What it is
LevelDB is a fast, lightweight, single-purpose key-value storage library written in C++. It provides ordered mapping from string keys to string values with support for batch writes, snapshots, and iterators.
LevelDB allows storing arbitrary byte arrays as keys and values, iterating over keys in sorted order, performing batch writes, and creating snapshots of the database state.
Installation
sudo apt install libleveldb-devGetting started
The smallest useful thing you can do with it, and what each part means.
#include <leveldb/db.h>
#include <iostream>
int main() {
leveldb::DB* db;
leveldb::Options options;
options.create_if_missing = true;
leveldb::Status status = leveldb::DB::Open(options, "testdb", &db);
if (!status.ok()) std::cerr << status.ToString() << std::endl;
status = db->Put(leveldb::WriteOptions(), "key1", "value1");
if (!status.ok()) std::cerr << status.ToString() << std::endl;
delete db;
return 0;
}#include <leveldb/db.h>
#include <iostream>
int main() {
leveldb::DB* db;
leveldb::Options options;
options.create_if_missing = false;
leveldb::DB::Open(options, "testdb", &db);
std::string value;
leveldb::Status status = db->Get(leveldb::ReadOptions(), "key1", &value);
if (status.ok()) std::cout << "Value: " << value << std::endl;
else std::cerr << status.ToString() << std::endl;
delete db;
return 0;
}Advanced usage
Where the library earns its place over a simpler alternative.
#include <leveldb/db.h>
#include <leveldb/write_batch.h>
leveldb::DB* db; // assume opened
leveldb::WriteBatch batch;
batch.Put("key2", "value2");
batch.Delete("key1");
db->Write(leveldb::WriteOptions(), &batch);#include <leveldb/db.h>
#include <iostream>
leveldb::DB* db; // assume opened
leveldb::Iterator* it = db->NewIterator(leveldb::ReadOptions());
for (it->SeekToFirst(); it->Valid(); it->Next()) {
std::cout << it->key().ToString() << " => " << it->value().ToString() << std::endl;
}
delete it;// leveldb::Snapshot* snapshot = db->GetSnapshot(); // read consistent view
// db->ReleaseSnapshot(snapshot);Errors and fixes
The failures you are most likely to hit, and what actually resolves them.
- leveldb::Status::IOError
- Indicates a file system or permission error. Check database path and permissions.
- leveldb::Status::NotFound
- Key does not exist in the database. Handle missing keys gracefully.
- leveldb::Status::Corruption
- Database corruption detected. Consider backup and restore or repair procedures.
Best practices
- Always check the return status of operations.
- Use batch writes for better performance.
- Close the database properly to ensure data integrity.
- Use snapshots for consistent reads in concurrent scenarios.
- Avoid storing very large values; LevelDB is optimized for many small key-value pairs.
Background
Why it exists, and what it was reacting to.
LevelDB was developed by Google to offer a high-performance key-value store optimized for SSDs and large datasets. It is widely used in applications requiring efficient storage and retrieval of key-value pairs, such as databases, caches, and embedded systems.