# Writing Functions

This will give you the basic overview of writing base level functions. You can also use higher level 
abstractions that make it easier such as [lambda](lambda/README.md).

## Code

The most basic code layout in any language is as follows, this is pseudo code and is not meant to run.

```ruby
# Read and parse from STDIN
body = JSON.parse(STDIN)

# Do something
return_struct = doSomething(body)

# Respond if sync: 
STDOUT.write(JSON.generate(return_struct))
# or update something if async
db.update(return_struct)
```

## Inputs

Inputs are provided through standard input and environment variables. We'll just talk about the default input format here, but you can find others [here](function-format.md). 
To read in the function body, just read from STDIN. 

You will also have access to a set of environment variables. 

* REQUEST_URL - the full URL for the request
* ROUTE - the matched route
* METHOD - the HTTP method for the request
* CONFIG_X - any configuration values you've set for the Application or the Route. Replace X with the upper cased name of the config variable you set. 
* HEADER_X - the HTTP headers that were set for this request. Replace X with the upper cased name of the header and replace dashes in the header with underscores. 

Warning: these may change before release.

## Logging

Standard out is where you should write response data for synchronous functions. Standard error 
is where you should write for logging, as [it was intended](http://www.jstorimer.com/blogs/workingwithcode/7766119-when-to-use-stderr-instead-of-stdout). 

So to write output to logs, simply log to STDERR. Here are some examples in a few languages. 

In Go, simply use the [log](https://golang.org/pkg/log/) package, it writes to STDERR by default. 

```go
log.Println("hi")
```

In Node.js:

```node
console.error("hi");
```

[More details for Node.js here](http://stackoverflow.com/a/27576486/105562).

In Ruby:

```ruby
STDERR.puts("hi")
```

## Next Steps

* [Packaging your function](packaging.md)