Error map
Puma: Address already in use - bind(2) for 0.0.0.0:3000 (Errno::EADDRINUSE) Rails/Puma: Errno::EADDRINUSE: Address already in use - bind(2) for "0.0.0.0" port 3000
Ruby / Rails · layer 4, Application & API
Centre: this error. First ring: errors it is linked with (11). 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 (11)
Docker: Bind for 0.0.0.0:80 failed: port is already allocated
Another container or host process already publishes that host port.
driver failed programming external connectivity on endpoint: iptables: No chain/target/match by that name
Docker’s iptables rules were wiped (firewalld/ufw reload, iptables flush) while Docker was running, so it can’t publish ports.
nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
Nginx (or another server) can’t listen on the port because another process already holds it — often Apache, another Nginx, or a s…
LOG: could not bind IPv4 address "0.0.0.0": Address already in use — Is another postmaster already running on port 5432?
Another process (often a second PostgreSQL install or a container) already listens on 5432.
Go: listen tcp :8080: bind: address already in use
The port your Go server wants is already taken by another process (or a previous instance of your app).
macOS: Address already in use — Port 5000 is in use by ControlCenter (AirPlay Receiver)
Since macOS Monterey the AirPlay Receiver listens on ports 5000 and 7000, colliding with dev servers like Flask or Docker registr…
Rails: ActiveRecord::IrreversibleMigration — This migration uses remove_column without a type; db:rollback can't reverse it
`rails db:rollback` can't undo the migration because it contains an operation Rails can't invert.
Rails: ActiveRecord::ConnectionNotEstablished: No connection pool for 'ActiveRecord::Base' found
ActiveRecord has no database connection configured or established for this process.
Rails: ActiveRecord::Deadlocked: PG::TRDeadlockDetected: ERROR: deadlock detected
Two transactions locked rows in opposite orders, so PostgreSQL aborted one.
Rails: A server is already running (pid: 12345, file: /app/tmp/pids/server.pid). Exiting
Rails thinks another server is running because a stale `server.pid` exists (often after a crash or in Docker).
Rails: ActiveRecord::AssociationTypeMismatch: User(#1234) expected, got "42" which is an instance of String(#5678)
You assigned the wrong kind of object (or an id string) to an association.
Two steps away (33)
Error: listen EADDRINUSE: address already in use :::3000
Another process is already listening on that port.
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
The docker CLI can’t talk to the Docker engine — it isn’t running, or your user can’t access the socket.
Conflict. The container name "/x" is already in use by container …
A container (running or stopped) already has that name; Docker won’t create a second one with the same name.
Cloudflare Error 1020: Access Denied
A Cloudflare firewall/WAF custom rule blocked the request — the visitor’s IP, country, user agent or request pattern matched a bl…
Web server failed to start. Port 8080 was already in use.
Spring Boot couldn’t bind its embedded server port because another process uses it.
Error: listen EACCES: permission denied 0.0.0.0:80
Node isn’t allowed to bind that port — ports below 1024 need root on Linux/macOS, and some Windows ports are reserved.
HTTP 403 Forbidden
The server understood the request but refuses to authorize it — you (or the server process) are not allowed to access this resour…
Nginx 444 No Response (connection closed without response)
An Nginx-specific code: the server deliberately closed the connection without sending anything — a rule is dropping the request.
Nginx 400: The SSL certificate error (495 / 496 client certificate)
Nginx rejected the request because the client certificate was invalid (495) or missing (496) on a site that requires mutual TLS.
HTTP Error 500.30 - ASP.NET Core app failed to start
IIS/Azure’s ASP.NET Core Module launched your app but it crashed during startup.
java.lang.ExceptionInInitializerError
A static initializer (static block or static field) threw an exception while the class was loading.
Job for mysql.service failed because the control process exited with error code
The MySQL/MariaDB service failed to start — the real reason is in the MySQL error log.
Error response from daemon: network X not found
The container or Compose project refers to a Docker network that doesn’t exist (any more).
Error response from daemon: Pool overlaps with other one on this address space
Docker tried to create a network whose subnet clashes with an existing Docker network (or a VPN/host route).
Go: cannot assign to struct field users["bob"].Age in map
Map values are not addressable, so you cannot modify a field of a struct stored directly in a map.
OSError: [Errno 98] Address already in use
The port your server wants to bind is already taken by another process.
Homebrew: Error: Permission denied @ dir_s_mkdir - /usr/local/Cellar / /opt/homebrew is not writable
Homebrew can't write to its prefix because the folders are owned by another user or root.
Homebrew: Error: No available formula with the name "x". Did you mean ...?
Homebrew can't find a formula or cask with that name in the taps it knows.
MySQL ERROR 1146 (42S02): Table 'db.table' doesn't exist
The query references a table that MySQL can’t find in that database.
PostgreSQL: ERROR: relation "x" does not exist
The table/view isn’t found — missing migration, wrong schema, or a case-sensitive quoted name.
MySQL ERROR 1022 (23000): Can't write; duplicate key in table
An ALTER/CREATE produced a duplicate key or constraint name — often a foreign key name that already exists in the database.
Rails: ActionController::InvalidAuthenticityToken (Can't verify CSRF token authenticity)
A non-GET request arrived without a valid CSRF token for the session.
Redis: Error: connect ECONNREFUSED 127.0.0.1:6379 / Could not connect to Redis at 127.0.0.1:6379: Connection refused
Nothing is listening on the Redis host/port your client uses.
Redis: Error: write EPROTO ... SSL routines:ssl3_get_record:wrong version number / Connection reset by peer (TLS vs plaintext)
The client and server disagree about TLS — plaintext client to a TLS port, or TLS client to a plaintext port.
Ruby: NameError: uninitialized constant UsersController::User (Did you mean? Users)
Ruby (or Rails autoloading) couldn't find a class or module with that name in the current scope.
Ruby: NameError: undefined local variable or method 'x' for main:Object
Ruby saw a bare identifier that is neither a local variable nor a method on the current object.
Rails: ActiveRecord::RecordNotFound: Couldn't find User with 'id'=42
`find`/`find_by!` didn't find a row, so Rails raises (and renders a 404 in production).
FATAL: the database system is starting up
PostgreSQL is still starting (or recovering) and not accepting connections yet.
FATAL: data directory "/var/lib/postgresql/data" has invalid permissions — Permissions should be u=rwx (0700) or u=rwx,g=rx (0750).
PostgreSQL refuses to start because the data directory is too open or owned by the wrong user.
FATAL: database files are incompatible with server — The data directory was initialized by PostgreSQL version 15, which is not compatible with this version 16
A newer (or older) PostgreSQL major version was started on an existing data directory.
Rails: ActiveRecord::PendingMigrationError — Migrations are pending. To resolve this issue, run: bin/rails db:migrate
The database schema is behind the migration files in the app.
Rails: ActiveRecord::RecordInvalid: Validation failed: Email can't be blank, Name is too short
`save!`/`create!`/`update!` raised because validations failed.
Rails: ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR: relation "users" does not exist
Rails queried a table that isn't in the connected PostgreSQL database.
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