'use strict';

var assert = require('assert-plus');
var ServiceUnavailableError = require('restify-errors').ServiceUnavailableError;

/**
 * The `inflightRequestThrottle` module allows you to specify an upper limit to
 * the maximum number of inflight requests your server is able to handle. This
 * is a simple heuristic for protecting against event loop contention between
 * requests causing unacceptable latencies.
 *
 * The custom error is optional, and allows you to specify your own response
 * and status code when rejecting incoming requests due to too many inflight
 * requests. It defaults to `503 ServiceUnavailableError`.
 *
 * This plugin should be registered as early as possibly in the middleware stack
 * using `pre` to avoid performing unnecessary work.
 *
 * @public
 * @function inflightRequestThrottle
 * @param {Object} opts - configure this plugin
 * @param {Number} opts.limit - maximum number of inflight requests the server
 *    will handle before returning an error
 * @param {Error} opts.err - A restify error used as a response when the
 *    inflight request limit is exceeded
 * @param {Function} opts.server - the instance of the restify server this
 *    plugin will throttle.
 * @returns {Function} middleware to be registered on server.pre
 * @example
 * var errors = require('restify-errors');
 * var restify = require('restify');
 *
 * var server = restify.createServer();
 * const options = { limit: 600, server: server };
 * options.res = new errors.InternalServerError();
 * server.pre(restify.plugins.inflightRequestThrottle(options));
 */
function inflightRequestThrottle(opts) {
    // Scrub input and populate our configuration
    assert.object(opts, 'opts');
    assert.number(opts.limit, 'opts.limit');
    assert.object(opts.server, 'opts.server');
    assert.func(opts.server.inflightRequests, 'opts.server.inflightRequests');

    if (opts.err !== undefined && opts.err !== null) {
        assert.ok(opts.err instanceof Error, 'opts.err must be an error');
        assert.optionalNumber(opts.err.statusCode, 'opts.err.statusCode');
    }

    var plugin = {};
    plugin._err = opts.err || new ServiceUnavailableError('resource exhausted');
    plugin._limit = opts.limit;
    plugin._server = opts.server;

    function onRequest(req, res, next) {
        var inflightRequests = plugin._server.inflightRequests();

        if (inflightRequests > plugin._limit) {
            req.log.trace(
                {
                    plugin: 'inflightRequestThrottle',
                    inflightRequests: inflightRequests,
                    limit: plugin._limit
                },
                'maximum inflight requests exceeded, rejecting request'
            );
            return next(plugin._err);
        }

        return next();
    }

    return onRequest;
}

module.exports = inflightRequestThrottle;