Rules

Rules in a chain describe the way the Scheduler should read the job description. A chain must have a proper starting point and end point. Steps in a chain are activated when the rules in the chain definition evaluate to true (as rules are Boolean) and mention the specific step names to be started.

Let's return to the backup example once more. We know there is only one location where it makes sense to start the job—the step that shuts down the database. How can we make the Scheduler know that? Let's start with an overview of what we know. The steps are:

This last rule makes sure that our example backup always ends. It returns an error code 0 when everything succeeds and returns an error code 1 when any step in the chain fails. The error code 0 means "no error", and any other value means "failure". In this short example, it is easy to make one rule that handles all FAILED situations. When the list of steps grows, it is more likely that we forget to mention a step. In the long run, it might be smarter to give every step its own FAILED rule. In doing so, we end up with a larger list of rules but they are easier to check for completeness and should help us to prevent getting stuck in CHAIN_STALLED situations.