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Installing the YARN Application

The YARN application listens for model deployment requests. Models are exposed as REST microservices that expose your model application as an endpoint. The YARN application takes the submitted request that specifies the model payload that includes a shell script and other model collateral which will start the microservice. Upon execution of the shell script that starts the model, the YARN application registers the endpoints in ZooKeeper.


If you are using or depending an API library in your model such as Flask and Jinja2, the library must be installed on every data node. This is because the model is executed by a shell script which must be able to run successfully on every node.

In order to know on which port that the REST service is listening, the model must create a file in the current working directory which indicates the URL for the model. Because you might have more than one copy of the model, it is a good idea to find an open port and bind to that. An example of how to do that in Python is as follows:

  sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.bind(('localhost', 0))
port = sock.getsockname()[1]
with open("endpoint.dat", "w") as text_file:
  text_file.write("{\"url\" : \"\"}" % port)

Install the YARN application by completing the following steps:

  1. As root, log into the host from which you run Metron.

  2. Create a directory called "sample" in the root user's home directory where you will put a very simple model.

  3. Now, you can create a simple shell script that will expose a REST endpoint called "echo" that will echo back the arguments passed to it. Create a file in the "sample" directory named "", and copy the following information into the file.

    Note that in this simple REST service, we are always binding to port 1500. In a real REST service which would expose your model, we would be more intelligent about the choice of the port.

    rm -f out
    mkfifo out
    trap "rm -f out" EXIT
    echo "{ \"url\" : \"http://localhost:1500\", \"functions\" : { \"apply\" : \"echo\" } }" > endpoint.dat
    while true
      cat out | nc -l 1500 > >( # parse the netcat output, to build the answer redirected to the pipe "out".
        export REQUEST=
        while read line
          line=$(echo "$line" | tr -d '[\r\n]')
          if echo "$line" | grep -qE '^GET /' # if line starts with "GET /"
            REQUEST=$(echo "$line" | cut -d ' ' -f2) # extract the request
          elif [ "x$line" = x ] # empty line / end of request
            HTTP_200="HTTP/1.1 200 OK"
            HTTP_404="HTTP/1.1 404 Not Found"
            # call a script here
            # Note: REQUEST is exported, so the script can parse it (to answer 200/403/404 status code + content)
            if echo $REQUEST | grep -qE '^/echo/'
                printf "%s\n%s %s\n\n%s\n" "$HTTP_200" "$HTTP_LOCATION" $REQUEST ${REQUEST#"/echo/"} > out
                printf "%s\n%s %s\n\n%s\n" "$HTTP_404" "$HTTP_LOCATION" $REQUEST "Resource $REQUEST NOT FOUND!" > out
  4. Change directories to $METRON_HOME.

  5. Start the MaaS service in bin/ -zq node1:2181.

    bash bin/ -zq node1:2181


    -c, --create

    Flag to indicate whether to create the domain specified with -domain.

    -d,--domain <arg>

    ID of the time line domain where the time line entities will be put

    -e,--shell_env <arg>

    Environment for shell script. Specified as env_key=env_val pairs.


    The help screen

    -j,--jar <arg>

    Jar file containing the application master

    -l,--log4j <arg>

    The log4j properties file to load

    -ma,--modify_acls <arg>

    Users and groups that allowed to modify the time line entities in the given domain

    -ma,--master_vcores <arg>

    Amount of virtual cores to be requested to run the application master


    Amount of memory in MB to be requested to run the application master

    -nle,--node_label_expression <arg>

    Node label expression to determine the nodes where all the containers of this application will be allocated, "" means containers can be allocated anywhere, if you don't specify the option, default node_label_expression of queue will be used.

    -q,--queue <arg>

    RM Queue in which this application is to be submitted

    -t,--timeout <arg>

    Application timeout in milliseconds

    -va,--view_acls <arg>

    Users and groups that allowed to view the time line entities in the given domain

    -zq,--zk_quorum <arg>

    ZooKeeper Quorum

    -zr,--zk_root <arg>

    ZooKeeper Root

  6. Test the configuration to ensure that the MaaS service is running correctly.

    For example, you would enter the following:

    1. Start one instance of a sample echo service (named 'sample' version '1.0') in a container of 500m:

      bin/ -lmp ~/sample -hmp /user/root/maas/sample -m 500 -mo ADD -n sample -ni 1 -v 1.0 -zq node1:2181
    2. Wait a couple seconds and then ensure that the service started by running the following command:

      curl -i http://localhost:1500/echo/foobar

      You should see a response foobar.

    3. List the active models and ensure that you see the sample model in the output.

      bin/ -mo LIST -n sample -zq node1:2181
    4. Remove one instance of the sample model.

      bin/ -mo REMOVE -n sample -ni 1 -v 1.0 -zq node1:2181
    5. After a couple seconds ensure that you cannot access the sample model any longer:

      curl -i http://localhost:1500/echo/foobar