There are several issues in the Admin SDK with respect to how it creates and uses threads:

1. Most public APIs are asynchronous, and returns Task instances. This tends to make implementing simple use cases unnecessarily complex (e.g. creating a custom token and sending back as a servlet response).
2. Main thread pool is initialized statically, and not configurable.
3. ThreadFactory is initialized statically, and not configurable.
4. Tasks API is visible to user code. A user can kick off arbitrary background tasks using this API, which will get executed on the main thread pool of the SDK, un-regulated.

In order to address these limitations, we can consider the following changes:

• Expose synchronous/blocking variants of public APIs where it makes sense (most notably at FirebaseAuth).

BlockingFirebaseAuth auth = FirebaseAuth.getBlockingInstance(); String customToken = auth.createCustomToken("user1"); 

• Make the thread pool and thread factory configurable via app options.
• Hide or decommission the Tasks API. We can consider using the ApiFutures from Google Cloud Commons (this is similar to Guava's ListenableFutures) as a replacement.

FirebaseAuth auth = FirebaseAuth.getInstance(); ApiFuture<String> customTokenFuture = auth.createCustomToken("user1"); // The following should not be supported // Tasks.call(new Callable(){...});