Block placement and Multi-tenancy – Where is your data?

Most folks tend to think of the Hadoop storage layer as a large hard drive. At a high level I guess this is a fair assumption. The real issue comes to light when one considers actual block placement in Hadoop. Many architects want to design systems for multitenancy using Hadoop as a core part of their design. An HDFS system has a very particular strategy for block placement. Within a single cluster blocks cannot be restricted to a set of hosts (using a default build of HDFS) or even a set of drives within a host. This means that even though users might feel that both HDFS POSIX ACLs level permissions protect data from unauthorized access this only applies to folks using the front door. As they say locks are for honest people. Only users attempting to access your data via Hadoop based methods will be blocked. The unscrupulous could still for instance directly access nodes and therefore the blocks placed in the Linux file system accessed by Hadoop. Hadoop itself leverages the Linux file system. Encryption of this data at rest is a partial answer to this dilemma but this is a very new feature of Hadoop 3 not yet adopted by most distribution vendors. The classical answer has been to engage a third party vendors for a more robust answer to the issue of Hadoop encryption at rest. At the end of the day the block could still be access (isolating direct log in to data node helps) and theoretically decrypted. Im sure this could easily be the topic or a least a sub plot of spy movie.

The other alternative is to simply NOT use HDFS. The MapR FS for example has none of these issues because it essentially is a true file system. Block are not the unit of replication, the Namenode meta data is not an issue as its distributed and one cannot access the MapR FS nor any of its components in any way other than the front door (via the MapR client). Along with the ability to place data not only on specific nodes but also on specific drives within a node MapR FS is really a more elegant solution. For these reasons true data isolation is really guaranteed with MapR FS. With HDFS the correct answer to guarantee data isolation is really an entirely alternate HDFS subsystem (another cluster). HDFS has the concept of Namespaces but that really addresses the small files issue and isnt an enforcement method for data placement on nodes or drives. The comparison on a features by feature basis tends to leave HDFS wanting.

The lesson here is to really study what is included and how components are configured prior to engaging in the use of Hadoop for a Multi-tenant infrastructure.

Hadoop Versioning and Ecosystem Abstraction – The MapR Angle

As a follow up to my previous article on Hadoop versioning I noticed that I left off MapR. There exists a similar logic and structure to the MapR versioning again with some nuance worth explaining. When looking for the version of MapR you are using a simple cat of MAPR_HOME/MapRBuildVersion will show you what you are working with.

This is a simple was to grab the version (5.0.0), the build (32987) and the release state (GA) of the distribution of MapR in use.

But what about Ecosystem products? 

MapR categorizes the packages in a distribution into core packages and ecosystem packages. Packages in the ecosystem would include a package like Hive for example. In RPM based Linux systems one can simply use Yum to investigate whats installed or available.

Yum cleanly separates installed vs available automatically and you can see core vs ecosystem by the repo column. Again the exact verssion is displayed in a similar context.

Core products use


with X.X.X being the major version, Y is the build and Z is the release state. The ecosystem packages are slightly different in that they are listed as


where X.X.X is the version number from the corresponding Apache project and Y is the date.

All that said its always good know not only what version of package you are using but do yourself a favor and be sure to read the release notes in the documentation.

One of the best things about MapR is the abstraction of the ecosystem layer from the core distribution. What does this mean in practice? This means that you have a range of ecosystem package versions to choose from within a version of the core MapR distro. This is often overlooked but is a great example of the forethought that has done into MapR that sometimes seen as “different”. This means if you have a mission critical application running on Hive 0.13 there is NO reason to install a whole new cluster to run Hive 1.0. Ultimately this type of flexibility leaves the user in charge of their own cluster.

MapR Yarn Log Aggregation

MapR sometimes takes some criticism for being “different” which is sometimes translated as “hard to use” because it doesn’t behave like Hadoop distribution brand X that I know and love. I get that. I also know that all software (like people) has warts. Sometimes those differences are intentional. For the most part this is the case with MapR. Lots of folks misunderstand the method to the madness initially. I can say that most users once armed with some understanding tend to love MapR.

That aside I wanted to point out one such difference. It has to to with Yarn Log Aggregation. I hear repeatedly folks saying they enabled log aggregation but for some reason are still seeing a message from the History Server telling them there is a problem.

What typically is happening in this case is that although folks remembered to turn on log aggregation by setting yarn.log-aggregation-enable to true in yarn-site.xml they for about Running is important when installing or changing the configuration of your cluster. It does a handful of things but most importantly it reads xml configuration file changes. There is a log kept for under MAPR_HOME/logs/configure.log showing what has been run and when. The other problem seen is that users forget to pass -“HS” to This lets MapR know where the history server is running and on what port. Without which you will likely get error messages similar to the one above. Once completed you should be able to use the History server web interface to drill into logs OR you can use the yarn logs command line.

The configure script is a small MapR nuance but is critical for proper cluster configuration.  Any time you add or remove software or reconfigure things make sure you take a look at You will see it across all the docs for the installation of Hadoop ecosystem products in MapR.

NOSASL- Hive 1.0 with JDBC connection timeout

So there I was using the new MapR 5.0 sandbox with Hive 1.0. A quick test of JDBC connectivity using beeline

Follow by an indefinite hang. So off I go to check logs. Hive.log, HS2 log and so on. No real output of consequence. I took a look at and turned the log level to DEBUG.

Again followed by an indefinite hang. Everything appeared to be running correctly. Nothing in the logs…To google with you! I found HIVE-6852 which describes an issue on the client with the SASL setting. The work around is described as adding “;auth=noSasl” which seemed to resolve the problem. Since so many folks I know use JDBC I thought this was worth a quick writeup.

NOSASL is picked up in hive-site.xml via the hive.server2.authentication property. It has three basic options NONE, NOSASL and KERBEROS. Setting NONE for hive.server.2.authenticaion in hive-site.xml would also make auth=noSasl not required.

From a  MapR perspective there is the concept of secure mode which when enabled secures connections via maprsasl mode (which is distinct from kerberos mode). Since the sandbox is configured without secure mode or kerberos setting the authentication mode is required.

Connecting to MapR via JDBC

I recently needed to do some connection testing via JDBC to Hiveserver 2 in MapR 4.0.2 in a secure and kerberized cluster. Since this was new to me I thought it would be worthwhile to write-up my notes as I will sure forget half of this in less than a few weeks. Just in case you are needing to connect via JDBC to a Kerberized cluster here are some tips.

MapR has its own login system called maprlogin for use in a secure cluster setup that requires authentication via password or Kerberos to use the cluster.

Once completed users can act on the cluster via the command line normally via the hadoop commands (or through the Linux command line in NFS mounted MapR-FS).

For JDBC connections users must use the principal configured by the Kerberos and Hadoop Admins for the hive service (not their user principal – that is used in the previous step). See the setting “hive.server2.authentication.kerberos.principal” along with the matching keytab specified in “hive.server2.authentication.kerberos.keytab” which in MapR is typically under /opt/mapr/conf/mapr.keytab.

As a quick refresher here is what we are dealing with in terms of a Kerberos principal for a JDBC connection string:


HS2.FQDN – Hive server 2 fully qualified Domain name and port – Default is 10000

primary – username – user must exist on Hiveserver2 node. You will normally see a user principal that matches the service name – i.e., hive, yarn etc.

Instance – use a FQDN that is resolvable both forward and reverse. This is a property of Apache Hive not specific to the MapR distribution (Hive version 0.13 was used in this testing against MapR 4.0.2). Hive attempts to resolve the hostname (in green above) when connecting via Kerberos.

Realm – correct Kerberos Realm name – see your local /etc/krb5.conf or consult your Kerberos Administrator.

And here are some examples of how to connect. Beeline is a Hive service you can use that is built into Hive. There are many tools that use a similar methods of connection that can be used.

You may need to either call the beeline client with a fake user name and password or hit enter twice interactively. This is a known bug.

Java Client



HDFS ACLs – Managing Data from the ground up.

HDFS and the permissions placed upon it are the first line of authorization in Hadoop. Every other service that relies upon HDFS in Hadoop must adhere to the permissions enforced by HDFS. This is the place to start when concerned about the security of your Hadoop system. One simply needs to apply the principle of least privilege just like with everything else in IT. Many people fret about Kerberos for authorization and encryption of data at rest and in motion, all of which are worthy discussions but ignore the basic principles of the file system (permissions and quotas) as one of the easiest way to both restrict and selectively share information.

A cool new feature in HDFS inclusion of  ACLs.The full documentation is provided here but I always like to write the guide for the impatient aka quick start articles. I wont visit every detail of this feature. Using classic users and groups from Linux (POSIX) you could restrict access to files and directories in HDFS using a pretty standard permissions model with a few exceptions like the execute bit (no executable files in HDFS) and setuid/setgid but the sticky bit is still there.


Once defined, two users and two unique groups in Linux (do this on the Namenode – this is where the Namenode picks up group membership) a quick investigation of the ACLs could be  launched. Many people ask “How do I enable complex access multiple groups and users?” HDFS was recently extended to support POSIX to support more complex use cases. So when user Bob attempts to write to user Joe’s home directory access is denied. Notice the plus sign now indicating some extended ACLs have been set on a directory.

Although you can see the actual ACLs in the permission denied error a slightly closer look at the extended ACLs shows the reason more clearly:

That directory is owned by user Joe and even though Joe is in poc_group2 that group does not have write permissions. That group can list the contents of the Joe’s home but unless you are in poc_group1 with full access or poc_group2 with read access you can see anything in Joe’s home (group other is set to no access). Another cool thing to notice is use of the mask setting. One could also set mask that overrides the other settings such as:

In this case you end up with an effective setting of read only since the mask now overrides the permissions of the ACL. The other interesting point is that the “default” settings have to do with the behavior of the child objects.

So what? What does all this have to do with anything. Well for starters it’s a great way to exert basic control over your HDFS layout. If you want to stop folks from simply filling up your file system with junk this is great way to stop that (along with Quotas). Lots of folks in the market talk about adopting Hadoop as a challenge due to lack of control over the environment. Hadoop permission and ACLs are the lowest levels of control one has over what is placed in Hadoop and probably one of the least well understood.


Hadoop Encryption at Rest

At Rest, as in not motion, not REST as in web services. For a long time the only real answer Hadoop had for encryption at rest was to leverage a third-party tool or consider the use of LUKS for whole disk encryption. What I see customers asking for these days is really encryption in motion (aka wire encryption), encryption at rest (at a HDFS layer) plus policies that will eliminate data from specific directories in HDFS based upon some business rule. The good news is that as of Hadoop 2.6 we now have HDFS-6134 in play so there is light at the end of the security tunnel.

The implementation of this new transparent encryption is supported via the normal Hadoop Filesystem Java API, the libhdfs C API and WebHDFS (REST) API. The great news is that once it is set up normal HDFS ACL control access to reading and writing so while there is some administration upfront from a user perspective there is not a terribly large new burden. This essentially means that third-party integration work should be largely left intact.

There is now a Key Management Server (KMS) used to create keys for the encryption process of “encryption zones” also know as directories in HDFS.

So how does all this happen? The design doc describes both the read and write action processes. Illustrated here is the read process:


So how does one functionally use encryption zones. Cloudera has a great docs page talking about how to create encryption zones based upon the technology used (over hdfs).

Like most newly invented technology the design doc also calls out some potential issues with the design. While there are some potential vulnerabilities called out in the spec I would still say this is a massive step in the right direction. I also noted this first version really only uses AES-CTR.

It might seem like this is small matter but in the larger context of a security discussion native encryption at rest is an important part of the Hadoop puzzle.


Hadoop and the mystery of the version number

When I’m working with people on Hadoop I ask what you would think is a simple question. What version of Hadoop are you using? The answer normally is one of several attempts to explain what’s installed including –


Answer Translation
Hortonworks/Cloudera This is my Hadoop Distribution.
Hortonworks 2 I know we aren’t using version 1.
Hadoop 2 I dont know my distro but I’m using Hadoop 2.
Apache someone else is working this. I have no idea.

In reality though it’s not as straight forward as you might think. I think the easiest way to get the most bang for your buck is to simply take a look at the version number of the package installed. So on yum based systems you could simply do

and get back of list of whats installed and whats available. You could also simply query the rpm database:

If you run SLES you will need to do zypper and on windows look at your add/remove programs dialog on most major newer versions of windows. In the end you are still left with this cryptic string to decode. If you look closely there is a method to the madness and it helps to know this level of detail when working in an area like Hadoop where minor version numbers or a build number could make all the difference.

For example:
package nameversionarchitecture

The version number in this case is from a Hortonworks distribution so  we have a seven digit (8 places) version number.

package versionHDP Versionbuild number 385

It’s important to know both the version of Hadoop and the version of the package you are working on. For example if someone says “I’m working on Hive”. You really need to know what hive version AND what Hadoop version because the two are intimately linked. If someone gives you the hive package string:

It’s really not enough information for you to tell what version of Hadoop someone is using. You know they are using HDP so one either asks for the same information on the Hadoop package installed OR goes to the release notes for the distro to decode the distribution version number into the Apache Hadoop version. Each distribution uses a different combination of packages and it pays to know EXACTLY what you are getting when you download a distro. Cloudera has exactly the same issues and their packaging may in fact be even more forthcoming in that they tell you how many patches were applied. Hortonworks does this in the context of their release notes.

package namepackage version+CDH version+patches


Hopefully now you have a better understanding of Hadoop package versions.


Hive with JSON data

I stumbled across this and thought it would be helpful to write this up to save everyone else some time. So I went to use JSON with Hive 13 for what I thought was a pretty simple use case of creating a table with JSON data. I was looking for the right SerDe and stumbled across this blog entry stating that we should use the code from this github repo to make a jar that works with Hive 0.13. So here we go…

Sigh…so after some searching I stumbled across another few blog posts and finally a github repo fork that I cloned and built to create a jar that works with Hive 13 and Hadoop 2.4.

Ahhhh. So much better. I am using the latest HDP 2.1 sandbox for writing code so my packages are:

I will create another blog post (and link it here) to explain the version numbers of the packages in HDP.

Many Thanks to KunBetter who saved the day for us in our work at a recent customer.


This saved us many hours of aggravation. Open Source works. Give it a try. Someday someone on the other side of the planet may have the answer you need.