Sqlalchemy.exc.operationalerror: (cx_oracle.operationalerror) ora-03114: not connected to oracle

Question

New to python, web development and therefore bulbs. I have a small application that accepts excel attachments, extracts data and writes it to oracle. My stack includes flask-sqlalchemy, cx_oracle. When I upload a small excel file about 10K lines in size, it works fine. When I drop really large files on it up to a million lines, it twitches, and I get:

sqlalchemy.exc.operationalerror: (cx_oracle.operationalerror) ora-03114: not connected to oracle

This is my view function:

@theapp.route('/upload', methods=('GET', 'POST'))
def upload():
    start_time = time.time()
    form = UploadForm()
    if form.validate_on_submit():
        file_name = secure_filename(form.upload.data.filename)
        form.upload.data.save(os.path.join(theapp.config['UPLOAD_FOLDER'], file_name))
        input_file = os.path.join(theapp.config['UPLOAD_FOLDER'], file_name)
        wb = load_workbook(input_file, data_only = True, read_only=True, use_iterators=True)
        sheet = wb.worksheets[0]
        allrows = sheet.iter_rows() #this is a generator
        headerobject = next(allrows) # row object for headerobject

        # Column names from excel file
        headerlist = []
        for c in headerobject:
            if c.value is not None: # If user uses delete button to delete contents of header cell, ignore it
                if isinstance(c.value, str) is False: # if not a string
                    return bad_request("Column names have to be strings without spaces. Value in cell {} is not a string".format(c.coordinate))
                if not c.value.strip(): # Empty string in header
                    return bad_request("Cell {} has line space(s). Delete the contents of the cell".format(c.coordinate))
                headerlist.append(c.value.strip())
        #print("Excel {}".format(headerlist))

        duplicate_elements = [k for k,v in Counter(headerlist).items() if v >1]
        if duplicate_elements: # A falsy. If there are duplicate elements
            return bad_request("Column(s) {} are duplicates".format(duplicate_elements))

        tableinfo = form.targettableinfo.data # tableinfo object in the databse order
        tablecolumns = []
        for col in tableinfo.fields:
            tablecolumns.append(col.name)
        #print("Table columns {}".format(tablecolumns))

        # Check if the columns match
        if not set(headerlist) == set(tablecolumns):
            foreign_columns = [x for x in headerlist if x not in tablecolumns]
            if foreign_columns:
                return bad_request("Column(s) {} in your excel file are not defined".format(foreign_columns))
            unfulfilled_columns = [x for x in tablecolumns if x not in headerlist]
            if unfulfilled_columns:
                return bad_request("Column(s) {} are expected by the table but not found in your excel file.".format(unfulfilled_columns))

        # Sort the tablecolumns object in the order of the headerlist
        sortedtablecolumnsobject = sorted(tableinfo.fields, key=lambda x: headerlist.index(x.name))

        # create a list of dicts to hold data to be written
        output_data = []
        for row in allrows:
            row_dict = {}
            for fobject,cobject in zip(sortedtablecolumnsobject,row):
            ######################################
            ##Validation & assignment
            ######################################
                if fobject.type.name == "Text":
                    if cobject.value == None:
                        row_dict[fobject.name] = cobject.value
                    else:
                        row_dict[fobject.name] = removeNonAscii(str(cobject.value))
                        #row_dict[fobject.name] = cobject.value.encode('utf-8')
                        #print(str(cobject.value))
                elif fobject.type.name == "Number":
                    if dao.isnumber(cobject.value):
                        row_dict[fobject.name] = float(cobject.value)
                        #print(float(cobject.value))
                    elif cobject.value is None:
                        row_dict[fobject.name] = cobject.value
                        #print(cobject.value)
                    else:
                        return bad_request("Cell {} has value {}. A numeric value is expected".format(cobject.coordinate, cobject.value))
                elif fobject.type.name == "Date":
                    if type(cobject.value) == datetime.datetime:
                        row_dict[fobject.name] = cobject.value
                    elif cobject.value is None:
                        row_dict[fobject.name] = cobject.value
                    else:
                        return bad_request("Cell {} does not contain a valid date".format(cobject.coordinate))
            ##############################################################

            output_data.append(row_dict)

        #print(output_data)
        m = db.MetaData()
        t = db.Table(tableinfo.table_name,m,autoload = True, autoload_with = db.engine)
        db.engine.execute(t.insert(),output_data)

        print("{} seconds" .format(time.time() - start_time))

        return redirect(url_for('summary'))

    return render_template('upload.html', title = 'Upload', form = form)

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