Package org.apache.hadoop.hbase.coprocessor

Interface WALObserver

@LimitedPrivate("Coprocesssor")
@Evolving
public interface WALObserver

It's provided to have a way for coprocessors to observe, rewrite, or skip WALEdits as they are being written to the WAL. Note that implementers of WALObserver will not see WALEdits that report themselves as empty via WALEdit.isEmpty(). RegionObserver provides hooks for adding logic for WALEdits in the region context during reconstruction. Defines coprocessor hooks for interacting with operations on the WAL. Since most implementations will be interested in only a subset of hooks, this class uses 'default' functions to avoid having to add unnecessary overrides. When the functions are non-empty, it's simply to satisfy the compiler by returning value of expected (non-void) type. It is done in a way that these default definitions act as no-op. So our suggestion to implementation would be to not call these 'default' methods from overrides.

Exception Handling

For all functions, exception handling is done as follows:

• Exceptions of type IOException are reported back to client.
• For any other kind of exception:
  • If the configuration CoprocessorHost.ABORT_ON_ERROR_KEY is set to true, then the server aborts.
  • Otherwise, coprocessor is removed from the server and DoNotRetryIOException is returned to the client.

Method Summary

Modifier and Type	Method	Description
default void	postWALRoll(ObserverContext<? extends WALCoprocessorEnvironment> ctx, org.apache.hadoop.fs.Path oldPath, org.apache.hadoop.fs.Path newPath)	Called after rolling the current WAL
default void	postWALWrite(ObserverContext<? extends WALCoprocessorEnvironment> ctx, RegionInfo info, WALKey logKey, WALEdit logEdit)	Called after a WALEdit is writen to WAL.
default void	preWALRoll(ObserverContext<? extends WALCoprocessorEnvironment> ctx, org.apache.hadoop.fs.Path oldPath, org.apache.hadoop.fs.Path newPath)	Called before rolling the current WAL
default void	preWALWrite(ObserverContext<? extends WALCoprocessorEnvironment> ctx, RegionInfo info, WALKey logKey, WALEdit logEdit)	Called before a WALEdit is writen to WAL.

Method Details

preWALWrite

default void preWALWrite(ObserverContext<? extends WALCoprocessorEnvironment> ctx,
 RegionInfo info,
 WALKey logKey,
 WALEdit logEdit)
                  throws IOException

Called before a WALEdit is writen to WAL.

The method is marked as deprecated in 2.0.0, but later we abstracted the WALKey interface for coprocessors, now it is OK to expose this to coprocessor users, so we revert the deprecation. But you still need to be careful while changing WALEdit, as when reaching here, if you add some cells to WALEdit, it will only be written to WAL but no in memstore, but when replaying you will get these cells and there are CP hooks to intercept these cells.

See HBASE-28580.

Throws:

IOException

postWALWrite

default void postWALWrite(ObserverContext<? extends WALCoprocessorEnvironment> ctx,
 RegionInfo info,
 WALKey logKey,
 WALEdit logEdit)
                   throws IOException

Called after a WALEdit is writen to WAL.

The method is marked as deprecated in 2.0.0, but later we abstracted the WALKey interface for coprocessors, now it is OK to expose this to coprocessor users, so we revert the deprecation. But you still need to be careful while changing WALEdit, as when reaching here, if you add some cells to WALEdit, it will only be written to WAL but no in memstore, but when replaying you will get these cells and there are CP hooks to intercept these cells.

See HBASE-28580.

Throws:

IOException

preWALRoll

default void preWALRoll(ObserverContext<? extends WALCoprocessorEnvironment> ctx,
 org.apache.hadoop.fs.Path oldPath,
 org.apache.hadoop.fs.Path newPath)
                 throws IOException

Called before rolling the current WAL

Parameters:

oldPath - the path of the current wal that we are replacing

newPath - the path of the wal we are going to create

Throws:

IOException

postWALRoll

default void postWALRoll(ObserverContext<? extends WALCoprocessorEnvironment> ctx,
 org.apache.hadoop.fs.Path oldPath,
 org.apache.hadoop.fs.Path newPath)
                  throws IOException

Called after rolling the current WAL

Parameters:

oldPath - the path of the wal that we replaced

newPath - the path of the wal we have created and now is the current

Throws:

IOException