Cluster

Redis CLUSTER SET-CONFIG-EPOCH

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Redis CLUSTER SET-CONFIG-EPOCH

CLUSTER SET-CONFIG-EPOCH은 current epoch을 지정한 값으로 setting하는 명령이다.

이 문서는 버전 3.2.0을 기준으로 만들었습니다.

설명

This command sets a specific config epoch in a fresh node. It only works when:
수행 조건

The nodes table of the node is empty.
cluster nodes 명령으로 조회했을때 myself만 나와야 한다.
The node current config epoch is zero.
cluster info 명령으로 조회했을때 cluster_current_epcoh이 0 이어야 한다.

These prerequisites are needed since usually, manually altering the configuration epoch of a node is unsafe, we want to be sure that the node with the higher configuration epoch value (that is the last that failed over) wins over other nodes in claiming the hash slots ownership.

However there is an exception to this rule, and it is when a new cluster is created from scratch. Redis Cluster config epoch collision resolution algorithm can deal with new nodes all configured with the same configuration at startup, but this process is slow and should be the exception, only to make sure that whatever happens, two more nodes eventually always move away from the state of having the same configuration epoch.

So, using CONFIG SET-CONFIG-EPOCH, when a new cluster is created, we can assign a different progressive configuration epoch to each node before joining the cluster together.

소스 코드

cluster.c

void clusterCommand(client *c) {
	...
    if (!strcasecmp(c->argv[1]->ptr,"set-config-epoch") && c->argc == 3) {
        /* CLUSTER SET-CONFIG-EPOCH <epoch>
         *
         * The user is allowed to set the config epoch only when a node is
         * totally fresh: no config epoch, no other known node, and so forth.
         * This happens at cluster creation time to start with a cluster where
         * every node has a different node ID, without to rely on the conflicts
         * resolution system which is too slow when a big cluster is created. */
        long long epoch;

        if (getLongLongFromObjectOrReply(c,c->argv[2],&epoch,NULL) != C_OK)
            return;

        if (epoch < 0) {
            addReplyErrorFormat(c,"Invalid config epoch specified: %lld",epoch);
        } else if (dictSize(server.cluster->nodes) > 1) {
            addReplyError(c,"The user can assign a config epoch only when the "
                            "node does not know any other node.");
        } else if (myself->configEpoch != 0) {
            addReplyError(c,"Node config epoch is already non-zero");
        } else {
            myself->configEpoch = epoch;
            serverLog(LL_WARNING,
                "configEpoch set to %llu via CLUSTER SET-CONFIG-EPOCH",
                (unsigned long long) myself->configEpoch);

            if (server.cluster->currentEpoch < (uint64_t)epoch)
                server.cluster->currentEpoch = epoch;
            /* No need to fsync the config here since in the unlucky event
             * of a failure to persist the config, the conflict resolution code
             * will assign an unique config to this node. */
            clusterDoBeforeSleep(CLUSTER_TODO_UPDATE_STATE|
                                 CLUSTER_TODO_SAVE_CONFIG);
            addReply(c,shared.ok);
        }
    } 
}

명령문

CLUSTER SET-CONFIG-EPOCH integer

이 명령은 version 3.0.0 부터 사용할 수 있다.
논리적 처리 소요시간은 O(1)이다.

Clients for Java	Jedis, Lettuce
Clients for C	Hiredis

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