> For the complete documentation index, see [llms.txt](https://tarmac.gitbook.io/tarmac-framework/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://tarmac.gitbook.io/tarmac-framework/capabilities/callbacks.md).

# Callbacks

Powered by Web Assembly Procedure Calls

Tarmac, at its core, is powered by the Web Assembly Procedure Call (waPC) Project. The Web Assembly Procedure Call Project defines imported and exported functions between a WASM host and a guest WASM function. A WASM runtime host like Tarmac and a WASM Function running within Tarmac can communicate back and forth using these functions.

A prime example of this is the `HostCall()` function used by guest WASM functions. This `HostCall()` is a callback function that enables WASM functions to pass back data to the Tarmac host for the explicit goal of executing host-level functionality.

This ability to perform a Host Callback is what sets Tarmac apart from most other serverless runtimes. A WASM function can use the Host Callback functionality to access a full suite of standard functionality that would traditionally be too heavy for a serverless function.

Essentially, Host Callbacks allow Tarmac to provide developers the functionality of a standard Microservice Framework along with the convenience of a serverless runtime.

## Using Host Callbacks

Calling a Host Callback is relatively straightforward for WASM functions. As outlined in the language guides, each WASM function must import a waPC compliant guest library. This guest library will allow users to access a `HostCall()` function for their language of choice.

The example below is an example of calling the Host Callback function in Go.

```golang
_, err := wapc.HostCall("tarmac", "logger", "debug", payload)
```

The `HostCall()` function takes three parameters. The first is the namespace which developers should always set to `tarmac`. The second is the capability requested, such as `logger` or `kvstore`. The third is the function to execute; for a `kvstore` capability, we may want to perform a `get` or a `set`.

This section of documentation outlines all of the various host-level capabilities Tarmac provides. Each unit will outline the capabilities, functions, and input/output data.