Apache ZooKeeper is a distributed service that provides configuration information storage, naming, synchronization, coordination, and group services.

Version 4 of ANother Tool for Language Recognition (ANTLR), a flexible lexer/parser generator. ANTLR4 features an enhanced adaptive LL(*) parsing algorithm, that improves on the simpler LL(*) algorithm used in ANTLR3.

Amazon CloudWatch provides monitoring for AWS cloud resources and the applications customers run on AWS. Use this tag when asking about CW features such as metrics or alarms. Use amazon-cloudwatchlogs instead when asking about log groups, log streams, and metric filters.

The asset pipeline provides a framework to concatenate and minify or compress JavaScript and CSS assets. It also adds the ability to write these assets in other languages and pre-processors such as CoffeeScript, Sass and ERB.

Related to Apache Kafka's built-in stream processing engine called Kafka Streams, which is a Java library for building distributed stream processing apps using Apache Kafka.

AWS framework for rapidly developing backends for mobile and web applications.

For questions regarding Facebook messages, JavaScript messages, mobile messages (Android, iOS, windows-phone), etc. Not for use for error messages. Include additional tags to indicate messaging platform, programming language, etc.

QUESTIONS ABOUT LICENSING ARE OFF-TOPIC. You may ask questions about open source licensing on https://opensource.stackexchange.com. There are no good reasons to use this tag on Stack Overflow

A system call is used by programs to request services from the operating system's kernel.

Immutability is the inability to modify data after it has been created. Modifications are instead made by copying the data. A property of immutable data is that it is *referentially transparent*.