CouchBase异常。 该文件发生了变异

Question

I have CouchBase server.

And have a question about concurrent document mutations: http://developer.couchbase.com/documentation/server/4.0/developer-guide/cas-concurrency.html

---

Code example with saving data in CouchBase:

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) {

}

---

But less than 0.002% from all messages I receive Exception:

CouchBase exception. The document was mutated.

Find this in documentation:

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

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But still can't understand, what this mutation means?

• Why if CAS values match, then the mutation operation succeeds, isn't it just rewrite message data?

• Why if values differ, then the mutation operation fails?

• Why I receive this Exception?

Answer 1

You can think about CAS as "revision number", which describes the document, but these "revision numbers" are not ordered, you allowed only to tell if two revisions are the same or not. For every change of the document the server will generate new CAS value (even if you rewrite the body with the same contents, set expiration time or lock the key).

So you might seen the CAS mismatch errors when document change occurred, but the content still the same, I can guess it from how you calculate md5 from the body, without CAS.