Building your first module: Hello, World

Distributed web applications can resemble a creature from the imagination of Mary Shelley. "You do what with Postgres?" "Kafka’s doing that in your cluster?" And the appeal is understandable; no one ever accused Dr. Frankenstein of living a dull life.

Still, not everyone is cut out for the mad genius lifestyle. Perhaps you yourself have wondered what it would be like to live a more predictable life, to use a single tool built from the ground up to handle your distributed needs without requiring two tons of duct tape and shell scripts.

Whether you’re building the next Twitter or bootstrapping a micro-niche SaaS, Rama is a distributed-first scalable programming platform that you can use to build the entire data layer of your application.

As you learn to build Rama applications, you might find your instincts fighting the design decisions you encounter. To ease your transition, this tutorial takes pains to identify differences with standard development practice and to show the rationale for the choices. Some of the differences won’t fully make sense until later in the tutorial. But like astronauts adjusting to zero-gravity, you’ll soon find yourself adjusting to your new environment and feeling more powerful than ever.

Let’s start learning. To begin, you’ll learn the basics of writing and running a Rama application. Just as Rama is new and exciting, your application will do something new and exciting: print hello world.

Setting up your sandbox

To get started, you’ll need to clone the rama-examples project on Github.

The rama-examples project uses Maven to manage dependencies. Starting a process and loading all the Rama classes takes a little while (30 seconds to a minute), so the easiest way to try out the examples is via an interactive shell. rama-examples is configured so you can easily launch a Groovy shell like so:

mvn compile
mvn gplus:shell

You must run mvn compile first so the examples are available inside the shell. From this shell you’ll be able to easily run the code examples in this tutorial.

Running a module on a cluster

Now that you’re all set up, let’s work with some code! Here’s how to run an application that prints hello world:

package example;

import com.rpl.rama.*;
import com.rpl.rama.module.*;
import com.rpl.rama.ops.*;
import com.rpl.rama.test.*;

public class HelloWorldModule implements RamaModule {
  public void define(Setup setup, Topologies topologies) {
    setup.declareDepot("*depot", Depot.random());
    StreamTopology s ="s");
     .each(Ops.PRINTLN, "*data");

  public static void main(String[] args) throws Exception {
    try (InProcessCluster cluster = InProcessCluster.create()) {
      cluster.launchModule(new HelloWorldModule(), new LaunchConfig(1, 1));

      Depot depot = cluster.clusterDepot(HelloWorldModule.class.getName(), "*depot");
      depot.append("Hello, world!!");

To run the example, try this command from the Groovy shell:

groovy:000> rama.examples.tutorial.HelloWorldModule.main(null)

The first time you run an InProcessCluster, the JVM will take 30 seconds to a minute loading all the Rama classes. Once everything has finished loading, you should see Hello, world!! printed. Subsequent examples you run in the same shell will run very quickly.

At a macro level, here is what the code does:

  1. It creates an in-process cluster. Clusters are the computation environment.

  2. It defines a Rama module, HelloWorldModule. Modules are executables.

  3. It launches the module on the cluster.

  4. It appends data to the module’s depot.

  5. The appended data is sourced from the depot in a streaming topology.

  6. The topology performs an operation on the data, which results in the string being printed.

By the end of this tutorial you’ll have a precise idea of what all this means. The first step to getting there is explaining the larger context in which Rama applications live.

The big picture

Rama integrates and generalizes data ingestion, processing, indexing, and querying. The "integrates" aspect means it encompasses a suite of responsibilities that aren’t normally packaged in a single tool. The "generalizes" part means Rama is unrestricted in the kinds of applications it can build, including applications that require huge scale. This means Rama can build complex application backends on its own in a small amount of code that previously required dozens of tools and a lot of code. In many cases the code difference is over 100x.

It can help to think of Rama as a combination of operating system and compiler. The metaphor is not perfect but hopefully proves a useful frame of reference as you examine the individual pieces.

As a developer, you write code that produces some kind of artifact that directs computation in some kind of execution environment. Your code is tailored to the execution environment, making use of whatever resource abstractions the environment provides – filesystem, sockets, signals and other whatnots.

Execution environments include operating systems, the JVM, the browser, etc. All of these environments provide virtual resources, abstracting away the details of resource management so that you can program at a higher level. A filesystem is a virtual layer on top of IO devices; an OS process is a virtual resource for managing execution; the DOM is a virtual resource, too.

Rama was created in response to the proliferation of low-cost distributed computing resources. The Rama platform provides an execution environment and a compiler for that environment that enables you to write distributed applications without having to manage the underlying resources. It exposes a pure Java API for that purpose.

In the same way that you can write read(f, buf, size) in C to read from a file stream without having to know anything about how the OS interacts with hard drives, Rama lets you write computations at a high level and seamlessly distributes them across machines and processes.

With this metaphor in place, let’s look at the Rama execution environment: the cluster.


The example code’s main method starts an in-process cluster with InProcessCluster cluster = InProcessCluster.create(). There are a couple things to unpack here: what’s a cluster, and what’s an in-process cluster?

Let’s start by exploring what we mean by cluster, both conceptually and concretely. The term is used with slightly different meanings in different contexts, so let’s define it precisely.

Conceptually, a cluster is Rama’s execution environment. Similar to how you start a program by telling an operating system to run it, you start a Rama module by telling a cluster to launch it (launching modules is covered in the next section).

Concretely, a cluster consists of a group of cooperating, networked machines (nodes); this is the general usage of the term "cluster" you’ve probably encountered. A Rama cluster refines this notion by specifying three different roles for the processes running on the machines in a cluster:

  • Conductor

  • Supervisor

  • Worker

Worker processes are responsible for, well, doing work – they perform the compute operations you specify. There can be multiple workers per machine. Workers are managed by Supervisors.

Supervisor processes monitor worker health and start or stop worker processes according to the specification you’ve given for how work should be distributed. There’s one supervisor per machine.

The Conductor orchestrates the steps involved in launching and updating modules.

Here’s what a Rama cluster could look like with two modules deployed, "SocialGraphModule" and "TimelineModule":

cluster overview

One Conductor exists for the whole cluster, and each other node has a Supervisor responsible for all the worker processes on it. Each module is partitioned across one or more workers in order to parallelize its work. Everything a module does, such as data ingestion via "depots", data processing via "topologies", and indexing via "PStates", is colocated within the same set of worker processes.

Rama gives you tools for creating clusters in a data center, on the cloud, or on your local machine. You can also create in-process clusters.

An in-process cluster (IPC) is a virtual cluster that runs within a single JVM process. Starting and stopping a fleet of machines and their conductor, supervisors, and worker processes takes time. By using an in-process cluster instead of a real cluster, you get faster feedback, which is better for learning. In-process clusters are also used for unit testing the behavior of modules.

In the example above, we create an in-process cluster and launch a module on it with:

InProcessCluster cluster = InProcessCluster.create()
cluster.launchModule(new HelloWorldModule(), new LaunchConfig(1, 1));

You’ll continue to see this as we work through examples. Just keep in mind that IPCs are a development tool, and that you’re unlikely to use them outside of tests and tutorials. In a real project, you wouldn’t have main methods on your modules and your use of IPCs would be in tests.

To sum up: a Rama cluster is a group of cooperating machines and processes responsible for performing the work defined in Rama modules. Modules run on clusters. We use in-process clusters for learning and development because they provide tight feedback loops.

All of these topics (conductors, workers, supervisors, modules, work) will be explained fully throughout this tutorial, but hopefully this gives you a decent idea of what’s going on.


Modules are where we access data and manipulate it. If a cluster is the execution environment, a module is the executable. You create modules to do work. We can understand modules in terms of their definitions and their runtime instantiation.

To define a module, you create a class that implements the RamaModule interface, which consists of the define method. define takes two arguments, setup and topologies, which you’ll learn more about in the next section. In the Hello World example, we define a module named HelloWorldModule.

Different execution environments expose different IO resources and interfaces. *nix operating systems, for example, employ the universal IO model of files and file operations (read, write, open, close). In Rama, the core IO resource is the depot.

Depots exist at the boundary between the external world and your system: clients append data to depots, and module ETLs read the data, perform work with it, and index it. (ETL stands for extract-transform-load, which refers to the general process of reading data from data stores, performing computations on it, and storing the result.) Queries read and transform stored data for consumption by user interfaces, APIs, and other systems that rely on your data.

Depots are defined within and belong to modules. The Hello World module defines a depot with setup.declareDepot("*depot", Depot.random());. The first argument "*depot" is the depot’s name. The * at the beginning of the name is important: strings beginning with * are variables that can be referred to later on in topology code. A depot’s name always begins with a *. The second argument, Depot.random(), specifies the depot’s partitioning scheme; more on that later.

The next few lines define a topology:

StreamTopology s ="s");
 .each(Ops.PRINTLN, "*data");

This creates a stream topology. Topologies are a world unto themselves that we’ll dedicate time and space to, but for now you can think of this topology as a kind of process with streaming performance and consistency characteristics: it operates on data as the data is received, rather than periodically performing batch operations. The overall effect is that, as data is appended to "*depot", it’s processed by the topology such that every append is printed.

There are other kinds of ETLs with different performance and consistency characteristics, and we’ll learn about those in a later section.

So that’s what’s involved in defining a module. Now let’s look at their runtime instantiations.

Launching a module

This section explains how to launch modules on an in-process cluster. Launching modules on a real cluster is done through Rama’s command-line client. The concepts are the same, though there are more options for launch on the command-line like replication factor and config overrides.

After crafting your beautiful new module, you’ll want to run it. To do that, you launch it on a cluster. When you launch a module, you register it with a cluster and you give the cluster some specifications around resource allocation. In the example we do that with this code:

cluster.launchModule(new HelloWorldModule(), new LaunchConfig(1, 1));

cluster is an instance of an InProcessCluster and its launchModule method takes two arguments, a module instance and a LaunchConfig instance. The launch config specifies how many partitions a module should have, as well as the number of workers and threads that should be dedicated to those partitions. In upcoming sections you’ll learn more about what it means to specify a launch config and how the choices you make in a launch config affect the runtime characteristics of your module.

It’s worth pointing out that the LaunchConfig strains the "execution environment / executable" metaphor a little bit, but you can consider it to be similar to how you can specify the max heap size and other resource constraints when starting a Java process. Runtime environments can provide knobs for you to tune an executable’s resource consumption, it’s just that in Rama you’re required to specify these settings.

Launching a module on a real cluster results in Java processes getting created on your cluster’s machines – these are the worker processes mentioned earlier. These worker processes run the module’s code, and they’ll start an event loop that listens for depot appends to kick off ETLs to process those appends. (In an IPC these worker processes are simulated as threads – otherwise IPC would be incredibly slow!)

Most programming languages specify some kind of entry point for a program, some variation of main that tells the runtime environment what sequence of steps to take. Your module has no such entry point; instead, you create a kind of event-driven dataflow pipeline that processes data as added.

Appending data to depots

After you’ve launched a module you can start appending data to its depots. Appending data to depots is the main means of conveying information from the outside world into your system. Modules can also perform further depot appends, both to their own depots and to depots belonging to other modules.

In the example, we appended data to the depot thusly:

String moduleName = HelloWorldModule.class.getName();
Depot depot = cluster.clusterDepot(moduleName, "*depot");
depot.append("Hello, world!!");

Here, we’re retrieving a depot object by looking it up with cluster.clusterDepot, and then appending to it directly with depot.append. After you append data to a depot, the ETLs sourcing that depot will receive the new data and process it.

Processing data with ETLs

When you define an ETL, you specify which depots it should source – you can see this in the example with s.source("*depot"). When an ETL sources a depot, it effectively listens for new depot appends and reactively performs operations on that data. In the example, we only perform the Ops.PRINTLN operation.

In real ETLs, you usually produce indexes that can satisfy application queries. Indexes are called "partitioned states" (PStates) in Rama and they can be fine-tuned to precisely the shapes needed for handling all your querying needs. You’ll learn a lot more about this later.

We’re going to spend a lot of time covering exactly how to define ETLs. They embody a programming style that may be new to you, so they’ll take some time getting used to. This can be daunting, but it’s also exciting: ETLs provide immense expressive power, allowing you to concisely describe distributed operations at a level of abstraction that isn’t possible with any other tool. By learning to write them, you’ll not only become proficient at Rama, you’ll expand your mental programming toolkit. In the same way that learning about declarative programming is useful if you’ve only ever been exposed to imperative programming, learning about how Rama ETLs work will give you a new way to think about how to solve problems in your work, even outside of using Rama.

Querying data

While processing and indexing data can be a life-enriching activity in and of itself, most of the time you’ll need to then retrieve the data so you can use it. Ultimately, you’re building a system that takes source data and processes it for consumption. ETLs perform some of that processing, indexing the results, and queries might perform further computation.

As in other systems, you make tradeoffs between precomputing some results and deriving some results at query time. Rama has two ways of doing queries: point queries, where you ask for one piece of information from one index on one module, and predefined queries, where your query is itself a full distributed computation that can talk to many indexes on many modules. Predefined queries are defined using Rama’s query topology API. Rama differs from other systems in that you’ll use the same computation API for both ETLs and query topologies. Being able to use the same API and concepts in both contexts is another way that simplifies the process of developing distributed applications, as you’ll see. We’ll look at querying PStates more on the next page.


In this section, you learned about Rama basics:

  • Rama applications run on clusters

  • Modules are Rama’s executables

  • You launch a module on a cluster

  • Modules contain depots, ETLs, PStates, and query topologies

  • You append data to depots

  • ETLs source data from depots and perform operations on that data