SSDB is a NoSQL database implemented in C++. The goal of SSDB is to substitute, or work together with Redis. It is compatible with Redis APIs, and has a larger storage space than Redis. SSDB supports multiple data structures, including key-value pairs, hashmap, sorted set, and list. Its low-level storage engine is based on Google LevelDB. SSDB supports a wide range of programming languages.
SSDB was originally an individual open-source project. It was started by Zuyang Wu in 2013, aiming to work as an alternative for Redis. Most of the features were implemented in 2013 and 2014. The last update to SSDB was one year ago.
SSDB supports data compression using Snappy. Snappy is an open-source data compression library developed by Google. It is used in both Google's internal tools and several famous open-source databases, including Cassandra, MongoDB, and LevelDB. SSDB naturally supports Snappy as it uses LevelDB for low-level storage. In LevelDB, developers should explicitly set parameters to use Snappy in the compile time. Since version 1.6.2, SSDB has changed this feature by directly integrating the source code of Snappy to the project. By default, SSDB will use Snappy, but users can explicitly choose to use a non-compression version. SSDB with Snappy compression is compatible with non-compression data.
SSDB does not support concurrency control, because operations are executed individually, and there is no transaction. However, unlike Redis, SSDB supports multi-threads to avoid write operations blocking read operations (since 1.5.2). All write operations are executed in a single thread while read operations are executed in other threads.
SSDB supports four main data types, which are key-value, hashmap, sorted set, and list. The key-value data type is used to store data with no strong relations. The hashmap is used to store data in different sets with counting and sorting based on the key value. Sorted set sorts all entries based on an integer of score. Unlike the key-value and hashmap data types, sorted set cannot be used to store large data. Besides, list data type stores data as a queue.
SSDB uses LevelDB as its storage engine. SSDB explicitly calls LevelDB's interfaces to insert, update, and delete data. In LevelDB, a Log-Structured Merge Tree is used for indexes. Log-Structured Merge Tree provides a higher write throughput than the B+ tree. For each write operation, it first writes to a data structure called memtable, which will be flushed to disk later. In a memtable, LevelDB uses Skip List to find a position.
SSDB supports logical logging for its operations. It has multiple logging levels. The log file will change its name and create a new log file when exceeding the size limitation.
No query compilation is found in SSDB.
SSDB implements three separate iterator classes for key-value, hashmap, and sorted set.
Firstly, SSDB supports a set of shell commands to execute queries. Secondly, SSDB provides custom APIs for a wide range of programming languages, including C#, C++, Erlang, Go, Java, Lua, PHP, Python, Ruby, and Swift. Besides, developers can use the Redis client to connect the server with Redis Serialization Protocol. The command set of SSDB is similar to that in Redis.
SSDB is a single-node database management system. Initially, the server does not share anything with other nodes. However, as SSDB supports Redis API, developers can directly use the Twemproxy to manage a number of SSDB servers. Besides, it is also workable to use a combination of Redis and SSDB.