Hello Igniters, I would like to start a discussion about simplifying NodeFilter functionality. At the moment NodeFilter's are used to control Caches and Services distribution across nodes. In most cases, NodeFIlter implementation seeks for a specific attribute in NodeAttributes map. If the attribute is found on a node, Cache or Service is deployed on that node. However, current NodeFilter interface gives a user much more ways to adjust such distribution. This gives more flexibility for configuration on the one hand but it increases complexity and understanding of the API. Because NodeFilter is a functional interface and configured on the user side there are problems with serialization, class loading and consistency check of such objects. Here is a couple of problems we have with NodeFilter: 1. User-defined node filter classes must be deployed on all nodes whether or nor they required on them. This increases the complexity of resolving problems like in IGNITE-1903. 2. Part of consistency checking of CacheConfigurations based on NodeFilter classes comparison, not on objects. User may use the same class for NodeFilter but with different constructor parameters and this can lead to inconsistent behavior of the same node filter on different nodes while consistency check will pass. 3. We can resolve p.2 using objects equality approach, but we can't force users to properly implement .equals() method on NodeFilter class. We can only recommend doing that thing. If the user forgot to implement .equals() or did it incorrectly we can't deal anything with it. All of those problems can lead to cluster instability and unpredictable behavior.
Instead of continuing using NodeFilter we can give more safe and simpler way to implement the same feature. I propose the following approach that is used in many other distributed systems: User may tag every Ignite node configuration with a specific label that will be placed in NodeAttributes map. NodeFilter interface replaced with just a string label. If a node NodeAttributes map contains such label a Cache or Service will be deployed on that node and not deployed if the label doesn't exist. I would like to add this change to Ignite 3.0 scope because it's an important and major change in public API. WDYT?
