server.views.populate_image uses a cached image, with no way to clear the cache

[kylemcleland-zz]

I have a series of dashboards with a live SQL connection. The workflow for these dashboards is to run a nightly refresh of the underlying data table, take a screenshot of the dashboard, and email it out to users.

The issue is that after refreshing the SQL connection, server.views.populate_image returns an image of the dashboard from before it is refreshed. (e.g., if I have data for 1/1/2020, truncate that table overnight, insert data for 1/2/2020, then use views.populate_image, it will still show 1/1/2020 data, even though that data is truncated from the underlying data table as Tableau is caching the image of the dashboard).

Here is a snippet of the offending code:

import logging
import tableauserverclient as TSC

workbook_name = "name of workbook containing view"
view_name = "view to print"
report_filter = "dashboard filter value"
filter_name = "dashboard filter name"
report_export_file_name = "file path + file name of image to be generated" 
tableau_auth = [censored]

with server.auth.sign_in_with_personal_access_token(tableau_auth):
    all_workbooks = TSC.Pager(server.workbooks.get)
    workbook = [
        wb
        for wb in all_workbooks
        if wb.name == workbook_name and wb.project_name == workbook_project
    ][0]
    logging.info(f"Found workbook: {workbook_name}")

    # request views from workbook and find first matching view
    server.workbooks.populate_views(workbook)
    view = [v for v in workbook.views if v.name == view_name][0]
    logging.info(f"Downloaded view to memory: {view_name}")

    # request image of view and download it locally
    # maxage = max age of image cached in minutes
    # shortest interval that you can use is one minute
    image_req_option = TSC.ImageRequestOptions(maxage=1)
    if report_filter:
        image_req_option.vf(filter_name, report_filter)

    server.views.populate_image(view, image_req_option)

    # save file
    with open(report_export_file_name, "wb") as view_png:
        view_png.write(view.image)
    logging.info(f"Saved view to {report_export_file_name}")

What I've tried:

Setting maxage to 1 minute, doesn't seem to work as intended, as even running this script >1 minutes after the SQL update sometimes shows a cached view.

Using workbooks.refresh method. Doesn't work as this seems to be expecting a data extract, not live SQL connection.

[divinorum-webb]

Here's a way you can toss out the cache and force a fresh PNG / PDF download:

1. Add a parameter to your workbook. Let's assume you name it 'cache_buster'. You might even reference this parameter value in a hidden sheet on your dashboard so you ensure it gets used.
2. In the code you have that downloads the PNG or PDF for your visual, specify 'cache_buster' as one of your filters
3. here is the important part: be sure to change the value set for 'cache_buster' in your code each time you want to void the old cached version and get a fresh copy.

So if you run your code the first time, maybe your cache_buster=1. If you download the image again 30 minutes later, you may receive a cached version of what you just pulled.
But, if you make cache_buster=2, then you will trick Tableau into thinking it is passing in a new filter value and it voids the cache on your visual.

That parameter doesn't really do anything in your workbook -- it's simply there to invalidate the unwanted cache on your visual.

[maxwqx]

@divinorum-webb I am new to the TSC. Can you give a sample code about how to add the parameter of "cache_buster".

[efarmun]

@divinorum-webb nice workaround advice but can you advise the way to update parameter on server side using tsc (python) ?
Thanks.
https://community.tableau.com/s/question/0D54T00000C6AwkSAF/is-there-a-way-to-edit-a-parameter-and-its-values-within-tableau-server

[jacalata]

For this workaround, you would have to add the update parameter while editing the workbook normally - either in Desktop or Web Authoring. It can't be edited through tsc.