poison
A high-performance, standards-compliant JSON library for Elixir, built for speed and correctness.
What is poison?
Poison is a pure Elixir JSON library designed to deliver exceptional performance while maintaining simplicity, completeness, and strict adherence to standards. It achieves its speed through optimized techniques like sub binary matching, a hand-rolled parser, and IO list encoding, making it one of the fastest JSON libraries available for the Elixir ecosystem.
Elixir developers who need high-performance JSON encoding and decoding, particularly those building web services, APIs, or applications where JSON processing speed is critical.
Developers choose Poison for its combination of being one of the fastest JSON libraries for Elixir, full standards compliance, and modular design that allows independent use of encoder, decoder, and parser components.
Overview
An incredibly fast, pure Elixir JSON library
Use Cases
Best For
- High-performance JSON processing in Elixir web applications and APIs.
- Ensuring strict JSON standards compliance (RFC 8259, ECMA 404) in Elixir projects.
- Encoding Elixir structs to JSON with customizable attribute selection or exclusion.
- Preventing duplicate keys in JSON objects with optional strict key validation.
- Parsing JSON without decoding, using the standalone parser for maximum speed.
- Benchmarking and comparing JSON library performance in the Elixir ecosystem.
Not Ideal For
- Projects requiring JSON streaming or incremental parsing for large datasets
- Teams that prioritize NIF-based libraries for absolute maximum speed over pure Elixir safety
- Applications where JSON processing is minimal and a simpler, less optimized dependency is sufficient
- Environments with strict memory constraints, due to risks of unbounded atom creation with key parsing
Pros & Cons
Pros
Uses sub binary matching, a hand-rolled parser optimized for BeamAsm JIT, and IO list encoding, making it one of the fastest Elixir JSON libraries, with benchmarks close to jiffy.
Fully conforms to RFC 8259 and ECMA 404, and passes the JSONTestSuite, ensuring correctness and reliability in JSON handling.
Encoder, decoder, and parser components are usable independently, allowing developers to integrate only what they need for specific tasks.
Provides derivable protocols for encoding Elixir structs with options like :only or :except for attribute selection, simplifying API response generation.
Cons
Using keys: :atoms for parsing creates atoms that are not garbage-collected, risking VM crashes if atom limits are exceeded, as warned in the README.
Enabling strict key validation to prevent duplicate keys causes a small performance penalty, which can impact high-throughput scenarios.
The README does not mention support for streaming JSON or incremental parsing, limiting its use for large file handling without custom workarounds.
Frequently Asked Questions
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