Glossary of local development terms
These are the terms that come up when you run projects on your own machine. Each entry opens with a definition short enough to quote on its own, then explains what the term means, how it works, and a common confusion, with a small example where one helps. Where Rig treats a term in a specific way, the entry says so in a separate section.
A to Z
- .env file
A .env file is a plain text file of KEY=value pairs that a library, framework, or tool loads into a process’s environment at startup. Each tool defines its own rules, so quoting, expansion, and precedence depend on whatever reads it.
- EADDRINUSE
EADDRINUSE is the error code for “address already in use”. A program gets it when it tries to bind a socket to an address and port that another socket already holds, which usually means a server is already listening on that port.
- Environment variable
An environment variable is a named string value that a process receives from the process that started it. Programs read them for settings such as ports, URLs, and credentials, so the same code can behave differently on each machine.
- Health check
A health check is a small, repeatable request that reports whether a service is alive and able to do its job. A process manager or orchestrator uses the answer to restart a process, hold back traffic, or decide what starts first.
- localhost
localhost is the reserved hostname for the machine you are on. Name resolution is expected to map it to a loopback address, usually 127.0.0.1 for IPv4 and ::1 for IPv6, so connections to it stay on your own computer.
- Loopback address
A loopback address is an IP address that sends traffic back to the same machine. IPv4 reserves the 127.0.0.0/8 block for it, usually used as 127.0.0.1, and IPv6 defines the single address ::1. Such traffic is meant to stay on the host.
- Model Context Protocol
The Model Context Protocol (MCP) is an open-source standard for connecting AI applications to external systems. A host runs one client per server, and each pair exchanges JSON-RPC 2.0 messages over stdio or Streamable HTTP.
- Network port
A network port is a 16-bit number, 0 to 65535, that identifies an endpoint on a host for a transport protocol such as TCP or UDP. Together with the address it lets one machine run many network services at once.
- Process manager
A process manager starts programs, tracks them, captures their output, and decides what happens when they exit: restart, stay stopped, or report the failure. Many run in the background, so a server keeps going without an open terminal tab.
- Procfile
A Procfile is a text file named Procfile that declares an app’s process types, one per line, as a type name, a colon, and the command that runs it. Heroku defines the format, and local runners such as Foreman read it too.
- Reverse proxy
A reverse proxy is a server that accepts requests on behalf of one or more backend servers, forwards them, and returns the responses. In local development, one common use is to put HTTPS in front of a plain HTTP dev server.
- Sidecar process
A sidecar is a helper that runs beside a main application and supports it. In Kubernetes it is a container in the same Pod. In local development the word is often used for a separate process, such as a queue worker, started next to the web server.
How entries are written and checked
Every definition rests on the specification or official documentation of the thing it defines, such as an RFC, a vendor manual, or a language reference. Each entry lists its sources with the date they were checked. Statements about Rig come from Rig’s source code and appear only in the section about Rig.
How the glossary is kept current
Every entry records the date of its last substantive update. An entry that is still a draft is not published and does not appear in the list. An entry found to be out of date is marked as needing an update until someone has re-checked it against its sources.
Related
Section links
- .env file: definition and meaning
- EADDRINUSE: definition and meaning
- Environment variable: definition and meaning
- Health check: definition and meaning
- localhost: definition and meaning
- Loopback address: definition and meaning
- Model Context Protocol: definition and meaning
- Network port: definition and meaning
- Process manager: definition and meaning
- Procfile: definition and meaning
- Reverse proxy: definition and meaning
- Sidecar process: definition and meaning