On Error Messages

Here’s a pet peeve of mine: Customers who don’t read the error messages. The usual symptom is a belief that there is just on error: “Doesn’t work”, and that all forms of “doesn’t work” are the same. So if you tried something, got an error, your changed something and you are still getting an error, nothing changed.

I hope everyone who reads this blog understand why this behavior makes any troubleshooting nearly impossible. So I won’t bother to explain why I find this so annoying and so self defeating. Instead, I’ll explain what can we, as developers, can do to improve the situation a bit. (OMG, did I just refer to myself as a developer? I do write code that is then used by customers, so I may as well take responsibility for it)

Here’s what I see as main reasons people don’t read error messages:

  1. Error message is so long that they don’t know where to start reading. Errors with multiple Java stack dumps are especially fun. Stack traces are useful only to people who look at the code, so while its important to get them (for support), in most cases your users don’t need to see all that very specific information.
  2. Many different errors lead to the same message. The error message simply doesn’t indicate what the error may be, because it can be one of many different things. I think Kerberos is the worst offender here, so many failures look identical. If this happens very often, you tune out the error message.
  3. The error is so technical and cryptic that it gives you no clue on where to start troubleshooting.  “Table not Found” is clear. “Call to localhost failed on local exception” is not.

I spend a lot of time explaining to my customers “When <app X> says <this> it means that <misconfiguration> happened and you should <solution>”.

To get users to read error messages, I think error messages should be:

  1. Short. Single line or less.
  2. Clear. As much as possible, explain what went wrong in terms your users should understand.
  3. Actionable. There should be one or two actions that the user should take to either resolve the issue or gather enough information to deduce what happened.

I think Oracle are doing a pretty good job of it. Every one of their errors has an ID number, a short description, an explanation and a proposed solution. See here for example: http://docs.oracle.com/cd/B28359_01/server.111/b28278/e2100.htm#ORA-02140

If we don’t make our errors short, clear and actionable – we shouldn’t be surprised when our users simply ignore them and then complain that our app is impossible to use (or worse – don’t complain, but also don’t use our app).




Kafka or Flume?

A question that keeps popping up is “Should we use Kafka or Flume to load data to Hadoop clusters?”

This question implies that Kafka and Flume are interchangeable components. It makes as much sense to me as “Should we use cars or umbrellas?”. Sure, you can hide from the rain in your car and you can use your umbrella when moving from place to place. But in general, these are different tools intended for different use-cases.

Flume’s main use-case is to ingest data into Hadoop. It is tightly integrated with Hadoop’s monitoring system, file system, file formats, and utilities such a Morphlines. A lot of the Flume development effort goes into maintaining compatibility with Hadoop. Sure, Flume’s design of sources, sinks and channels mean that it can be used to move data between other systems flexibly, but the important feature is its Hadoop integration.

Kafka’s main use-case is a distributed publish-subscribe messaging system. Most of the development effort is involved with allowing subscribers to read exactly the messages they are interested in, and in making sure the distributed system is scalable and reliable under many different conditions. It was not written to stream data specifically for Hadoop, and using it to read and write data to Hadoop is significantly more challenging than it is in Flume.

To summarize:
Use Flume if you have an non-relational data sources such as log files that you want to stream into Hadoop.
Use Kafka if you need a highly reliable and scalable enterprise messaging system to connect many multiple systems, one of which is Hadoop.

Quick Tip on Using Sqoop Action in Oozie

Another Oozie tip blog post.

If you try to use Sqoop action in Oozie, you know you can use the “command” format, with the entire Sqoop configuration in a single line:

<pre><workflow-app name="sample-wf" xmlns="uri:oozie:workflow:0.1">
    <action name="myfirsthivejob">
        <sqoop xmlns="uri:oozie:sqoop-action:0.2">
            <command>import  --connect jdbc:hsqldb:file:db.hsqldb --table TT --target-dir hdfs://localhost:8020/user/tucu/foo -m 1</command>
        <ok to="myotherjob"/>
        <error to="errorcleanup"/>

This is convenient, but can be difficult to read and maintain. I prefer using the “arg” syntax, with each argument in its own line:

<workflow-app name="sample-wf" xmlns="uri:oozie:workflow:0.1">
    <action name="myfirsthivejob">
        <sqoop xmlns="uri:oozie:sqoop-action:0.2">
        <ok to="myotherjob"/>
        <error to="errorcleanup"/>

As you can see, each argument here is in its own “arg” tag. Even two arguments that belong together like “–table” and “TT” go in two separate tags.
If you’ll try to put them together for readability, as I did, Sqoop will throw its entire user manual at you. It took me a while to figure out why this is an issue.

When you call Oozie from the command line, all the arguments you pass are sent as a String[] array, and the spaces separate the arguments into array elements. So if you call Sqoop with “–table TT” it will be two elements, “–table” and “TT”.
When using “arg” tags in Oozie, you are basically generating the same array in XML. Oozie will turn the XML argument list into an array and pass it to Sqoop just the way it would in the command line. Then Sqoop parses it in exactly the same way.
So every item separated with space in the command line must be in separate tags in Oozie.

Its simple and logical once you figure out why 🙂
If you want to dig a bit more into how Sqoop parses its arguments, it is using Apache Commons CLI with GnuParser. You can read all about it.

Why Oozie?

Thats a really frequently asked question. Oozie is a workflow manager and scheduler. Most companies already have a workflow schedulers – Activebatch, Autosys, UC4, HP Orchestration. These workflow schedulers run jobs on all their existing databases – Oracle, Netezza, MySQL. Why does Hadoop need its own special workflow scheduler?

As usual, it depends. In general, you can keep using any workflow scheduler that works for you. No need to change, really.
However, Oozie does have some benefits that are worth considering:

  1. Oozie is designed to scale in a Hadoop cluster. Each job will be launched from a different datanode. This means that the workflow load will be balanced and no single machine will become overburdened by launching workflows. This also means that the capacity to launch workflows will grow as the cluster grows.
  2. Oozie is well integrated with Hadoop security. This is especially important in a kerberized cluster. Oozie knows which user submitted the job and will launch all actions as that user, with the proper privileges. It will handle all the authentication details for the user as well.
  3. Oozie is the only workflow manager with built-in Hadoop actions, making workflow development, maintenance and troubleshooting easier.
  4. Oozie UI makes it easier to drill down to specific errors in the data nodes. Other systems would require significantly more work to correlate jobtracker jobs with the workflow actions.
  5. Oozie is proven to scale in some of the world’s largest clusters. The white paper discusses a deployment at Yahoo! that can handle 1250 job submissions a minute.
  6. Oozie gets callbacks from MapReduce jobs so it knows when they finish and whether they hang without expensive polling. No other workflow manager can do this.
  7. Oozie Coordinator allows triggering actions when files arrive at HDFS. This will be challenging to implement anywhere else.
  8. Oozie is supported by Hadoop vendors. If there is ever an issue with how the workflow manager integrates with Hadoop – you can turn to the people who wrote the code for answers.

So, should you use Oozie? If you find these benefits compelling, then yes. Step out of your comfort zone and learn another new tool. It will be worth it.

Parameterizing Hive Actions in Oozie Workflows

Very common request I get from my customers is to parameterize the query executed by a Hive action in their Oozie workflow.
For example, the dates used in the query depend on a result of a previous action. Or maybe they depend on something completely external to the system – the operator just decides to run the workflow on specific dates.

There are many ways to do this, including using EL expressions, capturing output from shell action or java action.
Here’s an example of how to pass the parameters through the command line. This assumes that whoever triggers the workflow (Human or an external system) has the correct value and just needs to pass it to the workflow so it will be used by the query.

Here’s what the query looks like:

insert into test select * from test2 where dt=${MYDATE}

MYDATE is the parameter that allows me to run the query on a different date each time. When running this query in hive, I’d use something like “set MYDATE=’2011-10-10′” before running the query. But when I run it from Oozie, I need to pass the parameter to the Hive action.

Lets assume I saved the query in a file hive1.hql. Here’s what the Oozie workflow would look like:

<workflow-app name="cmd-param-demo" xmlns="uri:oozie:workflow:0.4">
	<start to="hive-demo"/>
	<action name="hive-demo">
		<hive xmlns="uri:oozie:hive-action:0.2">
		<ok to="end"/>
		<error to="kill"/>
	<kill name="kill">
		<message>Action failed, error message[${wf:errorMessage(wf:lastErrorNode())}]</message>
	<end name="end"/>

The important line is “MYDATE=${MYDATE}”. Here I translate an Oozie parameter to a parameter that will be used by the Hive script. Don’t forget to copy hive-site.xml and hive1.hql to HDFS! Oozie actions can run on any datanode and will not read files on local file system.

And here’s how you call Oozie with the commandline parameter:
oozie job -oozie http://myserver:11000/oozie -config ~/workflow/job.properties -run -verbose -DMYDATE=’2013-11-15′

Thats it!

Using Oozie in Kerberized Cluster

In general, most Hadoop ecosystem tools work rather transparently in a kerberized cluster. Most of the time things “just work”. This includes Oozie. Still, when things don’t “just work”, they tend to fail with slightly alarming and highly ambiguous error messages. Here are few tips for using Oozie when your Hadoop cluster is kerberized. Note that this is a client/user guide. I assume you already followed the documentation on how to configure the Oozie server in the kerberized cluster (or you are using Cloudera Manager, which magically configures it for you).

    1. As always, use “kinit” to authenticate with Kerberos and get your tgt before trying to run oozie commands. Verify with klist. Failure to do this will result in “No valid credentials provided (Mechanism level: Failed to find any Kerberos tgt)”
    2. I always enable security debug messages. This helps me troubleshoot, and also helps when I need to ask support/mailing list for help.
      export HADOOP_ROOT_LOGGER=TRACE,console;
      export HADOOP_JAAS_DEBUG=true;
      export HADOOP_OPTS="-Dsun.security.krb5.debug=true"
    3. Your Oozie command typically contains a URL. Something like “oozie -url http://myserver:11000/oozie -conf job.properties -run” The server name in the URL must match an existing principal name in Kerberos. If your principals are actually “myserver.mydomain.com” make sure you use that in the URL.
    4. If you decide to use CURL to connect to your Oozie server, either for troubleshooting or for using the REST API, don’t forget to use “–negotiate -u foo:bar”. The actual username and password don’t matter (you are authenticating with your Kerberos ticket), but CURL throws a fit if they don’t exist.
    5. If you have Hive action in your Oozie workflow, you need to define and use credentials. Here’s an example:
      <workflow-app xmlns="uri:oozie:workflow:0.2.5" name="example-wf">
                      <credential name='hive_credentials' type='hcat'>
      <start to="hive-example"/>
      <action name="hive-example" cred="hive_credentials">
              <hive xmlns="uri:oozie:hive-action:0.2">
              <ok to="end"/>
              <error to="fail"/>
      <kill name="fail">
              <message>Workflow failed, error message[${wf:errorMessage(wf:lastErrorNode())}]</message>
      <end name="end"/>
    6. To make step #5 actually work (i.e. allow Oozie to run Hive actions), you will also need to do the following:
      In CM:
    7. go to “HDFS Service->Configuration->Service-Wide->Advanced->Cluster-wide Configuration Safety Valve for core-site.xml” and add:


      – go to “Oozie service->Configuration->Oozie Server(default)->Advanced-> Oozie Server Configuration Safety Valve for oozie-site.xml” and add:


      – Deploy client configuration and restart Hive service and Oozie service.

    8. Oozie doesn’t kinit a user for you on the node its launching the action on, and it doesn’t move principles and tickets around. Instead it uses delegation tokens. If you want to authenticate to Hadoop inside a shell or java action, you’ll need to use the same tokens.

      In a shell action, it will be something like:

      	hive -e "select x from test" -S
      	hive -e "SET mapreduce.job.credentials.binary=$HADOOP_TOKEN_FILE_LOCATION; select x from test" -S

      In Java it will be:

      if (System.getenv("HADOOP_TOKEN_FILE_LOCATION") != null) {
                  jobConf.set("mapreduce.job.credentials.binary", System.getenv("HADOOP_TOKEN_FILE_LOCATION"));

    Hope this helps! Feel free to comment with questions, especially if you ran into errors that I did not address. I’ll be happy to add more tips to the list.

Three Things To Do Before Starting Hadoop Project

I spent the last 6 month helping customers design, implement and deploy successful Hadoop systems. Over time, you start seeing patterns. Certain things that if the customer gets right before the project event starts increase the probability that the project will finish successfully and on-time.

Lets assume you already took the most important step – you have an actual use-case or a problem to solve, and you think  Hadoop could be the right technology to use in this case. What now?

  1. Learn how to automate production deployments: 
    Yes, Cloudera Manager would do a lot for you. But occasionally you’ll need to run the same command on 20+ servers. Thats what large clusters are all about. So get used to the idea from the beginning. Learn how to write loops in bash, how to ssh into remote machines to run commands, how to distribute files and restart services.
    You can build your own tools (I had for years), but these days you can just pick your favorite automation tool and use that instead. My favorite is Ansible. It works exactly the way I would have written a cluster automation tool, so learning it never felt like an effort and its usage is never surprising or unintuitive to me. Others prefer Puppet, Chef or Cfengine. It doesn’t matter what you use, but when I show up at your office as your Cloudera Solutions Architect and ask you to update sysctl.conf on your 50 node cluster, I don’t want you too look surprised, alarmed or tell me it will take few hours.
  2. Don’t try to boil the ocean:
    Hadoop implementation is often the first chance the dev/ops teams get to do something completely new. There is a blank slate, white sheet of paper, and you can design the perfect system. Fixing every problem the old system had and building functionality you always dreamed of.Better security!  Machine learning! Open source!
    I say – Using Hadoop successfully is a large and challenging project. Changing organizational processes and culture toward better security and processes is a large and challenging project. Creating a data driven organization is a huge project. Mixing them doesn’t give you three projects for the price of one. It isn’t even just three times more challenging than one project, I’d say the risk is an order of magnitude higher, and the risk of just implementing Hadoop is high enough. Especially in the early stages. Which brings us to… 
  3. Do a POC:
     Pay Cloudera or do it yourself. Either way, you need a POC. 
    If you start with a 12 month project, you will have to do a lot of design upfront. At a time when you have too little information. At the beginning of a large project, you won’t know for sure how the system will be used and you probably won’t know enough about Hadoop. Sure, you can call Cloudera Services and discuss the design with us, but even we can sometimes (rarely!) get things wrong. With 12 month projects you will be very deep in the project before you’ll find that limitation we completely forgot to mention.
    Be agile (really agile, 6 month project with daily scrum doesn’t count): Solve the smallest useful problem first. Implement just a single workflow, single statistical analysis, parse and search data from one source. Whatever is useful for your users – do it first. Learn in the process and build from there. This will allow you to build experience and iron-out issues at the system’s usefulness, load and importance grow.
  4. (Bonus tip) Get the most out of the POC: 
    Not all POCs are created equal. Sometimes the customer hires us to “prove that Hadoop can do X”. We get very specific requirements and very short time-frame, and we need to build a system that does X. I can see why customers need proof that their vendor can deliver. But this approach is of limited value. Because at the end of the POC you are left with a non-production system and you don’t know more Hadoop than you knew before. I love teaching, but when the attitude is “prove us this works” and the requirements are inflexible, there isn’t much time for discussions and casual learning. Most of the knowledge transfer will only happen in the delivery document, which is not the same as lively discussions.
    Better POC happens when a customer brings in Cloudera to help them build their first Hadoop project. There are still time and scope constraints, but now the POC is not about “Prove us this works” but rather “Help us make it work”. We work together as a team. We will brainstorm design possibilities with you and share best practices. We will teach you how to build the system, how to configure it and troubleshoot it. You get a chance to learn all our little tricks of development and deployment that makes life easier. At the end of the POC, your team will have real Hadoop expertise, relevant to your specific system, problems, culture, data and requirements. I see this as the best investment you can make in Hadoop for your organization. 
    But I may be a bit biased.


Control Charts

Last week, while working on customer engagement, I learned a new method of quantifying behavior of time-series data. The method is called “Control Chart” and credit to Josh Wills, our director of data science, for pointing it out. I thought I’ll share it with my readers as its easy to understand, easy to implement, flexible and very useful in many situations.

The problem is ages old – you collect measurements over time and want to know when your measurements indicate abnormal behavior. “Abnormal” is not well defined, and thats on purpose – we want our method to be flexible enough to match what you define as an issue.

For example, lets say Facebook are interested in tracking usage trend for each user, catching those with decreasing use

There are few steps to the Control Chart method:

  1. Collect all relevant data points. In our case, number of minutes of Facebook use per day for each user.
  2. Calculate a baseline – this can be average use for each user or average use for similar demographics.  Even adaptive average of the type calculated by Oracle Enterprise Manager, to take into account decreased Facebook use over the weekend.
  3. Calculate “zones” one, two and three standard deviations around the baseline

Those zones can be used to define rules for normal and abnormal behaviors of the system. These rules are what makes the system valuable.
Examples of rules that define abnormal behavior can be:

  1. Any point 3 standard deviations above the baseline. This will indicate extreme sudden increase.
  2. 7 consecutive measurements more than one standard deviation over the baseline. This indicates a sustained increase.
  3. 9 consecutive measurement each higher than previous one. This indicates steady upward trend.
  4. 6 consecutive measurements each more than two standard deviations away from baseline, each one of different side of the baseline than the previous measurement. This indicates instability of the system.

There are even sets of standard rules used in various industries, best practices of sorts. Western Electric rules and Nelson rules are particularly well know.

Note how flexible the method is – you can use any combination of rules that will highlight abnormalities you are interested in highlighting.
Also note that while the traditional use indeed involves charts, the values and rules are very easy to calculate programmatically and visualization can be useful but not mandatory.
If you measure CPU utilization on few servers, visualizing the chart and actually seeing the server behavior can be useful. If you are Facebook and monitor user behavior, visualizing a time series of every one of their millions of users is hopeless. Calculating baselines, standard deviations and rules for each use is trivial.

Also note how this problem is “embarrassingly parallel“. To calculate behavior for each user, you only need to look at data for that particular user. Parallel, share-nothing platform like Hadoop can be used to scale the calculation indefinitely simply by throwing increasing number of servers on the problem. The only limit is the time it takes to calculate the rules for a single user.

Naturally, I didn’t dive into some of the complexities in using Control Charts.  Such as how to select a good baseline, how to calculate standard deviation (or whether to use another statistic to define zones) and how many measurements should be examined before a behavior signals a trend. If you think this tool is useful for you, I encourage you to investigate more deeply.


Big Data News from Oracle OpenWorld 2013

Only a week after Oracle OpenWorld concluded and I already feel like I’m hopelessly behind on posting news and impressions. Behind or not, I have news to share!

The most prominent feature announced at OpenWorld is the “In-Memory Option”  for Oracle Database 12c.  This option is essentially a new part of the SGA that caches tables in column formats. This is expected to make data warehouse queries significantly faster and more efficient. I would have described the feature in more details, but Jonathan Lewis gave a better overview in this forum discussion, so just go read his post.

Why am I excited about a feature that has nothing to do with Hadoop?

First, because I have a lot of experience with large data warehouses. So I know that big data often means large tables, but only few columns used in each query. And I know that in order to optimize these queries and to avoid expensive disk reads every time each query runs, we build indexes on those columns, which makes data loading slow. In-memory option will allow us to drop those indexes and just store the columns we need in memory.

Second, because I’m a huge fan of in-memory data warehouses, and am happy that Oracle is now making these feasible. Few TB of memory in a large server are no longer science fiction, which means that most of your data warehouse will soon fit in memory. Fast analytics for all! And what do you do with the data that won’t fit in memory? Perhaps store it in your Hadoop cluster.

Now that I’m done being excited about the big news, lets talk about small news that you probably didn’t notice but you should.

Oracle announced two cool new features for the Big Data Appliance. Announced may be a big word, Larry Ellison did not stand up on stage and talked about them. Instead the features sneaked quietly into the last upgrade and appeared in the documentation.

Perfect Balance – If you use Hadoop as often as I do, you know how data skew can mess with query performance. You run a job with several reducers, each aggregates data for a subset of keys. Unless you took great care in partitioning your data, the data will not be evenly distributed between the reducers, usually because it wasn’t evenly distributed between the keys. As a result, you will spend 50% of the time waiting for that one last reducer to finish already.

Oracle’s Perfect Balance makes the “took great case in partitioning your data” part much much easier. This blog post is just a quick overview, not an in-depth blog post, so I won’t go into details of how this works (wait for my next post on this topic!). I’ll just mention that Perfect Balance can be used without any change to the application code, so if you are using BDA, there is no excuse not to use it.

And no excuse to play solitaire while waiting for the last reducer to finish.

Oracle XQuery for Hadoop – Announced but not officially released yet, which is why I’m pointing you at an Amis blog post. For now thats the best source of information about this feature. This feature, combined with the existing Oracle Loader for Hadoop will allow running XQuery operations on XMLs stored in Hadoop, pushing down the entire data processing bit to Map Reduce on the Hadoop cluster. Anyone who knows how slow, painful and CPU intensive XML processing can be on an Oracle database server will appreciate this feature. I wish I had it a year ago when I had to ingest XMLs at a very high rate. It is also so cool that I’m a bit sorry that we never developed more awesome XQuery capabilities for Hive and Impala. Can’t wait for the release so I can try that!

During OpenWorld there was also additional exposure for existing, but perhaps not very well known Oracle Big Data features – Hadoop for ODI, Hadoop for OBIEE and using GoldenGate with Hadoop. I’ll try to write more about those soon.

Meanwhile, let me know what you think of In-Memory, Perfect Balance and OXH.

My Oracle OpenWorld 2013 Presentations

Oracle OpenWorld was fantastic, as usual. The best show in San Francisco. This is the seventh year in a row that I’m attending – 3 times as HP employee, 3 times as Pythian employee, and now as a Clouderan. My life changes, but the event and people are always fantastic.

There will be a separate blogpost about what I learned at the event, new exciting products and my thoughts of them. But first, let me follow up on what I taught.

On Sunday afternoon, and then again on Thursday afternoon, I presented “Data Wrangling with Oracle Connectors for Hadoop”. I presented it twice because both Oracle and IOUG liked my abstract. I was surprised to discover that both audiences had no idea what “Data Wrangling” is! I appreciate the attendees, they trusted me enough to attend without even being sure what I’m planning to talk about. In both sessions I had people come up with excellent questions, mentioning that they are current or future Cloudera customers. I absolutely loved it, what a great opportunity to connect with Hadoopers from all industries.

You can find the slides here: Data Wrangling with Oracle Connectors for Hadoop

On Monday, at OakTable World, I presented ETL on Hadoop. I presented it at Surge earlier this year, but this time I think I misjudged the fit of the content to the audience – I gave pretty technical tips of how to implement ETL on Hadoop to an audience with very little experience with Hadoop. They were smart people and mostly followed along, but I should have kept my content to more introductory level.

You can find the slides here: Scaling ETL with Hadoop

On Wednesday, I was fortunate to present with my former colleague Marc Fielding on SSDs and their use in Exadata. The topic is not very Hadoop related, but I love SSDs regardless and presenting with Marc was fun and the audience was highly engaged. I did get a lot of questions on SSDs and Hadoop, so I’ll consider writing about the topic in the future.

Marc has the latest version of the slides, but you can find an approximation here: Databases in a Solid State World.

Thanks again to everyone who attended, to all the customers who stopped to say hello and to everyone who was friendly and made the event fun. I hope to see you again next year.