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.
So you may ask, how does HBase provide low-latency reads and writes? In this blog post, we explain this by describing the write path of HBase — how data is updated in HBase.
The write path is how an HBase completes put or delete operations. This path begins at a client, moves to a region server, and ends when data eventually is written to an HBase data file called an HFile. Included in the design of the write path are features that HBase uses to prevent data loss in the event of a region server failure.
Each HBase table is hosted and managed by sets of servers which fall into three categories: One active master server One or more backup master servers Many region servers Region servers contribute to handling the HBase tables.
Because HBase tables can be large, they are broken up into partitions called regions. Each write ahead log hbase hadoop server handles one or more of these regions. Note that because region servers are the only servers that serve HBase table data, a master server crash cannot cause data loss.
HBase data is organized similarly to a sorted map, with the sorted key space partitioned into different shards or regions.
An HBase client updates a table by invoking put or delete commands. However, programmatically, a client can cache the changes in the client side, and flush these changes to region servers in a batch, by turning the autoflush off.
Since the row key is sorted, it is easy to determine which region server manages which key. A change request is for a specific row. Each row key belongs to a specific region which is served by a region server.
From the root region server, the client finds out the location of the region server hosting the -META- region. From the meta region server, then we finally locate the actual region server which serves the requested region.
This is a three-step process, so the region location is cached to avoid this expensive series of operations. This allows searching for random rows efficiently when reading the data. Data cannot be randomly inserted into the HFile.
Instead, the change must be written to a new file. If each update were written to a file, many small files would be created. Such a solution would not be scalable nor efficient to merge or read at a later time. Therefore, changes are not immediately written to a new HFile.
Instead, each change is stored in a place in memory called the memstore, which cheaply and efficiently supports random writes. Data in the memstore is sorted in the same manner as data in a HFile. Although writing data to the memstore is efficient, it also introduces an element of risk: Information stored in memstore is stored in volatile memory, so if the system fails, all memstore information is lost.
To help mitigate this risk, HBase saves updates in a write-ahead-log WAL before writing the information to memstore. WAL may be disabled, but this should only be done if the risk of data loss is not a concern.
If you choose to disable WAL, consider implementing your own disaster recovery solution or be prepared for the possibility of data loss. WAL files contain a list of edits, with one edit representing a single put or delete.
The edit includes information about the change and the region to which the change applies. Edits are written chronologically, so, for persistence, additions are appended to the end of the WAL file that is stored on disk.
Because WAL files are ordered chronologically, there is never a need to write to a random place within the file. Once a WAL file is rolled, no additional changes are made to the old file.
You can configure the multiplier using parameter: The intent is to eventually write all changes from each WAL file to disk and persist that content in an HFile.
After this is done, the WAL file can be archived and it is eventually deleted by the LogCleaner daemon thread. Note that WAL files serve as a protective measure.HBase: The Hadoop Database HBase is a distributed column-oriented database which is built on top of the Hadoop file system.
It is an open-source project and is horizontally scalable. The Write Ahead Log files in HBase are not cleaned up, instead are accumulated in WAL directory. A force flushing of Regions also fails. The following is displayed in the Region Server. A Write Ahead Log (WAL) provides service for reading, writing waledits.
This interface provides APIs for WAL users (such as RegionServer) to use the WAL (do append, sync, etc). Note that some internals, such as log rolling and performance evaluation tools, will use grupobittia.com to determine if they have already seen a given WAL.
Roll the log writer. That is, start writing log messages to a new file. Because a log cannot be rolled during a cache flush, and a cache flush spans two method calls, a special lock needs to be obtained so that a cache flush cannot start when the log is being rolled and the log cannot be rolled during a cache flush.
Get details on HBase’s architecture, including the storage format, write-ahead log, background processes, and more Integrate HBase with Hadoop's MapReduce framework for massively parallelized data processing jobs. An In-Depth Look at the HBase Architecture. Blog NoSQL Current Post. Share.
Share. Share. Contributed by. Carol McDonald. The Hadoop DataNode stores the data that the Region Server is managing. All HBase data is stored in HDFS files. Write Ahead Log is a file on the distributed file system. The WAL is used to store new data that hasn't.