I have implemented a working API for DT3 along with Zapier integration. The concepts are mine – with help from a developer to implement it. To me this solves a long-running goal to be able to access my data when away from my main computer (beyond what DT3 Server and Devonthink to Go can do) and to more easily create custom reports and otherwise integrate DT3 with other apps. It lets me keep the unique and unparalleled benefits of a desktop DT3 app and local data storage but also have the benefits of a web app.
I am glad to share this with others - with the request that this be for non-commercial use and that you share back any edits or tweaks or tricks you develop with it.
The link to the Github repository for the API is below. I have also included a link for you to request access to the installed Zapier integration; note that the Zapier integration lets you enter the URL and other credentials for your own API and Zapier does not share that with me so there is no security issue beyond what you have with any other Zapier app. If you would prefer to install the Zapier integration as your own app, just let me know and I will add you to the private Github repository where that is stored.
You can use the API by itself; if you want to use the Zapier integration then you must also have the API installed.
In simplest form, the API itself has a bulit-in web app which lets you upload or download files to/from DT3 via the Internet. But if you set up a MongoDB database (which can be done on the Mongo free tier) then the API will do a 1-way sync of your DT3 metadata to MongoDB; that then opens up the floodgates to infinite possibilities for building your own web app to access your local DT3 database.
Personally I have been using www.retool.com which is a terrific low-code front-end builder for creating a GUI front-end to access MongoDB and other databases. But you have many other options.
The Zapier integration does 3 things - you can Trigger it via a webhook using the sample Applescript provided and a SmartRule (it uses tags to select which Zap you want to trigger); then you can pass data to any Zapier app. So for example you can have a DT3 smart rule which activates Zapier to copy a file to Dropbox/Google Drive, add an item and its X-Devonthink Link and URL to a Google Spreadsheet, or whatever else you can imagine with Zapier integration.
As an Action, the Zapier integration can either upload or download files and/or nested groups to/from DT3.
One helpful benefit of the API is that Shortcuts on iOS can access an API. So there is a sample Shortcut for iOS which takes as its input an X-Devonthink link and then downloads it from DT3 and sends it to the Share Sheet location of your choice.
I have been using this for a few weeks and have found it to be really useful in my work with DT3 but clearly there are infinite ways to integrate it with Zapier, with Retool low-code apps, or with any other web app framework you prefer.
I can answer some questions on getting it working and the big-picture capabilities of the software. @8isnothing developed the software and can provide more specifics if there are bugs or technical / coding issues.
A few screenshots to give you a flavor of what it does and looks like: