The API (short for Application Programming Interface) is the primary way to communicate with a running Zettelstore. Most integration with other systems and services is done through the API. The web user interface is just an alternative, secondary way of interacting with a Zettelstore.
Background
The API is HTTP-based and uses plain text and symbolic expressions as its main encoding formats for exchanging messages between a Zettelstore and its client software.
There is an overview zettel that shows the structure of the endpoints used by the API and gives an indication about its use.
Authentication
If authentication is enabled, most API calls must include an access token that proves the identity of the caller.
Authenticate an user to obtain an access token
Renew an access token without costly re-authentication
Provide an access token when doing an API call
Zettel lists
List all zettel
Query the list of all zettel
Determine a tag zettel
Determine a role zettel
Working with zettel
Create a new zettel
Retrieve metadata and content of an existing zettel
Retrieve metadata of an existing zettel
Retrieve evaluated metadata and content of an existing zettel in various encodings
Retrieve parsed metadata and content of an existing zettel in various encodings
Update metadata and content of a zettel
Rename a zettel
Delete a zettel
Various helper methods
Retrieve administrative data
Execute some commands
Check for authentication
Refresh internal data