MongooseServerSelectionError — get MongoDB connected
Quick answer: the driver could not select a MongoDB server to talk to within its timeout. Most often Mongo is not reachable at the host/port - it is not running, the host is wrong, or you used localhost from inside Docker - or, for Atlas, your server's IP is not in the Network Access allowlist. Confirm reachability, fix the host, and allowlist the IP.
What the error looks like
The reason after it tells you which connectivity problem you have:
MongooseServerSelectionError: connect ECONNREFUSED 127.0.0.1:27017
at ... (mongodb/lib/sdam/topology.js)
# Atlas / DNS variants:
MongooseServerSelectionError: getaddrinfo ENOTFOUND cluster0.xxxx.mongodb.net
MongooseServerSelectionError: connection timed outECONNREFUSED = something refused the connection (not running, wrong host/port). ENOTFOUND = DNS could not resolve the cluster host. timed out = often a firewall or Atlas allowlist silently dropping packets.
Why it happens
Mongo not running / wrong host
No mongod at that host:port, or the connection string points somewhere wrong.
localhost inside Docker
localhost in a container is the container itself - use the Compose service name as the host.
Atlas IP allowlist
Your server's IP is not in Atlas Network Access, so the connection is blocked.
Bad SRV / connection string
A wrong mongodb+srv host, missing TLS, or DNS that cannot resolve the cluster.
Diagnose it in three steps
Is MongoDB reachable?
nc -zv 127.0.0.1 27017 # local
mongosh "mongodb+srv://user:[email protected]/db" # AtlasFor Atlas, check Network Access
# In Atlas: Network Access -> add your server's public IP.
# Confirm the server IP:
curl -s ifconfig.meIn Docker, use the service name
# app connects to host 'mongo' (the compose service), not localhost
MONGO_URI=mongodb://mongo:27017/myappMake Mongo reachable, then connect with sane timeouts
Ensure MongoDB is running and reachable from the app - the right host (a service name in Docker), the IP allowlisted in Atlas, and the correct connection string. Then set a sensible selection timeout so failures are fast and clear.
// Mongoose with explicit options
mongoose.connect(process.env.MONGO_URI, {
serverSelectionTimeoutMS: 5000, // fail fast with a clear error
});
# Docker compose - app waits for and points at the mongo service
MONGO_URI=mongodb://mongo:27017/myapp
# Atlas - add the server IP under Network Access (not 0.0.0.0/0 in prod)If the app starts before Mongo is ready, add a short connection retry, or order startup so the database is reachable before the app accepts traffic.
Order startup, health-check the database
An app that connects before its database is reachable is a startup-ordering problem. On your own servers, Infraveil health-checks dependencies and coordinates how services start, so the app does not go live before MongoDB is reachable - and connection failures surface with context, with recovery approval-gated and recorded.
Frequently asked questions
What does MongooseServerSelectionError mean?
The driver could not find or reach a suitable MongoDB server within the serverSelectionTimeoutMS window - a connectivity problem (unreachable host/port, DNS failure, or a blocked IP), not a query problem.
Why connect ECONNREFUSED 127.0.0.1:27017?
Nothing is listening there: Mongo is not running locally, or inside Docker you used localhost, which is the app container itself. Use the Compose service name as the host, e.g. mongodb://mongo:27017.
Why does Atlas time out from my server but work locally?
Your server's public IP is not in the Atlas Network Access allowlist. Add it under Network Access. Use 0.0.0.0/0 only for short-lived testing, never in production.
How do I connect to MongoDB from Docker?
Use the Compose service name as the host - mongodb://mongo:27017/db - not localhost, which inside a container points at the container itself rather than the database.
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