/* Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "mod_proxy.h"
#include "scoreboard.h"
#include "ap_mpm.h"
#include "apr_version.h"
#include "ap_hooks.h"

module AP_MODULE_DECLARE_DATA lbmethod_byrequests_module;

static int (*ap_proxy_retry_worker_fn)(const char *proxy_function,
        proxy_worker *worker, server_rec *s) = NULL;

/*
 * The idea behind the find_best_byrequests scheduler is the following:
 *
 * lbfactor is "how much we expect this worker to work", or "the worker's
 * normalized work quota".
 *
 * lbstatus is "how urgent this worker has to work to fulfill its quota
 * of work".
 *
 * We distribute each worker's work quota to the worker, and then look
 * which of them needs to work most urgently (biggest lbstatus).  This
 * worker is then selected for work, and its lbstatus reduced by the
 * total work quota we distributed to all workers.  Thus the sum of all
 * lbstatus does not change.(*)
 *
 * If some workers are disabled, the others will
 * still be scheduled correctly.
 *
 * If a balancer is configured as follows:
 *
 * worker     a    b    c    d
 * lbfactor  25   25   25   25
 *
 * And b gets disabled, the following schedule is produced:
 *
 *    a c d a c d a c d ...
 *
 * Note that the above lbfactor setting is the *exact* same as:
 *
 * worker     a    b    c    d
 * lbfactor   1    1    1    1
 *
 * Asymmetric configurations work as one would expect. For
 * example:
 *
 * worker     a    b    c    d
 * lbfactor   1    1    1    2
 *
 * would have a, b and c all handling about the same
 * amount of load with d handling twice what a or b
 * or c handles individually. So we could see:
 *
 *   b a d c d a c d b d ...
 *
 */

static proxy_worker *find_best_byrequests(proxy_balancer *balancer,
                                request_rec *r)
{
    int i;
    int total_factor = 0;
    proxy_worker **worker;
    proxy_worker *mycandidate = NULL;
    int cur_lbset = 0;
    int max_lbset = 0;
    int checking_standby;
    int checked_standby;

    if (!ap_proxy_retry_worker_fn) {
        ap_proxy_retry_worker_fn =
                APR_RETRIEVE_OPTIONAL_FN(ap_proxy_retry_worker);
        if (!ap_proxy_retry_worker_fn) {
            /* can only happen if mod_proxy isn't loaded */
            return NULL;
        }
    }

    ap_log_error(APLOG_MARK, APLOG_DEBUG, 0, r->server, APLOGNO(01207)
                 "proxy: Entering byrequests for BALANCER (%s)",
                 balancer->s->name);

    /* First try to see if we have available candidate */
    do {
        checking_standby = checked_standby = 0;
        while (!mycandidate && !checked_standby) {
            worker = (proxy_worker **)balancer->workers->elts;
            for (i = 0; i < balancer->workers->nelts; i++, worker++) {
                if (!checking_standby) {    /* first time through */
                    if ((*worker)->s->lbset > max_lbset)
                        max_lbset = (*worker)->s->lbset;
                }
                if (
                    ((*worker)->s->lbset != cur_lbset) ||
                    (checking_standby ? !PROXY_WORKER_IS_STANDBY(*worker) : PROXY_WORKER_IS_STANDBY(*worker)) ||
                    (PROXY_WORKER_IS_DRAINING(*worker))
                    ) {
                    continue;
                }

                /* If the worker is in error state run
                 * retry on that worker. It will be marked as
                 * operational if the retry timeout is elapsed.
                 * The worker might still be unusable, but we try
                 * anyway.
                 */
                if (!PROXY_WORKER_IS_USABLE(*worker))
                    ap_proxy_retry_worker_fn("BALANCER", *worker, r->server);
                /* Take into calculation only the workers that are
                 * not in error state or not disabled.
                 */
                if (PROXY_WORKER_IS_USABLE(*worker)) {
                    (*worker)->s->lbstatus += (*worker)->s->lbfactor;
                    total_factor += (*worker)->s->lbfactor;
                    if (!mycandidate || (*worker)->s->lbstatus > mycandidate->s->lbstatus)
                        mycandidate = *worker;
                }
            }
            checked_standby = checking_standby++;
        }
        cur_lbset++;
    } while (cur_lbset <= max_lbset && !mycandidate);

    if (mycandidate) {
        mycandidate->s->lbstatus -= total_factor;
        ap_log_error(APLOG_MARK, APLOG_DEBUG, 0, r->server, APLOGNO(01208)
                     "proxy: byrequests selected worker \"%s\" : busy %" APR_SIZE_T_FMT " : lbstatus %d",
                     mycandidate->s->name, mycandidate->s->busy, mycandidate->s->lbstatus);

    }

    return mycandidate;
}

/* assumed to be mutex protected by caller */
static apr_status_t reset(proxy_balancer *balancer, server_rec *s)
{
    int i;
    proxy_worker **worker;
    worker = (proxy_worker **)balancer->workers->elts;
    for (i = 0; i < balancer->workers->nelts; i++, worker++) {
        (*worker)->s->lbstatus = 0;
    }
    return APR_SUCCESS;
}

static apr_status_t age(proxy_balancer *balancer, server_rec *s)
{
    return APR_SUCCESS;
}

/*
 * How to add additional lbmethods:
 *   1. Create func which determines "best" candidate worker
 *      (eg: find_best_bytraffic, above)
 *   2. Register it as a provider.
 */
static const proxy_balancer_method byrequests =
{
    "byrequests",
    &find_best_byrequests,
    NULL,
    &reset,
    &age
};

static void register_hook(apr_pool_t *p)
{
    /* Only the mpm_winnt has child init hook handler.
     * make sure that we are called after the mpm
     * initializes and after the mod_proxy
     */
    ap_register_provider(p, PROXY_LBMETHOD, "byrequests", "0", &byrequests);
}

AP_DECLARE_MODULE(lbmethod_byrequests) = {
    STANDARD20_MODULE_STUFF,
    NULL,       /* create per-directory config structure */
    NULL,       /* merge per-directory config structures */
    NULL,       /* create per-server config structure */
    NULL,       /* merge per-server config structures */
    NULL,       /* command apr_table_t */
    register_hook /* register hooks */
};