Scylla is an open-source distributed NoSQL database. It is a C++ rewritten implementation of Apache Cassandra. Scylla is compatible with Cassandra, and uses the same protocols (Cassandra Query Language and Thrift) and same file formats (SSTable). It is optimized for workloads that requires low latency and high throughput, in addition to Apache Cassandra's high availability, scalability and fault-tolerance guarantee. Scylla uses a shared-nothing model and shard-per-core architecture, where each thread executes on its own CPU core, memory, and multi-queue network interface controller. Cross-core communication is carried out by explicit asynchronous, message passing with the Seastar networking library.
Scylla supports Materialized Views in version 2.0 as an experimental feature. Whenever the base table is updated, the materialized view table will be automatically updated. Materialized View tables are distributed as normal tables and scale as well as normal tables. However, there are still limitations in the current experimental release, including but not limited to lack of local locking and local batch log.
Multi-version Concurrency Control (MVCC)
Scylla does not support ACID transactions as in RDBMS. However, CQL has a BATCH
statement that allows multiple update statements belonging to a given partition key be applied in isolation (note that batches are not a full analogue for SQL transactions). Besides, in UPDATE
, INSERT
, and DELETE
statements, modifications belonging to the same partition key are performed atomically and in isolation. Scylla implements Multi-Version Concurrency Control (MVCC) for partition mutation. Internally, versions are represented by an ordered list of states, where each state is a delta of current mutation.
Scylla does not support ACID transactions as in RDBMS. However, CQL has a BATCH
statement that allows multiple update statements belonging to a given partition key be applied in isolation (note that batches are not a full analogue for SQL transactions). Besides, in UPDATE
, INSERT
, and DELETE
statements, modifications belonging to the same partition key are performed atomically and in isolation. Scylla has a roadmap for supporting CQL Light-Weight Transactions (LWT) in 3.x.
Scylla supports both primary key and secondary key indexes. For primary index, Scylla hashes the key and finds the corresponding partition in the consistent hashing ring; within the partition, Scylla finds the row in a sorted data structure (SSTable). For secondary indexes, Scylla maintains an index table for the secondary index keys, where the value for each key is the (primary) partition keys associated with the secondary key. Whenever a secondary index is queried, Scylla first retrieves the partition keys using the secondary index, then retrieves the records with those partition keys returned by the first step.
Scylla supports non-blocking checkpoints through per-node backup procedures, which include full backup/snapshots and incremental backup. Snapshots are taken by the snapshot operation provided by the nodetool utility, while the incremental backup option can be configured in the configuration file. Automatic unnecessary backup cleaning is not implemented.
Scylla uses the same data model as Apache Cassandra, which represents data in key-value pairs (like row in RDBMS), and organizes a collection of rows as a column family (like table in RDBMS). One or more column families are contained in a keyspace (like database in RDBMS). It is encouraged that one application should use one keyspace.
Scylla uses a shared-nothing model. Nodes in the cluster are organized in a decentralized consistent hashing ring and data is partitioned into shards by the key across all nodes. Scylla uses a shard-per-core architecture, where each thread for a shard executes on its own CPU core, memory, and multi-queue network interface controller. Cross-core communication is carried out by explicit message passing. Scylla also uses replicas for fault-tolerance.
https://github.com/scylladb/scylla
ScyllaDB Inc.
2014
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