context.Rmd

---
title: "Using invocation context"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Using invocation context}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)
```

Context is metadata associated with each invocation. If the handler function
accepts a `context` argument then it will automatically receive at runtime a
named list consisting of these values along with the arguments in the body (if
any). For example, a function such as `my_func(x, context)` will receive the
context argument automatically. The `context` argument must be named (`...`
will not work).

The following context is made available **as a named list**:

* `aws_request_id` - The identifier of the invocation request
* `invoked_function_arn` – The Amazon Resource Name (ARN) that's used to
  invoke the function. Indicates if the invoker specified a version number or
  alias.
* `function_name`
* `function_version`
* `memory_limit_in_mb`
* `log_group_name`
* `log_stream_name`

## Event class-specific context

This section concerns future functionality for the `lambdr` package. It is not
yet implemented.

A particular event _class_ (determined by invocation method) can implement an `extract_context` method. This could be used to say, include details about the
HTTP request if an invocation is coming via an API Gateway.

In all cases the context should be a named list. The new context is appended to
the default values listed above.