Using the Host API
In this section
- Using the Host API (this page)
- Cell Introspection — finding out info about the DNA, zome, agent, and calling context
- Cryptography functions — key generation, signatures, hashing, and encryption
- Miscellaneous host functions — system time, logging, randomness
Holochain hosts back-end code (zomes) in a WebAssembly virtual machine and provides a host API to them, which the HDK library makes easy to use. It allows agents to read and write data, interact with other cells, and access various other host features.
Accessing the host
Holochain creates a sandbox for zomes to run in, so they can’t access the host computer’s functionality directly. It exposes a minimal interface for data access, introspection, peer-to-peer interactions, and interop between code modules. You use the HDK (or a minimal subset, the HDI library) to access the host API. The sandbox is implemented as a WebAssembly host, and your zome is a WebAssembly guest.
Holochain doesn't expose standard WASM host APIs
Holochain’s host API is a small feature set written explicitly for interacting with Holochain’s feature set. It doesn’t include the WebAssembly JavaScript API or the WebAssembly System Interface (WASI).
A lot of this is covered elsewhere already — Working with Data and Validation talk about data access functions, and Connecting the Parts talks about interop.
There are a few other functions that don’t fall into those categories, so we’ll cover them here in three categories: introspection, cryptography, and miscellaneous functions.
