If your team is building a big data pipeline to ingest data all the way to data processing/analytics, it is crucial to use something like Oozie to manage the entire workflow. Without a workflow management, it is difficult to manage and monitor the data pipeline. It will be prone to error and mismanagement. Without Oozie, you will most likely fall back to use shell scripting and cron to trigger, run and monitor each individual step, eg Hive query, MapReduce job, pre and post validation steps etc. Writing/maintaining these scripts is a cumbersome task. Soon enough, if you are not careful, the data pipeline turns against you and becomes this unmanageable and unwieldy monster. I have recently overhauled a complex data pipeline and also built new ones to use Oozie as the workflow manager and the difference is like day and night. Not only it allows us to piece together and coordinate different processing steps easily, it also makes the flow super easy to explain to others (eg. Developer, QA, Product Owner). The built in email notification action allows us to detect error every step along the pipeline and notify the appropriate team in charge. In addition, it is easy to parameterize various arguments passed to your job.
You can even build pre and post validation actions to perform simple sanity check of your input and output, eg. making sure your Hive output table or output files is not empty to a full blown abnormality detection pipeline.

In summary, workflow management tool like Oozie allows

1) Easy orchestration and coordination of complex data pipeline consists of hive query, java map reduce job, pig, sqoop and shell scripts.
2) Easy configurability of various processing steps. Eg. Use job.properties to configure and parameterize arguments.
3) Monitoring of complex pipeline. Trigger email notification if error do happen.

Oozie is not without any pain point but it is an essential tool for building and managing a robust data pipeline.

Oozie is a web application built on top of Apache Tomcat Catalina. Take a look at its web.xml and you can find the servlet listener called org.apache.oozie.servlet.ServicesLoader that initializes the Oozie’s services.

<listener>
<listener-class>org.apache.oozie.servlet.ServicesLoader</listener-class>
</listener>


<?xml version="1.0" encoding="UTF-8"?>
<!--
  Licensed to the Apache Software Foundation (ASF) under one
  or more contributor license agreements.  See the NOTICE file
  distributed with this work for additional information
  regarding copyright ownership.  The ASF licenses this file
  to you under the Apache License, Version 2.0 (the
  "License"); you may not use this file except in compliance
  with the License.  You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.
-->
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd" [
    <!ENTITY web-common SYSTEM "web-common.xml">
]>

<web-app>

    <!--
      ========================================================================
      IMPORTANT: ANY CHANGES TO THE SERVLETS, SERVLET MAPPINGS, LISTENERS, ETC
                 MUST BE REFLECTED IN distro/src/main/tomcat/ssl-web.xml
                 AS WELL.
      ========================================================================
    -->

    <display-name>OOZIE</display-name>

    <!-- Listeners -->
    <listener>
        <listener-class>org.apache.oozie.servlet.ServicesLoader</listener-class>
    </listener>

    <!-- Servlets -->
    <servlet>
        <servlet-name>versions</servlet-name>
        <display-name>WS API for Workflow Instances</display-name>
        <servlet-class>org.apache.oozie.servlet.VersionServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet>
        <servlet-name>v0admin</servlet-name>
        <display-name>Oozie admin</display-name>
        <servlet-class>org.apache.oozie.servlet.V0AdminServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet>
        <servlet-name>v1admin</servlet-name>
        <display-name>Oozie admin</display-name>
        <servlet-class>org.apache.oozie.servlet.V1AdminServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet>
        <servlet-name>v2admin</servlet-name>
        <display-name>Oozie admin</display-name>
        <servlet-class>org.apache.oozie.servlet.V2AdminServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet>
        <servlet-name>callback</servlet-name>
        <display-name>Callback Notification</display-name>
        <servlet-class>org.apache.oozie.servlet.CallbackServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet>
        <servlet-name>v0jobs</servlet-name>
        <display-name>WS API for Workflow Jobs</display-name>
        <servlet-class>org.apache.oozie.servlet.V0JobsServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet>
        <servlet-name>v1jobs</servlet-name>
        <display-name>WS API for Workflow Jobs</display-name>
        <servlet-class>org.apache.oozie.servlet.V1JobsServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet>
        <servlet-name>v0job</servlet-name>
        <display-name>WS API for a specific Workflow Job</display-name>
        <servlet-class>org.apache.oozie.servlet.V0JobServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet>
        <servlet-name>v1job</servlet-name>
        <display-name>WS API for a specific Workflow Job</display-name>
        <servlet-class>org.apache.oozie.servlet.V1JobServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet>
        <servlet-name>v2job</servlet-name>
        <display-name>WS API for a specific Workflow Job</display-name>
        <servlet-class>org.apache.oozie.servlet.V2JobServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet>
        <servlet-name>sla-event</servlet-name>
        <display-name>WS API for specific SLA Events</display-name>
        <servlet-class>org.apache.oozie.servlet.SLAServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet>
        <servlet-name>v2sla</servlet-name>
        <display-name>WS API for specific SLA Events</display-name>
        <servlet-class>org.apache.oozie.servlet.V2SLAServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <!-- servlet-mapping -->
    <servlet-mapping>
        <servlet-name>versions</servlet-name>
        <url-pattern>/versions</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>v0admin</servlet-name>
        <url-pattern>/v0/admin/*</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>v1admin</servlet-name>
        <url-pattern>/v1/admin/*</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>v2admin</servlet-name>
        <url-pattern>/v2/admin/*</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>callback</servlet-name>
        <url-pattern>/callback/*</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>v0jobs</servlet-name>
        <url-pattern>/v0/jobs</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>v1jobs</servlet-name>
        <url-pattern>/v1/jobs</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>v1jobs</servlet-name>
        <url-pattern>/v2/jobs</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>v0job</servlet-name>
        <url-pattern>/v0/job/*</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>v1job</servlet-name>
        <url-pattern>/v1/job/*</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>v2job</servlet-name>
        <url-pattern>/v2/job/*</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>sla-event</servlet-name>
        <url-pattern>/v1/sla/*</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>v2sla</servlet-name>
        <url-pattern>/v2/sla/*</url-pattern>
    </servlet-mapping>

    <!-- welcome-file -->
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>

    <filter>
        <filter-name>hostnameFilter</filter-name>
        <filter-class>org.apache.oozie.servlet.HostnameFilter</filter-class>
    </filter>

    <filter>
        <filter-name>authenticationfilter</filter-name>
        <filter-class>org.apache.oozie.servlet.AuthFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>hostnameFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>authenticationfilter</filter-name>
        <url-pattern>/versions/*</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>authenticationfilter</filter-name>
        <url-pattern>/v0/*</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>authenticationfilter</filter-name>
        <url-pattern>/v1/*</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>authenticationfilter</filter-name>
        <url-pattern>/v2/*</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>authenticationfilter</filter-name>
        <url-pattern>/index.jsp</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>authenticationfilter</filter-name>
        <url-pattern>/admin/*</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>authenticationfilter</filter-name>
        <url-pattern>*.js</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>authenticationfilter</filter-name>
        <url-pattern>/ext-2.2/*</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>authenticationfilter</filter-name>
        <url-pattern>/docs/*</url-pattern>
    </filter-mapping>

</web-app>

ServicesLoaader implements the ServletContextListener interface. During initialization, it basically creates and initializes the Services singleton instance which will then load all the different services used in Oozie. For example, ActionService, AuthorizationService, ConfigurationService, CoordinatorService, JPAService, and etc.

After the initialization, servlets in the Oozie will be able to use the above created services to fullfill the submitted requests. For example, JsonRestServlet, the base class for Oozie web service API utilizes the InstrumentationService and ProxyUserService.

package org.apache.oozie.servlet;

import org.apache.oozie.service.Services;

import javax.servlet.ServletContextListener;
import javax.servlet.ServletContextEvent;

/**
 * Webapp context listener that initializes Oozie {@link Services}.
 */
public class ServicesLoader implements ServletContextListener {
    private static Services services;
    private static boolean sslEnabled = false;

    /**
     * Initialize Oozie services.
     *
     * @param event context event.
     */
    public void contextInitialized(ServletContextEvent event) {
        try {
            String ssl = event.getServletContext().getInitParameter("ssl.enabled");
            if (ssl != null) {
                sslEnabled = true;
            }

            services = new Services();
            services.init();
        }
        catch (Throwable ex) {
            System.out.println();
            System.out.println("ERROR: Oozie could not be started");
            System.out.println();
            System.out.println("REASON: " + ex.toString());
            System.out.println();
            System.out.println("Stacktrace:");
            System.out.println("-----------------------------------------------------------------");
            ex.printStackTrace(System.out);
            System.out.println("-----------------------------------------------------------------");
            System.out.println();
            System.exit(1);
        }
    }

    /**
     * Destroy Oozie services.
     *
     * @param event context event.
     */
    public void contextDestroyed(ServletContextEvent event) {
        services.destroy();
    }

    public static boolean isSSLEnabled() {
        return sslEnabled;
    }
}

If you would like to take a look at Oozie’s services layer, check out my blog post https://weishungchung.com/2013/11/24/a-look-into-oozie-internals/