Hello Igniters, I would like to start a discussion on evolving IEP-73 [1]. Now it covers a narrow topic about components dependencies but it makes sense to cover in the IEP a broader question: how different components should be initialized to support different modes of an individual node or a whole cluster.
There is an idea to borrow the notion of run-levels from Unix-like systems, and I suggest the following design to implement it. 1. To start and function at a specific run-level node needs to start and initialize components in a proper order. During initialization components may need to notify each other about reaching a particular run-level so other components are able to execute their actions. Orchestrating of this process should be a responsibility of a new component. 2. Orchestration component doesn't manage the initialization process directly but uses another abstraction called scenario. Examples of run-levels in the context of Ignite 2.x may be Maintenance Mode, INACTIVE-READONLY-ACTIVE states of a cluster, and each level is reached when a corresponding scenario has executed. So the responsibility of the orchestrator will be managing scenarios and providing them with infrastructure of spreading notification events between components. All low-level details and knowledge of existing components and their dependencies are encapsulated inside scenarios. 3. Scenarios allow nesting, e.g. a scenario for INACTIVE cluster state can be "upgraded" to READONLY state by executing diff between INACTIVE and READONLY scenarios. I see several advantages of this design compared to existing model in Ignite 2.x (mostly implemented in IgniteKernal and based on two main methods: start and onKernalStart): 1. More flexible model allows implementing more diverse run-levels for different needs (already mentioned Maintenance Mode, cluster state modes like ACTIVE-INACTIVE and smart strategies for cache warmup on node start). 2. Knowledge of components and their dependencies is encapsulated inside scenarios which makes it easier to create new scenarios. Open questions: 1. As I see right now it is hard to standardize initialization events components notify each other with. 2. It is not clear if run-levels should be organized into one rigid hierarchy (when the first run-level should always precede the second and so on) or they should be more independent. What do you think? [1] https://cwiki.apache.org/confluence/display/IGNITE/IEP-73%3A+Node+startup
