sentinel

Sentinel

Triggered templating and command execution for etcd.

How It Works

Sentinel allows you to trigger template generation and command execution off of key changes in etcd. Sentinel defines a watcher to wait on one or more watch keys. The watcher then defines templates, context, and a command to execute when a watch key changes. The command is executed only when one or more of the destination templates files changed. If no templates are provided to a watcher then the command will always be executed. A watcher may also be defined with one ore more templates and no command. The context is is a list of keys which are retrieved from etcd and passed into each template for rendering.

Installing

This is a go-gettable package. It should be as simple as:

go get github.com/BlueDragonX/sentinel
go install github.com/BlueDragonX/sentinel

Configuring

The sentinel binary takes two optional arguments: -config and -exec. The -config argument takes as option the path to the Sentinel configuration file. This defaults to config.yml in the current directory. The -exec argument causes Sentinel to execute a watcher's actions directly instead of running continuously and waiting for its watchers. The -exec argument may be provided multiple times to execute multiple watchers.

The config file is written in YAML. It is structure into four sections: etcd, watchers, and logging.

etcd

This section configures the connection to etcd. Available parameters are:

• uri - The URI to connect to etcd at. Defaults to http://172.17.42.1:4001/.
• uris - Connect to multiple etcd nodes. Used as an alternative to uri when redundancy is called for.
• prefix - etcd key paths will be prefixed with this value. Defaults to an empty string.
• tls-key - The path to the TLS private key to use when connecting. Must be provided to enable TLS.
• tls-cert - The path to the TLS certificate to use when connecting. Must be provided to enable TLS.
• tls-ca-cert - The path to the TLS CA certificate to use when connecting. Must be provided to enable TLS.

watchers

This section defines watchers to trigger off of etcd key changes. The watchers section is a mapping of watcher names to their configuration. Available watcher paremeters are:

• watch - A list of etcd keys to wait watch for changes. These are automatically prefixed with the the value of etcd.prefix.
• context - A list of keys whose values will be retrieved and passed to the templates to render. These values are retrieved recursively. Key values will have dashes (-) replaces with underscored (_) so as to be accessible in the templates.
• templates - A list of templates to render. Each template is a mapping containing a src and dest value. The src is the template source code and the dest is the place where the rendered template will be written to. Directories under dest will be created if necessary.
• command - The command to execute. If templates are provided then this command will be only be executed when one or more template destinations are changed. The command may be one of two forms: a string or an array of arguments. The first form will cause the command to be executed in a bash shell. The second will cause it to be executed directly.

logging

This section controls how Beacon outputs logging. Available parameters are:

• console - Whether or not to log to the console. Defaults to true.
• syslog - Whether or not to log to syslog. Defaults to false.
• level - The log level. Valid values are debug, info, notice, warn, error, or fatal.

Beacon Example

Beacon discovers services running in Docker and registers them in etcd. The following Sentinel configuration maintains a list of registry endpoints:

etcd:
  uri: http://localhost:4001
  prefix: beacon

watchers:
  registries:
    watch:
    - registry/_index
    context:
    - registry
    templates:
    - src: /tmp/registries.tpl
      dest: /tmp/registries.yml

The registries.tpl template looks like this:

registries:
{{range $containerId, $container := .registry}}- http://{{$container.host_name}}:{{$container.host_port}}/{{end}}

Assuming there is one container listening on port 2002 whose hostname is docker.example.net the output of sentinel -exec registries would look like this:

registries:
- http://docker.example.net:2002/

License

Copyright (c) 2014 Ryan Bourgeois. Licensed under BSD-Modified. See the LICENSE file for a copy of the license.