AlwaysSerialized is a marker interface similar to java.io.Serializable. The
engine stores variables using persistence rules as described completely in
the Flux documentation. When storing a persistent variable that must always
be serialized, even if it has public fields with recognized types, tag that
class by implementing the AlwaysSerialized interface. All persistent
variables that implement AlwaysSerialized are indeed always serialized to the
database.
In general, it is bad form to serialize job data to the database, due to a
variety of technical reasons. However, sometimes it cannot be avoided.