An application's database: an embedded sql-file database, opened
from configuration, with its query cache, scheduled-job support and
backups.

This namespace needs sql-file, which playbook doesn't bring with it
(it's a provided dependency): an application that uses it lists
com.mschaef/sql-file among its own dependencies. Applications without
a database don't load this namespace and don't need sql-file.

Configuration:

[:db :subname] - The database name; defaults to [:app :name]. The
   db.subname system property takes precedence over both.
[:db :enable-query-cache] - Whether the query cache is on.
[:db :backup-path] - Directory for scheduled backups (the :db-backup
   job); with none, there are no backups.

A background mailer. Messages are queued and sent, one at a time,
through the SMTP server in the :smtp configuration.

Running an application as a service: its scheduled jobs, and the
body of code that is the application itself.

Markdown templates with {{name}} variables, rendered to HTML with
their metadata.