Sometimes your boxes in PlantUML end up next to each other. In this case, PlantUML by default adds a curved line which doesn't look very tidy.
To fix this, you can use the skinparam linetype
as below
Add ortho linetype:
skinparam linetype ortho
Add polyline linetype:
skinparam linetype polyline
Full plantuml:
@startuml
skinparam linetype polyline
database "MySQL" as mysql #grey
database "Oracle" as oracle #grey
rectangle "Container" as ws #grey {
rectangle "Test job" as testjob #yellow
}
oracle -left-> testjob
testjob <-right- mysql
@enduml