Error map
pymongo.errors.ServerSelectionTimeoutError PyMongo: pymongo.errors.ServerSelectionTimeoutError: localhost:27017: [Errno 111] Connection refused, Timeout: 30s
MongoDB · layer 5, Data
Centre: this error. First ring: errors it is linked with (8). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.
Directly related (8)
MongoDB: MongoParseError: Invalid scheme, expected connection string to start with "mongodb://" or "mongodb+srv://"
The connection string is missing, malformed, or the environment variable holding it is undefined.
MongoDB: MongoServerSelectionError: connect ECONNREFUSED 127.0.0.1:27017
The driver couldn't connect to a MongoDB server on localhost:27017 — nothing is listening there.
MongoDB Atlas: Error: querySrv ENOTFOUND _mongodb._tcp.cluster0.abcde.mongodb.net
The driver couldn't resolve the `mongodb+srv://` SRV record for your cluster.
MongoServerSelectionError: Could not connect to any servers in your MongoDB Atlas cluster. One common reason is that you're trying to access the database from an IP that isn't whitelisted
Atlas rejects connections from IP addresses that aren't in the project's Network Access list.
MongoServerError: Exceeded memory limit for $group, but didn't allow external sort. Pass allowDiskUse:true to opt in
An aggregation stage needed more than 100 MB of RAM and spilling to disk wasn't enabled.
MongoServerError: BSONObj size: 17825792 (0x1100000) is invalid. Size must be between 0 and 16793600(16MB)
A single document (or an aggregation result row) grew past MongoDB's 16 MB limit.
PyMongo: pymongo.errors.BulkWriteError: batch op errors occurred
A bulk insert/update had one or more failed operations; details are in `e.details`.
PyMongo: NotImplementedError: Database objects do not implement truth value testing or bool(). Please compare with None instead: database is not None
You used a Database/Collection object in an `if` (`if db:`), which PyMongo forbids.
Two steps away (22)
HTTP 413 Content Too Large (Request Entity Too Large)
The request body is bigger than the server is willing to accept — usually a file upload over the configured limit.
OAuth Error: redirect_uri_mismatch
The redirect URI in your OAuth request doesn’t exactly match one registered for the client in the provider’s console.
WordPress: Error Establishing a Database Connection
WordPress can’t connect to its MySQL/MariaDB database with the credentials in wp-config.php, or the database server is down.
MongooseServerSelectionError: connect ECONNREFUSED 127.0.0.1:27017
The MongoDB driver couldn’t find a reachable server for the connection string.
MongoDB: MongoServerError: bad auth : Authentication failed (code 8000 / 18)
MongoDB (often Atlas) rejected the username/password in the connection string.
MongoDB: connection refused / timed out from a remote host — mongod bindIp defaults to 127.0.0.1
MongoDB only listens on localhost by default, so remote clients can't connect.
ACM certificate stuck in Pending validation
ACM is waiting to see the DNS (or email) validation record — it wasn’t created, was created in the wrong DNS zone, or has a typo.
dial tcp: lookup registry-1.docker.io: no such host / Temporary failure in name resolution during docker build
DNS isn’t working for the Docker daemon or inside build containers, so pulls and package downloads fail.
DNS not working inside a Kubernetes pod (could not resolve host / nslookup: can't resolve)
Pods can’t resolve service names or external hosts — CoreDNS is down, blocked by NetworkPolicy, or the upstream resolver fails.
Error: connect ECONNREFUSED 127.0.0.1:5432
Nothing is listening at that host:port — the target service is down or you’re connecting to the wrong address.
Error [ERR_STREAM_PREMATURE_CLOSE]: Premature close
A stream ended or was destroyed before it finished — often a client disconnect or upstream aborting a download.
TypeError: fetch failed (cause: ConnectTimeoutError / ECONNREFUSED / UND_ERR)
Node’s built-in fetch (undici) couldn’t complete the request; the real reason is in error.cause.
Kubernetes: OOMKilled (exit code 137)
The container used more memory than its limit, and the kernel killed it.
Segmentation fault (core dumped)
A program accessed memory it wasn’t allowed to, and the kernel killed it with SIGSEGV (exit code 139).
Killed / Out of memory: Killed process (Linux OOM killer)
The kernel ran out of memory and killed a process to recover — the program just prints “Killed” (exit code 137).
Too many open files (EMFILE)
The process hit its file-descriptor limit — sockets, files and pipes all count.
MongooseError: Operation `users.find()` buffering timed out after 10000ms
Mongoose queued a query waiting for a connection that never opened.
CastError: Cast to ObjectId failed for value "x" (type string) at path "_id"
A value that isn’t a valid 24-hex-character ObjectId was used in an _id query.
MongoServerError: E11000 duplicate key error collection: db.users index: email_1 dup key
An insert/update violated a unique index in MongoDB.
MongoDB: MongoServerError: Document failed validation (code 121)
The collection has a JSON Schema validator and the inserted/updated document doesn’t satisfy it.
MongoServerError: bad auth : Authentication failed. (Atlas user / special characters in password)
Atlas rejected the username/password in the connection string.
TypeError: 'NoneType' object is not subscriptable
You used `[ ]` on a value that is `None` — typically a function that returned nothing.
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