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The watchdog provides an unmanged and generic interface between the outside world and your function. Its job is to marshal a HTTP request accepted on the API Gateway and to invoke your chosen appliaction. The watchdog is a tiny Golang webserver - see the diagram below for how this process works.
> Above: a tiny web-server or shim that forks your desired process for every incoming HTTP request
Every function needs to embed this binary and use it as its `ENTRYPOINT` or `CMD`, in effect it is the init process for your container. Once your process is forked the watchdog passses in the HTTP request via `stdin` and reads a HTTP response via `stdout`. This means your process does not need to know anything about the web or HTTP.
The easiest way to create a function is to use a template and the FaaS CLI. The CLI allows you to abstract all Docker knowledge away, you just have to write a handler file in one of the supported programming languages.
* [Your first serverless Python function with OpenFaaS](https://blog.alexellis.io/first-faas-python-function/)
* [Read a tutorial on the FaaS CLI](https://github.com/alexellis/faas-cli)
## Delve deeper
**Package your function**
Here's how to package your function if you don't want to use the CLI or have existing binaries or images:
- [x] Use an existing or a new Docker image as base image `FROM`
- [x] Add the fwatchdog binary from the [Releases page](https://github.com/alexellis/faas/releases) via `curl` or `ADD https://`
- [x] Set an `fprocess` environmental variable with the function you want to run for each request
- [x] Expose port 8080
- [x] Set the `CMD` to `fwatchdog`
```
FROM alpine:3.5
ADD https://github.com/alexellis/faas/releases/download/v0.5-alpha/fwatchdog /usr/bin
RUN chmod +x /usr/bin/fwatchdog
# Define your UNIX binary here
ENV fprocess="/bin/cat"
CMD ["fwatchdog"]
```
A Docker Healthcheck is not required but is best practice. It will make sure that the watchdog is ready to accept a request before forwarding requests via the API Gateway. If the function or watchdog runs into an unrecoverable issue Swarm will also be able to restart the container.
Here is an example of the `echo` function implementing a healthcheck with a 5-second checking interval.
```
FROM functions/alpine
ENV fprocess="cat /etc/hostname"
HEALTHCHECK --interval=5s CMD [ -e /tmp/.lock ] || exit 1
```
The watchdog process creates a .lock file in `/tmp/` on starting its internal Golang HTTP server. `[ -e file_name ]` is shell to check if a file exists. With Windows Containers this is an invalid path so you may want to set the `suppress_lock` environmental variable.
* [Test-drive Docker Healthcheck in 10 minutes](http://blog.alexellis.io/test-drive-healthcheck/)
The watchdog can be configured through environmental variables. You must always specifiy an `fprocess` variable.
| Option | Usage |
|------------------------|--------------|
| `fprocess` | The process to invoke for each function call. This must be a UNIX binary and accept input via STDIN and output via STDOUT. |
| `cgi_headers` | HTTP headers from request are made available through environmental variables - `Http_X_Served_By` etc. See section: *Handling headers* for more detail. Enabled by default. |
| `marshal_requests` | Instead of re-directing the raw HTTP body into your fprocess, it will first be marshalled into JSON. Use this if you need to work with HTTP headers and do not want to use environmental variables via the `cgi_headers` flag. |
| `content_type` | Force a specific Content-Type response for all responses. |
| `write_timeout` | HTTP timeout for writing a response body from your function (in seconds) |
| `read_timeout` | HTTP timeout for reading the payload from the client caller (in seconds) |
| `suppress_lock` | The watchdog will attempt to write a lockfile to /tmp/ for swarm healthchecks - set this to true to disable behaviour. |
| `exec_timeout` | Hard timeout for process exec'd for each incoming request (in seconds). Disabled if set to 0. |
Headers and other request information are injected into environmental variables in the following format:
The `X-Forwarded-By` header becomes available as `Http_X_Forwarded_By`
* `Http_Method` - GET/POST etc
* `Http_Query` - Querystring value
> This behaviour is enabled by the `cgi_headers` environmental variable which is enabled by default.
Here's an example of a POST request with an additional header and a query-string.
```
$ cgi_headers=true fprocess=env ./watchdog &
2017/06/23 17:02:58 Writing lock-file to: /tmp/.lock
$ curl "localhost:8080?q=serverless&page=1" -X POST -H X-Forwarded-By:http://my.vpn.com
```
This is what you'd see if you had set your `fprocess` to `env` on a Linux system:
```
Http_Method=POST
Http_Query=q=serverless&page=1
```
You can also use the `GET` verb:
```
$ curl "localhost:8080?action=quote&qty=1&productId=105"
```
The output from the watchdog would be:
```
Http_Accept=*/*
Http_Method=GET
Http_Query=action=quote&qty=1&productId=105
```
You can now use HTTP state from within your application to make decisions.
**HTTP methods**
The HTTP methods supported for the watchdog are:
With a body:
* POST, PUT, DELETE, UPDATE
Without a body:
* GET
> The API Gateway currently supports the POST route for functions.
**Content-Type of request/response**
By default the watchdog will match the response of your function to the "Content-Type" of the client.
* If your client sends a JSON post with a Content-Type of `application/json` this will be matched automatically in the response.
* If your client sends a JSON post with a Content-Type of `text/plain` this will be matched automatically in the response too
To override the Content-Type of all your responses set the `content_type` environmental variable.
**I don't want to use the watchdog**
This is an unsupported use-case for the OpenFaaS project however if your container exposes HTTP POST on port 8080 then the OpenFaaS API gateway and other tooling will manage your container.
**Tuning auto-scaling**
Auto-scaling starts at 1 replica and steps up in blocks of 5:
* 1->5
* 5->10
* 10->15
* 15->20
You can override the upper limit of auto-scaling by setting the following label on your container:
```
com.faas.max_replicas: "10"
```
If you want to disable scaling, set the `com.faas.max_replicas` value to `"1"`.