This forwarding service forwarder.tcl serves mainly as an example code for the Docker API implementation in Tcl. The service is able to attach to one or several docker containers and send their output to remote URLs. By default, the HTTP operation used is a POST, MIME type application/octet-stream. The containers to listen to and their destinations URLs is controlled by the option -mapper which should contain a space-separated list. The list contains any alternating number of container name, destination URLs and plugin call specification (see below). When the plugin specification is empty or a dash, the data is simply posted to the remote URL (or sent using the default operation). The forwarding service continuously listens for container presence, meaning that it will be able to pickup new containers matching the name and/or reattach to containers that would be restarted or recreated.

A plugin specification should take the form of a procedure call followed by an @ sign, followed by a file specification (with no spaces inbetween). The file should be found in the plugin directory specified by the -exts option to the program. In short, if the plugin specification looked like [email protected],

• the file myplugin.tcl would be looked up in the plugin directory
• myplugin.tcl would be sourced in a safe interpreter
• and the procedure myproc would be called with the content of the line captured on the component's stdout whenever it was captured.

Being placed in a safe interpreter, the procedure will be able to call most of the regular Tcl commands, but will not have an I/O capabilities. However, it can call a command called send which takes the following arguments (in order): the data to be sent, an additional (and optional) path or query argument string to append to the URL specified, an optional HTTP operation to perform (GET, POST, DELETE, etc.) and finally an optional MIME type. Passing empty strings for the HTTP operation or the MIME type will pick the defaults one from the program options. It is only possible to append path/queries to the URL for security reasons, i.e. so as to avoid side-effects for plugins and allow users to fully specify the (root) URL where to send data to.

It is possible to pass arguments to the procedure. The procedure specification part of the plugin specification (i.e. the string before the @ sign) should then contain a number of ! separated tokens. The first token will be the procedure name, and the following ones will be arguments. These arguments are passed further to the procedure after the data captured from the component.