[OASIS Issue Tracker] Commented: (MQTT-60) Are topic strings normalized at all?

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    [ http://tools.oasis-open.org/issues/browse/MQTT-60?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=34370#action_34370 ] 

Peter Niblett commented on MQTT-60:
-----------------------------------

It seems to me that the underlying question here is whether the spec allows empty elements in the topic path or not. This was basically Raph's question, which is whether a//b is legal or not. I would personally prefer it not to be legal, as that would keep our topic strings consistent with other path-like syntaxes, such as XPath, or WS-Topics.  However reading between the lines of the Input Spec, it seems that that spec permits empty elements.

It certainly allows them at the start of a topic expression, since it says that a leading / creates a new topic, e.g.   /finance is distinct from finance  and that /finance matches +/+ but does not match +.  That implies to me that there's an empty element at the start of /finance.

The input spec doesn't discuss trailing / but I imagine most people have assumed the same applies there, e.g. finance/  matches +/+ but not +,  and it is different from finance which matches + but not +/+

Having accepted the presence of empty levels at beginning and end, it's hard to argue that they should be banned in the middle of a topic expression, e.g finance//stock should be permitted and will be different from finance/stock.


However for ease of interoperability with other pub/sub systems, I would suggest that we recommend that people don't code these empty levels, In other words

1. Don't start with a /
2. Don't end with a /
3. Don't use //  in the middle


> Are topic strings normalized at all?
> ------------------------------------
>
>                 Key: MQTT-60
>                 URL: http://tools.oasis-open.org/issues/browse/MQTT-60
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Improvement
>          Components: core
>            Reporter: Allan Stockdill-Mander 
>
> Should multiple adjacent topic level separators in a topic name being subscribed to/unsubscribed from be reduced to a single topic level separator.
> Ie: is ////some////over/slashed///topic/etc/etc the same as /some/over/slashed/topic/etc/etc

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