继 HBase 协处理器 (一)
3.7 The RegionObserver Class
用于 region level, 注意，这个类的所有调用方法的第一个参数为 ObserverContext<RegionCoprocessorEnvironment> ctx8, 提供访问 context 实例。
操作可以被划分为两组： region 生命周期变化和客户端 API 调用。两种类型都有通用的回调操作：
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 中的这些方法在任何可能的 Operation 发生时被调用。
3.7.1 处理 region 生命周期事件 (Handling Region Life-Cycle Events)
-----------------------------------------------------------------------------------------------------------------------------------------
observer 可以钩入到 pending open, open, 和 pending close 状态变化。每一个钩子被框架隐式调用。
● State: pending open
-------------------------------------------------------------------------------------------------------------------------------------
region 要被打开时，处于这个状态。监听的协处理器可以搭载这个过程，或者使这个过程失效。为此，下列回调按次序调用：
postLogReplay(...)
preOpen(...)
preStoreFileReaderOpen(...)
postStoreFileReaderOpen(...)
preWALRestore(...) / postWALRestore(...)
postOpen(...)
这些方法只在 region 打开之前被调用，在存储文件打开之前和之后调用，WAL 准备重放时调用，以及在打开 region 之后调用。用户自己的协处理器
实现可以利用这些回调，例如，指示框架在 preOpen() 中，终止打开过程。或者钩入到 postOpen() 调用来触发一个缓存准备操作。
第一个事件，postLogReplay() 的触发取决于 WAL 恢复模式(WAL recovery mode)的配置：分布式日志切分(distributed log splitting),或者日志重
放(log replay)，由 hbase.master.distributed.log.replay 属性配置。前者运行于一个 region 打开之前，因此会首先触发这个调用。后者打开一个
region ，然后重放编辑日志，在 region 打开事件之后，触发该回调。
在这两种模式中，但取决于哪一种模式是活动的， region server 从 write-ahead-log (WAL) 应用记录。因此按次序调用监听者的 preWALRestore()
或 postWALRestore() 方法。在使用分布式日志切分模式下(distributed log splitting), 这会发生在 pending open 之后，但在 open state 之前。
否则，该调用在 open 事件之后，因为日志被重放。钩入到 WAL 调用可以控制在日志重放过程中应用了什么数据操作(mutation).可以访问编辑日志(
edit record), 可以用来观察应用了什么操作。
● State: open
-------------------------------------------------------------------------------------------------------------------------------------
当一个 region 被部署到一个 region 服务器上并且可以正常工作时，这个 region 被认为处于 open 状态。例如，region 的内存存储可以刷写到磁盘
上，或者在 region 变得太大时可以被拆分。可能的钩子如下：
preFlushScannerOpen(...)
preFlush(...) / postFlush(...)
preCompactSelection(...) / postCompactSelection(...)
preCompactScannerOpen(...)
preCompact(...) / postCompact(...)
preSplit(...)
preSplitBeforePONR(...)
preSplitAfterPONR(...)
postSplit(...)
postCompleteSplit(...) / preRollBackSplit(...) / postRollBackSplit(...)
这些方法名称上看都非常直接：pre 调用在各自的操作之前调用，而 post 调用在操作之后调用。例如，利用 preSplit() 钩子，可以有效地禁止内置
的 region 拆分过程，并手动执行这些操作。有些调用只有 pre 钩子，有些则只有 post 钩子。
flush, compact, 和 split 这三种类型的钩子直接连接到匹配 region 内部处理函数。也有一些更特殊的钩子，作为这三种函数的部分调用。例如，
preFlushScannerOpen() 钩子在 memstore 设置的时候被调用，这只在刷写实际发生之前调用。
类似地，对于压缩(compaction)，首先服务器选择包含的文件，该功能被封装到协处理器回调中。之后存储扫描器打开，最后，实际的压缩发生。
对于 split, 有几个回调反应当前的状态，通过一个状态之间特定的 point-of-no-return (PONR) 概念。这发生在 split process 开始之后，但在最后
的动作发生之前。拆分的处理类似于一个内部的事务，当事务要提交时，preSplitBeforePONR() 被调用，而之后 preSplitAfterPONR() 被调用。也有
对最终的 completed 或 rollback 的调用，通知用户 split 事务的结果。
● State: pending close
-------------------------------------------------------------------------------------------------------------------------------------
对于观察者的最后一组钩子是在 region 在进入 pending close 状态时调用的。这组调用在 region 从 open 变为 close 时发生。只是在 region 关闭
之前和之后，执行如下钩子：
preClose(..., boolean abortRequested)
postClose(..., boolean abortRequested)
abortRequested 参数指出一个 region 关闭的原因。通常情况下，region 正常操作关闭，如，由于负载均衡的原因，该 region 移动到一个不同的region
server 上。但也有可能由于某个 region server 出了问题而终止这个 region 以避免影响任何一端的处理结果。当这类情况发生时，所有存储在该服务器
上的 region 都会终止，用户可以通过这个给定的参数判断发生了什么情况。
除此之外，这个类也继承了 start() 和 stop() 方法，允许分配、释放生命周期资源。
3.7.2 处理客户端 API 事件 (Handling Client API Events)
-----------------------------------------------------------------------------------------------------------------------------------------
与生命周期事件相对，所有客户端 API 调用显式地从客户端应用发送到 region 服务器。用户可以钩入到这些调用，在应用之前或之后回调。下表列出可用
的 API 调用和相关的回调方法：
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顺序的，如果有多个事件发生，发生的顺序从上至下。斜线 "/" 表示可能发生的事件为多个事件中的一个，具体会是哪个事件，
取决于调用中具体包含的操作。各个事件回调方法的详细信息参考 API 文档或源代码。
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
-----------------------------------------------------------------------------------------------------------------------------------------
提供给实现 RegionObserver 接口的协处理器的环境实例基于 RegionCoprocessorEnvironment 类，该类实现 CoprocessorEnvironment 接口。除了已提供的
方法，RegionCoprocessorEnvironment 还提供了一些特殊的，面向 region 子类的方法，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() 调用可用于获取存本机上的 HRegion 实例的引用，并可以调用 HRegion 类提供的方法。如果需要有关该 region 的信息，可以调用
getRegionInfo() 来获取 HRegionInfo 实例。此类有非常有用的方法可以获取所包含的 key 的访问，region 的名称，以及状态标志：
byte[] getStartKey()
byte[] getEndKey()
byte[] getRegionName()
boolean isSystemTable()
int getReplicaId()
...
另外，用户代码可以访问共享的 region server 服务实例，通过调用 getRegionServerServices() 方法并返回 RegionServerServices 实例。该类提供了
很多高级方法，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
+-------------------------------+-----------------------------------------------------------------------------------------
没有必要自己实现 RegionObserver 接口，基于这个接口，可以利用 BaseRegionObserver 类来只实现所需要回调方法。
3.7.4 The BaseRegionObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
这个类可用作观察者类型协处理器的基类。它有 RegionObserver 接口要求的所有方法的空实现，因此默认什么有不做。必须重写所有感兴趣的回调来添加
自己的功能。
示例： Example region observer checking for special get requests
public static final byte[] FIXED_ROW = Bytes.toBytes("@@@GETTIME@@@");
@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
-------------------------------------------------------------------------------------------------------------------------------------
将如下属性添加到 hbase-site.xml 配置文件以启用这个协处理器
<property>
<name>hbase.coprocessor.user.region.classes</name>
<value>coprocessor.RegionObserverExample</value>
</property>
3.8 The MasterObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
讨论 Coprocessor 的第二个子类是为了处理 master server 启动的所有回调方法。它们被划归为数据定义操作(data-manipulation operations), 类似于
关系数据库系统中的 DDL。MasterObserver 类提供如下钩子：
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
-----------------------------------------------------------------------------------------------------------------------------------------
类似于 RegionCoprocessorEnvironment, 与 RegionObserver 协处理器紧紧关联，MasterCoprocessorEnvironment 封装到 MasterObserver 实例中。它也
实现 CoprocessorEnvironment 接口，如此，例如可以在自己的实现中通过 getTable() 调用访问数据。除了 CoprocessorEnvironment 接口提供的方法，也
提供了一些特殊的，面向 master 子类的方法，如下表所示：
Specific method provided by the MasterCoprocessorEnvironment class
+---------------------------+---------------------------------------------------------------------------------------+
| Method | Description |
+---------------------------+---------------------------------------------------------------------------------------+
| getMasterServices() | Provides access to the shared MasterServices instance |
+---------------------------+---------------------------------------------------------------------------------------+
用户代码中可以访问共享的 master services 实例，该服务类公开了很多 Master admin API, 大致分类如下：
table 相关的方法：
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 相关的方法：
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简短描述：
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
-----------------------------------------------------------------------------------------------------------------------------------------
用户可以凭自己的能力直接实现 MasterObserver 接口，或继承 BaseMasterObserver 类来实现。BaseMasterObserver 实现了 MasterObserver 接口，只是
所有的回调都为空实现。如果直接使用这个类，不会有任何实际的交互反应。
通过重写适当的事件回调方法，可以添加其它功能。可以将自己的代码钩入到 pre 或 post 回调方法中。下面示例利用 post 钩子在一个 table 创建之后
执行额外的任务。
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
-------------------------------------------------------------------------------------------------------------------------------------
将如下属性添加到 hbase-site.xml 配置文件以在 master 进程启动时载入这个协处理器
<property>
<name>hbase.coprocessor.master.classes</name>
<value>coprocessor.MasterObserverExample</value>
</property
修改后，重启 HBase 以使配置生效。
一旦激活了这个协处理器，它会在关注的事件上监听，并自动触发用户代码的执行。
可以使用如下 shell 命令触发事件：
hbase(main):001:0> create 'testtable3', 'colfam1'
0 row(s) in 0.6740 seconds
创建了一个表，随后会调用协处理器的 postCreateTable() 方法，通过 Hadoop 命令行工具验证结果：
$ bin/hadoop dfs -ls
Found 1 items
drwxr-xr-x - larsgeorge supergroup 0 ... testtable3-
blobs
3.8.3 The BaseMasterAndRegionObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
HBase 还提供了另外一个相关的基类，BaseMasterAndRegionObserver. 它联合了 BaseRegionObserver 类 和 MasterObserver 接口。
public abstract class BaseMasterAndRegionObserver
extends BaseRegionObserver implements MasterObserver {
...
}
在效果上，它像是组合了 BaseRegionObserver 和 BaseRegionObserver 为一个类。由于这个类提供了一个 region server 和 master server 实现，因此
只用于运行在 HBase Master 上。这对将系统表直接放在 master 上有用。
3.9 The RegionServerObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
这个类是 region 服务器级别的，它公开了与服务器功能相关联的定义良好的钩子方法，也就是说，它们是跨多个 region 和 table 的。钩子方法如下：
● postCreateReplicationEndPoint(...)
-------------------------------------------------------------------------------------------------------------------------------------
在服务器创建了一个 replication endpoint (不要与 coprocessor endpoint 混淆)之后调用。
● preMerge(...), postMerge(...)
-------------------------------------------------------------------------------------------------------------------------------------
两个 region 合并时调用
● preMergeCommit(...), postMergeCommit(...)
-------------------------------------------------------------------------------------------------------------------------------------
与上面的相同，但更小的作用域。在 preMerge() 之后和 postMerge() 之前调用。
● preRollBackMerge(...), postRollBackMerge(...)
-------------------------------------------------------------------------------------------------------------------------------------
这两个方法在一个 region 合并失败，并且合并事务回滚时被调用。
● preReplicateLogEntries(...), postReplicateLogEntries(...)
-------------------------------------------------------------------------------------------------------------------------------------
关联到 WAL 项重放过程，可以对每一个日志项进行特殊的处理。
● preRollWALWriterRequest(...), postRollWALWriterRequest(...)
-------------------------------------------------------------------------------------------------------------------------------------
关联到 WAL 文件回滚，基于文件大小，时间，或手工请求时调用
● preStopRegionServer(...)
-------------------------------------------------------------------------------------------------------------------------------------
这是个 pre-only 的钩子，当继承自 Stoppable 接口的 stop() 方法调用时被调用。
3.9.1 The RegionServerCoprocessorEnvironment Class
-----------------------------------------------------------------------------------------------------------------------------------------
也实现 CoprocessorEnvironment 接口，因此可以在自己的实现中访问 getTable() 调用以访问数据。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 接口的空实现，继承该类，可以将精力集中在真正需要处理的问题上，只重写必要的方法，简化实现。
3.10 The WALObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
与 write-ahead log(WAL for short) 相关的观察者类。它提供了易于处理的回调，如下：
preWALWrite(...), postWALWrite(...)
封装了正在写入到 WAL 的 log entry, 允许访问全部编辑记录。因为可以在这两个方法上接收到整个日志记录，因此用户代码可以影响写入日志的内容。
例如，在一些高级应用场景，可以向日志编辑记录添加额外的 cell, 这样在日志重放时，这个额外的 cell 可以帮助微调数据重建过程。
3.10.1 The WALCoprocessorEnvironment Class
-----------------------------------------------------------------------------------------------------------------------------------------
作为回调的一部分提供的环境，这里是 WALCoprocessorEnvironment 实例，它也扩展自 CoprocessorEnvironment，因此也可以在自己的实现代码中通过
getTable() 方法访问数据。
Specific method provided by the WALCoprocessorEnvironment class
+-----------------------------------+-------------------------------------------------------------------------------------
| Method | Description
+-----------------------------------+-------------------------------------------------------------------------------------
| getWAL() | Provides access to the shared WAL instance.
+-----------------------------------+-------------------------------------------------------------------------------------
利用 WAL 的引用，可以滚动当前的写入器，换句话说，关闭当前的日志文件，并创建一个新的日志文件。也可以调用 sync() 方法来强制编辑
日志写入到持久层。下面是 WAL 接口提供的方法：
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)
这些都是非常底层的方法，详细信息参考 API 文档
3.10.2 The BaseWALObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
BaseWALObserver 类实现 WALObserver 接口，用户可以继承这个类，或者直接实现接口。
3.11 The BulkLoadObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
这个观察者类用于在 bulk loading 操作期间，由 HBase 提供的 completebulkload 工具触发。包含在服务器 JAR 文件中。使用 Hadoop JAR 支持，可以
查看工具列表：
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 工具运行，它会尝试将所有分阶段的 bulk 载入。在此操作期间，触发可用的回调：
prePrepareBulkLoad(...)
-------------------------------------------------------------------------------------------------------------------------------------
在 bulk load 操作发生之前调用.
preCleanupBulkLoad(...)
-------------------------------------------------------------------------------------------------------------------------------------
当 bulk load 完成并且执行清除任务时调用。
3.12 The EndPointObserver Class
-----------------------------------------------------------------------------------------------------------------------------------------
这个类没有自己的环境，使用共享的 RegionCoprocessorEnvironment, 这很容易理解，因为 endpoint 运行在一个 region 的 context 中。
可用的回调方法如下：
preEndpointInvocation(...), postEndpointInvocation(...)
当一个 endpoint 方法从客户端调用时触发，这些回调封装了客户端的执行。
客户端可以替换(对于 pre 钩子) 或修改 (对于 post 钩子)给定的 Message 实例来修改 endpoint 方法的输出结果。如果在 pre 钩子调用期间发生了异常，
那么服务器端的调用完全终止。
（本篇完）
系列目录：
HBase 协处理器 (一)
HBase 协处理器 (二)
参考：
《HBase - The Definitive Guide - 2nd Edition》Early release —— 2015.7 Lars George