Issue
Heroku Postgres is unable to archive write-ahead logs fast enough, resulting in a risk of database shutdown and data loss.
Resolution
The connection limit for the default credential, and all of your user created credentials, has been temporarily significantly reduced to prevent the WAL storage from filling up and causing your database to crash.
Heroku Postgres will automatically remove this connection limitation once WAL archiving is able to keep up with database writes.
Consider taking the following action.
Reduce database load
If you have recently kicked off a process that is producing a large amount of write volume, such as an ETL process, consider stopping the process and batching the process, spreading it out over a larger window of time. Please be aware of database migrations that may update a very large table or tables with data (such as a new column with a NOT NULL constraint).
Ensure writes are not redundant
Unnecessary writes (where values are being updated to the same value) can be a cause – ensure that you are writing a true delta of changes.
Disable triggers
If the table(s) that you are writing to have triggers that also produce writes, you may be suffering from write amplification due to the triggers. Consider disabling the triggers until your write process is complete.
Prefer DROP and TRUNCATE for deletion
If possible, instead of using DELETE FROM in order to delete data, it is preferable to TRUNCATE or DROP entire tables at once in order to reduce write volume.
For more information, see https://devcenter.heroku.com/articles/postgres-write-ahead-log-usage