liblmdb platform packages -->
<dependency>
<groupId>org.deephacks.lmdbjni</groupId>
<artifactId>lmdbjni-linux64</artifactId>
<version>${lmdbjni.version}</version>
</dependency>
<dependency>
<groupId>org.deephacks.lmdbjni</groupId>
<artifactId>lmdbjni-osx64</artifactId>
<version>${lmdbjni.version}</version>
</dependency>
<dependency>
<groupId>org.deephacks.lmdbjni</groupId>
<artifactId>lmdbjni-win64</artifactId>
<version>${lmdbjni.version}</version>
</dependency>
<dependency>
<groupId>org.deephacks.lmdbjni</groupId>
<artifactId>lmdbjni-android</artifactId>
<version>${lmdbjni.version}</version>
</dependency>
Recommended package imports.
import org.fusesource.lmdbjni.*;
import static org.fusesource.lmdbjni.Constants.*;Opening and closing the database.
try (Env env = new Env("/tmp/mydb")) {
try (Database db = env.openDatabase()) {
... // use the db
}
}Putting, getting, and deleting key/values.
db.put(bytes("Tampa"), bytes("rocks"));
String value = string(db.get(bytes("Tampa")));
db.delete(bytes("Tampa"));Iterating and seeking key/values forward and backward.
try (EntryIterator it = db.iterate()) {
for (Entry next : it.iterable()) {
}
}
try (EntryIterator it = db.iterateBackward()) {
for (Entry next : it.iterable()) {
}
}
byte[] key = bytes("London");
try (EntryIterator it = db.seek(key)) {
for (Entry next : it.iterable()) {
}
}
try (EntryIterator it = db.seekBackward(key))) {
for (Entry next : it.iterable()) {
}
}Performing transactional updates.
try (Transaction tx = env.createWriteTransaction()) {
db.delete(tx, bytes("Denver"));
db.put(tx, bytes("Tampa"), bytes("green"));
db.put(tx, bytes("London"), bytes("red"));
tx.commit(); // if commit is not called, the transaction is aborted
}Working against a snapshot view of the database.
// create a read-only transaction...
try (Transaction tx = env.createReadTransaction()) {
// All read operations will now use the same
// consistent view of the data.
... = db.db.openCursor(tx);
... = db.get(tx, bytes("Tampa"));
}Set a custom key comparison function for a database.
db.setComparator(tx, new Comparator<byte[]>() {
@Override
public int compare(byte[] key1, byte[] key2) {
// do compare
}
});Atomic hot backup.
env.copy(backupPath);Using a memory pool to make native memory allocations more efficient:
Env.pushMemoryPool(1024 * 512);
try {
// .. work with the DB in here,
} finally {
Env.popMemoryPool();
}The safest (and least efficient) approach for interacting with LMDB JNI is using buffer copy using JNI as shown above. BufferCursor is an advanced, more efficient, zero copy mode. This mode is not available on Android.
There is also DirectBuffer which is even more advanced but users should avoid interacting directly with these and use the BufferCursor API instead. Otherwise take extra care of buffer memory address+size and byte ordering. Mistakes may lead to SIGSEGV or unpredictable key ordering etc.
// read only
try (Transaction tx = env.createReadTransaction();
BufferCursor cursor = db.bufferCursor(tx)) {
// iterate from first item and forwards
cursor.first();
while(cursor.next()) {
// read a position in buffer
cursor.keyByte(0);
cursor.valByte(0);
}
// iterate from last item and backwards
cursor.last();
while(cursor.prev()) {
// copy entire buffer
cursor.keyBytes();
cursor.valBytes();
}
// find first key greater than or equal to specified key.
cursor.seek(bytes("London"));
// read utf-8 string from position until NULL byte
cursor.keyUtf8(0);
cursor.valUtf8(0);
}
// open for write
try (Transaction tx = env.createWriteTransaction();
BufferCursor cursor = db.bufferCursor(tx)) {
cursor.first();
// write utf-8 ending with NULL byte
cursor.keyWriteUtf8("England");
cursor.valWriteUtf8("London");
// overwrite existing item if any. Data is not written
// into database before this operation is called and
// no updates are visible outside this transaction until
// the transaction is committed
cursor.overwrite();
cursor.first();
// delete current cursor position
curstor.delete();
// commit changes or try-with-resources will auto-abort
tx.commit();
} This project is licensed under the Apache License, Version 2.0 but the binary jar it produces also includes liblmdb library version 0.9.14 of the OpenLDAP project which is licensed under the The OpenLDAP Public License.