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Increase fixed delay with small randomness in failover delay logic to avoid same election #1669

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Increase fixed delay with small randomness in failover delay logic to avoid same election #1669

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0f5f0be
Improve delay logic for the same epoch election
Jan 31, 2025
7c47d17
address comment
Feb 25, 2025
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13 changes: 7 additions & 6 deletions src/cluster_legacy.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3260,6 +3260,7 @@ int clusterProcessPacket(clusterLink *link) {
sender_claimed_config_epoch = ntohu64(hdr->configEpoch);
if (sender_claimed_current_epoch > server.cluster->currentEpoch)
server.cluster->currentEpoch = sender_claimed_current_epoch;

/* Update the sender configEpoch if it is a primary publishing a newer one. */
if (sender_claims_to_be_primary && sender_claimed_config_epoch > sender->configEpoch) {
sender->configEpoch = sender_claimed_config_epoch;
Expand Down Expand Up @@ -4860,7 +4861,7 @@ void clusterHandleReplicaFailover(void) {
if (auth_age > auth_retry_time) {
server.cluster->failover_auth_time = now +
500 + /* Fixed delay of 500 milliseconds, let FAIL msg propagate. */
random() % 500; /* Random delay between 0 and 500 milliseconds. */
random() % 100; /* Random delay between 0 and 100 milliseconds. */
server.cluster->failover_auth_count = 0;
server.cluster->failover_auth_sent = 0;
server.cluster->failover_auth_rank = clusterGetReplicaRank();
Expand All @@ -4869,10 +4870,10 @@ void clusterHandleReplicaFailover(void) {
* less updated replication offset, are penalized. */
server.cluster->failover_auth_time += server.cluster->failover_auth_rank * 1000;
/* We add another delay that is proportional to the failed primary rank.
* Specifically 0.5 second * rank. This way those failed primaries will be
* Specifically (1 second + random % 100) * rank. This way those failed primaries will be
* elected in rank to avoid the vote conflicts. */
server.cluster->failover_failed_primary_rank = clusterGetFailedPrimaryRank();
server.cluster->failover_auth_time += server.cluster->failover_failed_primary_rank * 500;
server.cluster->failover_auth_time += server.cluster->failover_failed_primary_rank * (1000 + random() % 100);
/* However if this is a manual failover, no delay is needed. */
if (server.cluster->mf_end) {
server.cluster->failover_auth_time = now;
Expand Down Expand Up @@ -4917,7 +4918,7 @@ void clusterHandleReplicaFailover(void) {

int new_failed_primary_rank = clusterGetFailedPrimaryRank();
if (new_failed_primary_rank != server.cluster->failover_failed_primary_rank) {
long long added_delay = (new_failed_primary_rank - server.cluster->failover_failed_primary_rank) * 500;
long long added_delay = (new_failed_primary_rank - server.cluster->failover_failed_primary_rank) * (1000 + random() % 100);
server.cluster->failover_auth_time += added_delay;
server.cluster->failover_failed_primary_rank = new_failed_primary_rank;
serverLog(LL_NOTICE, "Failed primary rank updated to #%d, added %lld milliseconds of delay.",
Expand All @@ -4941,8 +4942,8 @@ void clusterHandleReplicaFailover(void) {
if (server.cluster->failover_auth_sent == 0) {
server.cluster->currentEpoch++;
server.cluster->failover_auth_epoch = server.cluster->currentEpoch;
serverLog(LL_NOTICE, "Starting a failover election for epoch %llu, node config epoch is %llu",
(unsigned long long)server.cluster->currentEpoch, (unsigned long long)nodeEpoch(myself));
serverLog(LL_NOTICE, "Starting a failover election for epoch %llu, node config epoch is %llu, failover primary rank is %llu",
(unsigned long long)server.cluster->currentEpoch, (unsigned long long)nodeEpoch(myself), (unsigned long long)server.cluster->failover_failed_primary_rank);
clusterRequestFailoverAuth();
server.cluster->failover_auth_sent = 1;
clusterDoBeforeSleep(CLUSTER_TODO_SAVE_CONFIG | CLUSTER_TODO_UPDATE_STATE | CLUSTER_TODO_FSYNC_CONFIG);
Expand Down
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