BigQuery schema change not propagated prior to running next step in Apache Beam pipeline

Question

I am using apache beam to achieve 3 steps in one pipeline.

1. Read data from mongodb
2. Update the schema in bigquery if there are new columns from mongodb data, and build the new schema for beam.io.WriteToBigQuery.
3. Save the data into bigquery

I noticed that step 3 executed before I updated the schema in step 2, and I got errors saying "Error while reading data, error message: JSON table encountered too many errors, giving up". However, if I run the same code again, the data could be successfully saved. It looks like that the first time step3 executed before the complete schema has been updated in step2.

I am new to Apache beam. Could you please help with it? Thanks. My code is attached below.

dim_seller_etl_executor = (
        p1
        | "read" >> beam.io.ReadFromMongoDB(uri='mongodb:///',
                                            db='',
                                            coll='',
                                            bucket_auto=True,
                                            extra_client_params={"username": "",
                                                                 "password": ""})

        | "transform" >> beam.Map(transform_doc)
        | 'save' >> beam.io.Write((beam.io.WriteToBigQuery("table_id",
                                                           schema=table_schema_for_beam,
                                                           create_disposition=beam.io.BigQueryDisposition.CREATE_IF_NEEDED,
                                                           write_disposition=beam.io.BigQueryDisposition.WRITE_APPEND)))
)

def transform_doc(document):
    global table_schema_for_beam
    global column_name_type
    new_columns = []
    for name, value in document.items():
        if name not in column_name_type:
            # some ways to get the column type
            new_columns.append((name, column_type))
        else:
            column_type = column_name_type[name]
        data[name] = document[name] if document[name] is not None else None
    # if new columns appear, update the schema in bigquery and the schema used in beam.io.WriteToBigQuery
    if new_columns:
        bigquery_schema.add_columns(new_columns)
        table_schema_for_beam, column_name_type = bigquery_schema.get_table_schema_for_beam()
    return data

Answer 1

I can see two issues:

1. BigQuery schema changes do not propagate immediately, but can take up to minutes to propagate https://cloud.google.com/bigquery/docs/error-messages#metadata-errors-for-streaming-inserts
2. The table_schema_for_beam variable is evaluated during pipeline construction. Mutating this variables while the pipeline is running may have unknown effects, or no effect at all.