DDevelopment & Design

Hadoop HA setup

With the advent of Hadoop’s 2.x version, there finally is a working
High-Availability solution. Even two of those. Now it really is easy to
configure and use those solutions. It no longer require external
components, like
DRBD.
It all is just neatly packed into Cloudera Hadoop distribution – the
precursor of this solution.

Read on to find out how to use it.

The most important weakness of previous Hadoop releases was the
single-point-of-failure, which happend to be NameNode. NameNode as a key
component of every Hadoop cluster, is responsible for managing
filesystem namespace information and block location. Loosing its data results in loosing all the data
stored on DataNodes. HDFS is no longer able to reach for specific files,
or its blocks. This renders your cluster inoperable.

So it is crucial to be able to detect and counter problems with NameNode.
The most desirable behavior is to have a hot backup, that would ensure
a no-downtime cluster operation. To achieve this, the second NameNode
need to have up-to-date information on filesystem metadata and it needs
to be also up and running. Starting NameNode with existing set of data
may easily take many minutes to parse the actual filesystem state.

Previously used solution – depoying SecondaryNameNode – was somewhat
flawed. It took long time to recover after failure. It was not a
hot-backup solution, which also added to the problem. Some other
solution was required.

So, what needed to be made redundant is the edits dir contents and
sending block location maps from each of the DataNodes to NameNodes –
in case of HA deployment – to both NameNodes. This was accomplished in
two steps. The first one with the release of CDH 4 beta – solution based
on sharing edits directory. Than, with CDH 4.1 came quorum based solution.

Find out how to configure those on your cluster.

Shared edits directory solution

For this kind of setup, there is an assumption, that in a cluster exists
a shared storage directory. It should be deployed using some kind of
network-based filesystem. You could try with NFS or GlusterFS.


  fs.default.name/name>
  hdfs://example-cluster




  dfs.nameservices
  example-cluster




  dfs.ha.namenodes.example-cluster
  nn1,nn2


  dfs.namenode.rpc-address.example-cluster.nn1
  master1:8020


  dfs.namenode.rpc-address.example-cluster.nn2
  master2:8020


  dfs.namenode.http-address.example-cluster.nn1
  0.0.0.0:50070


  dfs.namenode.http-address.example-cluster.nn2
  0.0.0.0:50070




  dfs.namenode.shared.edits.dir
  file:///mnt/filer1/dfs/ha-name-dir-shared




  dfs.client.failover.proxy.provider.example-cluster
  org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider




  dfs.ha.fencing.methods
  sshfence

 
  dfs.ha.fencing.ssh.private-key-files
  /home/user/.ssh/id_dsa





  dfs.ha.automatic-failover.enabled
  true


  ha.zookeeper.quorum
  zk1:2181,zk2:2181,zk3:2181

This setup is quite OK, as long as you’re comfortable with maintaining a
separate service (network storage) for handling the HA state. It seems
error prone to me, because it adds another service which high
availability should be ensured. NFS seems to be a bad choice here,
because AFAIK it does not offer HA out of the box.

On the other hand, we have GlusterFS, which is a distributed filesystem,
you can deploy on multiple bricks and increase the replication level.

Nevertheless, it still brings additional burden of another service to
maintain.

Quorum based solution

With the release of CDH 4.1.0 we are now able to use a much better
integrated solution called JournalNode. Now all the updates are
synchronized through a JournalNode. Each JournalNode have the same data
and all the NameNodes are able to recive filesystem state updates from
that daemons.

This solution is much more consistent with Hadoop ecosystem.

Please note, that the config is almost identical to the one needed for
shared edits directory solution. The only difference is the value for
dfs.namenode.shared.edits.dir. This now points to all the journal
nodes deployed in our cluster.


  fs.default.name/name>
  hdfs://example-cluster



  dfs.nameservices
  example-cluster




  dfs.ha.namenodes.example-cluster
  nn1,nn2


  dfs.namenode.rpc-address.example-cluster.nn1
  master1:8020


  dfs.namenode.rpc-address.example-cluster.nn2
  master2:8020


  dfs.namenode.http-address.example-cluster.nn1
  0.0.0.0:50070


  dfs.namenode.http-address.example-cluster.nn2
  0.0.0.0:50070




  dfs.namenode.shared.edits.dir
  qjournal://node1:8485;node2:8485;node3:8485/example-cluster




  dfs.client.failover.proxy.provider.example-cluster
  org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider




  dfs.ha.fencing.methods
  sshfence

 
  dfs.ha.fencing.ssh.private-key-files
  /home/user/.ssh/id_dsa





  dfs.ha.automatic-failover.enabled
  true


  ha.zookeeper.quorum
  zk1:2181,zk2:2181,zk3:2181

Infrastructure

In both cases you need to run Zookeeper-based Failover Controller
(hadoop-hdfs-zkfc). This daemon negotiates which NameNode should
become active and which standby.

But that’s not all. Depending on the way you’ve choosen to deploy HA you
need to do some other things:

Shared edits dir

With shared edits dir you need to deploy networked filesystem, and mount
it on your NameNodes. After that you can run your cluster and be happy
with your new HA.

Quroum based

For QJournal to operate you need to install one new package called
hadoop-hdfs-journalnode. This provides startup scripts for Journal
Node daemons. Choose at least three nodes that will be responsible for
handling edits state and deploy journal nodes on them.

Conclusion

Thanks to guys from Cloudera we now can use an enterprise grade High
Availability features for Hadoop. Eliminating the single point of
failure in your cluster is essential for easy maintainability of your
infrastructure.

Given the above choices, I’d suggest using QJournal setup, becasue of
its relatively small impact on the overal cluster architecture. It’s
good performance and fairly simple setup enable the users to easily
start using Hadoop in HA setup.

Are you using Hadoop with HA? What are your impressions?

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Udana Warsjawa V - 100. spotkanie WJUG

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DDevelopment & Design

Phonegap / Cordova and cross domain ssl request problem on android.

In one app I have participated, there was a use case:
  • User fill up a form.
  • User submit the form.
  • System send data via https to server and show a response.
During development there wasn’t any problem, but when we were going to release production version then some unsuspected situation occurred. I prepare the production version accordingly with standard flow for Android environment:
  • ant release
  • align
  • signing
During conduct tests on that version, every time I try to submit the form, a connection error appear. In that situation, at the first you should check whitelist in cordova settings. Every URL you want to connect to, must be explicit type in: 
res/xml/cordova.xml
If whitelist looks fine, the error is most likely caused by inner implementation of Android System. The Android WebView does not allow by default self-signed SSL certs. When app is debug-signed the SSL error is ignored, but if app is release-signed connection to untrusted services is blocked.



Workaround


You have to remember that secure connection to service with self-signed certificate is risky and unrecommended. But if you know what you are doing there is some workaround of the security problem. Behavior of method 
CordovaWebViewClient.onReceivedSslError
must be changed.


Thus add new class extended CordovaWebViewClient and override ‘onReceivedSslError’. I strongly suggest to implement custom onReceiveSslError as secure as possible. I know that the problem occours when app try connect to example.domain.com and in spite of self signed certificate the domain is trusted, so only for that case the SslError is ignored. 

public class MyWebViewClient extends CordovaWebViewClient {

   private static final String TAG = MyWebViewClient.class.getName();
   private static final String AVAILABLE_SLL_CN 
                               = "example.domain.com";

   public MyWebViewClient(DroidGap ctx) {
       super(ctx);
   }

   @Override
   public void onReceivedSslError(WebView view, 
                                  SslErrorHandler handler, 
                                  android.net.http.SslError error) {

       String errorSourceCName = error.getCertificate().
                                  getIssuedTo().getCName();

       if( AVAILABLE_SLL_CN.equals(errorSourceCName) ) {
           Log.i(TAG, "Detect ssl connection error: " + 
                       error.toString() + 
                       „ so the error is ignored”);

           handler.proceed();
           return;
       }

       super.onReceivedSslError(view, handler, error);
   }
}
Next step is forcing yours app to  use custom implementation of WebViewClient. 

public class Start extends DroidGap
{
   private static final String TAG = Start.class.getName();

   @Override
   public void onCreate(Bundle savedInstanceState)
   {
       super.onCreate(savedInstanceState);
       super.setIntegerProperty("splashscreen", R.drawable.splash);
       super.init();

       MyWebViewClient myWebViewClient = new MyWebViewClient(this);
       myWebViewClient.setWebView(this.appView);

       this.appView.setWebViewClient(myWebViewClient);
       
       // yours code

   }
}
That is all ypu have to do if minSdk of yours app is greater or equals 8. In older version of Android there is no class 
android.net.http.SslError
So in class MyCordovaWebViewClient class there are errors because compliator doesn’t see SslError class. Fortunately Android is(was) open source, so it is easy to find source of the class. There is no inpediments to ‘upgrade’ app and just add the file to project. I suggest to keep original packages. Thus after all operations the source tree looks like:

Class SslError placed in source tree. 
 Now the app created in release mode can connect via https to services with self-signed SSl certificates.
DDevelopment & Design

Using WsLite in practice

TL;DR

There is a example working GitHub project which covers unit testing and request/response logging when using WsLite.

Why Groovy WsLite ?

I’m a huge fan of Groovy WsLite project for calling SOAP web services. Yes, in a real world you have to deal with those - big companies have huge amount of “legacy” code and are crazy about homogeneous architecture - only SOAP, Java, Oracle, AIX…

But I also never been comfortable with XFire/CXF approach of web service client code generation. I wrote a bit about other posibilites in this post. With JAXB you can also experience some freaky classloading errors - as Tomek described on his blog. In a large commercial project the “the less code the better” principle is significant. And the code generated from XSD could look kinda ugly - especially more complicated structures like sequences, choices, anys etc.

Using WsLite with native Groovy concepts like XmlSlurper could be a great choice. But since it’s a dynamic approach you have to be really careful - write good unit tests and log requests. Below are my few hints for using WsLite in practice.

Unit testing

Suppose you have some invocation of WsLite SOAPClient (original WsLite example):

def getMothersDay(long _year) {
    def response = client.send(SOAPAction: action) {
       body {
           GetMothersDay('xmlns':'http://www.27seconds.com/Holidays/US/Dates/') {
              year(_year)
           }
       }
    }
    response.GetMothersDayResponse.GetMothersDayResult.text()
}

How can the unit test like? My suggestion is to mock SOAPClient and write a simple helper to test that builded XML is correct. Example using great SpockFramework: 

void setup() {
   client = Mock(SOAPClient)
   service.client = client
}

def "should pass year to GetMothersDay and return date"() {
  given:
      def year = 2013
  when:
      def date = service.getMothersDay(year)
  then:
      1 * client.send(_, _) >> { Map params, Closure requestBuilder ->
            Document doc = buildAndParseXml(requestBuilder)
            assertXpathEvaluatesTo("$year", '//ns:GetMothersDay/ns:year', doc)
            return mockResponse(Responses.mothersDay)
      }
      date == "2013-05-12T00:00:00"
}

This uses a real cool feature of Spock - even when you mock the invocation with “any mark” (_), you are able to get actual arguments. So we can build XML that would be passed to SOAPClient's send method and check that specific XPaths are correct:

void setup() {
    engine = XMLUnit.newXpathEngine()
    engine.setNamespaceContext(new SimpleNamespaceContext(namespaces()))
}

protected Document buildAndParseXml(Closure xmlBuilder) {
    def writer = new StringWriter()
    def builder = new MarkupBuilder(writer)
    builder.xml(xmlBuilder)
    return XMLUnit.buildControlDocument(writer.toString())
}

protected void assertXpathEvaluatesTo(String expectedValue,
                                      String xpathExpression, Document doc) throws XpathException {
    Assert.assertEquals(expectedValue,
            engine.evaluate(xpathExpression, doc))
}

protected Map namespaces() {
    return [ns: 'http://www.27seconds.com/Holidays/US/Dates/']
}

The XMLUnit library is used just for XpathEngine, but it is much more powerful for comparing XML documents. The NamespaceContext is needed to use correct prefixes (e.g. ns:GetMothersDay) in your Xpath expressions.

Finally - the mock returns SOAPResponse instance filled with envelope parsed from some constant XML:

protected SOAPResponse mockResponse(String resp) {
    def envelope = new XmlSlurper().parseText(resp)
    new SOAPResponse(envelope: envelope)
}

Request and response logging

The WsLite itself doesn’t use any logging framework. We usually handle it by adding own sendWithLogging method:

private SOAPResponse sendWithLogging(String action, Closure cl) {
    SOAPResponse response = client.send(SOAPAction: action, cl)
    log(response?.httpRequest, response?.httpResponse)
    return response
}

private void log(HTTPRequest request, HTTPResponse response) {
    log.debug("HTTPRequest $request with content:\n${request?.contentAsString}")
    log.debug("HTTPResponse $response with content:\n${response?.contentAsString}")
}

This logs the actual request and response send through SOAPClient. But it logs only when invocation is successful and errors are much more interesting… So here goes withExceptionHandler method:

private SOAPResponse withExceptionHandler(Closure cl) {
    try {
        cl.call()
    } catch (SOAPFaultException soapEx) {
        log(soapEx.httpRequest, soapEx.httpResponse)
        def message = soapEx.hasFault() ? soapEx.fault.text() : soapEx.message
        throw new InfrastructureException(message)
    } catch (HTTPClientException httpEx) {
        log(httpEx.request, httpEx.response)
        throw new InfrastructureException(httpEx.message)
    }
}
def send(String action, Closure cl) {
    withExceptionHandler {
        sendWithLogging(action, cl)
    }
}

XmlSlurper gotchas

Working with XML document with XmlSlurper is generally great fun, but is some cases could introduce some problems. A trivial example is parsing an id with a number to Long value:

def id = Long.valueOf(edit.'@id' as String)

The Attribute class (which edit.'@id' evaluates to) can be converted to String using as operator, but converting to Long requires using valueOf.

The second example is a bit more complicated. Consider following XML fragment:

<edit id="3">
   <params>
      <param value="label1" name="label"/>
      <param value="2" name="param2"/>
   </params>
   <value>123</value>
</edit>
<edit id="6">
   <params>
      <param value="label2" name="label"/>
      <param value="2" name="param2"/>
   </params>
   <value>456</value>
</edit>

We want to find id of edit whose label is label1. The simplest solution seems to be:

def param = doc.edit.params.param.find { it['@value'] == 'label1' }
def edit = params.parent().parent()

But it doesn’t work! The parent method returns multiple edits, not only the one that is parent of given param…

Here’s the correct solution:

doc.edit.find { edit ->
    edit.params.param.find { it['@value'] == 'label1' }
}

Example

The example working project covering those hints could be found on GitHub.