A Tomcat server worker thread replying to an incoming request must access at least one of several possible cloud API services (Google Cloud Storage in particular; non-modifying, that is, only performing Reads). Tomcat offers out-of-the-box pooling for JDBC and JNDI compatible DB's and services. What is the best strategy for pooling connections to multiple cloud services to be shared across worker threads?
As you mentioned Tomcat allows defining JDBC/JNDI resources that can be used by web applications. If there is a JDBC driver available for the service you want to use then you can use the same approach. For example, a quick search for GCS JDBC driver returns this.
For pooling, you could define HikariCP as a tomcat resource and configure it to connect to GCS using the third-party driver using the driverClassName property.
Alternatively, applications can use the HTTP-based SDKs that cloud providers usually make available.