[HBASE-10378] Divide HLog interface into User and Implementor specific interfaces

• Type: Sub-task
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.0.0
• Component/s: wal
• Hadoop Flags: Incompatible change, Reviewed

Release Note

HBase internals for the write ahead log have been refactored. Advanced users of HBase should be aware of the following changes.

Public Audience
  - The Admin API for asking a region server to roll WAL files has changed from a synchronous command that returns a set of regions the WAL implementation would like flushed into an asynchronous command that returns nothing. Older clients relying on the former behavior will still be able to interact with newer servers, but the response body will always contain an empty list of regions to flush.
  - The shell command "hlog_roll" has been deprecated. Operators should use the "wal_roll" command instead. This command is subject to the changes described above for the Admin API to roll WAL files.
  - The command for analyzing write ahead logs has been renamed from 'hlog' to 'wal'. The old usage is deprecated and will be removed in a future version.
  - Some utility methods in the HBaseTesetingUtility related to testing write-ahead-logs were changed in incompatible ways. No functionality has been removed, but method names and arguments have changed. See the HBaseTestingUtility javadoc for details.
  - The WALPlayer utility has deprecated the configuration keys used for advanced customization. Users should switch to the updated configuration keys. See the usage information on the WALPlayer tool for details.
  - The HLogInputFormat utility class for processing logs with MapReduce has been deprecated and will be removed in a future version. Users should switch to the WALInputFormat.
  - The labeling of server metrics on the region server status pages changed. Previously, the number of backing files for the write ahead log was labeled 'Num. HLog Files'. If you wish to see this statistic now, please look for the label 'Num. WAL Files.' If you rely on JMX for these metrics, their location has not changed.

LimitedPrivate(COPROC) Audience, LimitedPrivate(PHOENIX)
  - The RegionObserver API has been updated. The changes are both binary and source backwards compatible for coprocessors that use the BaseRegionObserver class. For those that implement RegionObserver directly the changes are binary backwards compatible. Depending on the internals of future HBase versions, coprocessors using the deprecated API may not see all WAL related events. Users are strongly encouraged to update their use of the API; see the RegionObserver javadoc for details.
  - Classes related to reading WAL entries (ReaderBase, ProtobufLogReader, SequenceFileLogReader) have changed in a backwards incompatible way. Users who referenced HLog.Reader directly or HLog.Entry will have to update. These changes do not impact compatibility with extant wal files.
  - The WALObserver API has been updated. The changes are both binary and source backwards compatible for coprocessors that use the BaseWALObserver class. For those that implement WALObserver directly the changes are binary backwards compatible. Depending on the internals of future HBase versions, coprocessors using the deprecated API may not see all WAL related events. Users are strongly encouraged to update their use of the API; see the WALObserver javadoc for details.
 - The WALCoprocessorEnvironment has changed in a backwards incompatible way. WALObserver coprocessors that relied on retrieving an object representing the write ahead log instance will have to be updated.

LimitedPrivate(REPLICATION) Audience
 - The WALEntryFilter API has changed in a backwards incompatible way. Implementers will have to be updated.
 - The ReplicationEndpoint.ReplicateContext API has changed in a backwards incompatible way. Implementers who use this interface will have to be updated. These changes do not impact wire compatibility for replicating between clusters.
 - The HLogKey API is deprecated in favor of the WALKey API. Additionally, the HLogKey API has changed in a backwards incompatible way by changing from implementing WriteableComparable<HLogKey> to implementing Writeable and Comparable<WALKey>.

• Assignee: Sean Busbey
• Reporter: Himanshu Vashishtha
• Created: 19/Jan/14 00:43
• Updated: 16/Jun/22 17:01
• Resolved: 20/Nov/14 23:57

Description

HBASE-5937 introduces the HLog interface as a first step to support multiple WAL implementations. This interface is a good start, but has some limitations/drawbacks in its current state, such as:
1) There is no clear distinction b/w User and Implementor APIs, and it provides APIs both for WAL users (append, sync, etc) and also WAL implementors (Reader/Writer interfaces, etc). There are APIs which are very much implementation specific (getFileNum, etc) and a user such as a RegionServer shouldn't know about it.
2) There are about 14 methods in FSHLog which are not present in HLog interface but are used at several places in the unit test code. These tests typecast HLog to FSHLog, which makes it very difficult to test multiple WAL implementations without doing some ugly checks.

I'd like to propose some changes in HLog interface that would ease the multi WAL story:

1) Have two interfaces WAL and WALService. WAL provides APIs for implementors. WALService provides APIs for users (such as RegionServer).
2) A skeleton implementation of the above two interface as the base class for other WAL implementations (AbstractWAL). It provides required fields for all subclasses (fs, conf, log dir, etc). Make a minimal set of test only methods and add this set in AbstractWAL.
3) HLogFactory returns a WALService reference when creating a WAL instance; if a user need to access impl specific APIs (there are unit tests which get WAL from a HRegionServer and then call impl specific APIs), use AbstractWAL type casting,
4) Make TestHLog abstract and let all implementors provide their respective test class which extends TestHLog (TestFSHLog, for example).

Issue Links

• breaks: HBASE-12532 TestFilter failing occasionally with ExitCodeException doing chmod since HBASE-10378
• is depended upon by: HBASE-12437 HLogFactory should be able to accept multiple custom WALReaders in case of failure to read WAL logs due to upgrade/migrate of hbase
• is depended upon by: HBASE-5699 Run with > 1 WAL in HRegionServer
• is depended upon by: HBASE-12522 Backport WAL refactoring to branch-1
• is duplicated by: HBASE-6935 Rename HLog interface to WAL
• supercedes: HBASE-8610 Introduce interfaces to support MultiWAL
• links to: reviewboard