Chains and steps

As mentioned earlier, a chain consists of multiple steps that are logically connected together. We cannot say anything about the behavior of these steps in the job chain. In order to know when these steps are going to be executed, we have to know what rules are applied to them. All steps have a name. We are going to reference these names of steps in our rules, which we will define later. Now, let's return to the backup example and list the steps there:

Without rules, nothing much will happen. The Scheduler will be looking for a step to start, but will be unable to find it. As soon as the Scheduler finds itself in a situation where it is given a chain and it cannot determine what to do next, the job gets a special job status—CHAIN_STALLED. This status means that the Scheduler does not know how to handle the chain. The chain will probably remain in this state indefinitely, unless we manually take over. If we ever find our job in such a state, it means that we have not taken into account all possible combinations of outcomes that the steps can have. Defining a good set of rules will mostly prevent us from getting in this state. Let's go to the rules now.