php - CouchBase 异常。文档被篡改

Question

我有 CouchBase 服务器。

还有关于并发文档突变的问题:http://developer.couchbase.com/documentation/server/4.0/developer-guide/cas-concurrency.html

---

在 CouchBase 中保存数据的代码示例:

try {
  Yii::$app->Couch->set($key, $data, 0, '', 1);
    
} catch (\Exception $e) {

    $already_saved = Yii::$app->Couch->get($key);

    Yii::$app->Logger->alert(
        'CouchBase exception',
        [
            'exception' => $e->getMessage(),
            'key' => $key,
            'need_saved' => $data,
            'already_saved' => $already_saved,
            'equal' => md5($already_saved)==md5(json_encode($data))
        ]
    );
}

/**
 * Store a document in the cluster.
 *
 * The set operation stores a document in the cluster. It differs from
 * add and replace in that it does not care for the presence of
 * the identifier in the cluster.
 *
 * If the $cas field is specified, set will <b>only</b> succeed if the
 * identifier exists in the cluster with the <b>exact</b> same cas value
 * as the one specified in this request.
 *
 * @param string $id the identifier to store the document under
 * @param object|string $document the document to store
 * @param integer $expiry the lifetime of the document (0 == infinite)
 * @param string $cas a cas identifier to restrict the store operation
 * @param integer $persist_to wait until the document is persisted to (at least)
 *                            this many nodes
 * @param integer $replicate_to wait until the document is replicated to (at least)
 *                            this many nodes
 * @return string the cas value of the object if success
 * @throws CouchbaseException if an error occurs
 */
function set($id, $document, $expiry = 0, $cas = "", $persist_to = 0, $replicate_to = 0) {

}

---

但我收到的所有消息中不到 0.002% 异常:

CouchBase exception. The document was mutated.

在文档中找到这个:

CAS is an acronym for Compare And Swap, and is known as a form of optimistic locking. The CAS can be supplied by applications to mutation operations ( insert, upsert, replace). When applications provide the CAS, server will check the application-provided version of CAS against its own version of the CAS:

• If the two CAS values match (they compare successfully), then the mutation operation succeeds.

• If the two CAS values differ, then the mutation operation fails

---

但是还是不明白，这个突变是什么意思？

• 为什么CAS值匹配，那么变异操作就成功了，不就是重写了消息数据吗？

• 为什么如果值不同，变异操作就会失败？

• 为什么我会收到此异常？

Answer 1

你可以把CAS看成是描述文档的“修订号”，但是这些“修订号”是没有顺序的，你只允许判断两个修订版是否相同。对于文档的每次更改，服务器都会生成新的 CAS 值(即使您用相同的内容重写正文，设置过期时间或锁定 key )。

所以当文档发生变化时你可能会看到 CAS 不匹配错误，但内容仍然是一样的，我可以从你如何从正文中计算 md5 来猜测，没有 CAS。