Junos offers a flexible model for you to decide when a new configuration should be active, and it’s called the commit model. The concept comes from revision control systems which are used for software version control, whereby you edit the configuration (as shown previously) and then, once the configuration is complete, you commit it to the system. Upon commit, several things happen.
First, the configuration is validated by using a commit check. A
commit check reviews the configuration for any syntax errors or disallowed
statements. It’s a sanity check to ensure that a bad configuration is not
installed. It’s also possible to issue a commit check manually by using
commit check command. If an error is encountered,
Junos responds and specifies where the error occurs and what the error
set interfaces fe-2/0/3.0 family inet filter input bad-filter root@SRX210-A#
commit[edit interfaces fe-2/0/3 unit 0 family inet] 'filter' Referenced filter 'bad-filter' is not defined error: configuration check-out failed  root@SRX210-A#
In the preceding output, a commit error is highlighted. The
configuration was attempting to reference a firewall filter that did not
exist and the commit check provided an error which prevented the
configuration from being installed. The hierarchy in which the error
occurred was returned to the user,
interfaces fe-2/0/3 unit 0 family inet], so it is easy to track down from where the error was issued. In some ...