Write ahead log in hbase interview

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Write ahead log in hbase interview

In my previous post we had a look at the general storage architecture of HBase. This post explains how the log works in detail, but bear in mind that it describes the current version, which is 0. I will address the various plans to improve the log for 0. For the term itself please read here.

This is important in case something happens to the primary storage. So if the server crashes it can effectively replay that log to get everything up to where the server should have been just before the crash. It also means that if writing the record to the WAL fails the whole operation must be considered a failure.

Let"s look at the high level view of how this is done in HBase. First the client initiates an action that modifies data. This is currently a call to put Putdelete Delete and incrementColumnValue abbreviated as "incr" here at times.

And that also pretty much describes the write-path of HBase. Eventually when the MemStore gets to a certain size or after a specific time the data is asynchronously persisted to the file system.

In between that timeframe data is stored volatile in memory. We have a look now at the various classes or "wheels" working the magic of the WAL. First up is one of the main classes of this contraption. What you may have read in my previous post and is also illustrated above is that there is only one instance of the HLog class, which is one per HRegionServer.

HDFS only writes data, does not update

It is what is called when the above mentioned modification methods are invoked One thing to note here is that for performance reasons there is an option for putdeleteand incrementColumnValue to be called with an extra parameter set: If you invoke this method while setting up for example a Put instance then the writing to WAL is forfeited!

That is also why the downward arrow in the big picture above is done with a dashed line to indicate the optional step. By default you certainly want the WAL, no doubt about that.

But say you run a large bulk import MapReduce job that you can rerun at any time. You gain extra performance but need to take extra care that no data was lost during the import.

The choice is yours.

Lineland: HBase Architecture - Write-ahead-Log

Another important feature of the HLog is keeping track of the changes. This is done by using a "sequence number".

write ahead log in hbase interview

It uses an AtomicLong internally to be thread-safe and is either starting out at zero - or at that last known number persisted to the file system.HBase is a distributed, persistent, strictly consistent storage system with near-optimal write—in terms of I/O channel saturation—and excellent read performance, and it makes efficient use of disk space by supporting pluggable compression algorithms that can be selected based on the nature of the data in specific column families.

Answer: When data is updated it is first written to a commit log, called a write-ahead log (WAL) in HBase, and then stored in the in-memory memstore. Once the data in memory has exceeded a given maximum value, it is flushed as an HFile to disk. Apache Hadoop HDFS Architecture Introduction: In this blog, I am going to talk about Apache Hadoop HDFS Architecture.

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write ahead log in hbase interview
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