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Yeah filtering by app is one way, zappl currently does it in that way. The most efficient way is a lib or a advancement in steem-js and current libraries that would allow data to be sorted by app default. There is certain limitations to the current blockchain that makes it kinda unfriendly to apps that only want to show their own content but allow the use of all tags.

but currently this is limited by providing meta-data per application and other applications that are (not) aware of the meta-data being already used could (unconsciously) abuse the same meta-data. so currently the application (for example zappl) has to track in a separate database which entries were really submitted by itself to filter out unwanted content.

or will there be another possiblity to achieve that in future? do you know about plans to solve this issue (for example by introducing namespaces for each application)?

What would hope a multiple support for namespace would be allowed. So apps could have more than just a community token it could have its own list of tags and so and on the launch could have a smart contract to copy all content made on the main steem chain so apps wouldn't lose data or have to pull from multiple namespaces.

Zappl is curious to see how this evolves , because steem currently is not friendly to short form applications.