SPARK-5063 error

[ramondalmau]

Dear all

Many thanks for this nice contribution. Sparktorch is exactly what I was looking for! :) I am trying to attach a trained Pytorch network to a fitted ML PipelineModel. My attempt is:

trained_net = ... (a trained Pytorch nn.Module)
pipeline_model = ... (a fitted ML PipelineModel)

pipeline_with_net = attach_pytorch_model_to_pipeline(
    trained_net,
    pipeline_model = pipeline, 
    inputCol = 'features',
    predictionCol = 'predicted',
    useVectorOut = False
)

Unfortunately, I get the following error:

Exception: It appears that you are attempting to reference SparkContext from a broadcast variable, action, or transformation. SparkContext can only be used on the driver, not in code that it run on workers. For more information, see SPARK-5063.

I also tried with the following (simple) neural network and command, and I receive EXACTLY the same error

class Net(nn.Module):
    def __init__(self):
        super().__init__()
        self.fc1 = nn.Linear(10, 20)
        self.fc2 = nn.Linear(20, 1)

    def forward(self, x):
        x = self.fc1(x)
        x = F.relu(x)
        x = self.fc2(x)
        return x

create_spark_torch_model(
    Net(),
    useVectorOut = False
)

However, the following code runs smooth, without errors:

network = nn.Sequential(
    nn.Linear(784, 256),
    nn.ReLU(),
    nn.Linear(256, 256),
    nn.ReLU(),
    nn.Linear(256, 10)
)

create_spark_torch_model(
    network,
    useVectorOut = True
)

That is, it seems that the error is not due to my specific neural network, nor the pipeline. I am using Databricks with Apache Spark 3.0.1. Do you know how to solve this issue? Many thanks in advance

Ramon

[gunjan075]

getting same error It appears that you are attempting to reference SparkContext from a broadcast variable, action, or transformation. SparkContext can only be used on the driver, not in code that it run on workers

[gunjan075]

@ramondalmau what was the solution you found 2 years ago.

[srigas]

@gunjan075 Admittedly, I'm kind of late to the party but I just stumbled upon this problem myself, so here's the workaround. First of all, if you check out the documentation of SparkTorch (README page), you will see the following note:

NOTE: One thing to remember is that if your network is not a sequential, it will need to be saved in a separate file and available in the python path.

From my experience, it appears that using a Sequential network is not sufficient in itself, because simply adding a forward pass also leads to this problem.

So, the workaround in Databricks is to upload a separate file locally or by using the Repos functionality. One rather straightforward way of doing this would be to upload the file in DBFS (this can be done manually while in your Databricks notebook by selecting File -> Upload data to DBFS...) and then inform spark with a command similar to

spark.sparkContext.addPyFile("dbfs:/FileStore/mymodel.py"),

supposing that your file is named mymodel.py and was uploaded in this DBFS directory. This file must contain the code that defines your NN model (as a class, as a function, etc.). Then, if for example the NN model is defined through the MyNN class, you would simply add

from mymodel import MyNN

to your code. And that's pretty much it.