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classic-watchdog

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Classic Watchdog for OpenFaaS

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Watchdog
==========

The watchdog provides an interface between the outside world and your function. It starts a new process for each request and uses STDIO for communication.

![Classic watchdog architecture](https://pbs.twimg.com/media/DGScDblUIAAo4H-.jpg:large)

>  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.

### Next-gen: of-watchdog

Are you looking for more control over your HTTP responses, "hot functions", persistent connection pools or to cache a machine-learning model in memory? Then check out the *http mode* of the new [of-watchdog](https://github.com/openfaas-incubator/of-watchdog).

## Create a new function the easy way

**Create a function via the CLI**

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/openfaas/faas-cli)

## Delve deeper

**Package your function**

Example Dockerfile for an `echo` function:

```Dockerfile
FROM ghcr.io/openfaas/classic-watchdog:0.3.4 AS watchdog

FROM alpine:3.22.1

RUN mkdir -p /home/app

COPY --from=watchdog /fwatchdog /usr/bin/fwatchdog
RUN chmod +x /usr/bin/fwatchdog

# Add non root user
RUN addgroup -S app && adduser app -S -G app
RUN chown app /home/app

WORKDIR /home/app

USER app

# Populate example here - i.e. "cat", "sha512sum" or "node index.js"
ENV fprocess="cat"
# Set to true to see request in function logs
ENV write_debug="false"

EXPOSE 8080

CMD ["fwatchdog"]
```

## Environment variable overrides:

The watchdog can be configured through environment variables. You must always specifiy an `fprocess` variable.

| Option                 | Usage             |
|------------------------|--------------|
| `fprocess`             | The process to invoke for each function call (function process). 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_request`     | 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) |
| `healthcheck_interval` | Interval (in seconds) for HTTP healthcheck by container orchestrator i.e. kubelet. Used for graceful shutdowns. |
| `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 |
| `write_debug`          | Write all output, error messages, and additional information to the logs. Default is false |
| `combine_output`       | True by default - combines stdout/stderr in function response, when set to false `stderr` is written to the container logs and stdout is used for function response |
| `max_inflight`         | Limit the maximum number of requests in flight |
| `jwt_auth`                       | For OpenFaaS for Enterprises customers only. When set to `true`, the watchdog will require a JWT token to be passed as a Bearer token in the Authorization header. This token can only be obtained through the OpenFaaS gateway using a token exchange using the `http://gateway.openfaas:8080` address as the authority. |
| `jwt_auth_debug`                 | Print out debug messages from the JWT authentication process (OpenFaaS for Enterprises only). |
| `jwt_auth_local`                 | When set to `true`, the watchdog will attempt to validate the JWT token using a port-forwarded or local gateway running at `http://127.0.0.1:8080` instead of attempting to reach it via an in-cluster service name  (OpenFaaS for Enterprises only). |

## Metrics

| Name                            | Description             | Type                   |
|---------------------------------|-------------------------|------------------------|
| http_requests_total             | Total number of requests | Counter               |
| http_request_duration_seconds   | Duration of requests    | Histogram              |
| http_requests_in_flight         | Number of requests in-flight | Gauge             |

## Advanced / tuning

### (New) of-watchdog and HTTP mode

* of-watchdog

Forking a new process per request has advantages such as process isolation, portability and simplicity. Any process can be made into a function without any additional code. The of-watchdog and its "HTTP" mode is an optimization which maintains one single process between all requests.

A new version of the watchdog is being tested over at [openfaas-incubator/of-watchdog](https://github.com/openfaas-incubator/of-watchdog).

This re-write is mainly structural for on-going maintenance. It will be a drop-in replacement for the existing watchdog and also has binary releases available.

### Graceful shutdowns

The watchdog is capable of working with health-checks to provide a graceful shutdown.

When a `SIGTERM` signal is detected within the watchdog process a Go routine will remove the `/tmp/.lock` file and mark the HTTP health-check as unhealthy and return HTTP 503. The code will then wait for the duration specified in `write_timeout`. During this window the container-orchestrator's health-check must run and complete.

Now the orchestrator will mark this replica as unhealthy and remove it from the pool of valid HTTP endpoints.

Now we will stop accepting new connections and wait for the value defined in `write_timeout` before finally allowing the process to exit.

### Working with HTTP headers

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
* `Http_ContentLength` and `Http_Content_Length` - gives the total content-length of the incoming HTTP request received by the watchdog, see note below
* `Http_Transfer_Encoding` - only set when provided, if set to `chunked` the Content-Length will be `-1` to show that it does not apply

> This behaviour is enabled by the `cgi_headers` environmental variable which is enabled (`true`) 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_User_Agent=curl/7.43.0
Http_Accept=*/*
Http_X_Forwarded_By=http://my.vpn.com
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_User_Agent=curl/7.43.0
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.

classic-watchdog

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