前幾天在生產環境上redis建立連線方面的故障，分析過程中對ServiceStack.Redis的連線建立和連線池機制有了進一步瞭解。問題分析結束後，通過此文系統的將學習到的知識點整理出來。
從連線池獲取RedisClient的流程
業務程式中通過PooledRedisClientManager物件的GetClient()方法獲取客戶端物件，就以此處的原始碼作為入口：
檢視程式碼
public IRedisClient GetClient()
{
RedisClient redisClient = null;
DateTime now = DateTime.Now;
for (; ; )
{
if (!this.deactiveClientQueue.TryPop(out redisClient))
{
if (this.redisClientSize >= this.maxRedisClient)
{
Thread.Sleep(3);
if (this.PoolTimeout != null && (DateTime.Now - now).TotalMilliseconds >= (double)this.PoolTimeout.Value)
{
break;
}
}
else
{
redisClient = this.CreateRedisClient();
if (redisClient != null)
{
goto Block_5;
}
}
}
else
{
if (!redisClient.HadExceptions)
{
goto Block_6;
}
List<RedisClient> obj = this.writeClients;
lock (obj)
{
this.writeClients.Remove(redisClient);
this.redisClientSize--;
}
RedisState.DisposeDeactivatedClient(redisClient);
}
}
bool flag2 = true;
if (flag2)
{
throw new TimeoutException("Redis Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use.");
}
return redisClient;
Block_5:
this.writeClients.Add(redisClient);
return redisClient;
Block_6:
redisClient.Active = true;
this.InitClient(redisClient);
return redisClient;
}此方法的主體是死迴圈，主要實現了這幾項功能：
- this.deactiveClientQueue代表空閒的Client集合，是ConcurrentStack<RedisClient>型別的。
- 當this.deactiveClientQueue能夠Pop出redisClient時，則跳轉到Block_6分支：標記redisClient.Active屬性，並執行this.InitClient(redisClient)，然後將redisClient例項返回。
- 當this.deactiveClientQueue沒有可以Pop的元素時，首先執行Client數量上限的判斷this.redisClientSize >= this.maxRedisClient；
- 如果未到達上限，則執行redisClient = this.CreateRedisClient();
- 如果達到上限，則先休眠3毫秒，然後判斷是否超過連線池超時時間this.PoolTimeout，單位毫秒。超時的話直接break中斷迴圈，不超時的話繼續下一次for迴圈。
上述流程就是從連線池獲取Client的主要流程，其中this.deactiveClientQueue相當於“Client池”。需要注意this.PoolTimeout的含義是當連線池耗盡時呼叫方等待的時間。
上述過程通過流程圖表示為：
建立新Client的過程：CreateRedisClient()
原始碼如下：
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private RedisClient CreateRedisClient()
{
if (this.redisClientSize >= this.maxRedisClient)
{
return null;
}
object obj = this.lckObj;
RedisClient result;
lock (obj)
{
if (this.redisClientSize >= this.maxRedisClient)
{
result = null;
}
else
{
Random random = new Random((int)DateTime.Now.Ticks);
RedisClient newClient = this.InitNewClient(this.RedisResolver.CreateMasterClient(random.Next(100)));
newClient.OnDispose += delegate()
{
if (!newClient.HadExceptions)
{
List<RedisClient> obj2 = this.writeClients;
lock (obj2)
{
if (!newClient.HadExceptions)
{
try
{
this.deactiveClientQueue.Push(newClient);
return;
}
catch
{
this.writeClients.Remove(newClient);
this.redisClientSize--;
RedisState.DisposeDeactivatedClient(newClient);
}
}
}
}
this.writeClients.Remove(newClient);
this.redisClientSize--;
RedisState.DisposeDeactivatedClient(newClient);
};
this.redisClientSize++;
result = newClient;
}
}
return result;
}基於併發的考慮，建立新Client的流程需要增加併發鎖限制，即lock (obj)處。此時如果多個執行緒都進入CreateRedisClient()方法，則只有一個執行緒實際執行，其它執行緒阻塞等待鎖釋放。這個現象可以通過windbg的syncblk、clrstack命令分析檢視。其餘的部分就是繼續呼叫this.InitNewClient(this.RedisResolver.CreateMasterClient(random.Next(100)))建立物件，並對newClient的OnDispose事件增加了處理邏輯。需要說明的是此處OnDispose事件並不是傳統意義的析構，而是呼叫方用完此RedisClient物件後，用於將其回收到連線池的操作，即：newClient物件沒有異常的前提下， 將其Push到this.deactiveClientQueue棧裡，連線池就是此處回收擴充的。
this.InitNewClient()方法解讀
此處是對新建立的RedisClient物件初始化，包括Id、Active等，並繼續呼叫this.InitClient()進一步初始化。
this.RedisResolver.CreateMasterClient()解讀
this.redisResolver是IRedisResolver介面型別，原始碼中有三種實現，如下截圖。此處以生產常見的哨兵模式為例進行分析。
RedisSentinelResolver類對應的就是哨兵模式，其相關操作原始碼如下：
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public RedisClient CreateMasterClient(int desiredIndex)
{
return this.CreateRedisClient(this.GetReadWriteHost(desiredIndex), true);
}
public RedisEndpoint GetReadWriteHost(int desiredIndex)
{
return this.sentinel.GetMaster() ?? this.masters[desiredIndex % this.masters.Length];
}
public virtual RedisClient CreateRedisClient(RedisEndpoint config, bool master)
{
RedisClient result = this.ClientFactory(config);
if (master)
{
RedisServerRole redisServerRole = RedisServerRole.Unknown;
try
{
using (RedisClient redisClient = this.ClientFactory(config))
{
redisClient.ConnectTimeout = 5000;
redisClient.ReceiveTimeout = 5000;
redisServerRole = redisClient.GetServerRole();
if (redisServerRole == RedisServerRole.Master)
{
this.lastValidMasterFromSentinelAt = DateTime.UtcNow;
return result;
}
}
}
catch (Exception exception)
{
Interlocked.Increment(ref RedisState.TotalInvalidMasters);
using (RedisClient redisClient2 = this.ClientFactory(config))
{
redisClient2.ConnectTimeout = 5000;
redisClient2.ReceiveTimeout = 5000;
if (redisClient2.GetHostString() == this.lastInvalidMasterHost)
{
object obj = this.oLock;
lock (obj)
{
if (DateTime.UtcNow - this.lastValidMasterFromSentinelAt > this.sentinel.WaitBeforeForcingMasterFailover)
{
this.lastInvalidMasterHost = null;
this.lastValidMasterFromSentinelAt = DateTime.UtcNow;
RedisSentinelResolver.log.Error("Valid master was not found at '{0}' within '{1}'. Sending SENTINEL failover...".Fmt(redisClient2.GetHostString(), this.sentinel.WaitBeforeForcingMasterFailover), exception);
Interlocked.Increment(ref RedisState.TotalForcedMasterFailovers);
this.sentinel.ForceMasterFailover();
Thread.Sleep(this.sentinel.WaitBetweenFailedHosts);
redisServerRole = redisClient2.GetServerRole();
}
goto IL_16E;
}
}
this.lastInvalidMasterHost = redisClient2.GetHostString();
IL_16E:;
}
}
if (redisServerRole != RedisServerRole.Master && RedisConfig.VerifyMasterConnections)
{
try
{
Stopwatch stopwatch = Stopwatch.StartNew();
for (;;)
{
try
{
RedisEndpoint master2 = this.sentinel.GetMaster();
using (RedisClient redisClient3 = this.ClientFactory(master2))
{
redisClient3.ReceiveTimeout = 5000;
redisClient3.ConnectTimeout = this.sentinel.SentinelWorkerConnectTimeoutMs;
if (redisClient3.GetServerRole() == RedisServerRole.Master)
{
this.lastValidMasterFromSentinelAt = DateTime.UtcNow;
return this.ClientFactory(master2);
}
Interlocked.Increment(ref RedisState.TotalInvalidMasters);
}
}
catch
{
}
if (stopwatch.Elapsed > this.sentinel.MaxWaitBetweenFailedHosts)
{
break;
}
Thread.Sleep(this.sentinel.WaitBetweenFailedHosts);
}
throw new TimeoutException("Max Wait Between Sentinel Lookups Elapsed: {0}".Fmt(this.sentinel.MaxWaitBetweenFailedHosts.ToString()));
}
catch (Exception exception2)
{
RedisSentinelResolver.log.Error("Redis Master Host '{0}' is {1}. Resetting allHosts...".Fmt(config.GetHostString(), redisServerRole), exception2);
List<RedisEndpoint> list = new List<RedisEndpoint>();
List<RedisEndpoint> list2 = new List<RedisEndpoint>();
RedisClient redisClient4 = null;
foreach (RedisEndpoint redisEndpoint in this.allHosts)
{
try
{
using (RedisClient redisClient5 = this.ClientFactory(redisEndpoint))
{
redisClient5.ReceiveTimeout = 5000;
redisClient5.ConnectTimeout = RedisConfig.HostLookupTimeoutMs;
RedisServerRole serverRole = redisClient5.GetServerRole();
if (serverRole != RedisServerRole.Master)
{
if (serverRole == RedisServerRole.Slave)
{
list2.Add(redisEndpoint);
}
}
else
{
list.Add(redisEndpoint);
if (redisClient4 == null)
{
redisClient4 = this.ClientFactory(redisEndpoint);
}
}
}
}
catch
{
}
}
if (redisClient4 == null)
{
Interlocked.Increment(ref RedisState.TotalNoMastersFound);
string message = "No master found in: " + string.Join(", ", this.allHosts.Map((RedisEndpoint x) => x.GetHostString()));
RedisSentinelResolver.log.Error(message);
throw new Exception(message);
}
this.ResetMasters(list);
this.ResetSlaves(list2);
return redisClient4;
}
return result;
}
return result;
}
return result;
}其中GetReadWriteHost()方法的邏輯是：優先使用this.sentinel.GetMaster()得到的主節點資訊。如果GetMaster()失敗，則從現有的主節點集合masters中隨機選擇一個進行連線。
然後進入CreateRedisClient()方法內：
- 首先通過this.ClientFactory()工廠建立物件redisClient，工廠內部實現了計數和new RedisClient()操作。沒有太多內容。
- 然後是執行redisClient.GetServerRole()，代表向伺服器核實當前連線的節點確實是Master角色。如果確認，則直接返回給呼叫方。【如果傳送查詢請求的過程出現異常，且符合一定條件，則會發起故障轉移請求，即this.sentinel.ForceMasterFailover();】
- 如果當前連線的不是Master角色的節點，則多次呼叫this.sentinel.GetMaster()查詢Master節點資訊並重新例項化RedisClient物件；
- 如果超時仍然未能連線到Master節點，則會進入catch異常處理流程，遍歷this.allHosts全部節點並更新對應的節點角色。
至此，通過上述的流程，最終能夠得到master節點的RedisClient物件，並返回給呼叫方。
上述過程中，還有幾處方法的實現比較重要和複雜，下面對其一一解釋說明：
RedisSentinel類的GetMaster()實現原理解析
呼叫處很簡單，但是此方法的實現操作挺多，RedisSentinel類 原始碼如下：
檢視程式碼
public RedisEndpoint GetMaster()
{
RedisSentinelWorker validSentinelWorker = this.GetValidSentinelWorker();
RedisSentinelWorker obj = validSentinelWorker;
RedisEndpoint result;
lock (obj)
{
string masterHost = validSentinelWorker.GetMasterHost(this.masterName);
if (this.ScanForOtherSentinels && DateTime.UtcNow - this.lastSentinelsRefresh > this.RefreshSentinelHostsAfter)
{
this.RefreshActiveSentinels();
}
result = ((masterHost != null) ? ((this.HostFilter != null) ? this.HostFilter(masterHost) : masterHost).ToRedisEndpoint(null) : null);
}
return result;
}
private RedisSentinelWorker GetValidSentinelWorker()
{
if (this.isDisposed)
{
throw new ObjectDisposedException(base.GetType().Name);
}
if (this.worker != null)
{
return this.worker;
}
RedisException innerException = null;
while (this.worker == null && this.ShouldRetry())
{
try
{
this.worker = this.GetNextSentinel();
this.GetSentinelInfo();
this.worker.BeginListeningForConfigurationChanges();
this.failures = 0;
return this.worker;
}
catch (RedisException ex)
{
if (this.OnWorkerError != null)
{
this.OnWorkerError(ex);
}
innerException = ex;
this.worker = null;
this.failures++;
Interlocked.Increment(ref RedisState.TotalFailedSentinelWorkers);
}
}
this.failures = 0;
Thread.Sleep(this.WaitBetweenFailedHosts);
throw new RedisException("No Redis Sentinels were available", innerException);
}
private RedisSentinelWorker GetNextSentinel()
{
object obj = this.oLock;
RedisSentinelWorker result;
lock (obj)
{
if (this.worker != null)
{
this.worker.Dispose();
this.worker = null;
}
int num = this.sentinelIndex + 1;
this.sentinelIndex = num;
if (num >= this.SentinelEndpoints.Length)
{
this.sentinelIndex = 0;
}
result = new RedisSentinelWorker(this, this.SentinelEndpoints[this.sentinelIndex])
{
OnSentinelError = new Action<Exception>(this.OnSentinelError)
};
}
return result;
}
private void OnSentinelError(Exception ex)
{
if (this.worker != null)
{
RedisSentinel.Log.Error("Error on existing SentinelWorker, reconnecting...");
if (this.OnWorkerError != null)
{
this.OnWorkerError(ex);
}
this.worker = this.GetNextSentinel();
this.worker.BeginListeningForConfigurationChanges();
}
}先通過GetValidSentinelWorker()獲得RedisSentinelWorker物件。此方法的實現包含了重試機制的控制，最終是通過this.GetNextSentinel()方法給this.worker欄位，即RedisSentinelWorker物件例項。
而GetNextSentinel()方法內部包含了同步鎖、呼叫this.worker.Dispose()、隨機選擇哨兵節點、例項化RedisSentinelWorker物件等操作。
後面是對validSentinelWorker進行加鎖，然後繼續執行string masterHost = validSentinelWorker.GetMasterHost(this.masterName);
對應的RedisSentinelWorker類的程式碼如下：
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internal string GetMasterHost(string masterName)
{
string result;
try
{
result = this.GetMasterHostInternal(masterName);
}
catch (Exception obj)
{
if (this.OnSentinelError != null)
{
this.OnSentinelError(obj);
}
result = null;
}
return result;
}
private string GetMasterHostInternal(string masterName)
{
List<string> list = this.sentinelClient.SentinelGetMasterAddrByName(masterName);
if (list.Count <= 0)
{
return null;
}
return this.SanitizeMasterConfig(list);
}
public void Dispose()
{
new IDisposable[]
{
this.sentinelClient,
this.sentinePubSub
}.Dispose(RedisSentinelWorker.Log);
}注意GetMasterHost()方法內：當發生異常時，會觸發this物件的OnSentinelError事件，顧名思義這個事件用於哨兵異常的後續處理。通過原始碼搜尋，只有GetNextSentinel()方法內對OnSentinelError事件增加了處理程式-->即RedisSentinel內的private void OnSentinelError(Exception ex)方法。而這個方法內部對列印日誌和觸發事件this.OnWorkerError後，又呼叫GetNextSentinel()重新給this.worker欄位賦值。
需要注意：Dispose()方法實際是分別呼叫了this.sentinelClient和this.sentinePubSub的登出操作。
RedisNativeClient類的相關功能和實現
接著呼叫了RedisNativeClient類的SentinelGetMasterAddrByName()方法：
這個類裡的幾個方法的含義綜合起來就是：將哨兵客戶端的查詢指令通過Socket傳送到服務端，並將返回結果格式化為所需的RedisEndpoint型別。
在方法SendReceive()內還包含了Socket連線、重試、頻率控制、超時控制等機制。
檢視程式碼
public List<string> SentinelGetMasterAddrByName(string masterName)
{
List<byte[]> list = new List<byte[]>
{
Commands.Sentinel,
Commands.GetMasterAddrByName,
masterName.ToUtf8Bytes()
};
return this.SendExpectMultiData(list.ToArray()).ToStringList();
}
protected byte[][] SendExpectMultiData(params byte[][] cmdWithBinaryArgs)
{
return this.SendReceive<byte[][]>(cmdWithBinaryArgs, new Func<byte[][]>(this.ReadMultiData), (this.Pipeline != null) ? new Action<Func<byte[][]>>(this.Pipeline.CompleteMultiBytesQueuedCommand) : null, false) ?? TypeConstants.EmptyByteArrayArray;
}
protected T SendReceive<T>(byte[][] cmdWithBinaryArgs, Func<T> fn, Action<Func<T>> completePipelineFn = null, bool sendWithoutRead = false)
{
int num = 0;
Exception ex = null;
DateTime utcNow = DateTime.UtcNow;
T t;
for (;;)
{
try
{
this.TryConnectIfNeeded();
if (this.socket == null)
{
throw new RedisRetryableException("Socket is not connected");
}
if (num == 0)
{
this.WriteCommandToSendBuffer(cmdWithBinaryArgs);
}
if (this.Pipeline == null)
{
this.FlushSendBuffer();
}
else if (!sendWithoutRead)
{
if (completePipelineFn == null)
{
throw new NotSupportedException("Pipeline is not supported.");
}
completePipelineFn(fn);
t = default(T);
t = t;
break;
}
T t2 = default(T);
if (fn != null)
{
t2 = fn();
}
if (this.Pipeline == null)
{
this.ResetSendBuffer();
}
if (num > 0)
{
Interlocked.Increment(ref RedisState.TotalRetrySuccess);
}
Interlocked.Increment(ref RedisState.TotalCommandsSent);
t = t2;
}
catch (Exception ex2)
{
RedisRetryableException ex3 = ex2 as RedisRetryableException;
if ((ex3 == null && ex2 is RedisException) || ex2 is LicenseException)
{
this.ResetSendBuffer();
throw;
}
Exception ex4 = ex3 ?? this.GetRetryableException(ex2);
if (ex4 == null)
{
throw this.CreateConnectionError(ex ?? ex2);
}
if (ex == null)
{
ex = ex4;
}
if (!(DateTime.UtcNow - utcNow < this.retryTimeout))
{
if (this.Pipeline == null)
{
this.ResetSendBuffer();
}
Interlocked.Increment(ref RedisState.TotalRetryTimedout);
throw this.CreateRetryTimeoutException(this.retryTimeout, ex);
}
Interlocked.Increment(ref RedisState.TotalRetryCount);
Thread.Sleep(RedisNativeClient.GetBackOffMultiplier(++num));
continue;
}
break;
}
return t;
}總結
本文著重以Redis連線建立、獲取為線索，對SDK內部的實現機制有了更深入的瞭解。在此基礎上，分析生產環境Redis SDK相關故障時更加得心應手。