Starting with version 4.1, Infinispan distribution contains a server module that implements Infinispan's custom binary protocol called Hot Rod. The protocol was designed to enable faster client/server interactions compared to other existing text based protocols and to allow clients to make more intelligent decisions with regards to load balancing, failover and even data location operations.
See the Infinispan Server page for details.
In order to provide a more consistent experience when interacting with Hot Rod servers and avoid issues related to lazily started cache instances, on startup, Hot Rod server starts all defined caches in the Infinispan configuration file including the default cache. Any request to an undefined cache will be rejected by the server.
So, as user, this means that any caches you interact with must be defined in the Infinispan configuration file. Besides, as explained the 'Cache Name' section of the Hot Rod protocol, you can also interact with the default cache by passing an empty cache name.