# Topic proc (world)

Source: https://ref.dm-lang.org/world/proc/Topic

### Format
> `Topic(T,Addr,Master,Keys)`

### When
> Called when a message is received from another server by using world.Export(). If a file is expected, world.Import() may be called to get it. The return value of Topic() will be passed back to the remote server.

### Args
- `T`: The topic text string specified by the remote server (everything following ? in the URL).
- `Addr`: The address of the remote server.
- `Master`: 1 if remote server is the server which started this one.
- `Keys`: List of keys belonging to users who are logged in on the remote server


### Default action
> The topic "ping" returns a true value (number of players plus one), which may be useful for telling if a server is alive. The topics "Reboot" and "Del" will call world.Reboot() and world.Del() respectively if the message was sent by the master server.

```dm /Topic/
world/Topic(T)
  if(findtext(T,"shout:") == 1)
    world << copytext(T,7)
```

This example allows other servers to send this server topic text of the form "shout:msg" and will broadcast the message to all the players in this world.

The Keys argument is either null, or a list of user keys. Any keys in the list are logged in to the remote server.

> [!danger]
> 
> Always validate the input in `Topic()` calls to make sure it's correct and the query you're recieving is legitimate.

### See also
- [Del proc (world)](https://ref.dm-lang.org/world/proc/Del.md)
- [Export proc (world)](https://ref.dm-lang.org/world/proc/Export.md)
- [Import proc (client)](https://ref.dm-lang.org/client/proc/Import.md)
- [Import proc (world)](https://ref.dm-lang.org/world/proc/Import.md)
- [Reboot proc (world)](https://ref.dm-lang.org/world/proc/Reboot.md)