Runners

When you deploy an application with Velociraptor, it runs in a container. Those containers are set up and launched by a ‘runner’, which is a small command line application on the application host.

You can do some neat things using runners:

  • Run a container on one host that will uptest a proc on another host (by setting the ‘host’ and ‘port’ keys in your local proc.yaml appropriately).
  • Get a shell on a production host in the exact same environment in which your production instance is running.
  • Easily start up a local instance of a proc that exactly matches what’s running production.

The interface between Velociraptor and its runners is specified in terms of command line arguments and a yaml file.

A runner is launched with two command line arguments:

  1. A command. (‘setup’, ‘run’, ‘uptest’, ‘teardown’, or ‘shell’)
  2. The path to a proc.yaml file containing all the information necessary to set up the container with the application’s build and config.

Using the ‘setup’ command might look like this:

some_runner setup /home/dave/myproc.yaml

The proc.yaml file

The proc.yaml file contains the following keys:

  • port: The network port on which the application should listen, if applicable.
  • user: The name of the user account under which the application should be run. The application’s files will have their owner set to this user.
  • group: The application’s files will have their group set to this.
  • env: A dictionary of environment variables to be set before launching the application.
  • settings: A dictionary of config values to be written to settings.yaml inside the container. The location of that file will be passed to the application using the APP_SETTINGS_YAML environment variable.
  • cmd OR proc_name: If there is a ‘cmd’ key in proc.yaml, it will be used as the command to launch inside the container. If there is no ‘cmd’ key, then the ‘proc_name’ key should have an entry like ‘web’ or ‘worker’ that points to a line in the application’s Procfile.
  • build_url: The HTTP URL of the build tarball.
  • build_md5: Optional. If supplied, and the runner sees that the build tarball has already been downloaded, its md5sum will be checked against build_md5 from the proc.yaml.

The file doesn’t actually have to be named proc.yaml. It can have any name you like.

Note: String config values that start with an @ sign and also contain single quote characters get serialized in a special way by the underlying YAML library. For instance, “@How’s it going?” gets serialized as '@How''s it going?’.

Commands

Runners support the following commands:

Setup

Example:

some_runner setup /home/dave/myproc.yaml

The setup command will read the proc.yaml file, download the build (if necessary), and create necessary scripts, directories and LXC config files for the container.

If proc.yaml contains a ‘cmd’ key, this will be written into the startup script created during setup. If there is no ‘cmd’ key, the runner will use the ‘proc_name’ key to determine which line from the application’s Procfile should be executed.

This command locks the proc.yaml file so other locking runner commands cannot run on this file at the same time.

Teardown

Example:

some_runner teardown /home/dave/myproc.yaml

The teardown command should remove the proc folder and related files from the filesystem. If the runner has done other changes to the host, such as creating special network interfaces for the container, it should clean those up too.

Note: It is permissible for teardown to leave a copy of the build tarball in /apps/builds even after teardown is called. (There’s no way for teardown to know whether you have other containers based on the same build.)

The teardown command locks the proc.yaml file while running.

Run

Example:

some_runner run /home/dave/myproc.yaml

The run command starts your process inside the container. The process should not daemonize. When the process exists, the container will stop with it.

The run command locks the proc.yaml while running.

Uptest

Example:

some_runner uptest /home/dave/myproc.yaml

The uptest command relies on the presence of a proc_name key in proc.yaml. It looks for any scripts in <app_dir>/uptests/<proc_name> and will execute each one, passing host and port on the command line (as specified in the uptests spec). The host and port settings passed to the uptests will be pulled from the host and port keys in the proc.yaml.

The results from the uptests will be written to stdout as a JSON array of objects (one object for each uptest result). The uptest command must not emit any other output besides the JSON results.

Uptests should be run in an environment identical to the proc being tested (same os, build, settings, environment variables, etc.).

The uptest command does not lock the proc.yaml while running.

Shell

Example:

some_runner shell /home/dave/myproc.yaml

The shell command creates and starts a container identical to the one running for the proc, but starts /bin/bash in it instead of the proc’s command. This is useful for debugging pesky problems that only seem to show up in production.

The shell command does not lock the proc.yaml while running.

Runner Variants

Velociraptor provides two runner implementations.

vrun_precise

The vrun_precise runner is specific to Ubuntu 12.04 (Precise) hosts. It creates bind mounts of the host’s essential system folders inside the container. This matches Velociraptor’s original container implementation.

vrun

The vrun runner supports specifying an OS image tarball to be used inside the container. It uses the following additional keys in proc.yaml:

  • image_name: This should be a filesystem-safe name for the image to be used in the container. Example: ubuntu-core-12.04.3-amd64
  • image_url: An http URL from which the image tarball can be downloaded.
  • image_md5 (optional): If provided, this checksum will be used to determine whether an already-downloaded tarball is correct. If there’s a mismatch, the image will be re-downloaded.

Here’s a working example of those three proc.yaml lines:

image_url: http://cdimage.ubuntu.com/ubuntu-core/releases/12.04/release/ubuntu-core-12.04.3-core-amd64.tar.gz
image_md5: ea978e31902dfbf4fc0dac5863d77988
image_name: ubuntu-core-12.04.3-amd64

(That Ubuntu core image is only 34MB!)

Image tarballs must be compressed with either gzip or bzip2 compression, and use the appropriate extension in their filenames.

The vrun runner uses an overlayfs mount of the unpacked build inside each container, so the same image can be used by many containers without using any more disk space.

Other runner implementations may be added in the future, or created as separate projects.