Question about Memento

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Brian Townsend

Sep 1, 2021, 6:31:04 AM9/1/21
to Trellis LDP
Can you tell me please if Memento information is stored in the triple store in Trellis, or does Memento only apply to Non-RDF Resource? So, if I `replace` a Resource, is the previous version stored in the triple store as a memento?
Also, is the default impl. of triple store actually a quad store? I haven't been able to find specific documentation about the underlying store. I did see that someone created a Trellis Extension for Cassandra as a store. Very interesting! Is there a docker image with a Cassandra store or would I need to build one.

Also, I think I saw from your GH profile @Aaron that you work for Inrupt.
I also saw from a previous thread in this group that you said Trellis is LDP compliant and not necessarily Solid compliant. Do you know of any resource I can read that outlines the key differences? Solid implements the LDP spec...not sure if it 100% compliant. Are there significant `extras` in Solid?

I'm building a suite of applications for enterprise clients that will involve heavy use of linked data. I would like users from different organisations to be able to share certain content with each other - read/write access on specific triples. It is a heavily regulated industry so audit logs of data changes are essential. Having the ability to view a resource in it's various lifecycle states is a big bonus - Memento?

Thanks for this awesome work!

Aaron Coburn

Sep 1, 2021, 11:47:17 AM9/1/21
Hello Brian,

Thanks for your message. To answer your question about Mementos, no: the memento data are not stored in a triple store. The data in the triple store represent the current resource state. Because Mementos are immutable, they are persisted in a separate location where they cannot be changed. In the Triplestore-based application, those Mementos are stored at a file location. In a high availability scenario, you would likely want that file location to point to a shared volume mount. There is also an S3-based connector for storing Mementos. And mementos are stored for all resources: RDF and Non-RDF.

There is a cassandra-based docker container available from the Trellis-Extensions repository:

In terms of Solid and LDP, there is a lot of overlap, but they are somewhat different. Solid does not strictly require LDP, though many Solid server implementations are also LDP-conforming servers. The best place to keep track of the Solid protocol specification is at and the corresponding GH repository:

I do not believe there is a single document that outlines the differences between Solid and LDP. Perhaps the most significant differences relate to authentication and authorization, both of which are out of scope for LDP. Another area relates to notifications, but that, too, is out of scope for LDP. Trellis implements WebAccessControl, though that specification was recently revised, and I have not checked that the Trellis implementation still conforms. Furthermore, Solid only makes use of LDP's concept of basic containment; direct and indirect containment are not part of Solid. It is also important to note that it is possible to build a Solid server that does not strictly conform to LDP.


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Brian Townsend

Sep 2, 2021, 11:27:15 AM9/2/21
Hi again Aaron.
Thank you so much for the fast and detailed response! Some of the things that attracted me to Trellis (on my search for LDP conforming servers and Solid servers) are the support for Mementos, WAC, built-in Audit and K/V based triple/quad store.
It's a really amazing project and congrats on it! I'm hoping to start playing with it soon...

btw...I hope you don't mind me saying...
You look like a young James Coburn :)
Don't suppose you're related?

Kind regards,

Brian Townsend 

Founder & CEO

T:  +61 424 728 778     M:  +61 424 728 778

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