nginx upstream一致性哈希的实现

地址：http://wiki.nginx.org/HttpUpstreamConsistentHash

首先声明一个命令：

static ngx_command_t  ngx_http_upstream_consistent_hash_commands[] = {

{   ngx_string("consistent_hash"),
        NGX_HTTP_UPS_CONF|NGX_CONF_TAKE1,
        ngx_http_upstream_consistent_hash,
        0,
        0,
        NULL },

ngx_null_command
};

看下命令的处理函数：ngx_http_upstream_consistent_hash

ngx_str_t                                    *value;
    ngx_http_script_compile_t                     sc;
    ngx_http_upstream_srv_conf_t                 *uscf;
    ngx_http_upstream_consistent_hash_srv_conf_t *uchscf;

value = cf->args->elts;

uscf = ngx_http_conf_get_module_srv_conf(cf, ngx_http_upstream_module);
    uchscf = ngx_http_conf_upstream_srv_conf(uscf,
                                          ngx_http_upstream_consistent_hash_module);

ngx_memzero(&sc, sizeof(ngx_http_script_compile_t));

sc.cf = cf;
    sc.source = &value[1];
    sc.lengths = &uchscf->lengths;
    sc.values = &uchscf->values;
    sc.complete_lengths = 1;
    sc.complete_values = 1;

if (ngx_http_script_compile(&sc) != NGX_OK) {
        return NGX_CONF_ERROR;
    }

    uscf->peer.init_upstream = ngx_http_upstream_init_consistent_hash;

uscf->flags = NGX_HTTP_UPSTREAM_CREATE
        |NGX_HTTP_UPSTREAM_WEIGHT;

这个模块获取upstream模块的配置，设置了peer.init_upstream为函数ngx_http_upstream_init_consistent_hash，这个函数用来初始化upstream的，ngx_http_upstream_init_consistent_hash(ngx_conf_t *cf,
函数会根据服务器的IP和端口号进行一致性哈希计算，

ngx_int_t
ngx_http_upstream_init_consistent_hash(ngx_conf_t *cf,
        ngx_http_upstream_srv_conf_t *us)
{

……

us->peer.init = ngx_http_upstream_init_consistent_hash_peer;

……

for (i = 0; i < us->servers->nelts; i++) {
        for (j = 0; j < server[i].naddrs; j++) {
            for (k = 0; k < ((MMC_CONSISTENT_POINTS * server[i].weight) / server[i].naddrs); k++) {
                ngx_snprintf(hash_data, HASH_DATA_LENGTH, "%V-%ui%Z", &server[i].addrs[j].name, k);
                continuum->nodes[continuum->nnodes].sockaddr = server[i].addrs[j].sockaddr;
                continuum->nodes[continuum->nnodes].socklen = server[i].addrs[j].socklen;
                continuum->nodes[continuum->nnodes].name = server[i].addrs[j].name;
                continuum->nodes[continuum->nnodes].name.data[server[i].addrs[j].name.len] = 0;
                continuum->nodes[continuum->nnodes].point = ngx_crc32_long(hash_data, ngx_strlen(hash_data));
                continuum->nnodes++;
            }
        }
    }

　　//排序
    qsort(continuum->nodes, continuum->nnodes,
            sizeof(ngx_http_upstream_consistent_hash_node),
            (const void*) ngx_http_upstream_consistent_hash_compare_continuum_nodes);

……

}

可以看到，是根据服务器的IP和端口号及索引做ngx_crc32_long计算；

这里也指定了ngx_http_upstream_init_consistent_hash_peer函数为peer的init函数，这个在每次连接的时候都会执行，作用是根据key来查找服务器，

static ngx_int_t
ngx_http_upstream_init_consistent_hash_peer(ngx_http_request_t *r,
        ngx_http_upstream_srv_conf_t *us)
{

……

uchscf = ngx_http_conf_upstream_srv_conf(us,
                                          ngx_http_upstream_consistent_hash_module);

uchpd = ngx_pcalloc(r->pool, sizeof(ngx_http_upstream_consistent_hash_peer_data_t));

r->upstream->peer.data = uchpd->peers;
    uchpd->peers = us->peer.data;
　 //编译变量
    if (ngx_http_script_run(r, &evaluated_key_to_hash,
                uchscf->lengths->elts, 0, uchscf->values->elts) == NULL)
    {
        return NGX_ERROR;
    }
　//根据key计算hash值
    uchpd->point =
        ngx_crc32_long(evaluated_key_to_hash.data, evaluated_key_to_hash.len);

r->upstream->peer.free = ngx_http_upstream_free_consistent_hash_peer;

//获取哪个服务器作计算节点
    r->upstream->peer.get = ngx_http_upstream_get_consistent_hash_peer;

……

}