An Apisix plugin to dynamically set upstream host:port based on URI, or Header or whatever. Very basic implementation, modify on your needs.

dynamic_uri_upstream.lua

local upstream = require("apisix.upstream")
local core = require("apisix.core")
local ipmatcher  = require("resty.ipmatcher")

local ngx = ngx

local schema = {
    type = "object",
    properties = {
    },
    additionalProperties = true,
}

local _M = {
    version = 0.1,
    priority = 80,
    name = 'dynamic_uri_upstream',
    schema = schema,
}

local function parse_domain(host)
    local ip = ""
    if not ipmatcher.parse_ipv4(host) and not ipmatcher.parse_ipv6(host)
    then
        local ip, err = core.resolver.parse_domain(host)
        if ip then
            return ip
        end

        if err then
            core.log.error("dns resolver domain: ", host, " error: ", err)
        end
    end
    return host
end

function _M.check_schema(conf)
    return core.schema.check(schema, conf)
end

function _M.access(conf, ctx)
    -- Get the request URI
    local uri = ngx.var.uri
    local args = ngx.var.args

    -- get upstream hostport and path based on uri
    local upstream_hostport, path = string.match(uri, "^/proxy/([^/]+)/?(.*)") -- change here on your needs !!!!!
    if not path then
        path = ""
    end

    local upstream_host, upstream_port = string.match(upstream_hostport, "([^:]+):?(%d*)")
    if upstream_port == "" then
        upstream_port = "80"
    end
    upstream_port = tonumber(upstream_port)

    -- core.log.warn("hostport: ", upstream_hostport)
    -- core.log.warn("path: ", path)

    -- set path
    ngx.req.set_uri("/" .. path)

    -- set upstream
    -- some params can be set from existing upstream config
    -- or can be set by plugin conf
    local up_conf = {
        timeout = {
            connect = 6,
            send = 300,
            read = 300
        },
        scheme = "http",
        type = "roundrobin",
        pass_host = "pass",
        keepalive_pool = {
            idle_timeout = 60,
            requests = 1000,
            size = 320
        },
        hash_on = "vars",
        nodes = {
            {
                priority = 0,
                port = upstream_port,
                host = parse_domain(upstream_host),
                weight = 1
            }
        }
    }

    local ok, err = upstream.check_schema(up_conf)
    if not ok then
        core.log.error("failed to validate generated upstream: ", err)
        return 500, err
    end

    local matched_route = ctx.matched_route
    up_conf.parent = matched_route
    local upstream_key = up_conf.type .. "#route_" .. matched_route.value.id
    core.log.info("upstream_key: ", upstream_key)

    upstream.set(ctx, upstream_key, ctx.conf_version, up_conf)
end

return _M

Note: in nginx, this can be done very simple

    location ~ /proxy\/(.*) {
        proxy_pass  http://$1?$args