userId

A user identifier (often written as userID, user ID, UID, or user_id) is a value that uniquely identifies a user account within a system or service. It can be numeric, alphanumeric, or a UUID, and is typically stable across changes to a username or display name. In many contexts the user ID should be treated as an internal, non-guessable key.

In Unix-like operating systems, the UID is a numeric identifier assigned to an account. The kernel uses it to enforce permissions and ownership. The root account commonly has UID 0. User accounts are stored in system databases such as /etc/passwd and /etc/shadow, or in centralized directories like LDAP or Active Directory; the exact storage depends on the system.

In software applications and web services, a userId often refers to a unique value assigned to a user for identification across resources. It may be exposed as an API field or stored in databases as the primary key. Unlike a username, the userId is generally not shown to end users and is designed to remain stable even if a person changes their display name.

Security and privacy considerations: exposing raw user IDs in URLs or API endpoints can enable tracking or enumeration; implement authorization checks on the server side and protect IDs from leakage; use tokens or session identifiers for authentication; consider using opaque identifiers or pseudonymization where appropriate.

Data modeling and formats: user IDs can be integers, UUIDs, or other opaque strings. When designing schemas, choose a type that scales, supports migrations, and preserves privacy. In distributed systems, using universally unique identifiers (UUIDs) or Snowflake-like IDs helps avoid collisions and improves sharding.