Discovery Settings

Heimdall discovery is a simple pattern used by frontend to discover basic application settings from a universal endpoint based on simple URL rules.
This is useful for retrieving settings such as GQL endpoints and language options.

Discovery settings are organized as a sequence of URL matching rules.
Each rule defines how a URL is matched and if matched, what the settings are.
There are several ways to match rules:

1. Exact match of URL
2. Regex match of URL
3. Exact match of host
4. Regex match of host
5. Exact match of path (with / prefix)
6. Regex match of path (with / prefix)

Regex are based on Python style regexes using re.search.
URL nomenclatures are based on definitions in RFC 3986: Uniform Resource Identifier (URI): Generic Syntax.
We normalize URI using our implementation loosely based on guidelines described in URI normalization (Wikipedia).
The actual implementation can be found in scalpel.common.uri code and examples of the URI normalization can be found in scalpel.tests.common.uri test cases.
URL, host, and path rules may be used together.

Rule settings are simple objects/dictionaries/maps with an opinionated convention based on segregation by applications.
Example:

hippocrates:
  gql_endpoint: https://example.com/gql

login:
  url: https://login.com/

care:
  language: id

When a URL match multiple rules, the individual rule settings are merged recursively in order of preference from the first matched rule to the last.
That is, rule settings objects are union-ized with earlier rule settings retaining their values.

The pseudo-code for generating the discovery setting is as follows:

url ← normalize_uri('https://subdomain.example.com/a/b/c')

matched_settings ← []
for each rule in rules
  if url matches rule
    matched_settings ← matched_settings + [rule settings]

return deep_merge(reversed(matched_settings))