Marked as helpful by the asker
The singleton helps within one function instance, but on Vercel you have many instances, each holding its own connections, straight to Postgres on 5432. Small Supabase instances only allow a limited number of direct connections, so 30 users can fill them.
Use the pooler for the app and keep the direct connection for migrations only:
# .env (Vercel)
DATABASE_URL="postgresql://postgres.abcd:[pw]@aws-0-eu-central-1.pooler.supabase.com:6543/postgres?pgbouncer=true&connection_limit=1"
DIRECT_URL="postgresql://postgres.abcd:[pw]@aws-0-eu-central-1.pooler.supabase.com:5432/postgres"
datasource db {
provider = "postgresql"
url = env("DATABASE_URL")
directUrl = env("DIRECT_URL")
}Port 6543 is transaction mode: connections go back to the pool after each transaction. pgbouncer=true stops Prisma from using prepared statements, which transaction mode does not support.
Switched, redeployed, 2 hours of traffic and zero errors. What does connection_limit=1 do exactly? 
wendy_l
wendy_l Each function instance opens at most 1 connection to the pooler. Serverless instances handle one request at a time anyway, so more would just be idle. hannah_reyes
hannah_reyes Bookmarking this, the pgbouncer=true part bit me last month with 'prepared statement already exists'. pawel_z
