Overview
Koios is a public API, a connection point that apps use to ask for data, that lets developers read blockchain records such as wallet balances, transactions, tokens and stake pools without running their own servers1. An app sends a question over the web and gets back a ready-to-use answer. Wallets, NFT tools and developer toolkits use it to show people what is happening on the chain2.
Koios serves Cardano. It covers the main network plus two test networks, preprod and preview, where developers try things before going live3. Koios DAO runs it: a group of long-term Cardano contributors who coordinate as a community rather than as one company (DAO stands for decentralized autonomous organization)4. Its core developers came from Guild Operators, a community that builds tools for stake pool operators5. Koios sits alongside the other API providers in the directory.
Key Features
- Many servers share the work. Community members run their own Koios servers, and a routing layer sends each request to a server that is online and up to date, so one server going down does not stop the service6.
- Start without signing up. The public tier needs no API key (a personal access code) and allows 5,000 requests a day. A free registered key raises the limit to 50,000, and paid plans go higher7.
- Many records in one request. A single call can return a batch of results, such as 25 transactions at once, instead of one object per call4.
- Ask for only the data you need. Requests can pick which fields come back, filter and sort the results, and page through long lists using options added to the web address8.
- Open query code. The database queries, API specification and tests behind Koios are published on GitHub under a Creative Commons license, so anyone can read or reuse them9.
What to Expect
Developers start in the Koios documentation, which lists the web address for each network and includes an interactive reference for trying requests in the browser3. Getting a key means signing in on the Koios profile page and sending the token with each request, and the same token works on mainnet and the test networks8,7. Community members have written client libraries for Python, Go, Java, Rust, .NET, PHP and other languages2. Mesh, a TypeScript toolkit for building Cardano apps, ships a ready-made Koios connector10. Teams weighing options can read CIP-0139, a Cardano Improvement Proposal for a common standard that wallets and apps can use to request blockchain data, whose annex compares Koios side by side with other providers11.
Stake pool operators and other people who run Cardano servers can join the network themselves. They can set up a full Koios server, which stacks a Cardano node, a database copy of the chain and a web layer12. A Lite Node option packages the same pieces as containers, self-contained software bundles, for smaller machines13. Each server can serve its owner privately or join the public group4.
Projects listed as Koios users include Eternl, VESPR, NMKR and the Cardano Foundation2. Some of those teams run their own Koios servers4. Koios has received Project Catalyst funding, including the Fund 6 proposal that built the original Koios API and a Fund 10 proposal for upgrades and maintenance, both marked complete14,15.
