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CCB-400 Examination questions (September)

Achieve New Updated (September) Cloudera CCB-400 Examination Questions 1-10

September 24, 2015

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Exam A

 

QUESTION 1

Your HBase cluster has hit a performance wall and doesn’t seem to be getting faster as you add

 

RegionServers. Adding an additional HMaster will:

 

A.

Have no effect on performance.

B.

Improve the performance of region writes but decrease the performance of metadata changes.

C.

Improve the performance of metadata chancier, but decrease the performance of region writes.

D.

Make the performance problem even worse, as operations will have to be replicated to multiple masters.

 

Answer: A

Explanation: You can add multiple HBase master nodes; however, only one HBase master node is active at a time. The active HBase master node changes only when the current active HBase master node is shut down or fails.

 

 

QUESTION 2

Yon are storing page view data for a large number of Web sites, each of which has many subdomains (www.example.com, archive.example.com, beta.example.com, etc.) Your reporting tool needs to retrieve the total number of page views for a given subdomain of a Web site. Which of the following rowkeys should you use?

 

A.

The reverse domain name (e.g., com.example.beta)

B.

The domain name followed by the URL

C.

The URL

D.

The URL followed by the reverse domain name

 

Answer: A

Explanation: Consider a table whose keys are domain names. It makes the most sense to list them in reverse notation (so “com.jimbojw.www” rather than “www.jimbojw.com”) so that rows about a subdomain will be near the parent domain row. Continuing the domain example, the row for the domain “mail.jimbojw.com” would be right

 

 

 

 

 

next to the row for “www.jimbojw.com” rather than say “mail.xyz.com” which would happen if the keys were regular domain notation.

 

Reference: Understanding HBase and BigTable

 

 

QUESTION 3

Data is written to the HLog in which of the following orders?

 

A.

In order of writes

B.

In order of writes, separated by region

C.

Ascending first by region and second by row key

D.

Descending first by region and second by row key

 

Answer: D

 

 

QUESTION 4

You have an average key-value pair size of 100 bytes. Your primary access is random needs on the table. Which of the following actions will speed up random reading performance on your cluster?

 

A.

Turn off WAL on puts

B.

Increase the number of versions kept

C.

Decrease the block size

D.

Increase the block size

 

Answer: C

Explanation: Larger block size is preferred if files are primarily for sequential access. Smaller blocks are good for random access, but require more memory to hold the block index, and may be slower to create

 

Reference: Could I improve HBase performance by reducing the hdfs block size?

 

 

 

 

 

 

QUESTION 5

You have two tables in an existing RDBMS. One table contains order information (item, quantity, price, etc.) and the other contains store information (address, phone, manager, etc). These two tables are not often accessed simultaneously. You would like to move this data into HBase. How would you design the schema?

 

A.

Create two tables each with a single column family

B.

Create a single table with one column family

C.

Create a single table with two column families

D.

Create two tables each with multiple column families

 

Answer: C

Explanation: Physically, all column family members are stored together on the filesystem. Because tunings and storage specifications are done at the column family level, it is advised that all column family members have the same general access pattern and size characteristics.

 

Reference:The Apache HBaseReference Guide,Column Family

 

 

QUESTION 6

From within an HBase application, you want to retrieve two versions of a row, if they exist. Where your application should configure the maximum number of versions to be retrieved?

 

A.

HTableDescriptor

B.

HTable

C.

Get or scan

D.

HColumnDescriptor

 

Answer: D

Explanation: maxVersions – Maximum number of versions to keep

 

Note:

*public HColumnDescriptor(byte[] familyName,

int maxVersions,

String compression,

 

 

 

 

 

boolean inMemory,

boolean blockCacheEnabled,

int timeToLive,

String bloomFilter)

 

*

An HColumnDescriptor contains 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. Once set, the parameters that specify a column cannot be changed without deleting the column and recreating it. If there is data stored in the column, it will be deleted when the column is deleted.

 

Reference:org.apache.hadoop.hbase,Class HColumnDescriptor

 

 

QUESTION 7

Your client application connects to HBase for the first time to perform a write. Which of the following sequences will it traverse to find the region serving the row range of interest?

 

A.

ZooKeeper -> RegionServer -> Region

B.

ZooKeeper ->.META. -> RegionServer -> Region

C.

ZooKeeper -> ROOT -> .META. -> RegionServer -> Region

D.

ZooKeeper -> HMaster -> -ROOT- -> .META. -> RegionServer -> Region

 

Answer: C

Explanation: The general flow is that a new client contacts the Zookeeper quorum (a separate cluster of Zookeeper nodes) first to find a particular row key. It does so by retrieving the server name (i.e. host name) that hosts the -ROOT- region from Zookeeper. With that information it can query that server to get the server that hosts the .META. table. Both of these two details are cached and only looked up once. Lastly it can query the .META. server and retrieve the server that has the row the client is looking for.

 

Note:

*The .META. table keeps a list of all regions in the system. The .META. table structure is as follows:

 

Key:

 

 

 

 

 

Region key of the format ([table],[region start key],[region id]) Values:

 

info:regioninfo (serialized HRegionInfo instance for this region) info:server (server:port of the RegionServer containing this region) info:serverstartcode (start-time of the RegionServer process containing this region)

 

Reference:HBase Architecture 101 – Storage

 

 

QUESTION 8

You have two standbys and one primary HMaster. Your primary HMaster fails. Which of the remaining HMasters becomes the new primary?

 

A.

Whichever HMaster first responds to ZooKeeper

B.

Whichever HMaster ZooKeeper randomly selects

C.

Whichever HMaster creates the znode first

D.

Whichever HMaster has the lower IP address

 

Answer: C

Explanation: * The Hbase master server creates the zookeeper znode /hbase . This is then used for hbase daemons to coordinate. Even the name of the active Hbase master is stored here. If the hbase master dies, the backup hbase master overwrites the contents of the znode so clients and region servers know about the new master. Apart from this, region info is maintained in zookeeper znodes as well.

* Multi-master feature introduced in 0.20.0 does not add cooperating Masters; there is still just one working Master while the other backups wait. For example, if you start 200 Masters only 1 will be active while the others wait for it to die. The switch usually takes zookeeper.session.timeout plus a couple of seconds to occur.

 

 

QUESTION 9

You need to create a “WebLogs” table in HBase. The table will consist of a single Column Family called “Errors” and two column qualifiers, “IP” and “URL”. The shell command you

 

 

 

 

should use to create the table is:

 

A.

create ‘WebLogs’, {NAME => ‘Errors:IP’, NAME =>’Errors:URL’}

B.

create ‘WebLogs’, ‘Errors’ {NAME => ‘IP’, NAME => ‘URL’}

C.

create ‘WebLogs’, ‘Errors:IP’, ‘Errors:URL’

D.

create ‘WebLogs’, ‘Errors’

 

Answer: C

Explanation: Columns in Apache HBase are grouped into column families. All column members of a column family have the same prefix. For example, the columns courses:history and courses:math are both members of the courses column family. The colon character (:) delimits the column family from the column qualifier . The column family prefix must be composed of printable characters. The qualifying tail, the column family qualifier, can be made of any arbitrary bytes. Column families must be declared up front at schema definition time whereas columns do not need to be defined at schema time but can be conjured on the fly while the table is up an running.

 

Physically, all column family members are stored together on the filesystem. Because tunings and storage specifications are done at the column family level, it is advised that all column family members have the same general access pattern and size characteristics.

 

 

QUESTION 10

You need to free up disk space on your HBase cluster. You delete all versions of your data that is older than one week. You notice your delete has had minimal impact on your storage availability. This is because:

 

A.

You have large store file indexes

B.

HBase has not flushed the MemStore

C.

HBase has not run a minor compaction

D.

HBase has not run a major compaction

 

Answer: D

Explanation: The actual deletion of the excess versions is done upon major compaction.

 

Note: HBase basically never overwrites data but only appends. The data files are rewritten once in while by a compaction process. A data file is basically a list of key-value pairs,

 

 

 

 

 

where the key is the composite {row key, column key, time}. Each time you do a put that writes a new value for an existing cell, a new key-value pair gets appended to the store. Even if you would specify an existing timestamp. Doing lots of updates to the same row in a short time span will lead to a lot of key-value pairs being present in the store. Depending on the garbage collection settings (see next), these will be removed during the next compaction.

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