Why does an arrow make a duplicate component?

Question

I'm very new to PlantUML, so this is likely something basic.

Sometimes when I add a arrow from one rectangle to another, the original component is not linked.  Instead, a new component is created and linked.  What am I doing wrong?

In my example, I have this fairly simple diagram:

@startuml
database "DB" {
  frame Rules {
    rectangle "Item 1"
    rectangle "Item 2"
  }
}

rectangle "App Server" {
  rectangle "My UI"
}

rectangle "System" {
  rectangle "Foo"
}

[My UI] --> [Item 1] : create and edit
[System] --> [Item 1] : extract
@enduml

And this is generated:

Note that the arrow from System to Item 1 is for a new component and not the existing "System" element.

What am I doing wrong?

Answer 1

You should use this instead :

@startuml
database "DB" {
  frame Rules {
    rectangle "Item 1"
    rectangle "Item 2"
  }
}

rectangle "App Server" {
  rectangle "My UI"
}

rectangle "System" {
  rectangle "Foo"
}

[My UI] --> [Item 1] : create and edit
System --> [Item 1] : extract
@enduml

http://www.plantuml.com/plantuml/uml/ROyn3u9038Nt-nKljpFew63G61C65uaJOQXSDIOSa5un4iD_5s4XwDZtldvfuw0aslhIE5AwKc3OmzxYRO2Ra6USsvB3E0F2XL9rBmSeKVPOsR_n-XltfZTcrkoQ1YdBasMopwfJXqjY5tpQ1MN_mnxhUYIpSIb756sHJSVasA0G9cLGvS3keMQoB2b-gL2X9kRA3Rz_0000

I agree that the syntax is not very convenient in that case. 

Comments

Thank you very much.

So it looks as though the brackets can be used when -- and only when -- the name contains a space.  I guess that makes sense.

---

No, it seems to be more to do with whether the thing you are referencing is a container or not.  So System contains Foo, and so can't be referenced using the [System] notation.  

Therefore I suspect there is no way of referencing the "App Server" container without giving it an alias, because of the space in the name.

As far as I understand, [x] is supposed to represent "the component^* called x".  And referring to it will automatically create a component^* called x if x doesn't already exist.  The two oddities to me are that it ignores containers (even component^* ones), and that it doesn't mind if x already exists as something other than a component^*.

^* I'm referring to the 'component' shape (the rectangle with the little icon in the top right).