ARMI (pronounced "army") is an alternative to Java's built-in RMI. Like RMI, ARMI can make synchronous remote method calls. But ARMI can also support asynchronous messaging. ARMI also works around common client/server headaches, like NAT and firewalls.

[January 17, 2012] ARMI is open-source.
Download at
http://hg.augursystems.com/armi/overview

Synchronous

When a program calls a method, it will wait (block) until the method returns with a value, or void. Remote methods work the same way. So a client application will block until the method on the remote server returns. This call/response scenario is called synchronous.

All method calls are synchronous by nature. Do not confuse this term with the synchronized modifier in Java. The word synchronous means "as the same time". So if you think of a method from the perspective of the caller, the method's return value is received by the caller at the same time the call is made, because time essentially stands still for the caller (the CPU no longer executes the caller's code) while the method is executing.

Sometimes this blocking nature of method calls is inconvenient. For example, what if your client application is supposed to display ever-changing data from a server, for example, stock quotes? To keep your client viewer up-to-date, you must periodically call a method on the server to get updates, probably every few seconds. This is a form of polling, also called client-pull.

Client-Pull is Not Great

If you ask the server for an update too often, then the server will often have nothing new to give you. Each time that happens, it wastes a method call, and all the associated network and CPU resources. But if you poll less often, then your client will often receive old news. Neither situation is ideal. It would be better if the server could automatically update your client immediately, only when new data is available.

Asynchronous

In asynchronous communication, the client subscribes to receive responses from the server by calling a remote method on the server, just once. This initial method call immediately returns normally (synchronously). The subscribed responses will come later (asynchronously).

In that initial method call, the client tells the server where to send response(s). In a stock viewer example, we might want responses to go to a StockListener.handleUpdate() method on the client. At some time later, the server can publish data by calling that method. This is also called also known as a subscribe/publish model, or server-push.

Can RMI Be Asynchronous?

You might wonder, "Can't I do this in RMI?" While server-push works fine with RMI in the lab, it fails in the real world, mainly due to NAT firewalls in the network that prevent connections in that direction. In general, a client can initiate communication with a server, but not vice versa. RMI does not provide any way around that limitation.

ARMI

ARMI solves the server-push problem by keeping the client's original socket open, as a permanent two-way communications link. NAT firewalls permit the client-to-server connection, and then ARMI uses the connection for server-to-client calls too.

No Port Chaos

That same socket is reused for all subsequent ARMI communications between the two JVMs, including other method calls and responses. So ARMI avoids a core problem of RMI: port management.

Every RMI Remote object (anything that extends UnicastRemoteObject) sets up its own network server socket port. This makes it difficult to configure firewalls to permit RMI traffic; there are so many ports to manage.

Instead of all that, ARMI multiplexes all method calls onto a single socket. And that socket's port number is fixed, so there's no need for a registry.

Performance

Every time you call an RMI remote method, a new socket connection is set up and torn down. ARMI avoids that overhead cost by maintaining one socket. (You might worry that ARMI's one socket might cause multiple calls to queue up, but ARMI handles each call in a separate thread.)

No Stubs

In RMI, stub classes are generated by an RMI compiler that you must run for each of your remote server classes. Then you must include those stubs in your client jar files. (Each stub handles the network connections and serialization when your client wants to call methods on the remote server object.) ARMI simply eliminates this unnecessary complexity. No stubs.

Asynchronous Data

In the subscribe/publish model (asynchronous mode), all kinds of data may be thrown at the ARMI hub. Your data-generating code does not need to know if anyone is listening, which keeps your code simple. You can just publish data, and ARMI routes is to the right subscribers.

Types and Flavors

You label published data with a type (for example "Stock Quote"), and optionally a flavor (for example, stock ticker symbol "IBM"). When each client subscribes, it specifies a preferred type and optionally a flavor. When new data matches a type and flavor, ARMI automatically routes it to all matching subscribers.

Filters

The subscriber can also specify a filter so that ARMI can be even more selective about which data is sent across the network. The filter is just an object that implements ARMI's Filter interface. The ARMI server will use the filter to further test all data that already matched the subscribed type and flavor. For example, a client might subscribe to the "Stock Quote" data type, with a null flavor (match all ticker symbols). Then a filter might contain a list of several stocks to match.

Since a filter is Java code that you write, you can get very fancy. For example, a complex filter might perform a statistical analysis or reference an external database before approving data.

>>Small<<

The entire ARMI library currently fits in a 32 kb jar.

Why?

Our flagship Augur product uses RMI extensively. RMI has served us well, especially once we finally figured out how to work around NAT issues via some tricky coding. Our next step was to reduce our port usage, which would require a bit of a re-write anyway. But we also wanted to asynchronously feed Augur's Alert Viewer so that app could be more efficient and time-sensitive. That required something new.

Why free? Well honestly, it's about time we gave something back to the software community. (We've benefitted greatly from so many freebies: Apache, Tomcat, Ant, SNMP4J, IDEs, and more!) Also, our very old and humble RMI Tips web page still consistently gets hits every day. (Perhaps that's how you stumbled upon ARMI too?) And the Google search terms that people use to get there have been very telling. It seems that we are not alone in our RMI difficulties. So we'd like to do something about it.

Open Source!

Download at:
http://hg.augursystems.com/armi/overview

Code Example

The following code snippets demonstrate some typical tasks. The examples refer to a stock quote viewer application just for illustration. A full tutorial is available at the ARMI project's home: http://hg.augursystems.com/armi/overview

Call a Regular (Synchronous) Method

import com.augursystems.armi.*;

// ...

Armi armi = new Armi(); // Instantiate an ARMI server

 

// Let's call market.getMarketIndex("DJX", "20") on a remote ARMI server...

Serializable[] params = new Serializable[] { "DJX", "20" }; // An array for the method's arguments

Serializable value = armi.call(remoteServer,"Market", "getMarketIndex", params); // The remote call

System.out.println("The Dow Jones index is" + value);

 

Register a Remote Service

armi.acceptRemoteClients(null, Armi.DEFAULT_PORT); // Turn on the server.

 

// Create your ARMI service object, e.g. a model of a stock market.

// No "Remote" interface necessary; all public methods are automatically accessible...

StockMarket market = new StockMarket();

// Register your service with ARMI...

armi.registerService("Market", market);

// "Market" is an instance name that remote callers can reference

Register a Client to Receive Asynchronous Data

// Subscribers must implement the Client.handlePacket() interface method

Client client = new Client()

{

  public void handlePacket(final Packet p)

  {

    try

    {

      Object quote = p.decodeInstance(); // Let ARMI de-serialize (unmarshal) the object for you

      System.out.println("Received stock quote: "+ quote);

    }

    catch (Exception e) { System.out.println("Problem unmarshalling a quote: "+e; }

  }

}

// Point to the remote server; use default port...

HostPort remoteServer = new HostPort("192.168.1.111");

// Ask the remoteServer to send "StockQuote" data types, with any flavor (null), and no filter (null), to our client

armi.subscribeRemote(remoteServer, StockQuote.class.getName(), null, null, client);

Publish Data

// The following code would presumably be running on the remote server that publishes data for clients...

StockQuote quote = new StockQuote("ACME", "$8.94"); // Create a datum representing a stock ticker value

 

// Publish the data so the server can transmit it to any subscribers...

armi.transmit(quote, "NASDAQ");

// The data type is automatically set to the class name via quote.getClass(), e.g. "com.bank.myapp.StockQuote"

// The flavor is specified as the "NASDAQ" stock exchange, which may be a useful classification

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