> "how to convert a generator from Keras to an input in estimator"

This is a bit of a mistaken question, because you would not "convert" a DataGenerator into an estimator input. Instead, you can just wrap the DataGenerator in a simple function that lazily outputs the next batch of training examples. Input functions for Estimators are just functions that accept no arguments and produce a 2-tuple, with first component of a dictionary of named inputs and second component of the target value. You can write your own wrapper functions that consumes from a DataGenerator and normalized the output to the format. I'm sure there will be a helper function to do this automatically in the future, but it's about as easy as can be to just wrap with a function anyway.

> "add class weights, custom loss functions"

This too seems mistaken, because this is part of the compiled Keras model, before ever converting anything to TensorFlow Estimator. You can use whatever you want for this and the Keras Model.compile function accepts dictionaries for loss and loss_weights, as well as custom add_loss usage in your own layers (even pass through layers that don't affect the computation graph).

> "plug-in various Keras-based callbacks as well"

This is admittedly slightly harder, but I think it's a little bit of an unfair question because Keras offers far more functionality in its Callbacks than TensorFlow offers with predefined hooks. "Penalizing" Keras because TensorFlow offers less functionality doesn't seem right.

Either way, this is also not too hard. For any Callback you want to use from Keras, you basically just write a tiny wrapper class that subclasses from session_run_hook.SessionRunHook from tensorflow, and then maps the TensorFlow naming conventions, like "begin" or "before_run" etc., to wrap the equivalent method from the Keras callback, like "on_train_begin", or "on_epoch_end".

The bigger point is that this headache is because of TensorFlow. Both because TF chose a really silly class design for the SessionRunHooks thing, making automatic conversion from Keras (which has the more established set of pre-existing callbacks) harder for no good reason, and also because TensorFlow lacks functionality that Keras gives you for free.

For orchestration, my team just uses a simple GPU cluster where the native device placement primitives with TensorFlow allow us to scale to as many GPUs as we've needed (max in the dozens).

For distributing and orchestrating over larger clusters, Keras provides some good alternatives right on its own FAQ page:

< https://keras.io/why-use-keras/#keras-has-strong-multi-gpu-s... >

In the end, I would not claim you can immediately translate every complex feature of Keras, like deep custom callbacks or something, over to TensorFlow ... but that's usually not a big deal. Most times, you just want to port a fairly standard model to the Estimator API, and for this, it "just works" directly and is easy to use for local, small-ish clusters of GPUs.

When you have a much rarer problem that needs a huge GPU cluster, then use the other suggests like dist-keras or Horovod, or write your own simple map-reduce-ish wrapper to put data on different nodes and deploy e.g. a containerized training application.

Also people need to definitely keep in mind that most of the limitations are TensorFlow's own fault for not designing things to be compatible with heavily used Keras features like Callbacks out of the box. TensorFlow has a history of doing this, and has been very developer-unfriendly in this way even when it has no downside or impact on performance or anything. The core TensorFlow designs suffer from an unfortunate "not invented here" kind of philosophy, even when dealing with Keras.