Is it possible to parameterize Post requests payload and then encode it to base64 in Jmeter - jmeter

Question

The background is that I want to make a REST to Google pub/sub. Where the data need to follow this format:
Link to documentation of Pub/Sub format
My current solution is that I uses an BeanShell PreProcessor script to encode the payload to base64 before I send the request to the endpoint. This solution works, but I would like to
parameterize the data in the payload, instead of having the whole payload inserted as test data in the csv-file.
BeanShell PreProcessor used to encode the message before it is being sent:
import org.apache.jmeter.protocol.http.util.Base64Encoder;
String csv_payload = vars.get("csv_payload");
String csv_payload_encoded = Base64Encoder.encode(csv_payload);
vars.put("csv_payload_encoded", csv_payload_encoded);
Payload populated from the csv-file in Post request:
{
"messages": [
{
"data":"${csv_payload_encoded}",
}
]
}
Example of payload data stored in the csv-file that is sent in the request:
{"identId":"123456","requestId":null,"payload":{"header":{"requestid":1,"timeStamp":1617873956,"version":"0.0.0.1","eventId":0001,"creatorId":0,"messageTTL":34560},"body":{"checkid":001,"checkData":{"diagnosticsData":{"troubleSource":0,"data":"[2020-01-01 16:00:53.707961][[lat[0]][long[0]][alt[0]][canbetrust[0]][mars[0]]][signal[5]][TEM2 wake up]"}}}}}
Example of the encoded payload that the request sends to google pub/sub:
{
"messages": [
{
"data":"eyJpZGVudElkIjoiMTIzNDU2IiwicmVxdWVzdElkIjpudWxsLCJwYXlsb2FkIjp7ImhlYWRlciI6eyJyZXF1ZXN0aWQiOjEsInRpbWVTdGFtcCI6MTYxNzg3Mzk1NiwidmVyc2lvbiI6IjAuMC4wLjEiLCJldmVudElkIjowMDAxLCJjcmVhdG9ySWQiOjAsIm1lc3NhZ2VUVEwiOjM0NTYwfSwiYm9keSI6eyJjaGVja2lkIjowMDEsImNoZWNrRGF0YSI6eyJkaWFnbm9zdGljc0RhdGEiOnsidHJvdWJsZVNvdXJjZSI6MCwiZGF0YSI6IlsyMDIwLTAxLTAxIDE2OjAwOjUzLjcwNzk2MV1bW2xhdFswXV1bbG9uZ1swXV1bYWx0WzBdXVtjYW5iZXRydXN0WzBdXVttYXJzWzBdXV1bc2lnbmFsWzVdXVtURU0yIHdha2UgdXBdIn19fX19",
}
]
}
If there are any feedback I would appreciate it or any other improvements suggestion
for how I would be able to proceed in order to parameterize and encode the payload to base64 eg:
Example of

Answer 1

Don't use Beanshell, since JMeter 3.1 you're supposed to be using JSR223 Test Elements and Groovy language for scripting
There is __base64Encode() function available via Custom JMeter Functions plugin bundle installable using JMeter Plugins Manager
If you CSV file has references to other JMeter Functions or Variables you can resolve them by wrapping into __eval() function like:
to read the file:
${__FileToString(somefile.csv,,)}
to read the file and resolve any functions or variables in it:
${__eval( ${__FileToString(somefile.csv,,)})
to read the file, resolve any functions or variables and Base64-encode the result:
${__base64Encode(${__eval( ${__FileToString(somefile.csv,,)})},)}
Demo: