Update: this scheduled maintenance has been complete.
The Contacts API will be undergoing maintenance between 5:00 PM and 5:30 PM ET on Sunday, September 29th. During this maintenance period, there will be a brief period where the Contacts API will be unavailable, and any requests to the API will result in 500 errors. We expect this period to last less than 5 minutes, but during this period any attempts to create, update, or fetch contacts will fail, and those requests will need to be retried.
If your integration retries on 5xx errors, then you shouldn't need to make any major changes for this, as long as your retry period lasts for at least 5 minutes.
If your integration does not currently retry on 5xx errors, then these requests will fail. Updates will not be queued or retried from HubSpot's side, so you will need to log these failed requests and replay them to make sure that all contacts get created or updated.
If you have any questions, please join the discussion in the community, where our developer support team will be available to answer your questions.