Emissary Dark Knight - some code just wants to watch the core burn

Table of Contents

Created by gh-md-toc


Emissary is a P2P based data-driven workflow engine that runs in a heterogenous possibly widely dispersed, multi-tiered P2P network of compute resources. Workflow itineraries are not pre-planned as in conventional workflow engines, but are discovered as more information is discovered about the data. There is typically no user interaction in an Emissary workflow, rather the data is processed in a goal oriented fashion until it reaches a completion state.

Emissary is highly configurable, but in this base implementation does almost nothing. Users of this framework are expected to provide classes that extend emissary.place.ServiceProviderPlace to perform work on emissary.core.IBaseDataObject payloads.

A variety of things can be done and the workflow is managed in stages, i.e. STUDY, ID, COORDINATE, TRANSFORM, ANALYZE, IO, REVIEW.

The classes responsble for directing the workflow are the emissary.core.MobileAgent and classes derived from it, which manage the path of a set of related payload objects through the workflow and the emissary.directory.DirectoryPlace which manages the available services, their cost and quality and keep the P2P network connected.


Code organization

Emissary is laid out in the following package structure:


There are two ways to run Emissary - through a "standalone" mode where a node runs in isolation and "cluster" where nodes connect and form P2P network. All startup goes through the emissary.Emissary class.


Emissary runs several threads to keep track of its processing. If you jstack <emissary pid> you will see all the running threads and what code they are currently executing.

Java threads

Typical Java threads

Jetty threads

Jetty threads used for handling requests

Emissary threads

Processing Flow

There are various adapters to abstract where data comes from. These sources ultimately end up in PickUpPlace broken out sessions. PickUpPlace then pulls a MobileAgent from the thread pool and hands off the session for processing.

Processing Routing

MobileAgent walks a given session through all the relevant places. This happens by MobileAgent#go() -> MobileAgent#agentControl() -> MobileAgent#getNextKey() -> MobileAgent#getNextKeyFromDirectory which ultimately hooks into DirectoryPlace#nextKeys().


The Emissary data driven workflow has stages. These are specified by emissary.core.Stage. The stages of the workflow are used to control certain aspects of unwrapping and processing and help to ensure that the workflow can always make progress on the task at hand.

There are guidelines on how the workflow stages should be used. These guidelines are not currently enforced but deviations from these guidelines might be an indication that more thought should be applied.


The name for this stage came from the idea behind Perl's Regex study method -- some work that can be done up-front to optimize or prepare for the remaining work to come.

This stage is designed to make no modifications to the data itself but can be used for


The Id phase of the workflow is for identification of data. Service places operating in this phase are expected to modify the currentForm and fileType of the payload. They are expected to not change the payload bytes of the data or extract any other metadata, unwrap child payload objects, or extract metadata.


The coordinate phase of the workflow is a natural fit for the emissary.place.CoordinationPlace which is designed to wrap any number of otherwise instantiated processing places and lock down the workflow among them without regard to cost. So that we don't have to create and remember an artificially low cost for the CoordinatePlace, setting into this phase of the work flow causes it to run before any Transform or PreTransform places that it might be coordinating for.


If there is the need to record provenance or export data for intermediate forms this is a place where that could happen.


The Transform phase of the workflow is expected to modify the bytes of the payload, set the current form to something new when appropriate, extract nested child payload objects, record metadata. After a Transform phase service place the workflow engine will go back to the ID phase and start evaluating for places that are interested in the currentForm.


Since the other primary phases of the workflow had "hook" stages in between them, it seemed good to have one here too. Nothing past this phase will cause the workflow engine to go back to the ID phase of the workflow, things must continue forward in the workflow stages from here.


The analyze phase is designed to collect metadata and add value in ways that do not affect the currentForm, fileType or bytes of the payload. In future versions Analyze places that apply may be done in parallel.


Little used, but serves as a catch-all point before the IO stage.


The IO stage is when data is available for output. This is a blocking point in the workflow as all items in the payload family tree must be prepared to transition to the IO stage before we can proceed.


Post IO most of the currentForms are stripped off as they are handled by the IO places. The itinerary is available and could be used in post-processing provenance events.


Decisions on how data gets sent to Places revolves around the concept of DirectoryEntry, which is also referred to as a Key within the code. A DirectoryEntry is of the form:


Routing uses a combination of the FORM, STAGE and EXPENSE to decide where to send the data. The STAGEs define an order for the processing flow within the framework. Places are bound to specific STAGEs and will not be run outside of that STAGE. The current STAGE list and order is:

Actual name Description Run in parallel?
STUDY prepare, coordinate idents No
ID identification phase No
COORDINATE Coordinate processing No
PRETRANSFORM before transform hook Yes
TRANSFORM transformation phase No
POSTTRANSFORM after transform hook Yes
ANALYZE analysis/metadata generation Yes
VERIFY verify for output No
IO output No
REVIEW finish off No

Parsers vs Places

Emissary Places are for processing, transforming, identifying, analyzing data of interest. As items are unwrapped and processed the results are all kept together in a "family tree". This structure is a List in the code. The list can be sorted in family order by using the emissary.util.ShortNameComparator.

Sometimes, for our own convenience, or for dealing with formats provided by up-stream systems, the data arrives packaged in various formats. These containers can have multiple items that should not be logically related in a family tree. The Emissary Parser framework deals with this situation.

Parsers need to make a quick identification of the format of the container. This is called out by the emissary.parser.ParserFactory and uses the emissary.parser.DataIdentifier as the engine to perform the identification. The engine is configured by name and can be replace by anything that meets the required interface. The name of the format identified should have parsers that meet the SessionParser interface configured into the factory.

Once the container is identified, items are parsed out of it, built into IBaseDataObject instances by the SessionParser appropriate for that data type, and handed over to MobileAgent instances for processing as they become available from the pool. It is important to have in mind that the Parser framework is operating on the single-threaded side of the system and handing items over to the multi-threaded (MobileAgent) side of the system. This implies that the parsers are fast enough to keep the number of configured threads busy.



Linux or MacOSX operating system

Currently, developing and running natively on Windows is not supported. Use a virtualized Linux OS if you are using Windows.

In terms of Linux distributions, the two most widely used on the Emissary codebase are CentOS 6.X and Ubuntu 12.X.

Java 1.8

Java 1.6 and Java 1.7 are no longer supported in this codebase only Java 1.8.
OpenJDK should work, although most testing is done with Oracle's JDK. Be sure to use a recent version of 1.8 as there are bugs in earlier versions.

Apache Maven 3.2

Download and install Apache Maven 3.2. Some maven commands are shown below. A great place to start with Maven is the 5 min tutorial. Another great resource explains the lifecycles. We cover configuring maven below with Emissary specific settings.

As you go through this guide, keep in mind the steps in the default lifecycle are cumulative. If you run test, Maven will run compile among other steps first. Similarly, when you run package, Maven will run compile and test along with other steps first.

We also use maven profiles. To activate specific functionality. To activate profile abc, you would add -Pabc. Mutliple profiles can be activated at once if you seperate them with a comma. Something like -Pprofile-1,profile-2.

Git 1.8+

In order to check out the code, you must install git. Github has a page with links to some good tutorials. The book by Scott Chacon is particularly good and available to read online.


Removing anything not being tracked by git

Not a Maven command, but useful to get rid of any file not tracked by git. This will remove the jflex generated files, as well as any files setup by your IDE.

git clean -dxf


Remove everything under 'project.build.directory' which will be target from the command line

mvn clean

Format code

The autoformat profile is run unless you use '-DskipFormat'. There is a java source code formatter that uses a [special file](link here) to format Java source code and formatter to sort pom files. The 2 formatter are attached to the process-sources lifecycle, which as you know from reading the lifecycles is run before compile. So typically you do not need to run this separatelty. But you could run the following to just format everything:

mvn clean process-sources

TODO: fill this out when we have a resources jar


Will compile all Java code under the src/main directory. Useful if you change something and want to run Emissary with the changes, but that is discussed in the section below about running.

mvn clean compile


Run all tests

mvn clean test

Inspect the output to see what tests have failed. More information about failed tests will be under target/surefire-tests should need to see more output for a failed test.

As configured, each run will execute the tests in random order. This is intentional to highlight dependencies between tests.

Currently, the POM used .5C for the forkCount when running tests. This means half the number of cores on your box. So if you have 4 cores, it will use 2 forks.

If you have a beefy box, you may see a speed up in tests by setting surefire.forkCount to something else. You can hardcode a number, like 4 or use #C to multiple by the number of cores. For example, but YMMV

mvn -Dsurefire.forkCount=2C clean test

Test one file

If you want to run just one test file from the command line, do this:

mvn clean test -Dtest=SomeClassTest

Skip tests

You should never do this, but if you need it, add the '-DskipTest' option to the Maven command to avoid running any tests


Create a jar with

mvn clean package


Currently there are no integration tests running. But when there are, those only run during the verify lifecycle, which is run before install. So if you just wanted to run up to the integration tests, do

mvn clean verify

Javadoc generation is also attached to verify phase.

Code quality

Code quality reports are also attached to the verify phase, but are in a couple of Maven profiles. To activate all the code quality profiles, run the following.

mvn clean package site -Dcode-quality site:stage -DstagingDirectory=${PWD}/target/staging

Because we are viewing locally, the stagingDirectory is necessary to tell Maven where to write the site. You may then view the reports by opening target/site/project-reports.html. The entire site is at target/staging/index.html

You can also run this to start a local server with the site

mvn clean package site -Dcode-quality site:run

Then hit http://localhost:9000

Code coverage

Since the whole site lifecycle can take some time, you can get just the code coverage with

mvn clean verify -Dcode-quality

Then open target/site/jacoco/index.html in your browser to see code coverage.

If you want to see more of the code quality outputs, you must run the full site generation from above


The install task runs after verify and copies any included artifacts made in the package phase to your local Maven repo. This repository is usually ~/.m2/repository unless you set to be somewhere else. See the settings.xml.sample file for details about moving that off. It is useful, for example, when your home directory is slow. You can move your Maven repo to a faster drive.

Run the install task with

mvn clean install

If any phase prior to install fails, no artifacts will be copied.


TODO: Write this documentation when we figure it out

Making a distribution

Full distributions create a zip bundle that you can move to another computer. To make a distribution, run:

mvn clean package -Pdist

After a successful build, there will be a bundle located in the target directory:


This zip file can be transferred to your cluster, extracted, configurations tweaked, then launched with the emissary script. Example to run a distribution:

tar -xvf emissary-<version>-dist.tar.gz
cd emissary-<version>
./emissary server -a 2

This starts a standalone emissary server. For more emissary options, see running emissary.

Running in Continuous Integration

If you wanted to run CI on a jenkins server or something similar, use the following command.

mvn clean install site -Dcode-quality -DskipFormat

The skipFormat is used because sort-pom and the source code formatter plugin rewrite files in the git working directory. This would not be good and could cause conflicts on the next checkout.

Remote debug a Maven test

It is straightforward to attach a remote debugger to your tests, as long as you tell the surefire plugin about it. Because Emissary runs tests in jvm forks, the mvnDebug command will not work. Here is an example of running. You have already seen the -Dtest= option. The -Dmaven.surefire.debug option will stop the JVM until a remote debugger attaches, and you can then step through the code.

mvn clean test -Dtest=ServerCommandTest -Dmaven.surefire.debug="-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000 -Xnoagent -Djava.compiler=NONE"

Potentially speed up the development cycle

The above commands all have 'clean' in the command. It is safest to do that. But it is possible to omit the 'clean' and maven will then only compile files that have changed. This can speed up things, but for Emissary it is unlikely to speed up much.

IDE integration

The poms are setup to use a different project.build.directory depending on the IDE you are using. The reason for this is so that running Maven on the command line and using your IDE will not interfere with each other. So keep in mind, your IDE will no longer build to the /target directory.

You can run/debug the emissary.Emissary class in your IDE. Just setup the same arguments you would use on the command line. Running tests in your IDE is also supported. IntelliJ and Netbeans will read the surefire plugin configuration and use it. Eclipse does not read the surefire configuration, so the process is explained below.


When Eclipse runs with the M2E, a system property named m2e.version is set. We can use that fact to activate the eclipse profile in the pom and then change the project.build.directory to target-eclipse. I will be a sibling to the target directory. The mvn clean command will not remove this directory, but cleaning from Eclipse should remove it.

The eclipse profile also configures some exclusions for the m2e plugin so it doesn't warn about not knowing how to handle some of the Maven plugins used.

When running tests in Eclipse, so you will need set the environment variable PROJECT_BASE to the directory eclipse is using to build the project. So in the run configuration of a test, add the following to the Run Configuration -> Environment -> New

PROJECT_BASE = ${project_loc}/target-eclipse

Unfortunately you will have to do this for every test, that's Eclipse with the M2E plugin for you.


When IntelliJ runs a Maven project, a system property named idea.version is set. We use this property to activate the intellij profile in the pom and change the project.build.directory to target-idea, which is a sibiling to the target directory. The mvn clean command will not remove this directory, but cleaning from IntelliJ should remove it.

Because we are using the jacoco plugin, we must put @argLine at the begining of the surefire plugin to allow jacoco to add what it needs. See the second answer here. But that can cause IntelliJ to report this if you run a test

Error: Could not find or load main class @{argLine}

The fix is to tell IntelliJ not to use that. Uncheck argLine from Preferences -> Build,Execution,Deployment -> Build Tools -> Maven -> Running Tests. See stackoverflow for more info


Unfortunately, I could not find a system property Netbeans sets when running a Maven project, so we are unable to automatically activate the netbeans profile like we can with the eclipse and intellij profile. Fortunately, it is easy to add a system property yourself.
Open Netbean's Preferences -> Java -> Maven and then add -Dnetbeans.ide into the Global Execution Options.
Afterwards, the netbeans profile will be activated in the pom and change the project.build.directory to target-netbeans, which is a sibiling to the target directory.
The mvn clean command will not remove this directory, but cleaning from Netbeans should remove it.

Running Emissary

There is one bash script in Emissary that runs everything. It is in the top level Emissary directory. This script setups up a few environment variables and Java system properties. It also sets up the classpath in a couple of ways. More on that in a minute. The script then runs the emissary.Emissary class which has several JCommander commands available to handle different functions.


The classpath is setup in one of 2 ways.


If the emissary script is a sibling to pom.xml, it is assumed you are running from the git checkout. If a file named .mvn-classpath exists, that file is used to setup the classpath. If that file does not exist, it created using a Maven command then the Emissary class is launched. NOTE if any dependencies are changed in the pom, then this file needs to be removed and regenerated.


If the emissary script does not have a pom.xml file as a sibling, then it is assuming you are running from distribution. If you run mvn clean package -Pdist, a tar.gz file will be placed in the target directory. Unpack that file, change into the directory and you will see the emissary but no pom, so it will load class from the lib directory

No arguments

If the emissary script is run without any arguments, you will get a listing of all the configuration subcommands and a brief description.



Running ./emissary help will give you the same output as running wiht no arguments. If you want to see more detailed information on a command, add the command name after help. For example, see all the arguments with descriptions for the what command, run:

./emissary help what

Common parameters

The rest of commands all have (-b or --projectBase) arguments that can be set, but it must match PROJECT_BASE. The config directory is defaulted to /config but can also be passed in with (-c or --config). When running from the git checkout, you should use target as the projectBase. Feel free to modify config files in target/config before you start.

Logging is handled by [logback](Logback Home). By default, the file logback.xml in the \<configDir> will be used. You can point to another file with the --logbackConfig argument. Feel free to modify the file in target/config if you want different logging. The logback.xml is configured to only log to the console currently.

See the help -c for each command to get more info.


This command will use the configured engines to identify the file. Emissary currently only comes with the SizeIdPlace, so the id will TINY or SMALL etc. See that class for more info. The -i or --input argument is required as well as -b. Here is how to run the command

./emissary what -i <path to some file>

Server (Standalone)

This command will start up an Emissary server and initialize all the places, a pickup place, and drop off filters that are configured. It will start in standalone mode if -m or --mode is not specified. By default, the number of MobileAgents is calculated based on the specs of the machine. On modern computers, this can be high. You can control the number of agents with -a or --agents. Here is an example run.

./emissary server -a 2

Without further configuration, it will start on http://localhost:8001. If you browse to that url, you will need to enter the username and password defined in target/config/jetty-users.properties, which is emissary and emissary123.

The default PickUpPlace is configured to read files from target/data/InputData. If you copy files into that directory, you will see Emissary process them. Keep in mind, only toUpper and toLower are configured, so the output will be to interesting.

Agents (Standalone)

The agents command shows you the number of MobileAgents for the configured host and what those agents are doing. By default, the port is 9001, but you can use -p or --port to change that.
Assuming you are running on 8001 from the server command above, try:

./emissary agents -p 8001

Pool (Standalone)

Pool is a collapsed view of agents for a node. It too defaults to port 9001. To run for the standalone server started above run

./emissary pool -p 8001

This command is more useful for a cluster as it a more digestable view of every node.


The Env Command requires a server to be running. It will ask the server for some configuration values, like PROJECT_BASE and BIN_DIR. With no arguments, it will dump an unformatted json response.

./emissary env

But you can also dump a response suitable for sourcing in bash.

./emissary env --bashable

Starting the Emissary server actually calls this endpoint and dumps out $PROJECT_BASE}/env.sh with the configured variables. This is done so that shell scripts can source $PROJECT_BASE}/env.sh and then have those variable available without having to worry with configuring them elsewhere.


The Run command is a simple command to execute the main method of the given class. For example

./emissary run emissary.config.ConfigUtil  <path_to_some_cfg_file>

If you need to pass flags to the main method, use -- to stop parsing flags and simply pass them along.

./emissary run emissary.config.ExtractResource -- -o outputdir somefile

Server (Cluster)

Emissary is fun in standalone, but running cluster is more appropriate for real work. The way to run clustered is similar to the standalone, but you need to -m cluster to tell the node to connect to other nodes. In clustered mode Emissary will also startup the PickUpClient instead of the PickUpPlace, so you will need to start a feeder.

Look at the target/config/peers.cfg to see the rendevous peers. In this case, there are 3. Nodes running on port 8001 and 9001 are just Emissary nodes. The node running on 7001 is the feeder. So let's start up 8001 and 9001 in two different terminals.

./emissary server -a 2 -m cluster
./emissary server -a 2 -m cluster -p 9001

Because these nodes all know about ports 8001, 9001 and 7001, you will see errors in the logs as they continue to try to connect.

Note, in real world deployments we don't run multiple Emissary processes on the same node. You can configure the hostname with -h.

Feed (Cluster)

With nodes started on port 8001 and 9001, we need to start the feeder. The feed command uses port 7001 by default, but we need to setup a directory that the feeder will read from. Files dropped into that directory will be available for worker nodes to take and the work should be distributed amongst the cluster. Let's startup the feed with

mkdir ~/Desktop/feed1
./emissary feed -i ~/Desktop/feed1/

You should be able to hit http://localhost:8001, http://localhost:9001 and http://localhost:7001 in the browser and look at the configured places. Drop some files in the ~/Desktop/feed1 and see the 2 nodes process them. It may take a minute for them to start processing

Agents (Cluster)

Agents in clustered mode again shows details about the mobileAgents. It starts at with the node you configure (localhost:9001 by default), then calls out to all nodes it knows about and gets the same information. Run it like so:

./emissary agents --cluster

Pool (Cluster)

Pool in clustered mode also does the same pool in standalone. It start at the node (locahost:9001) by default then goes to all the nodes it knows about and aggregrates a collapsed view of the cluster. Run it with

./emissary pool --cluster

Topology (Clustered)

The topology talks to the configured node (localhost:8001 by default) and talks to every it knows about. The response is what all those nodes know about, so you can build up a network topology of your cluster. Run it with

./emissary topology

Running server with SSL

The keystore and keystore password are in the emissary.client.EmissaryClient-SSL.cfg file. Included and configured by default is a sample keystore you can use for testing this functionality. It is not recommended to use in production environments. To use your own keystore, change configuration values in the emissary.client.EmissaryClient-SSL.cfg file.


./emissary server -p 8443 --ssl


./emissary server -p 8443 --ssl --mode cluster
./emissary server -p 9443 --ssl --mode cluster
mkdir ~/Desktop/feed1
./emissary feed -p 7443 --ssl -i ~/Desktop/feed1/

Debugging a command

The emissary script allows you to remote debug any command. Simply use DEBUG=true before the command and it will wait for a remote debugger to attach at port 8000. If you want to change that port, use DEBUG_PORT instead.
For example:

DEBUG=true ./emissary <command> <command_options>


DEBUG_PORT=<someport> ./emissary <command> <command_options>

You can also debug what the script is doing by prepending the following


This will output the command that is going to be run and show you how JCommander is parsing the arguments.

Setting up remote debugging in your IDE

For info on how to setup remote debugging in your IDE, reference the following articles:

Coding standards

Many of coding standards are defined the formatter config file. Here are some additional things to consider when developing.


Hanging tests

Emissary uses fake host names in some of the tests. If you happen to be on a Verizon network, you may get redirected for unknown hosts to To test this run

nslookup somehost

If you have this situation, then it is recommended to Google's DNS servers. Here are some instructions from Google.

Can't run tests in Eclipse

Did you read the Eclipse section carefully?

Running tests in IntelliJ throw an error "Error: Could not find or load main class @{argLine}"

Did you read the IntelliJ section carefully?