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Data at Scale: Mastering HBase Job Interviews with Confidence and Skill

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What are the differences between RDBMS and HBase data model?

The main differences between RDBMS and HBase data model are as follows: Schema: In RDBMS, there is a schema of tables, columns etc. In HBASE, there is no schema. Normalization: RDBMS data model is normalized as per the rule of Relation DB normalization. HBase data model does not require any normalization. Partition: In RDBMS we can choose to do partitioning. In HBase, partitioning happens automatically.

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Explain what is Hbase?

Hbase is a column-oriented database management system which runs on top of HDFS (Hadoop Distribute File System). Hbase is not a relational data store, and it does not support structured query language like SQL. In Hbase, a master node regulates the cluster and region servers to store portions of the tables and operates the work on the data.

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Explain why to use Hbase?

High capacity storage system Distributed design to cater large tables Column-Oriented Stores Horizontally Scalable High performance & Availability Base goal of Hbase is millions of columns, thousands of versions and billions of rows Unlike HDFS (Hadoop Distribute File System), it supports random real time CRUD operations

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Mention what are the key components of Hbase?

Zookeeper: It does the co-ordination work between client and Hbase Maser Hbase Master: Hbase Master monitors the Region Server RegionServer: RegionServer monitors the Region Region: It contains in memory data store(MemStore) and Hfile. Catalog Tables: Catalog tables consist of ROOT and META

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Explain what does Hbase consists of?

Hbase consists of a set of tables And each table contains rows and columns like traditional database Each table must contain an element defined as a Primary Key Hbase column denotes an attribute of an object

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Mention how many operational commands in Hbase?

Operational command in Hbases is about five types Get Put Delete Scan Increment

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Explain what is WAL and Hlog in Hbase?

WAL (Write Ahead Log) is similar to MySQL BIN log; it records all the changes occur in data. It is a standard sequence file by Hadoop and it stores HLogkey’s. These keys consist of a sequential number as well as actual data and are used to replay not yet persisted data after a server crash. So, in cash of server failure WAL work as a life-line and retrieves the lost data’s.

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When you should use Hbase?

Data size is huge: When you have tons and millions of records to operate Complete Redesign: When you are moving RDBMS to Hbase, you consider it as a complete re-design then mere just changing the ports SQL-Less commands: You have several features like transactions; inner joins, typed columns, etc. Infrastructure Investment: You need to have enough cluster for Hbase to be really useful

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In Hbase what is column families?

Column families comprise the basic unit of physical storage in Hbase to which features like compressions are applied.

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Explain what is the row key?

Row key is defined by the application. As the combined key is pre-fixed by the rowkey, it enables the application to define the desired sort order. It also allows logical grouping of cells and make sure that all cells with the same rowkey are co-located on the same server.

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Explain deletion in Hbase? Mention what are the three types of tombstone markers in Hbase?

When you delete the cell in Hbase, the data is not actually deleted but a tombstone marker is set, making the deleted cells invisible. Hbase deleted are actually removed during compactions. Three types of tombstone markers are there: Version delete marker: For deletion, it marks a single version of a column Column delete marker: For deletion, it marks all the versions of a column Family delete marker: For deletion, it marks of all column for a column family

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Explain how does Hbase actually delete a row?

In Hbase, whatever you write will be stored from RAM to disk, these disk writes are immutable barring compaction. During deletion process in Hbase, major compaction process delete marker while minor compactions don’t. In normal deletes, it results in a delete tombstone marker- these delete data they represent are removed during compaction. Also, if you delete data and add more data, but with an earlier timestamp than the tombstone timestamp, further Gets may be masked by the delete/tombstone marker and hence you will not receive the inserted value until after the major compaction.

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Explain what happens if you alter the block size of a column family on an already occupied database?

When you alter the block size of the column family, the new data occupies the new block size while the old data remains within the old block size. During data compaction, old data will take the new block size. New files as they are flushed, have a new block size whereas existing data will continue to be read correctly. All data should be transformed to the new block size, after the next major compaction.

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Mention the difference between Hbase and Relational Database?

Hbase Relational Database It is schema-less It is a column-oriented data store It is used to store de-normalized data It contains sparsely populated tables Automated partitioning is done in Hbase It is a schema based database It is a row-oriented data store It is used to store normalized data It contains thin tables There is no such provision or built-in support for partitioning

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What is HBaseFsck class?

There is a tool name called back is available in HBase, which is implemented by the HBaseFsck class. It offers several command-line switches that influence its behavior.

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What are the main key structures of HBase?

Row key and Column key are the two most important key structures using in HBase

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Discuss how you can use filters in Apache HBase

Filters In HBase Shell. It was introduced in Apache HBase 0.92 which helps you to conduct server-side filtering for accessing HBase over HBase shell or thrift.

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HBase support syntax structure like SQL yes or No?

No, unfortunately, SQL support for HBase is not available currently. However, by using Apache Phoenix, we can retrieve data from HBase through SQL queries.

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What is the meaning of compaction in HBase?

At the time of heavy incoming writes, it is impossible to achieve optimal performance by having one file per store. HBase helps you to combines all these HFiles to reduce the number of disk seeds for every read. This process is known as for as Compaction in HBase.

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How will you implement joins in HBase?

HBase, not support joins directly but uses MapReduce jobs join queries can be implemented by retrieving data with the help of different HBase tables.

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Explain JMX concerning HBSE

Java Management Extensions or JMX is an export status of Java applications is the standard for them.

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What is the use of MasterServer?

Master sever helps you to assign a region to the region server as well. It also helps you to handle the load balancing we use the MasterServer.

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Define the Term Thrift.

Apache Thrift is written in C++. It provides schema compilers for various programming languages like C++, Perl, PHP, Python, Ruby, and more.

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Why use HColumnDescriptor class?

The detail regarding column family such as compression settings, Number of versions, are stored .in HColumnDescriptor.

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What is a cell in HBase?

A cell in HBase is the smallest unit of an Hbase table. It helps you to holds a piece of data in the form of a tuple{row, column, version}

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What is a Bloom filter?

HBase supports Bloom Filter helps you to improve the overall throughput of the cluster. An HBase Bloom Filter is a space-efficient mechanism to test whether a HFile includes certain row or row-col cell.

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Tell me about the types of HBase Operations?

Two types of HBase Operations are: Read Operation Write Operation

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What is the use of HBase HMaster?

Main responsibilities of a master are: Coordinating the region servers Admin functions

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Which technique can you use in HBase to access HFile directly without the help of HBase?

To access HFile directly without using HBase, we use HFile.main() method.

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Can the region server will be located on all DataNodes?

Yes, Region Servers run on the same servers as a DataNodes

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Name the filter which accepts the page size as the parameter in HBase?

A filter named PageFilter accepts the page size as the parameter.

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When would you use HBase?

HBase is used in cases where we need random read and write operations and it can perform a number of operations per second on a large data sets. HBase gives strong data consistency. It can handle very large tables with billions of rows and millions of columns on top of commodity hardware cluster.

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What is the use of get method?

get() method is used to read the data from the table.

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Define the difference between Hive and HBase?

Apache Hive is a data warehousing infrastructure built on top of Hadoop. It helps in querying data stored in HDFS for analysis using Hive Query Language (HQL), which is a SQL-like language, that gets translated into MapReduce jobs. Hive performs batch processing on Hadoop. Apache HBase is NoSQL key/value store which runs on top of HDFS. Unlike Hive, HBase operations run in real-time on its database rather than MapReduce jobs. HBase partitions the tables, and the tables are further splitted into column families. Hive and HBase are two different Hadoop based technologies – Hive is an SQL-like engine that runs MapReduce jobs, and HBase is a NoSQL key/value database of Hadoop. We can use them together. Hive can be used for analytical queries while HBase for real-time querying. Data can even be read and written from HBase to Hive and vice-versa.

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Explain the data model of HBase.

HBase comprises of: Set of tables. Each table consists of column families and rows. Row key acts as a Primary key in HBase. Any access to HBase tables uses this Primary Key. Each column qualifier present in HBase denotes attributes corresponding to the object which resides in the cell.

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Define column families?

Column Family is a collection of columns, whereas row is a collection of column families.

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Define standalone mode in HBase?

It is a default mode of HBase. In standalone mode, HBase does not use HDFS—it uses the local filesystem instead—and it runs all HBase daemons and a local ZooKeeper in the same JVM process.

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What is decorating Filters?

It is useful to modify, or extend, the behavior of a filter to gain additional control over the returned data. These types of filters are known as decorating filter. It includes SkipFilter and WhileMatchFilter.

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What is RegionServer?

A table can be divided into several regions. A group of regions is served to the clients by a Region Server.

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What are the data manipulation commands of HBase?

Data Manipulation commands of HBase are: put – Puts a cell value at a specified column in a specified row in a particular table. get – Fetches the contents of a row or a cell. delete – Deletes a cell value in a table. deleteall – Deletes all the cells in a given row. scan – Scans and returns the table data. count – Counts and returns the number of rows in a table. truncate – Disables, drops, and recreates a specified table.

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What happens when you issue a delete command in HBase?

Once you issue a delete command in HBase for cell, column or column family, it is not deleted instantly. A tombstone marker in inserted. Tombstone is a specified data, which is stored along with standard data. This tombstone makes hides all the deleted data. The actual data is deleted at the time of major compaction. In Major compaction, HBase merges and recommits the smaller HFiles of a region to a new HFile. In this process, the same column families are placed together in the new HFile. It drops deleted and expired cell in this process. All the results from scan and get filters the deleted cells.

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What are different tombstone markers in HBase?

There are three types of tombstone markers in HBase: Version Marker: Marks only one version of a column for deletion. Column Marker: Marks the whole column (i.e. all version) for deletion. Family Marker: Marks the whole column family (i.e. all the columns in the column family) for deletion

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HBase blocksize is configured on which level?

The blocksize is configured per column family and the default value is 64 KB. This value can be changed as per requirements.

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Which command is used to run HBase Shell?

./bin/hbase shell command is used to run the HBase shell. Execute this command in HBase directory.

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Which command is used to show the current HBase user?

whoami command is used to show HBase user.

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What is the full form of MSLAB?

MSLAB stands for Memstore-Local Allocation Buffer. Whenever a request thread needs to insert data into a MemStore, it doesn’t allocates the space for that data from the heap at large, but rather allocates memory arena dedicated to the target region.

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Define LZO?

Lempel-Ziv-Oberhumer (LZO) is a lossless data compression algorithm that focuses on decompression speed.

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What is HBase Fsck?

HBase comes with a tool called hbck which is implemented by the HBaseFsck class. HBaseFsck (hbck) is a tool for checking for region consistency and table integrity problems and repairing a corrupted HBase. It works in two basic modes – a read-only inconsistency identifying mode and a multi-phase read-write repair mode.

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What is REST?

Rest stands for Representational State Transfer which defines the semantics so that the protocol can be used in a generic way to address remote resources. It also provides support for different message formats, offering many choices for a client application to communicate with the server.

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What is Nagios?

Nagios is a very commonly used support tool for gaining qualitative data regarding cluster status. It polls current metrics on a regular basis and compares them with given thresholds.

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What is the use of ZooKeeper?

The ZooKeeper is used to maintain the configuration information and communication between region servers and clients. It also provides distributed synchronization. It helps in maintaining server state inside the cluster by communicating through sessions. Every Region Server along with HMaster Server sends continuous heartbeat at regular interval to Zookeeper and it checks which server is alive and available. It also provides server failure notifications so that, recovery measures can be executed.

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Define catalog tables in HBase?

Catalog tables are used to maintain the metadata information.

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Define compaction in HBase?

HBase combines HFiles to reduce the storage and reduce the number of disk seeks needed for a read. This process is called compaction. Compaction chooses some HFiles from a region and combines them. There are two types of compactions.

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Explain what is Hbase?

Hbase is a column-oriented database management system which runs on top of HDFS (Hadoop Distribute File System). Hbase is not a relational data store, and it does not support structured query language like SQL. In Hbase, a master node regulates the cluster and region servers to store portions of the tables and operates the work on the data.

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Explain why to use Hbase?

High capacity storage system Distributed design to cater large tables Column-Oriented Stores Horizontally Scalable High performance & Availability Base goal of Hbase is millions of columns, thousands of versions and billions of rows Unlike HDFS (Hadoop Distribute File System), it supports random real time CRUD operations

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Mention what are Key Components of Hbase?

Zookeeper: It does the co-ordination work between client and Hbase Maser Hbase Master: Hbase Master monitors the Region Server RegionServer: RegionServer monitors the Region Region: It contains in memory data store(MemStore) and Hfile. Catalog Tables: Catalog tables consist of ROOT and META

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What is the Role of Master Server in Hbase?

The Master server assigns regions to region servers and handles load balancing in the cluster.

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Explain what does Hbase consists of?

Hbase consists of a set of tables And each table contains rows and columns like traditional database Each table must contain an element defined as a Primary Key Hbase column denotes an attribute of an object

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What is the Role of Zookeeper in Hbase?

The zookeeper maintains configuration information, provides distributed synchronization, and also maintains the communication between clients and region servers.

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Mention how many operational commands in Hbase?

Operational command in Hbases is about five types: + Get + Put + Delete + Scan + Increment

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When do we need to disable a table in Hbase?

In Hbase a table is disabled to allow it to be modified or change its settings. .When a table is disabled it cannot be accessed through the scan command.

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Explain what is Wal and Hlog in Hbase?

WAL (Write Ahead Log) is similar to MySQL BIN log; it records all the changes occur in data. It is a standard sequence file by Hadoop and it stores HLogkey’s. These keys consist of a sequential number as well as actual data and are used to replay not yet persisted data after a server crash. So, in cash of server failure WAL work as a life-line and retrieves the lost data’s.

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What are the different Types of Filters used in Hbase?

Filters are used to get specific data form a Hbase table rather than all the records. They are of the following types. Column Value Filter Column Value comparators KeyValue Metadata filters. RowKey filters.

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In Hbase what is Column Families?

Column families comprise the basic unit of physical storage in Hbase to which features like compressions are applied.

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Name three disadvantages Hbase has as compared to Rdbms?

Hbase does not have in-built authentication/permission mechanism The indexes can be created only on a key column, but in RDBMS it can be done in any column. With one HMaster node there is a single point of failure.

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Explain what is the Row Key?

Row key is defined by the application. As the combined key is pre-fixed by the rowkey, it enables the application to define the desired sort order. It also allows logical grouping of cells and make sure that all cells with the same rowkey are co-located on the same server.

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Is Hbase a scale out or scale up process?

Hbase runs on top of Hadoop which is a distributed system. Haddop can only scale uo as and when required by adding more machines on the fly. So Hbase is a scale out process.

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Explain deletion in Hbase?

When you delete the cell in Hbase, the data is not actually deleted but a tombstone marker is set, making the deleted cells invisible. Hbase deleted are actually removed during compactions. Three types of tombstone markers are there: + Version delete marker: For deletion, it marks a single version of a column + Column delete marker: For deletion, it marks all the versions of a column + Family delete marker: For deletion, it marks of all column for a column family

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What are the step in writing something into Hbase by a Client?

In Hbase the client does not write directly into the HFile. The client first writes to WAL(Write Access Log), which then is accessed by Memdtore. The Memstore Flushes the data into permanent memory from time to time.

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Explain how does Hbase actually delete a Row?

In Hbase, whatever you write will be stored from RAM to disk, these disk writes are immutable barring compaction. During deletion process in Hbase, major compaction process delete marker while minor compactions don’t. In normal deletes, it results in a delete tombstone marker- these delete data they represent are removed during compaction. Also, if you delete data and add more data, but with an earlier timestamp than the tombstone timestamp, further Gets may be masked by the delete/tombstone marker and hence you will not receive the inserted value until after the major compaction.

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What is compaction in Hbase?

As more and more data is written to Hbase, many HFiles get created. Compaction is the process of merging these HFiles to one file and after the merged file is created successfully, discard the old file.

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Explain what happens if you alter the Block Size of a column Family on an already occupied Database?

When you alter the block size of the column family, the new data occupies the new block size while the old data remains within the old block size. During data compaction, old data will take the new block size. New files as they are flushed, have a new block size whereas existing data will continue to be read correctly. All data should be transformed to the new block size, after the next major compaction.

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What are the different compaction types in Hbase?

There are two types of compaction. Major and Minor compaction. In minor compaction, the adjacent small HFiles are merged to create a single HFile without removing the deleted HFiles. Files to be merged are chosen randomly. In Major compaction, all the HFiles of a column are emerged and a single HFiles is created. The delted HFiles are discarded and it is generally triggered manually.

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What is a Cell in Hbase?

A cell in Hbase is the smallest unit of a Hbase table which holds a piece of data in the form of a tuple{row,column,version}

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What is the Scope of a Rowkey in Hbase?

Rowkeys are scoped to ColumnFamilies. The same rowkey could exist in each ColumnFamily that exists in a table without collision.

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What is the Role of the Class Hcolumndescriptor in Hbase?

This class is used to store information about a column family such as the number of versions, compression settings, etc. It is used as input when creating a table or adding a column.

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What is a Namespace in Hbase?

A Namespace is a logical grouping of tables . It is similar to a database object in a Relational database system.

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What is the Lower Bound of Versions in Hbase?

The lower bound of versions indicates the minimum number of versions to be stored in Hbase for a column. For example If the value is set to 3 then three latest version wil be maintained and the older ones will be removed.

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What is Hotspotting in Hbase?

Hotspotting is a situation when a large amount of client traffic is directed at one node, or only a few nodes, of a cluster. This traffic may represent reads, writes, or other operations. This traffic overwhelms the single machine responsible for hosting that region, causing performance degradation and potentially leading to region unavailability.

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What is Ttl in Hbase?

TTL is a data retention technique using which the version of a cell can be preserved till a specific time period.Once that timestamp is reached the specific version will be removed.

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Why do we Pre create empty Regions?

Tables in HBase are initially created with one region by default. Then for bulk imports, all clients will write to the same region until it is large enough to split and become distributed across the cluster. So empty regions are created to make this process faster.

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Does Hbase support Table Joins?

Hbase does not support table joins. But using a mapreduce job we can specify join queries to retrieve data from multiple Hbase tables.

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Which File in Hbase is designed after Sstable File of Bigtable?

The HFile in Habse which stores the Actual data(not metadata) is designed after the SSTable file of BigTable.

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What is a Hbase Store?

Hbase Store hosts a MemStore and 0 or more StoreFiles (HFiles). A Store corresponds to a column family for a table for a given region.

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What are the Two Types of Table Design Approach in Hbase?

They are: Short and Wide Tall and Thin

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When do we do Manual Region Splitting?

The manual region splitting is done we have an unexpected hotspot in your table because of many clients querying the same table.

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In which scenario should we consider creating a short and wide Hbase Table?

The short and wide table design is considered when there is There is a small number of columns There is a large number of rows

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In Hbase what is Log Splitting?

When a region is edited, the edits in the WAL file which belong to that region need to be replayed. Therefore, edits in the WAL file must be grouped by region so that particular sets can be replayed to regenerate the data in a particular region. The process of grouping the WAL edits by region is called log splitting.

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How does Hbase support Bulk Data Loading?

There are two main steps to do a data bulk load in Hbase: Generate Hbase data file(StoreFile) using a custom mapreduce job) from the data source. The StoreFile is created in Hbase internal format which can be efficiently loaded. The prepared file is imported using another tool like comletebulkload to import data into a running cluster. Each file gets loaded to one specific region.

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Why Multiwal is Needed?

With a single WAL per RegionServer, the RegionServer must write to the WAL serially, because HDFS files must be sequential. This causes the WAL to be a performance bottleneck.

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How does Hbase provide High Availability?

Hbase uses a feature called region replication. In this feature for each region of a table, there will be multiple replicas that are opened in different RegionServers. The Load Balancer ensures that the region replicas are not co-hosted in the same region servers.

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How does Wal help when a Regionserver Crashes?

The Write Ahead Log (WAL) records all changes to data in HBase, to file-based storage. if a RegionServer crashes or becomes unavailable before the MemStore is flushed, the WAL ensures that the changes to the data can be replayed.

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What is Hregionserver in Hbase?

HRegionServer is the RegionServer implementation. It is responsible for serving and managing regions. In a distributed cluster, a RegionServer runs on a DataNode.

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What are the different Block Caches in Hbase?

HBase provides two different BlockCache implementations: the default on-heap LruBlockCache and the BucketCache, which is (usually) off-heap.

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