This glossary is for developers of We also have a general Nextstrain glossary for users.

Terms used in the codebase, issues, documentation, and beyond. Arranged not alphabetically but conceptually since the terms are searchable and directly linkable.


An implementation concept encompassing both the place from which JSONs are fetched for a dataset and their authorization rules. Except for our sources for core, staging, community, and /fetch, sources currently have a 1:1 correspondence with groups. However, a group source and Group itself are not conceptually synonymous. Defined by code in src/sources/.


The primary general-interest datasets maintained by the Nextstrain team, shown first on the Nextstrain homepage. JSONs are fetched from the nextstrain-data S3 bucket.


Testing area for core. JSONs are fetched from the nextstrain-staging S3 bucket.


Datasets and builds maintained by community members on GitHub, with no coordination from the Nextstrain team required. JSONs are fetched from community-managed GitHub repositories. Some community builds are highlighted on the Nextstrain homepage.


An individual login account associated with one or more group. Managed in an AWS Cognito User Pool called Not to be confused with AWS IAM users.


also Nextstrain Group

A named set of users with access to see and update a set of datasets and narratives. Each group has a related source, which typically authorizes access based on the group. Managed in an AWS Cognito User Pool called Not to be confused with AWS IAM groups.


A URL path (e.g. /zika), possibly parameterized (e.g. /groups/:groupName/*), at which provides content. Routes map paths to resources (i.e. HTML pages, images, datasets, narratives, etc).

The process of finding a matching route, if any exists, is called routing.

The Express server performs server-side routing, which means it responds to HTTP requests for a matching path. Express route declarations include the stack of request handlers to use.

The Next.js app (sometimes referred to as the “static site”) performs client-side routing, which means it displays content based on the browser’s location.

request handler

also Express handler, route handler, or just handler.

Either an endpoint or middleware.


Function that sends the response to a request. Specific to an HTTP method and a resource or type of resource.


Function that either responds to a request or does some preparation and then hands processing to the next request handler in the stack.