host-interpolate-by-header

host-interpolate-by-header is a plugin for Kong and is used to dynamically update hostname of upstream service by interpolating url with values of request headers.

How does it work?

1. The plugin reads all the headers from the incoming request specified in the config as headers.
2. It transforms the value of the specified headers as per operation in the config.
3. It interpolates hostname of the request with above values before making upstream request.
4. The plugin also interpolates the environment variables in the host but does not apply any operation on it.

Example:

Operation = none

conf = {
    host = "service_<zone>_<shard>.com",
    headers = {"zone", "shard"},
    environment_variables = {},
    fallback_host = "service_fallback.com",
    operation = "none",
    modulo_by = 1
}

Now a request with headers:
zone: us-east-1
shard: z3e67
on kong will be routed to host = service_us-east-1_z3e67.com.

Operation = modulo

conf = {
    host = "service_<env>_shard_<user_id>.com",
    headers = {"user_id"},
    environment_variables = {"env"},
    fallback_host = "service_fallback.com",
    operation = "modulo",
    modulo_by = 3
}

Let's say the environment variable set in nginx worker is env: production.
Now a request with header: user_id: 13
on kong will be routed to host = service_production_shard_1.com as 13 % 3 = 1.
Note: Operation is not applied on environment variable interpolation.

Installation

luarocks

luarocks install host-interpolate-by-header

source

Clone this repo and run:

 luarocks make

---

You also need to set the KONG_PLUGINS environment variable:

 export KONG_PLUGINS=host-interpolate-by-header

Usage

Parameters

Parameter	Default	Required	description
host	hostname-<PLACE_HOLDER>.com	true	Hostname of upstream service
headers	{}	true	array of headers read from request for interpolation
environment_variables	{}	true	array of environment varibales for interpolation
operation	none	false	Operation to apply on header value (none/modulo)
modulo_by	1	false	Number to do modulo by when operation = modulo
fallback_host	-	false	Route to fallback_host if any of the headers is missing in request else error is returned with status code 422