Q: What is the difference between overriding a value by using SET vs APPEND?
A value set by using SET command will set the value for the key.Doing it over again will override the value. However , using append has the effect of set only when the key has no value earlier, but if there is a value already assigned to the key , then doing append on the key would lead to an appended value to the existing value of the key.

Q: What do you mean by data modeling in Redis.
Just like any other database, data modeling represents the storage pattern of how and which data structures are used to store the data to achieve a domain requirement. For example in relational database world, we use primary key to establish a relationship between two entities. Data stored in relational databases are in table format, where as in Redis there are set of data structures available, which are used to represent the domain data. There is certainly different design mindset needed to convert the relational data to a Redis dataset.