Question: What are some examples of open-source in-memory databases?

Answer

Open-source in-memory databases are data storage systems meant for high-speed querying, analytics, and real-time use cases. They store data in the main memory (RAM) of a computing machine as opposed to traditional databases that use disk-based storage. This results in significantly faster read/write operations. Below are a few examples:

1. Redis: Redis (Remote Dictionary Server) is an open-source, in-memory data structure store used as a database, cache, and message broker.

# To install Redis on Ubuntu
sudo apt update
sudo apt install redis-server

2. Memcached: Memcached is a distributed, high-performance, in-memory caching system originally designed by Danga Interactive for LiveJournal but now used by many other sites.

# To install Memcached on Ubuntu
sudo apt update
sudo apt install memcached

3. Apache Ignite: Apache Ignite is a memory-centric distributed database, caching, and processing platform for transactional, analytical, and streaming workloads delivering in-memory speeds at petabyte scale.

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4. Hazelcast: Hazelcast IMDG (In-Memory Data Grid) is an open-source data distribution platform for Java. It allows for the distribution of data across multiple nodes, thus enabling dynamic scaling.

// To create a new Hazelcast instance.
HazelcastInstance hz = Hazelcast.newHazelcastInstance();

5. VoltDB: VoltDB is an in-memory database designed by DBMS veterans Michael Stonebraker and Lawrence A. Rowe. It's optimized for high velocity applications and supports ACID-compliant transactions.

# To start Voltdb
voltdb init
voltdb start

Please note: the specifics of how you install and use these databases vary depending on the OS and specific use case. Therefore, always refer to the latest official documentation to get the most accurate information.