ES6.3.2 副本失敗處理
副本的失敗處理對理解ES的數據副本模型很有幫助。在ES6.3.2 index操作源碼流程的總結中提到：ES的寫操作會先寫主分片，然后主分片再將操作同步到副本分片。本文給出ES中的源碼片斷，分析副本執行操作失敗時，ES是如何處理的。
副本執行源碼：replicasProxy.performOn實現了副本操作，執行正常結束回調onResponse()，異常回調onFailure()
replicasProxy.performOn(shard, replicaRequest, globalCheckpoint, new ActionListener<ReplicaResponse>() {
@Override
public void onResponse(ReplicaResponse response) {
successfulShards.incrementAndGet();
try {
primary.updateLocalCheckpointForShard(shard.allocationId().getId(), response.localCheckpoint());//執行成功回調更新檢查點
primary.updateGlobalCheckpointForShard(shard.allocationId().getId(), response.globalCheckpoint());
} catch (final AlreadyClosedException e) {
// okay, the index was deleted or this shard was never activated after a relocation; fall through and finish normally
} catch (final Exception e) {
// fail the primary but fall through and let the rest of operation processing complete
final String message = String.format(Locale.ROOT, "primary failed updating local checkpoint for replica %s", shard);
primary.failShard(message, e);
}
decPendingAndFinishIfNeeded();//不管是正常的onResponse還是異常的onFailure,都會調用這個方法,代表已經完成了一個操作,pendingActions減1
}
@Override
public void onFailure(Exception replicaException) {
logger.trace(() -> new ParameterizedMessage(
"[{}] failure while performing [{}] on replica {}, request [{}]",
shard.shardId(), opType, shard, replicaRequest), replicaException);
// Only report "critical" exceptions - TODO: Reach out to the master node to get the latest shard state then report.
if (TransportActions.isShardNotAvailableException(replicaException) == false) {
RestStatus restStatus = ExceptionsHelper.status(replicaException);
shardReplicaFailures.add(new ReplicationResponse.ShardInfo.Failure(
shard.shardId(), shard.currentNodeId(), replicaException, restStatus, false));
}
String message = String.format(Locale.ROOT, "failed to perform %s on replica %s", opType, shard);
//---> failShardIfNeeded 具體執行何種操作要看 replicasProxy的真正實現類:如果是WriteActionReplicasProxy則會報告shard錯誤
replicasProxy.failShardIfNeeded(shard, message,
replicaException, ReplicationOperation.this::decPendingAndFinishIfNeeded,
ReplicationOperation.this::onPrimaryDemoted, throwable -> decPendingAndFinishIfNeeded());
}
});
}
執行正常結束回調onResponse()
successfulShards.incrementAndGet();，在返回的結果里面，_shards 字段里面就能看到 successful 數值。
更新 local checkpoint 和 global checkpoint：如果檢查點更新失敗，觸發：replica shard engine 關閉。
/**
* Fails the shard and marks the shard store as corrupted if
* <code>e</code> is caused by index corruption
*
* org.elasticsearch.index.shard.IndexShard#failShard
*/
public void failShard(String reason, @Nullable Exception e) {
// fail the engine. This will cause this shard to also be removed from the node's index service.
getEngine().failEngine(reason, e);
}
fail engine due to some error. the engine will also be closed.
The underlying store is marked corrupted iff failure is caused by index corruption
關於檢查點，可參考這篇文章：elasticsearch-sequence-ids-6-0
異常結束回調 onFailure()
replicasProxy.failShardIfNeeded(shard, message,
replicaException, ReplicationOperation.this::decPendingAndFinishIfNeeded,
ReplicationOperation.this::onPrimaryDemoted, throwable -> decPendingAndFinishIfNeeded());
failShardIfNeeded 可以做2件事情，具體是如何執行得看failShardIfNeeded的實現類。
-
onPrimaryDemoted
通知master primary stale（過時）了。index操作首先在primary shard執行成功了，然后同步給replica，但是replica發現此primary shard 的 primary term 比它知道的該索引的primary term 還小，於是replica就認為此primary shard是一個已經過時了的primary shard，因此就回調onFailure()拒絕執行，並執行onPrimaryDemoted通知master節點。
private void onPrimaryDemoted(Exception demotionFailure) { String primaryFail = String.format(Locale.ROOT, "primary shard [%s] was demoted while failing replica shard", primary.routingEntry()); // we are no longer the primary, fail ourselves and start over primary.failShard(primaryFail, demotionFailure); finishAsFailed(new RetryOnPrimaryException(primary.routingEntry().shardId(), primaryFail, demotionFailure)); }
-
decPendingAndFinishIfNeeded
計數。一個請求會由ReplicationGroup中的 多個分片執行，這些分片是否都已經執行完成了？就由pendingActions計數。不管是執行正常結束onResponse還是異常結束onFailure都會調用這個方法。
private void decPendingAndFinishIfNeeded() { assert pendingActions.get() > 0 : "pending action count goes below 0 for request [" + request + "]"; if (pendingActions.decrementAndGet() == 0) {//當所有的shard都處理完這個請求,client收到ACK(里面允許一些replica執行失敗), 或者是收到一個請求超時的響應 finish(); } }對於發起index操作的Client而言，該 index 操作會由primary shard 執行，也會由若干個replica執行。因此，pendingActions統計到底有多少個分片(既包括主分片也包括副本分片)執行完成(在某些副本分片上執行失敗也算執行完成)了。正是由於不管是 onResponse() 還是 onFailure()，都會執行decPendingAndFinishIfNeeded()方法，每執行一次，意味着有一個分片返回了響應，這時
if (pendingActions.decrementAndGet() == 0)就減1，直到減為0時，調用finish()方法給Client返回ACK響應。
private void finish() {
if (finished.compareAndSet(false, true)) {
final ReplicationResponse.ShardInfo.Failure[] failuresArray;
if (shardReplicaFailures.isEmpty()) {
failuresArray = ReplicationResponse.EMPTY;
} else {
failuresArray = new ReplicationResponse.ShardInfo.Failure[shardReplicaFailures.size()];
shardReplicaFailures.toArray(failuresArray);
}
primaryResult.setShardInfo(new ReplicationResponse.ShardInfo(
totalShards.get(),
successfulShards.get(),
failuresArray
)
);
resultListener.onResponse(primaryResult);
}
}
Client要么收到一個執行成功的ACK（默認情況下，只要primary shard執行成功，若存在 replica執行失敗，Client也會收到一個執行成功的ACK，只不過 返回的ACK里面 _shards參數下的 failed 不為0而已），如下：
{
"_index": "user",
"_type": "profile",
"_id": "10",
"_version": 1,
"result": "created",
"_shards": {
"total": 3,
"successful": 1,
"failed": 0
},
"_seq_no": 0,
"_primary_term": 1
}
另外，ES6.3.2 index操作源碼流程 的總結部分，詳細解釋了Client收到執行成功的ACK的原因。
要么收到一個超時ACK，如下：（這篇文章提到了如何產生一個超時的ACK）
{
"statusCode": 504,
"error": "Gateway Time-out",
"message": "Client request timeout"
}
failShardIfNeeded方法一共有2個具體實現，看類圖：
TransportReplicationAction.ReplicasProxy#failShardIfNeeded （默認實現）
@Override
public void failShardIfNeeded(ShardRouting replica, String message, Exception exception,Runnable onSuccess, Consumer<Exception> onPrimaryDemoted, Consumer<Exception> onIgnoredFailure) {
// This does not need to fail the shard. The idea is that this
// is a non-write operation (something like a refresh or a global
// checkpoint sync) and therefore the replica should still be
// "alive" if it were to fail.
onSuccess.run();
}
TransportResyncReplicationAction.ResyncActionReplicasProxy#failShardIfNeeded(副本resync操作的實現)
/**
* A proxy for primary-replica resync operations which are performed on replicas when a new primary is promoted.
* Replica shards fail to execute resync operations will be failed but won't be marked as stale.
* This avoids marking shards as stale during cluster restart but enforces primary-replica resync mandatory.
*/
class ResyncActionReplicasProxy extends ReplicasProxy {
@Override
public void failShardIfNeeded(ShardRouting replica, String message, Exception exception, Runnable onSuccess,
Consumer<Exception> onPrimaryDemoted, Consumer<Exception> onIgnoredFailure) {
shardStateAction.remoteShardFailed(replica.shardId(), replica.allocationId().getId(), primaryTerm, false, message, exception,
createShardActionListener(onSuccess, onPrimaryDemoted, onIgnoredFailure));
}
}
TransportWriteAction.WriteActionReplicasProxy#failShardIfNeeded(index 寫操作的實現)
/**
* A proxy for <b>write</b> operations that need to be performed on the
* replicas, where a failure to execute the operation should fail
* the replica shard and/or mark the replica as stale.
*
* This extends {@code TransportReplicationAction.ReplicasProxy} to do the
* failing and stale-ing.
*/
class WriteActionReplicasProxy extends ReplicasProxy {
@Override
public void failShardIfNeeded(ShardRouting replica, String message, Exception exception,Runnable onSuccess, Consumer<Exception> onPrimaryDemoted, Consumer<Exception> onIgnoredFailure) {
if (TransportActions.isShardNotAvailableException(exception) == false) {
logger.warn(new ParameterizedMessage("[{}] {}", replica.shardId(), message), exception);}
shardStateAction.remoteShardFailed(replica.shardId(), replica.allocationId().getId(), primaryTerm, true, message, exception,
createShardActionListener(onSuccess, onPrimaryDemoted, onIgnoredFailure));
}
總結
從上面代碼中可看出：副本resync操作、副本上 index 寫操作失敗都會導致 調用 onPrimaryDemoted() 方法，通知master節點判斷當前primary shard 是否已經過時(stale)。這可以說是：replica 檢驗 primary shard是否stale的方式。
另外，primary shard 和 各個replica之間也會通過 租約機制 進行故障檢測，以判斷對方是否stale，不過這不是本文要討論的內容了。
參考文章：