transform-lua

Lua JSON transform

Last modified: 2026-04-27

Demonstrates the lua_json transform. The script entrypoint is function modify_json(data, ctx) where data is the decoded JSON value (a Lua table), not a string. A self-contained static action seeds the input so the example runs offline. The script uppercases title, derives a word_count field from the body, drops body, and stamps transformed_by = "lua" before returning the modified table for the proxy to re-serialise to JSON. The origin is reached on 127.0.0.1:8080 via the lua.local Host header.

Run

sbproxy serve -f sb.yml

Try it

# Upstream body (what the static action emits internally):
# {
#   "id": 1,
#   "title": "lua transforms keep their cool",
#   "body": "the lua script will count these eight words",
#   "userId": 7
# }

# Client response after the lua_json transform
$ curl -s -H 'Host: lua.local' http://127.0.0.1:8080/ | jq
{
  "id": 1,
  "title": "LUA TRANSFORMS KEEP THEIR COOL",
  "userId": 7,
  "word_count": 8,
  "transformed_by": "lua"
}

# title is uppercased
$ curl -s -H 'Host: lua.local' http://127.0.0.1:8080/ | jq -r '.title'
LUA TRANSFORMS KEEP THEIR COOL

# body is gone, replaced by the derived word_count
$ curl -s -H 'Host: lua.local' http://127.0.0.1:8080/ | jq 'has("body"), .word_count'
false
8

# Provenance tag is added
$ curl -s -H 'Host: lua.local' http://127.0.0.1:8080/ | jq -r '.transformed_by'
lua

What this exercises

• lua_json transform with the canonical function modify_json(data, ctx) entrypoint
• Reading and mutating a parsed JSON table directly inside Lua (no string parsing)
• Field derivation (word_count from body), mutation (string.upper), and removal (data.body = nil)
• static action - inline JSON body so the example runs offline

See also

• docs/features.md - full feature reference
• docs/configuration.md - configuration schema
• docs/scripting.md - Lua, JavaScript, and CEL scripting reference