HBase 協處理器 (二)
3.7 The RegionObserver Class
用於 region level, 注意，這個類的所有呼叫方法的第一個引數為 ObserverContext<RegionCoprocessorEnvironment> ctx8, 提供訪問 context 例項。
操作可以被劃分為兩組： region 生命週期變化和客戶端 API 呼叫。兩種型別都有通用的回撥操作：
enum Operation {
ANY, GET, PUT, DELETE, SCAN, APPEND, INCREMENT, SPLIT_REGION,
MERGE_REGION, BATCH_MUTATE, REPLAY_BATCH_MUTATE, COMPACT_REGION
}
postStartRegionOperation(..., Operation operation)
postCloseRegionOperation(..., Operation operation)
RegionObserver 中的這些方法在任何可能的 Operation 發生時被呼叫。
3.7.1 處理 region 生命週期事件 (Handling Region Life-Cycle Events)
-----------------------------------------------------------------------------------------------------------------------------------------
observer 可以鉤入到 pending open, open, 和 pending close 狀態變化。每一個鉤子被框架隱式呼叫。
● State: pending open
-------------------------------------------------------------------------------------------------------------------------------------
region 要被開啟時，處於這個狀態。監聽的協處理器可以搭載這個過程，或者使這個過程失效。為此，下列回撥按次序呼叫：
postLogReplay(...)
preOpen(...)
preStoreFileReaderOpen(...)
postStoreFileReaderOpen(...)
preWALRestore(...) / postWALRestore(...)
postOpen(...)
這些方法只在 region 開啟之前被呼叫，在儲存檔案開啟之前和之後呼叫，WAL 準備重放時呼叫，以及在開啟 region 之後呼叫。使用者自己的協處理器
實現可以利用這些回撥，例如，指示框架在 preOpen() 中，終止開啟過程。或者鉤入到 postOpen() 呼叫來觸發一個快取準備操作。
第一個事件，postLogReplay() 的觸發取決於 WAL 恢復模式(WAL recovery mode)的配置：分散式日誌切分(distributed log splitting),或者日誌重
放(log replay)，由 hbase.master.distributed.log.replay 屬性配置。前者運行於一個 region 開啟之前，因此會首先觸發這個呼叫。後者開啟一個
region ，然後重放編輯日誌，在 region 開啟事件之後，觸發該回調。
在這兩種模式中，但取決於哪一種模式是活動的， region server 從 write-ahead-log (WAL) 應用記錄。因此按次序呼叫監聽者的 preWALRestore()
或 postWALRestore() 方法。在使用分散式日誌切分模式下(distributed log splitting), 這會發生在 pending open 之後，但在 open state 之前。
否則，該呼叫在 open 事件之後，因為日誌被重放。鉤入到 WAL 呼叫可以控制在日誌重放過程中應用了什麼資料操作(mutation).可以訪問編輯日誌(
edit record), 可以用來觀察應用了什麼操作。
● State: open
-------------------------------------------------------------------------------------------------------------------------------------
當一個 region 被部署到一個 region 伺服器上並且可以正常工作時，這個 region 被認為處於 open 狀態。例如，region 的記憶體儲存可以刷寫到磁碟
上，或者在 region 變得太大時可以被拆分。可能的鉤子如下：
preFlushScannerOpen(...)
preFlush(...) / postFlush(...)
preCompactSelection(...) / postCompactSelection(...)
preCompactScannerOpen(...)
preCompact(...) / postCompact(...)
preSplit(...)
preSplitBeforePONR(...)
preSplitAfterPONR(...)
postSplit(...)
postCompleteSplit(...) / preRollBackSplit(...) / postRollBackSplit(...)
這些方法名稱上看都非常直接：pre 呼叫在各自的操作之前呼叫，而 post 呼叫在操作之後呼叫。例如，利用 preSplit() 鉤子，可以有效地禁止內建
的 region 拆分過程，並手動執行這些操作。有些呼叫只有 pre 鉤子，有些則只有 post 鉤子。
flush, compact, 和 split 這三種類型的鉤子直接連線到匹配 region 內部處理函式。也有一些更特殊的鉤子，作為這三種函式的部分呼叫。例如，
preFlushScannerOpen() 鉤子在 memstore 設定的時候被呼叫，這隻在刷寫實際發生之前呼叫。
類似地，對於壓縮(compaction)，首先伺服器選擇包含的檔案，該功能被封裝到協處理器回撥中。之後儲存掃描器開啟，最後，實際的壓縮發生。
對於 split, 有幾個回撥反應當前的狀態，通過一個狀態之間特定的 point-of-no-return (PONR) 概念。這發生在 split process 開始之後，但在最後
的動作發生之前。拆分的處理類似於一個內部的事務，當事務要提交時，preSplitBeforePONR() 被呼叫，而之後 preSplitAfterPONR() 被呼叫。也有
對最終的 completed 或 rollback 的呼叫，通知使用者 split 事務的結果。
● State: pending close
-------------------------------------------------------------------------------------------------------------------------------------
對於觀察者的最後一組鉤子是在 region 在進入 pending close 狀態時呼叫的。這組呼叫在 region 從 open 變為 close 時發生。只是在 region 關閉
之前和之後，執行如下鉤子：
preClose(..., boolean abortRequested)
postClose(..., boolean abortRequested)
abortRequested 引數指出一個 region 關閉的原因。通常情況下，region 正常操作關閉，如，由於負載均衡的原因，該 region 移動到一個不同的region
server 上。但也有可能由於某個 region server 出了問題而終止這個 region 以避免影響任何一端的處理結果。當這類情況發生時，所有儲存在該伺服器
上的 region 都會終止，使用者可以通過這個給定的引數判斷髮生了什麼情況。
除此之外，這個類也繼承了 start() 和 stop() 方法，允許分配、釋放生命週期資源。
3.7.2 處理客戶端 API 事件 (Handling Client API Events)
-----------------------------------------------------------------------------------------------------------------------------------------
與生命週期事件相對，所有客戶端 API 呼叫顯式地從客戶端應用傳送到 region 伺服器。使用者可以鉤入到這些呼叫，在應用之前或之後回撥。下表列出可用
的 API 呼叫和相關的回撥方法：
Callbacks for client API functions
+-------------------------------+-------------------------------------------+--------------------------------------------
| API Call | Pre-Hook | Post-Hook
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.put() | prePut(...) | postPut(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.checkAndPut() | preCheckAndPut(...) | postPut(...)
| | preCheckAndPutAfterRowLock(...) | postCheckAndPut(...)
| | prePut(...) |
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.get() | preGetOp(...) | postGetOp(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.delete() | preDelete(...) | postDelete(...)
| Table.batch() | prePrepareTimeStampForDeleteVersion(...) |
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.checkAndDelete() | preCheckAndDelete(...) | postDelete(...)
| | preCheckAndDeleteAfterRowLock(...) | postCheckAndDelete(...)
| | preDelete(...) |
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.mutateRow() | preBatchMutate(...) | postBatchMutate(...)
| | prePut(...)/preGetOp(...) | postPut(...)/postGetOp(...)
| | | postBatchMutateIndispensably()
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.append() | preAppend(...) | postMutationBeforeWAL(...)
| | preAppendAfterRowLock() | postAppend(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.batch() | preBatchMutate(...) | postPut(...)/postGetOp(...)
| | prePut(...)/preGetOp(...)/preDelete(...) | postBatchMutate(...)
| | prePrepareTimeStampForDeleteVersion(...) |
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.checkAndMutate() | preBatchMutate(...) | postBatchMutate(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.getScanner() | preScannerOpen(...) | postInstantiateDeleteTracker(...)
| | preStoreScannerOpen(...) | postScannerOpen(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
| ResultScanner.next() | preScannerNext(...) | postScannerFilterRow(...)
| | | postScannerNext(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
| ResultScanner.close() | preScannerClose(...) | postScannerClose(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.increment(), | preIncrement(...) | postMutationBeforeWAL(...)
| Table.batch() | preIncrementAfterRowLock(...) | postIncrement(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.incrementColumnValue() | preIncrementColumnValue(...) | postIncrementColumnValue(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.getClosestRowBefore() | preGetClosestRowBefore(...) | postGetClosestRowBefore(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
| Table.exists() | preExists(...) | postExists(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
| completebulkload (tool) | preBulkLoadHFile(...) | postBulkLoadHFile(...)
+-------------------------------+-------------------------------------------+--------------------------------------------
上表中列出的事件是有順序的，如果有多個事件發生，發生的順序從上至下。斜線 "/" 表示可能發生的事件為多個事件中的一個，具體會是哪個事件，
取決於呼叫中具體包含的操作。各個事件回撥方法的詳細資訊參考 API 文件或原始碼。
示例： Observer collecting invocation statistics
@SuppressWarnings("deprecation") // because of API usage
public class ObserverStatisticsEndpoint
extends ObserverStatisticsProtos.ObserverStatisticsService
implements Coprocessor, CoprocessorService, RegionObserver {
private RegionCoprocessorEnvironment env;
private Map<String, Integer> stats = new LinkedHashMap<>();
// Lifecycle methods
@Override
public void start(CoprocessorEnvironment env) throws IOException {
if (env instanceof RegionCoprocessorEnvironment) {
this.env = (RegionCoprocessorEnvironment) env;
} else {
throw new CoprocessorException("Must be loaded on a table region!");
}
}
...
// Endpoint methods
@Override
public void getStatistics(RpcController controller,
ObserverStatisticsProtos.StatisticsRequest request,
RpcCallback<ObserverStatisticsProtos.StatisticsResponse> done) {
ObserverStatisticsProtos.StatisticsResponse response = null;
try {
ObserverStatisticsProtos.StatisticsResponse.Builder builder =
ObserverStatisticsProtos.StatisticsResponse.newBuilder();
ObserverStatisticsProtos.NameInt32Pair.Builder pair =
ObserverStatisticsProtos.NameInt32Pair.newBuilder();
for (Map.Entry<String, Integer> entry : stats.entrySet()) {
pair.setName(entry.getKey());
pair.setValue(entry.getValue().intValue());
builder.addAttribute(pair.build());
}
response = builder.build();
// optionally clear out stats
if (request.hasClear() && request.getClear()) {
synchronized (stats) {
stats.clear();
}
}
} catch (Exception e) {
ResponseConverter.setControllerException(controller, new IOException(e));
}
done.run(response);
}
/**
* Internal helper to keep track of call counts.
*
* @param call The name of the call.
*/
private void addCallCount(String call) {
synchronized (stats) {
Integer count = stats.get(call);
if (count == null) count = new Integer(1);
else count = new Integer(count + 1);
stats.put(call, count);
}
}
// All Observer callbacks follow here
@Override
public void preOpen(ObserverContext<RegionCoprocessorEnvironment> observerContext) throws IOException {
addCallCount("preOpen");
}
@Override
public void postOpen(ObserverContext<RegionCoprocessorEnvironment> observerContext) {
addCallCount("postOpen");
}
...
}
3.7.3 The RegionCoprocessorEnvironment Class
-----------------------------------------------------------------------------------------------------------------------------------------
提供給實現 RegionObserver 介面的協處理器的環境例項基於 RegionCoprocessorEnvironment 類，該類實現 CoprocessorEnvironment 介面。除了已提供的
方法，RegionCoprocessorEnvironment 還提供了一些特殊的，面向 region 子類的方法，如下表所示：
Specific methods provided by the RegionCoprocessor Environment class
+---------------------------+---------------------------------------------------------------------------------------+
| Method | Description |
+---------------------------+---------------------------------------------------------------------------------------+
| getRegion() | Returns a reference to the region the current observer is associated with. |
+---------------------------+---------------------------------------------------------------------------------------+
| getRegionInfo() | Get information about the region associated with the current coprocessor instance. |
+---------------------------+---------------------------------------------------------------------------------------+
| getRegionServerServices() | Provides access to the shared RegionServerServices instance. |
+---------------------------+---------------------------------------------------------------------------------------+
| getSharedData() | All the shared data between the instances of this coprocessor |
+---------------------------+---------------------------------------------------------------------------------------+
getRegion() 呼叫可用於獲取存本機上的 HRegion 例項的引用，並可以呼叫 HRegion 類提供的方法。如果需要有關該 region 的資訊，可以呼叫
getRegionInfo() 來獲取 HRegionInfo 例項。此類有非常有用的方法可以獲取所包含的 key 的訪問，region 的名稱，以及狀態標誌：
byte[] getStartKey()
byte[] getEndKey()
byte[] getRegionName()
boolean isSystemTable()
int getReplicaId()
...
另外，使用者程式碼可以訪問共享的 region server 服務例項，通過呼叫 getRegionServerServices() 方法並返回 RegionServerServices 例項。該類提供了
很多高階方法，如下表所示：
Methods provided by the RegionServerServices class
+-------------------------------+-----------------------------------------------------------------------------------------
| Method | Description
+-------------------------------+-----------------------------------------------------------------------------------------
| abort() | Allows aborting the entire server process, shutting down the instance with the given reason.
+-------------------------------+-----------------------------------------------------------------------------------------
| addToOnlineRegions() | Adds a given region to the list of online regions. This is used for internal bookkeeping.
+-------------------------------+-----------------------------------------------------------------------------------------
| getCompactionRequester() | Provides access to the shared CompactionRequestor instance. This can be used to initiate
| | compactions from within the coprocessor.
+-------------------------------+-----------------------------------------------------------------------------------------
| getConfiguration() | Returns the current server configuration.
+-------------------------------+-----------------------------------------------------------------------------------------
| getConnection() | Provides access to the shared connection instance.
+-------------------------------+-----------------------------------------------------------------------------------------
| getCoordinatedStateManager() | Access to the shared state manager, gives access to the TableStateManager, which in turn can
| | be used to check on the state of a table
+-------------------------------+-----------------------------------------------------------------------------------------
| getExecutorService() | Used by the master to schedule system-wide events.
+-------------------------------+-----------------------------------------------------------------------------------------
| getFileSystem() | Used by the master to schedule system-wide events.
+-------------------------------+-----------------------------------------------------------------------------------------
| getFlushRequester() | Provides access to the shared FlushRequester instance. This can be used to initiate memstore flushes.
+-------------------------------+-----------------------------------------------------------------------------------------
| getFromOnlineRegions() | Returns a HRegion instance for a given region, must be hosted by same server
+-------------------------------+-----------------------------------------------------------------------------------------
| getHeapMemoryManager() | Provides access to a manager instance, gives access to heap related information, such as occupancy.
+-------------------------------+-----------------------------------------------------------------------------------------
| getLeases() | Returns the list of leases, as acquired for example by client side scanners.
+-------------------------------+-----------------------------------------------------------------------------------------
| getMetaTableLocator() | The method returns a class providing system table related functionality.
+-------------------------------+-----------------------------------------------------------------------------------------
| getNonceManager() | Gives access to the nonce manager, which is used to generate unique IDs.
+-------------------------------+-----------------------------------------------------------------------------------------
| getOnlineRegions() | Lists all online regions on the current server for a given table.
+-------------------------------+-----------------------------------------------------------------------------------------
| getRecoveringRegions() | Lists all regions that are currently in the process of replaying WAL entries.
+-------------------------------+-----------------------------------------------------------------------------------------
| getRegionServerAccounting() | Provides access to the shared RegionServerAccounting instance. It allows you to check on what
| | the server currently has allocated—for example, the global memstore size.
+-------------------------------+-----------------------------------------------------------------------------------------
| getRegionsInTransitionInRS() | List of regions that are currently in-transition.
+-------------------------------+-----------------------------------------------------------------------------------------
| getRpcServer() | Returns a reference to the low-level RPC implementation instance
+-------------------------------+-----------------------------------------------------------------------------------------
| getServerName() | The server name, which is unique for every region server process.
+-------------------------------+-----------------------------------------------------------------------------------------
| getTableLockManager() | Gives access to the lock manager. Can be used to acquire read and write locks for the entire table.
+-------------------------------+-----------------------------------------------------------------------------------------
| getWAL() | Provides access to the write-ahead log instance.
+-------------------------------+-----------------------------------------------------------------------------------------
| getZooKeeper() | Returns a reference to the ZooKeeper watcher instance.
+-------------------------------+-----------------------------------------------------------------------------------------
| isAborted() | Flag is true when abort() was called previously
+-------------------------------+-----------------------------------------------------------------------------------------
| isStopped() | Returns true when stop() (inherited from Stoppable) was called beforehand
+-------------------------------+-----------------------------------------------------------------------------------------
| isStopping() | Returns true when the region server is stopping.
+-------------------------------+-----------------------------------------------------------------------------------------
| postOpenDeployTasks() | Called by the region server after opening a region, does internal housekeeping work.
+-------------------------------+-----------------------------------------------------------------------------------------
| registerService() | Registers a new custom service. Called when server starts and coprocessors are loaded.
+-------------------------------+-----------------------------------------------------------------------------------------
| removeFromOnlineRegions() | Removes a given region from the internal list of online regions.
+-------------------------------+-----------------------------------------------------------------------------------------
| reportRegionStateTransition() | Triggers a report chain when a state change is needed for a region. Sent to the Master
+-------------------------------+-----------------------------------------------------------------------------------------
| stop() | Stops the server gracefully
+-------------------------------+-----------------------------------------------------------------------------------------
沒有必要自己實現 RegionObserver 介面，基於這個介面，可以利用 BaseRegionObserver 類來只實現所需要回調方法。
3.7.4 The BaseRegionObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
這個類可用作觀察者型別協處理器的基類。它有 RegionObserver 介面要求的所有方法的空實現，因此預設什麼有不做。必須重寫所有感興趣的回撥來新增
自己的功能。
示例： Example region observer checking for special get requests
public static final byte[] FIXED_ROW = Bytes.toBytes("@@@
@Override
public void preGetOp(ObserverContext<RegionCoprocessorEnvironment> e, Get get, List<Cell> results) throws IOException {
//Check if the request row key matches a well known one.
if (Bytes.equals(get.getRow(), FIXED_ROW)) {
Put put = new Put(get.getRow());
put.addColumn(FIXED_ROW, FIXED_ROW,
Bytes.toBytes(System.currentTimeMillis()));
CellScanner scanner = put.cellScanner();
scanner.advance();
Cell cell = scanner.current();
results.add(cell);
}
}
}
NOTE
-------------------------------------------------------------------------------------------------------------------------------------
將如下屬性新增到 hbase-site.xml 配置檔案以啟用這個協處理器
<property>
<name>hbase.coprocessor.user.region.classes</name>
<value>coprocessor.RegionObserverExample</value>
</property>
3.8 The MasterObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
討論 Coprocessor 的第二個子類是為了處理 master server 啟動的所有回撥方法。它們被劃歸為資料定義操作(data-manipulation operations), 類似於
關係資料庫系統中的 DDL。MasterObserver 類提供如下鉤子：
Callbacks for master API functions
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| API Call | Shell Call | Pre-Hook | Post-Hook
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| createTable() | create | preCreateTable(...) | postCreateTable(...)
| | | preCreateTableHandler(...) |
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| deleteTable() | drop | preDeleteTable(...) | postDeleteTableHandler(...),
| deleteTables() | | preDeleteTableHandler(...) | postDeleteTable(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| modifyTable() | alter | preModifyTable(...) | postModifyTableHandler(...),
| | | preModifyTableHandler(...) | postModifyTable(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| modifyTable() | alter | preAddColumn(...) | postAddColumnHandler(...),
| | | preAddColumnHandler(...) | postAddColumn(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| modifyTable() | alter | preDeleteColumn(...) | postDeleteColumnHandler(...),
| | | preDeleteColumnHandler(...) | postDeleteColumn(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| modifyTable() | alter | preModifyColumn(...) | postModifyColumnHandler(...),
| | | preModifyColumnHandler(...) | postModifyColumn(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| enableTable() | enable | preEnableTable(...) | postEnableTableHandler(...),
| enableTables() | | preEnableTableHandler(...) | postEnableTable(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| disableTable() | disable | preDisableTable(...), | postDisableTableHandler(...),
| disableTables() | | preDisableTableHandler(...) | postDisableTable(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| flush() | flush | preTableFlush(...) | postTableFlush(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| truncateTable() | truncate | preTruncateTable(...) | postTruncateTableHandler(...),
| | | preTruncateTableHandler(...) | postTruncateTable(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| move() | move | preMove(...) | postMove(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| assign() | assign | preAssign(...) | postAssign(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| unassign() | unassign | preUnassign(...) | postUnassign(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| offline() | n/a | preRegionOffline(...) | postRegionOffline(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| balancer() | balancer | preBalance(...) | postBalance(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| setBalancerRunning() | balance_switch | preBalanceSwitch(...) | postBalanceSwitch(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| listTableNames() | list | preGetTableNames(...) | postGetTableNames(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| getTableDescriptors() | list | preGetTableDescriptors(...) | postGetTableDescriptors(...)
| listTables() | | |
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| createNamespace() | create_namespace | preCreateNamespace(...) | postCreateNamespace(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| deleteNamespace() | drop_namespace | preDeleteNamespace(...) | postDeleteNamespace(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| getNamespaceDescriptor() | describe_namespace | preGetNamespaceDescriptor(...) | postGetNamespaceDescriptor(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| listNamespaceDescriptors() | list_namespace | preListNamespaceDescriptors(...) | postListNamespaceDescriptors(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| modifyNamespace() | alter_namespace | preModifyNamespace(...) | postModifyNamespace(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| cloneSnapshot() | clone_snapshot | preCloneSnapshot(...) | postCloneSnapshot(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| deleteSnapshot(), | delete_snapshot | preDeleteSnapshot(...) | postDeleteSnapshot(...)
| deleteSnapshots() | delete_all_snapshot | |
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| restoreSnapshot() | restore_snapshot | preRestoreSnapshot(...) | postRestoreSnapshot(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| snapshot() | snapshot | preSnapshot(...) | postSnapshot(...)
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| shutdown() | n/a | preShutdown(...) | n/a
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| stopMaster() | n/a | preStopMaster(...) | n/a
+-------------------------------+-----------------------+-----------------------------------+-------------------------------
| n/a | n/a | preMasterInitialization(...) | postStartMaster(...)
+-------------------------------+-----------------------+-----------------------------------+------------------------------
3.8.1 The MasterCoprocessorEnvironment Class
-----------------------------------------------------------------------------------------------------------------------------------------
類似於 RegionCoprocessorEnvironment, 與 RegionObserver 協處理器緊緊關聯，MasterCoprocessorEnvironment 封裝到 MasterObserver 例項中。它也
實現 CoprocessorEnvironment 介面，如此，例如可以在自己的實現中通過 getTable() 呼叫訪問資料。除了 CoprocessorEnvironment 介面提供的方法，也
提供了一些特殊的，面向 master 子類的方法，如下表所示：
Specific method provided by the MasterCoprocessorEnvironment class
+---------------------------+---------------------------------------------------------------------------------------+
| Method | Description |
+---------------------------+---------------------------------------------------------------------------------------+
| getMasterServices() | Provides access to the shared MasterServices instance |
+---------------------------+---------------------------------------------------------------------------------------+
使用者程式碼中可以訪問共享的 master services 例項，該服務類公開了很多 Master admin API, 大致分類如下：
table 相關的方法：
createTable(HTableDescriptor, byte[][])
deleteTable(TableName)
modifyTable(TableName, HTableDescriptor)
enableTable(TableName)
disableTable(TableName)
getTableDescriptors()
truncateTable(TableName, boolean)
addColumn(TableName, HColumnDescriptor)
deleteColumn(TableName, byte[])
modifyColumn(TableName, HColumnDescriptor)
namespace 相關的方法：
createNamespace(NamespaceDescriptor)
deleteNamespace(String)
modifyNamespace(NamespaceDescriptor)
getNamespaceDescriptor(String)
listNamespaceDescriptors()
listTableDescriptorsByNamespace(String)
listTableNamesByNamespace(String)
下表列出更特定的方法及其簡短描述：
Methods provided by the MasterServices class
+-----------------------------------+-------------------------------------------------------------------------------------
| Method | Description
+-----------------------------------+-------------------------------------------------------------------------------------
| abort() | Allows aborting the entire server process, shutting down the instance with the given reason.
+-----------------------------------+-------------------------------------------------------------------------------------
| checkTableModifiable() | Convenient to check if a table exists and is offline so that it can be altered.
+-----------------------------------+-------------------------------------------------------------------------------------
| dispatchMergingRegions() | Flags two regions to be merged, which is performed on the region servers.
+-----------------------------------+-------------------------------------------------------------------------------------
| getAssignmentManager() | Gives you access to the assignment manager instance. It is responsible for all region
| | assignment operations, such as assign, unassign, balance, and so on
+-----------------------------------+-------------------------------------------------------------------------------------
| getConfiguration() | Returns the current server configuration.
+-----------------------------------+-------------------------------------------------------------------------------------
| getConnection() | Provides access to the shared connection instance.
+-----------------------------------+-------------------------------------------------------------------------------------
| getCoordinatedStateManager() | Access to the shared state manager, gives access to the TableStateManager, which in turn
| | can be used to check on the state of a table
+-----------------------------------+-------------------------------------------------------------------------------------
| getExecutorService() | Used by the master to schedule system-wide events
+-----------------------------------+-------------------------------------------------------------------------------------
| getMasterCoprocessorHost() | Returns the enclosing host instance.
+-----------------------------------+-------------------------------------------------------------------------------------
| getMasterFileSystem() | Provides you with an abstraction layer for all filesystem-related operations the master is
| | involved in—for example, creating directories for table files and logfiles.
+-----------------------------------+-------------------------------------------------------------------------------------
| getMetaTableLocator() | The method returns a class providing system table related functionality.
+-----------------------------------+-------------------------------------------------------------------------------------
| getServerManager() | Returns the server manager instance. With it you have access to the list of servers, live or
| | considered dead, and more
+-----------------------------------+-------------------------------------------------------------------------------------
| getServerName() | The server name, which is unique for every region server process
+-----------------------------------+-------------------------------------------------------------------------------------
| getTableLockManager() | Gives access to the lock manager. Can be used to acquire read and write locks for the entire table.
+-----------------------------------+-------------------------------------------------------------------------------------
| getZooKeeper() | Returns a reference to the ZooKeeper watcher instance
+-----------------------------------+-------------------------------------------------------------------------------------
| isAborted() | Flag is true when abort() was called previously
+-----------------------------------+-------------------------------------------------------------------------------------
| isInitialized() | After the server process is operational, this call will return true.
+-----------------------------------+-------------------------------------------------------------------------------------
| isServerShutdownHandlerEnabled() | When an optional shutdown handler was set, this check returns true.
+-----------------------------------+-------------------------------------------------------------------------------------
| isStopped() | Returns true when stop() (inherited from Stoppable) was called beforehand.
+-----------------------------------+-------------------------------------------------------------------------------------
| registerService() | Registers a new custom service. Called when server starts and coprocessors are loaded
+-----------------------------------+-------------------------------------------------------------------------------------
| stop() | Stops the server gracefully
+-----------------------------------+-------------------------------------------------------------------------------------
3.8.2 The BaseMasterObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
使用者可以憑自己的能力直接實現 MasterObserver 介面，或繼承 BaseMasterObserver 類來實現。BaseMasterObserver 實現了 MasterObserver 介面，只是
所有的回撥都為空實現。如果直接使用這個類，不會有任何實際的互動反應。
通過重寫適當的事件回撥方法，可以新增其它功能。可以將自己的程式碼鉤入到 pre 或 post 回撥方法中。下面示例利用 post 鉤子在一個 table 建立之後
執行額外的任務。
示例： Example master observer that creates a separate directory on the file system when a table is created.
public class MasterObserverExample extends BaseMasterObserver {
@Override
public void postCreateTable(ObserverContext<MasterCoprocessorEnvironment> ctx, HTableDescriptor desc, HRegionInfo[] regions)
throws IOException {
TableName tableName = desc.getTableName();
MasterServices services = ctx.getEnvironment().getMasterServices();
MasterFileSystem masterFileSystem = services.getMasterFileSystem();
FileSystem fileSystem = masterFileSystem.getFileSystem();
Path blobPath = new Path(tableName.getQualifierAsString() + "-
blobs");
fileSystem.mkdirs(blobPath);
}
}
NOTE
-------------------------------------------------------------------------------------------------------------------------------------
將如下屬性新增到 hbase-site.xml 配置檔案以在 master 程序啟動時載入這個協處理器
<property>
<name>hbase.coprocessor.master.classes</name>
<value>coprocessor.MasterObserverExample</value>
</property
修改後，重啟 HBase 以使配置生效。
一旦激活了這個協處理器，它會在關注的事件上監聽，並自動觸發使用者程式碼的執行。
可以使用如下 shell 命令觸發事件：
hbase(main):001:0> create 'testtable3', 'colfam1'
0 row(s) in 0.6740 seconds
建立了一個表，隨後會呼叫協處理器的 postCreateTable() 方法，通過 Hadoop 命令列工具驗證結果：
$ bin/hadoop dfs -ls
Found 1 items
drwxr-xr-x - larsgeorge supergroup 0 ... testtable3-
blobs
3.8.3 The BaseMasterAndRegionObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
HBase 還提供了另外一個相關的基類，BaseMasterAndRegionObserver. 它聯合了 BaseRegionObserver 類 和 MasterObserver 介面。
public abstract class BaseMasterAndRegionObserver
extends BaseRegionObserver implements MasterObserver {
...
}
在效果上，它像是組合了 BaseRegionObserver 和 BaseRegionObserver 為一個類。由於這個類提供了一個 region server 和 master server 實現，因此
只用於執行在 HBase Master 上。這對將系統表直接放在 master 上有用。
3.9 The RegionServerObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
這個類是 region 伺服器級別的，它公開了與伺服器功能相關聯的定義良好的鉤子方法，也就是說，它們是跨多個 region 和 table 的。鉤子方法如下：
● postCreateReplicationEndPoint(...)
-------------------------------------------------------------------------------------------------------------------------------------
在伺服器建立了一個 replication endpoint (不要與 coprocessor endpoint 混淆)之後呼叫。
● preMerge(...), postMerge(...)
-------------------------------------------------------------------------------------------------------------------------------------
兩個 region 合併時呼叫
● preMergeCommit(...), postMergeCommit(...)
-------------------------------------------------------------------------------------------------------------------------------------
與上面的相同，但更小的作用域。在 preMerge() 之後和 postMerge() 之前呼叫。
● preRollBackMerge(...), postRollBackMerge(...)
-------------------------------------------------------------------------------------------------------------------------------------
這兩個方法在一個 region 合併失敗，並且合併事務回滾時被呼叫。
● preReplicateLogEntries(...), postReplicateLogEntries(...)
-------------------------------------------------------------------------------------------------------------------------------------
關聯到 WAL 項重放過程，可以對每一個日誌項進行特殊的處理。
● preRollWALWriterRequest(...), postRollWALWriterRequest(...)
-------------------------------------------------------------------------------------------------------------------------------------
關聯到 WAL 檔案回滾，基於檔案大小，時間，或手工請求時呼叫
● preStopRegionServer(...)
-------------------------------------------------------------------------------------------------------------------------------------
這是個 pre-only 的鉤子，當繼承自 Stoppable 介面的 stop() 方法呼叫時被呼叫。
3.9.1 The RegionServerCoprocessorEnvironment Class
-----------------------------------------------------------------------------------------------------------------------------------------
也實現 CoprocessorEnvironment 介面，因此可以在自己的實現中訪問 getTable() 呼叫以訪問資料。除 CoprocessorEnvironment 提供的方法，也提供了
面向 server 的方法：
Specific method provided by the RegionServerCoprocessorEnvironment class
+-----------------------------------+-------------------------------------------------------------------------------------
| Method | Description
+-----------------------------------+-------------------------------------------------------------------------------------
| getRegionServerServices() | Provides access to the shared RegionServerServices instance.
+-----------------------------------+-------------------------------------------------------------------------------------
3.9.2 The BaseRegionServerObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
RegionServerObserver 介面的空實現，繼承該類，可以將精力集中在真正需要處理的問題上，只重寫必要的方法，簡化實現。
3.10 The WALObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
與 write-ahead log(WAL for short) 相關的觀察者類。它提供了易於處理的回撥，如下：
preWALWrite(...), postWALWrite(...)
封裝了正在寫入到 WAL 的 log entry, 允許訪問全部編輯記錄。因為可以在這兩個方法上接收到整個日誌記錄，因此使用者程式碼可以影響寫入日誌的內容。
例如，在一些高階應用場景，可以向日志編輯記錄新增額外的 cell, 這樣在日誌重放時，這個額外的 cell 可以幫助微調資料重建過程。
3.10.1 The WALCoprocessorEnvironment Class
-----------------------------------------------------------------------------------------------------------------------------------------
作為回撥的一部分提供的環境，這裡是 WALCoprocessorEnvironment 例項，它也擴充套件自 CoprocessorEnvironment，因此也可以在自己的實現程式碼中通過
getTable() 方法訪問資料。
Specific method provided by the WALCoprocessorEnvironment class
+-----------------------------------+-------------------------------------------------------------------------------------
| Method | Description
+-----------------------------------+-------------------------------------------------------------------------------------
| getWAL() | Provides access to the shared WAL instance.
+-----------------------------------+-------------------------------------------------------------------------------------
利用 WAL 的引用，可以滾動當前的寫入器，換句話說，關閉當前的日誌檔案，並建立一個新的日誌檔案。也可以呼叫 sync() 方法來強制編輯
日誌寫入到持久層。下面是 WAL 介面提供的方法：
void registerWALActionsListener(final WALActionsListener listener)
boolean unregisterWALActionsListener(final WALActionsListener listener)
byte[][] rollWriter() throws FailedLogCloseException, IOException
byte[][] rollWriter(boolean force) throws FailedLogCloseException, IOException
void shutdown() throws IOException
void close() throws IOException
long append(HTableDescriptor htd, HRegionInfo info, WALKey key, WALEdit edits,
AtomicLong sequenceId, boolean inMemstore, List<Cell> memstoreKVs)throws IOException
void sync() throws IOException
void sync(long txid) throws IOException
boolean startCacheFlush(final byte[] encodedRegionName)
void completeCacheFlush(final byte[] encodedRegionName)
void abortCacheFlush(byte[] encodedRegionName)
WALCoprocessorHost getCoprocessorHost()
long getEarliestMemstoreSeqNum(byte[] encodedRegionName)
這些都是非常底層的方法，詳細資訊參考 API 文件
3.10.2 The BaseWALObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
BaseWALObserver 類實現 WALObserver 介面，使用者可以繼承這個類，或者直接實現介面。
3.11 The BulkLoadObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
這個觀察者類用於在 bulk loading 操作期間，由 HBase 提供的 completebulkload 工具觸發。包含在伺服器 JAR 檔案中。使用 Hadoop JAR 支援，可以
檢視工具列表：
bin/hadoop jar /usr/local/hbase-1.0.0-bin/lib/hbaseserver-1.0.0.jar
An example program must be given as the first argument.
Valid program names are:
CellCounter: Count cells in HBase table
completebulkload: Complete a bulk data load.
copytable: Export a table from local cluster to peer cluster
export: Write table data to HDFS.
import: Import data written by Export.
importtsv: Import data in TSV format.
rowcounter: Count rows in HBase table
verifyrep: Compare the data from tables in two different clusters.
WARNING: It doesn't work for incrementColumnValues'd cells since the timestamp is changed after being appended to the log.
一旦 completebulkload 工具執行，它會嘗試將所有分階段的 bulk 載入。在此操作期間，觸發可用