doOnNext / doOnComplete / doOnError / doOnEach / doOnSubscribe / doOnDispose / doOnLifeCycle
코드가 어느 스레드에서 실행될 것인지를 지정하는 역할
delay / timeInterval / debounce / throttleFirst / throttleLast
amb / takeUntil / skipUntil / all
zip / zipWith / combineLatest / merge / concat
filter / map / concatMap / flatMap / switchMap / reduce / scan
