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测试Twemproxy集群，双主双活
向twemproxy集群做写操作时，发现key的分布不太理想。在测试节点故障时，也发现一些和预想不太一样的地方。
1、Key的一致性Hash
当尝试以a001,a002这样有规律且的key值写入的时候，在4节点的集群环境中，key主要分布在其中的2台节点，另外两台分配极少。对于一些应用来说，key值可能根据一定规则生成，所以有被定向分配的可能。
解决办法在key中使用hash_key：{}，hask_key使用8位随机数，测试结果分布的比较满意。
测试4节点中key的分布：
1： 12917
2： 10761
3： 8596
4： 14382
2： 10761
3： 8596
4： 14382
由于ketama的算法仍是使用了md5签名（具体后面说），又特意观察了比如有序数字生成的md5序列，结果并没有出现明显的有序或连序值。所以只能建议不使用连续的数据结尾key做一致性hash key。
2、ketama算法
twemproxy源码下载：https://github.com/twitter/twemproxy，命令：git clone https://github.com/twitter/twemproxy
关于ketama算法的代码在nc_ketama.c文件中，主要是四个方法：
- ketama_hash 计算某个主机，某个point的hash值
- ketama_item_cmp 比较两个连续区的值，用于在ketama_update 方法中排序
- ketama_update 更新server-pool的分配策略
- ketama_dispatch 找出给定hash值所在的连续区
2.1 连续区
说一下连续区(continuum)，参考下图。想象所有md5的值构成下面完整的“环”（没有起点），那么所有md5结果值在环上都有一个固定的位置。
按ketama的算法，在这个环上创建服务器数*160个点，这些点把环分成了同等数量的段。
那么，被插入数据的md5值也一定会落到环的某个区间，以此来判断数据应被写入哪台服务器。
参考：理想化的Redis集群
2.2 如何生成ketama_hash
再来看服务器+点的hash值是如何生成的：
alignment的值固定是4，ketama_hash是对由server名+索引组成的md5签名，从第16位开始取值，再重组一个32位值。
static uint32_t
ketama_hash(const char *key, size_t key_length, uint32_t alignment)
{
unsigned char results[16]; md5_signature((unsigned char*)key, key_length, results); return ((uint32_t) (results[3 + alignment * 4] & 0xFF) << 24)
| ((uint32_t) (results[2 + alignment * 4] & 0xFF) << 16)
| ((uint32_t) (results[1 + alignment * 4] & 0xFF) << 8)
| (results[0 + alignment * 4] & 0xFF);
}
下面是调用ketama_hash的代码：
for (x = ; x < pointer_per_hash; x++) {
value = ketama_hash(host, hostlen, x);
pool->continuum[continuum_index].index = server_index;
pool->continuum[continuum_index++].value = value;
}
每个服务器被分成160个point点，由服务器名+索引组成host值，x值等于160/索引。
这样计算出的服务器各点的值并不是有序的，所以进行排序。
qsort(pool->continuum, pool->ncontinuum, sizeof(*pool->continuum), ketama_item_cmp);
排序后的点值是连续的，但同一服务器的点并不一定连续。这时，所有的值构成了用于一致性hash的环。
2.3、分配Key
由ketama_dispatch实现key值的分配。
可见方法中使用二分法找到一个值在环中的对应区域。
uint32_t
ketama_dispatch(struct continuum *continuum, uint32_t ncontinuum, uint32_t hash)
{
struct continuum *begin, *end, *left, *right, *middle; ASSERT(continuum != NULL);
ASSERT(ncontinuum != ); begin = left = continuum;
end = right = continuum + ncontinuum; while (left < right) {
middle = left + (right - left) / ;
if (middle->value < hash) {
left = middle + ;
} else {
right = middle;
}
}
if (right == end) {
right = begin;
}
return right->index;
}
3、服务器的故障处理
从集群中摘除节点时，ketama的算法不会重新计算"环"。当需要写入故障节点时，会抛出异常。
仔细想一下是合理的，因为摘除的节点持有一部分数据，一般来说是需要恢复的，这是一个前提。
我们假设twemproxy可以感知节点故障，并重新计算分配策略。那么，故障后又有新的数据写入。这时，一部分原本要写入故障节点的数据会被分配到其它节点上。
随后，故障节点恢复，twemproxy又重新调整了分配策略。那么，后写入的那部分数据就不会再被找到（这个有点像内存泄露）。
nc_ketama.c 完整代码
/*
* twemproxy - A fast and lightweight proxy for memcached protocol.
* Copyright (C) 2011 Twitter, Inc.
*
* Licensed 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 <stdio.h>
#include <stdlib.h>
#include <math.h> #include <nc_core.h>
#include <nc_server.h>
#include <nc_hashkit.h> #define KETAMA_CONTINUUM_ADDITION 10 /* # extra slots to build into continuum */
#define KETAMA_POINTS_PER_SERVER 160 /* 40 points per hash */
#define KETAMA_MAX_HOSTLEN 86 static uint32_t
ketama_hash(const char *key, size_t key_length, uint32_t alignment)
{
unsigned char results[]; md5_signature((unsigned char*)key, key_length, results); return ((uint32_t) (results[ + alignment * ] & 0xFF) << )
| ((uint32_t) (results[ + alignment * ] & 0xFF) << )
| ((uint32_t) (results[ + alignment * ] & 0xFF) << )
| (results[ + alignment * ] & 0xFF);
} static int
ketama_item_cmp(const void *t1, const void *t2)
{
const struct continuum *ct1 = t1, *ct2 = t2; if (ct1->value == ct2->value) {
return ;
} else if (ct1->value > ct2->value) {
return ;
} else {
return -;
}
} rstatus_t
ketama_update(struct server_pool *pool)
{
uint32_t nserver; /* # server - live and dead */
uint32_t nlive_server; /* # live server */
uint32_t pointer_per_server; /* pointers per server proportional to weight */
uint32_t pointer_per_hash; /* pointers per hash */
uint32_t pointer_counter; /* # pointers on continuum */
uint32_t pointer_index; /* pointer index */
uint32_t points_per_server; /* points per server */
uint32_t continuum_index; /* continuum index */
uint32_t continuum_addition; /* extra space in the continuum */
uint32_t server_index; /* server index */
uint32_t value; /* continuum value */
uint32_t total_weight; /* total live server weight */
int64_t now; /* current timestamp in usec */ ASSERT(array_n(&pool->server) > ); now = nc_usec_now();
if (now < ) {
return NC_ERROR;
} /*
* Count live servers and total weight, and also update the next time to
* rebuild the distribution
*/
nserver = array_n(&pool->server);
nlive_server = ;
total_weight = ;
pool->next_rebuild = 0LL;
for (server_index = ; server_index < nserver; server_index++) {
struct server *server = array_get(&pool->server, server_index); if (pool->auto_eject_hosts) {
if (server->next_retry <= now) {
server->next_retry = 0LL;
nlive_server++;
} else if (pool->next_rebuild == 0LL ||
server->next_retry < pool->next_rebuild) {
pool->next_rebuild = server->next_retry;
}
} else {
nlive_server++;
} ASSERT(server->weight > ); /* count weight only for live servers */
if (!pool->auto_eject_hosts || server->next_retry <= now) {
total_weight += server->weight;
}
} pool->nlive_server = nlive_server; if (nlive_server == ) {
log_debug(LOG_DEBUG, "no live servers for pool %"PRIu32" '%.*s'",
pool->idx, pool->name.len, pool->name.data); return NC_OK;
}
log_debug(LOG_DEBUG, "%"PRIu32" of %"PRIu32" servers are live for pool "
"%"PRIu32" '%.*s'", nlive_server, nserver, pool->idx,
pool->name.len, pool->name.data); continuum_addition = KETAMA_CONTINUUM_ADDITION;
points_per_server = KETAMA_POINTS_PER_SERVER;
/*
* Allocate the continuum for the pool, the first time, and every time we
* add a new server to the pool
*/
if (nlive_server > pool->nserver_continuum) {
struct continuum *continuum;
uint32_t nserver_continuum = nlive_server + continuum_addition;
uint32_t ncontinuum = nserver_continuum * points_per_server; continuum = nc_realloc(pool->continuum, sizeof(*continuum) * ncontinuum);
if (continuum == NULL) {
return NC_ENOMEM;
} pool->continuum = continuum;
pool->nserver_continuum = nserver_continuum;
/* pool->ncontinuum is initialized later as it could be <= ncontinuum */
} /*
* Build a continuum with the servers that are live and points from
* these servers that are proportial to their weight
*/
continuum_index = ;
pointer_counter = ;
for (server_index = ; server_index < nserver; server_index++) {
struct server *server;
float pct; server = array_get(&pool->server, server_index); if (pool->auto_eject_hosts && server->next_retry > now) {
continue;
} pct = (float)server->weight / (float)total_weight;
pointer_per_server = (uint32_t) ((floorf((float) (pct * KETAMA_POINTS_PER_SERVER / * (float)nlive_server + 0.0000000001))) * );
pointer_per_hash = ; log_debug(LOG_VERB, "%.*s:%"PRIu16" weight %"PRIu32" of %"PRIu32" "
"pct %0.5f points per server %"PRIu32"",
server->name.len, server->name.data, server->port,
server->weight, total_weight, pct, pointer_per_server); for (pointer_index = ;
pointer_index <= pointer_per_server / pointer_per_hash;
pointer_index++) { char host[KETAMA_MAX_HOSTLEN]= "";
size_t hostlen;
uint32_t x; hostlen = snprintf(host, KETAMA_MAX_HOSTLEN, "%.*s-%u",
server->name.len, server->name.data,
pointer_index - ); for (x = ; x < pointer_per_hash; x++) {
value = ketama_hash(host, hostlen, x);
pool->continuum[continuum_index].index = server_index;
pool->continuum[continuum_index++].value = value;
}
}
pointer_counter += pointer_per_server;
} pool->ncontinuum = pointer_counter;
qsort(pool->continuum, pool->ncontinuum, sizeof(*pool->continuum),
ketama_item_cmp); for (pointer_index = ;
pointer_index < ((nlive_server * KETAMA_POINTS_PER_SERVER) - );
pointer_index++) {
if (pointer_index + >= pointer_counter) {
break;
}
ASSERT(pool->continuum[pointer_index].value <=
pool->continuum[pointer_index + ].value);
} log_debug(LOG_VERB, "updated pool %"PRIu32" '%.*s' with %"PRIu32" of "
"%"PRIu32" servers live in %"PRIu32" slots and %"PRIu32" "
"active points in %"PRIu32" slots", pool->idx,
pool->name.len, pool->name.data, nlive_server, nserver,
pool->nserver_continuum, pool->ncontinuum,
(pool->nserver_continuum + continuum_addition) * points_per_server); return NC_OK;
} uint32_t
ketama_dispatch(struct continuum *continuum, uint32_t ncontinuum, uint32_t hash)
{
struct continuum *begin, *end, *left, *right, *middle; ASSERT(continuum != NULL);
ASSERT(ncontinuum != ); begin = left = continuum;
end = right = continuum + ncontinuum; while (left < right) {
middle = left + (right - left) / ;
if (middle->value < hash) {
left = middle + ;
} else {
right = middle;
}
} if (right == end) {
right = begin;
} return right->index;
}