6. State of the World¶

AMPS State of the World (SOW) allows you to automatically keep and query the latest information about a topic on the AMPS server, without building a separate database. Using SOW lets you build impressively high-performance applications that provide rich experiences to users. The AMPS Java client lets you query SOW topics and subscribe to changes with ease.

Performing SOW Queries¶

To begin, we will look at a simple example of issuing a SOW query.

...

public void executeSOWQuery(Client client) {

    for (Message m : client.sow("messages-sow", "/symbol = 'ROL'")) {
        if (m.getCommand() == Message.Command.GroupBegin) {
            System.out.println("--- Begin SOW Results ---");
        }
        if (m.getCommand() == Message.Command.GroupEnd) {
           System.out.println("--- End SOW Results ---");
        }
        if (m.getCommand() == Message.Command.SOW) {
            System.out.println(m.getData());
        }
    }
}
...

Example 6.1: Basic SOW query

In listing Example 6.1 the executeSOWQuery() method invokes Client.sow() to initiate a SOW query on the orders topic, for all entries that have a symbol of ’ROL’.

As the query executes, the body of the loop processes each matching entry in the topic. Messages containing the data of matching entries have a Command of value sow; so as those arrive, we write them to the console. AMPS sends a begin_group message at the beginning of the results and an end_group message at the end of the results. We use those messages to delimit the results of the query.

As with subscribe, the sow command also provides an asynchronous version, as well as versions that accept a Command. For example, the listing below shows an asynchronous SOW command that specifies the batch size, or the maximum number of records that AMPS will return at a time.

public void executeSOWQuery(Client client) {

    Command command = new Command(Message.Command.SOW)
                       .setTopic("orders")
                       .setFilter("/symbol = 'ROL'")
                       .setBatchSize(100);

    client.executeAsync(command, new MessagePrinter());
}

...

public class MessagePrinter implements MessageHandler {
    public void invoke(Message m) {
        if (m.getCommand() == Message.Command.SOW) {
            System.out.println(m.getData());
        }
    }
}

Example 6.2: Asynchronous SOW query

Samples of Querying a Topic in the SOW¶

The Java client distribution includes the following samples that demonstrate how to query a topic in the State-of-the-World.

Sample Name	Demonstrates
`EX03AMPSSOWConsolePublisher.java`	Publishing messages to a State-of-the-World topic.
`EX04AMPSSOWConsoleSubscriber.java`	Querying messages from a State-of-the-World topic.

SOW and Subscribe¶

Imagine an application that displays real time information about the position and status of a fleet of delivery vans. When the application starts, it should display the current location of each of the vans along with their current status. As vans move around the city and post other status updates, the application should keep its display up to date. Vans upload information to the system by posting message to a van location topic, configured with a key of van_id on the AMPS server.

In this application, it is important to not only stay up-to-date on the latest information about each van, but also to ensure all of the active vans are displayed as soon as the application starts. Combining a SOW with a subscription to the topic is exactly what is needed, and that is accomplished by the AMPS sow_and_subscribe command. Now we will look at an example:

private void updateVanPosition(Message message) {
    switch (message.getCommand()) {
        case Message.Command.SOW:
        case Message.Command.Publish:
            /* For each of these messages, addOrUpdateVan() presumably adds the van
             * to our application’s display. As vans send updates to the AMPS server,
             * those are also received by the client because of the subscription
             * placed by sowAndSubscribe(). Our application does not need to distinguish
             * between updates and the original set of vans we found via the SOW
             * query, so we use addOrUpdateVan() to display the new position of vans
             * as well.
             */
            addOrUpdateVan(message);
            break;
        case Message.Command.OOF:
            removeVan(message);
        break;
    }
}

public void subscribeToVanLocation(Client client) {
    try {
        Command command = new Command("sow_and_subscribe")
                            .setTopic("van_location")
                            .setFilter("/status = 'ACTIVE'")
                            .setBatchSize(100)
                            .setOptions("oof");


        /* we issue a sowAndSubscribe() to begin receiving information about all of
         * the active delivery vans in the system. All of the vans in the system
         * now are returned as Messages whose getCommand() returns SOW.
         */
        for (Message message : client.execute(command)) {
                updateVanPosition(message);
        }
    }
    catch (AMPSException aex) {
        System.err.println("TestListener caught exception.");
    }
}

public void addOrUpdateVan(message) {

    // use information in the message to add the van or update
    // the van position

    ...

}

public void removeVan(message) {

   // use information in the message to remove information on
   // the van position

   ...
}

Example 6.3: Using sowAndSubscribe

Now we will look at an example that uses the asynchronous form of sowAndSubscribe:

public class VanPositionUpdater {

    public void invoke(Message message) {
        updateVanPosition(message);
    }

    private void updateVanPosition(Message message) {
        switch (message.getCommand()) {
            case Message.Command.SOW:
            case Message.Command.Publish:
                addOrUpdateVan(message);
                break;
            case Message.Command.OOF:
                removeVan(message);
                break;
        }
    }

    public void addOrUpdateVan(message) {
       // use information in the message to add the van or update
       // the van position
       ...
     }

     public void removeVan(message) {
      // use information in the message to remove information on
      // the van position
      ...
     }
}


public void subscribeToVanLocation(Client client) {
    try {

        VanPositionUpdater vp = new VanPositionUpdater();
        Command command = new Command("sow_and_subscribe")
                            .setTopic("van_location")
                            .setFilter("/status = 'ACTIVE'")
                            .setBatchSize(100)
                            .setOptions("oof_enabled");

        client.execute(command, vp);

    }
    catch (AMPSException aex) {
        System.err.println("TestListener caught exception.");
    }
}

Samples of Querying and Subscribing to a Topic in the SOW¶

The Java client distribution includes the following samples that demonstrate how to query and subscribe to a topic in the State-of-the-World.

Sample Name	Demonstrates
`EX05AMPSSOWandSubscribeConsoleSubscriber.java`	Querying a State-of-the-World topic and entering a subscription to the topic in a single atomic operation.
`EX06AMPSSowAndSubscribeWithOOF.java`	Querying a State-of-the-World topic and entering a subscription to the topic in a single atomic operation, while registering for notification that a previously-matching message is no longer a match for the subscription.
`EX07AMPSSOWUpdater.java`	Updating records in a State-of-the-World topic.

Setting Batch Size¶

The AMPS clients include a batch size parameter that specifies how many messages the AMPS server will return to the client in a single batch when returning the results of a SOW query. The 60East clients set a batch size of 10 by default. This batch size works well for common message sizes and network configurations.

Adjusting the batch size may produce better network utilization and produce better performance overall for the application. The larger the batch size, the more messages AMPS will send to the network layer at a time. This can result in fewer packets being sent, and therefore less overhead in the network layer. The effect on performance is generally most noticeable for small messages, where setting a larger batch size will allow several messages to fit into a single packet. For larger messages, a batch size may still improve performance, but the improvement is less noticeable.

In general, 60East recommends setting a batch size that is large enough to produce few partially-filled packets. Bear in mind that AMPS holds the messages in memory while batching them, and the client must also hold the messages in memory while receiving the messages. Using batch sizes that require large amounts of memory for these operations can reduce overall application performance, even if network utilization is good.

For smaller message sizes, 60East recommends using the default batch size, and experimenting with tuning the batch size if performance improvements are necessary. For relatively large messages (especially messages with sizes over 1MB), 60East recommends explicitly setting a batch size of 1 as an initial value, and increasing the batch size only if performance testing with a larger batch size shows improved network utilization or faster overall performance.

Example 6.4: Asynchronous sowAndSubscribe

Client-Side Conflation¶

In many cases, applications that use SOW topics only need the current value of a message at the time the message is processed, rather than processing each change that lead to the current value. On the server side, AMPS provides conflated topics to meet this need. Conflated topics are described in more detail in the AMPS User Guide, and require no special handling on the client side.

In some cases, though, it’s important to conflate messages on the client side. This can be particularly useful for applications that do expensive processing on each message, applications that are more efficient when processing batches of messages, or for situations where you cannot provide an appropriate conflation interval for the server to use.

A MessageStream has the ability to conflate messages received for a subscription to a SOW topic, view, or conflated topic. When conflation is enabled, for each message received, the client checks to see whether it has already received an unprocessed message with the same SowKey. If so, the client replaces the unprocessed message with the new message. The application never receives the message that has been replaced.

To enable client-side conflation, you call conflate() on the MessageStream, and then use the MessageStream as usual:

// Query and subscribe
MessageStream results =
    ampsClient.sowAndSubscribe("orders", "/symbol == 'ROL'");

// Turn on conflation
results.conflate();

// Process the results
foreach (Message m : results)
{
   // Process message here
}

Notice that if the MessageStream is used for a subscription that does not include SowKeys (such as a subscription to a topic that does not have a SOW), no conflation will occur.

When using client-side conflation with delta subscriptions, bear in mind that client-side conflation replaces the whole message, and does not attempt to merge deltas. This means that updates can be lost when messages are replaced. For some applications (for example, a ticker application that simply sends delta updates that replace the current price), this causes no problems. For other applications (for example, when several processors may be updating different fields of a message simultaneously), using conflation with deltas could result in lost data, and server-side conflation is a safer alternative.

Managing SOW Contents¶

AMPS allows applications to manage the contents of the SOW by explicitly deleting messages that are no longer relevant. For example, if a particular delivery van is retired from service, the application can remove the record for the van by deleting the record for the van.

The client provides the following methods for deleting records from the SOW.

sowDelete accepts a filter, and deletes all messages that match the filter
sowDeleteByKeys accepts a set of SOW keys as a comma-delimited string and deletes messages for those keys, regardless of the contents of the messages. A SOW key is provided in the header of a SOW message, and is the internal identifier AMPS uses for that SOW message
sowDeleteByData accepts a message, and deletes the record that would be updated by that message

The most efficient way to remove messages from the SOW is to use sowDeleteByKeys or sowDeleteByData, since those options allow AMPS to exactly target the message or messages to be removed. Many applications use sowDelete, since this is the most flexible method for removing items from the SOW when the application does not have information on the exact messages to be removed.

Regardless of the command used, AMPS sends an OOF message to all subscribers who have received updates for the messages removed, as described in the previous section.

The simple form of the sowDelete command returns a MessageStream that receives the response. The response is an acknowledgment message that contains information on the delete command. For example, the following snippet simply prints informational text with the number of messages deleted:

for (Message msg : client.sowDelete("sow_topic", "/id IN (42, 64, 37)")) {
    System.out.println("Got an " + msg.getCommand() +
                      " containing " + msg.getAckType() + ": " +
                      " deleted " + msg.getMatches() + " messages.");
}

You can also use client.execute to send a SOW delete command. As with the other SOW methods, the client provides an asynchronous versions of the SOW delete commands that require a message handler to be invoked.

Acknowledging messages from a queue uses a form of the sow_delete command that is only supported for queues. Acknowledgment is discussed in the chapter on queues.

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