"500 Backend Error" using Gmail API - safe to retry? - gmail

Question

I'm sending messages via the Gmail API. In particular, I am trying to send 5-7 emails from the same account to different users (1 each) within about 2 seconds.
About 8% of those emails are failing with this error:
&googleapi.Error{
Code:500,
Message:"Backend Error", Body:`{
"error": {
"errors": [
{
"domain": "global",
"reason": "backendError",
"message": "Backend Error"
}
],
"code": 500,
"message": "Backend Error"
}
}`,
Header:http.Header(nil),
Errors:[]googleapi.ErrorItem{
googleapi.ErrorItem{Reason:"backendError", Message:"Backend Error"}
}
}
It doesn't seem like it's specific to a particular account, as 6/7 emails may succeed.
I'm hesitant to retry this for fear of sending 2 emails to the same person.
Is there any way to tell whether this message is safe to retry?

Answer 1

"code": 500, "message": "Backend Error"
Is basically an issue with Google server. Either the request you are making took to long or the server preforming the request is busy and the request again took to long. It doesn't sound like what you are doing should be causing the problem.
Tips when not to run: Don't run on the hour you will be completing with everyone who has cron jobs set up also don't run at midnight (PDT) as this is when quotas reset and again you will be completing with everyone who blew out yesterdays quota.
Solution:
The normal solution is to wait a few seconds then send the same request again. (Implementing Exponential Backoff)
The flow for implementing simple exponential backoff is as follows.
Make a request to the API
Receive an error response that has a retry-able error code
Wait 1s + random_number_milliseconds seconds
Retry request
Receive an error response that has a retry-able error code
Wait 2s + random_number_milliseconds seconds
Retry request
Receive an error response that has a retry-able error code
Wait 4s + random_number_milliseconds seconds
Retry request
Receive an error response that has a retry-able error code
Wait 8s + random_number_milliseconds seconds
Retry request
Receive an error response that has a retry-able error code
Wait 16s + random_number_milliseconds seconds
Retry request
If you still get an error, stop and log the error.

Answer 2

Sometimes it can happen before send and sometimes after send.
I logged the "To" and "From" from five different email attempts that all received a 500 Backend Error. None of these attempts made it to the "Sent" folder of my inbox. I conclude that they were never sent, and it's safe to retry these messages. However, other people in the comments (see below) indicated that the messages actually made it to the remote mailbox, and it's not safe to retry.