Auto Sharding Index Algorithm

(Since v2.2)

This indexing algorithm is based on the DHT concept, where they keys are stored on different partition, based on the Murmur3 hash function.

Auto Sharding Index supports the following index types:

• UNIQUE_HASH_INDEX Does not allow duplicate keys, it fails when it encounters duplicates.
• NOTUNIQUE_HASH_INDEX Does allow duplicate keys.

Under the hood, this index creates multiple Hash Indexes, one per cluster. So if you have 8 clusters for the class "Employee", this index will create, at the beginning, 8 Hash Indexes.

Since this index is based on the Hash Index, it's able to perform index read operations in one I/O operation and write operations in a maximum of three I/O operations. The Hash Index algorithm is based on the Extendible Hashing algorithm. Despite not providing support for range queries, it is noticeably faster than SB-Tree Index Algorithms, (about twice as fast when querying through ten million records).

Usage

Create an index by passing "AUTOSHARDING" as index engine:

final OClass cls = db.getMetadata().getSchema().createClass("Log");
cls.createProperty("key", OType.LONG);
cls.createIndex("idx_LogKey", OClass.INDEX_TYPE.UNIQUE.toString(),
    (OProgressListener) null, (ODocument) null, "AUTOSHARDING", new String[] { "key" });

Performance

On multi-core hw, using this index instead of Hash Index gives about +50% more throughput on insertion on a 8 cores machine.

Distributed

The fully distributed version of this index will be supported in v3.0. In v2.2 each node has own copy of the index with all the partitions.

Internals

This is the algorithm for the put(key,value):

int partition = Murmur3_hash(key) % partitions;
getSubIndex(partition).put(key,value);

This is for the value = get(key):

int partition = Murmur3_hash(key) % partitions;
return getSubIndex(partition).get(key);