4. Subscriptions

Messages published to a topic on an AMPS server are available to other clients via a subscription. Before messages can be received, a client must subscribe to one or more topics on the AMPS server so that the server will begin sending messages to the client. The server will continue sending messages to the client until the client unsubscribes, or until the client disconnects. With content filtering, the AMPS server will limit the messages sent to only those messages that match a client-supplied filter. In this chapter, you will learn how to subscribe, unsubscribe, and supply filters for messages using the AMPS C# client.

Subscribing

Subscribe to an AMPS topic by calling Client.subscribe(). Here is a short example (error handling and connection details are omitted for brevity):

class MyApp
{
    public static void Main()
    {

        // Here, we create a Client. We protect the Client in a using block so that the
        // connection and subscriptions are properly cleaned up when dispose() is called.
        using(Client client = new Client("subscribe"))
        {

            client.connect("tcp://127.0.0.1/9007/amps");
            client.logon();


            // Here we subscribe to the topic messages. We do not provide a filter, so the
            // subscription receives all of the messages published to the topic, regardless of
            // content. The foreach loop, iterates over the messages returned by the message
            // stream. When we no longer need to subscribe, we can break out of the loop. When
            // the MessageStream is disposed, the client sends an unsubscribe command to AMPS
            // and stops receiving messages.
            foreach(Message m in client.subscribe("messages"))
            {

                // Within the loop, we process the message. In this case, we simply print the
                // contents of the message.
                System.Console.Writeline(m.getData());
            }
        }
    }
}

Example 4.1: Subscribing to a topic

AMPS creates a background thread that receives the messages and copies them into the MessageStream that you iterate over. This means that the client application as a whole can continue to receive messages while you are doing processing work.

The simple method described above is provided for convenience. The AMPS C# client provides convenience methods for the most common forms of the commands. The client also provides an interface that gives you precise control over the command. Using that interface, the example above becomes:

class MyApp
{
    public static void Main()
    {

        // Here, we create a Client. We protect the Client in a using block so that the
        // connection and subscriptions are properly cleaned up when dispose() is called.
        using(Client client = new Client("subscribe"))
        {
            client.connect("tcp://127.0.0.1/9007/amps");
            client.logon();

            // We create a Command object to subscribe to the messages topic.
            Command command = new Command("subscribe").setTopic("messages");

            // Here we execute the command and subscribe to the topic messages. This works
            // exactly the same way as the command in Example 3.1 We do not provide a filter,
            // so the subscription receives all of the messages published to the topic,
            // regardless of content.
            // The foreach loop, iterates over the messages returned by the message stream.
            // When we no longer need to subscribe, we can break out of the loop. When the
            // MessageStream is disposed, the client sends an unsubscribe command to AMPS
            // and stops receiving messages.
            foreach(Message m in client.execute(command))
            {
                // Within the loop, we process the message. In this case, we simply print the
                // contents of the message.
                System.Console.WriteLine(m.getData());
            }
        }
    }
}

Example 4.2: Subscribing to a topic

The Command interface allows you to precisely customize the commands you send to AMPS. For flexibility and ease of maintenance, 60East recommends using the Command interface (rather than a named method) for any command that will receive messages from AMPS. For publishing messages, there can be a slight performance advantage to using the named commands where possible.

Asynchronous Message Processing Interface

The AMPS C# client also supports an asynchronous interface. In this case, you add a message handler to the call to the subscribe. The client returns the command ID of the command submitted to AMPS, and returns once the server has acknowledged that the command has been processed. As messages arrive, AMPS calls your message handler directly on the background thread. This can be an advantage for some applications. For example, if your application is highly multithreaded and copies message data to a work queue processed by multiple threads, there is usually a performance benefit to enqueuing work directly from the background thread. See Understanding Threading for a discussion of threading considerations, including considerations for message handlers.

As with the simple interface, the AMPS client provides both convenience interfaces and interfaces that use a Command object. The following example shows how to use the asynchronous interface.

class MyApp
{
    public static void Main()
    {
        // Here, we create a Client. We protect the Client in a using block so that the
        // connection and subscriptions are properly cleaned up when dispose() is called.
        using(Client client = new Client("subscribe"))
        {
            client.connect("tcp://127.0.0.1/9007/amps");
            client.logon();

            Command command = new Command("subscribe").setTopic("messages");

            // Here, we call the executeAsync() method, specifying the command and the message
            // handler to invoke with messages received in response to the command.
            CommandId subscriptionId = c.executeAsync(command, (message) => Console.WriteLine(message.Data));

            // (message) => Console.WriteLine(message.Data) is a lambda function that acts
            // as our message handler. When a message is received, this lambda function is
            // invoked, and in this case, the Data property from message is printed to the
            // screen. message is of type AMPS.Client.Message.
        }
    }
}
tip In the asynchronous interface, the AMPS client resets and reuses the message object provided to this lambda function between calls. This improves performance in the client, but means that if your handler function needs to preserve information contained within the message, you must copy the information rather than just saving the message object. Otherwise, the AMPS client cannot guarantee the state of the object or the contents of the object when your program goes to use it.

Unsubscribing

The AMPS server continues a subscription until the client explicitly ends the subscription (that is, unsubscribes) or the connection to the client is closed.

With the synchronous message processing interface, AMPS automatically unsubscribes when the dispose() method for the MessageSTream is called. You can also explicitly call the close() method on the MessageStream to remove the subscription.

In the asynchronous message processing interface, when a subscription is successfully made, messages will begin flowing to the message hanlder, and the subscribe() or executeAsync() call will return the identifier for the newly created subscription. A Client can have any number of subscriptions, and this identifier is how AMPS designates messages intended for this particular subscription. To unsubscribe, we simply call unsubscribe with the subscription identifier:

Client c = ...;

Command subscribe_command = new Command("subscribe").setTopic("messages");

CommandId subscriptionId = c.executeAsync(subscribe_command,
    (message) => Console.WriteLine(message));

...

client.unsubscribe(subscriptionId);

Example 4.3: Unsubscribing from a topic

In this example, we use the executeAsync() method to create a subscription to the messages topic. When our application is done listening to this topic, it unsubscribes by executing an unsubscribe command that contains the subscriptionId returned when the subscription was created. After the subscription is removed, no more messages will flow into our (message) lambda function.

When an application calls unsubscribe(), the client sends an explicit unsubscribe command to AMPS. The AMPS server removes that subscription from the set of subscriptions for the client, and stops sending messages for that subscription. On the client side, the client unregisters the subscription so that the MessageStream or MessageHandler for that subscription will no longer receive messages for that subscription.

Notice that calling unsubscribe does not destroy messages that the server has already sent to the client. If there are messages on the way to the client for this subscription, the AMPS client must consume those messages. If a LastChanceMessageHandler is registered, the handler will receive the messages. Otherwise, they will be discarded since no message handler matches the subscription ID on the message.

Understanding Messages

So far, we have seen that subscribing to a topic involves working with objects of type AMPS.Client.Message. A Message represents a single message to or from an AMPS server. Messages are received or sent for every client/server operation in AMPS.

Header properties

There are two parts of each message in AMPS: a set of headers that provide metadata for the message, and the data that the message contains. Every AMPS message has one or more header fields defined. The precise headers present depend on the type and context of the message. There are many possible fields in any given message, but only a few are used for any given message. For each header field, the Message class contains a distinct property that allows for retrieval and setting of that field. For example, the Message.getCommandId() function corresponds to the commandId header field, the Message.getBatchSize() function corresponds to the BatchSize header field, and so on. For more information on these header fields, consult the AMPS User Guide and AMPS Command Reference.

To work with header fields, a Message contains getXxx()/setXxx() methods corresponding to the header fields. 60East does not recommend attempting to parse header fields from the raw data of the message.

In AMPS, fields sometimes need to be set to a unique identifier value. For example, when creating a new subscription, or sending a manually constructed message, you’ll need to assign a new unique identifier to multiple fields such as CommandId and SubscriptionId. For this purpose, Message provides newXxx() methods for each field that generates a new unique identifier and sets the field to that new value.

Data Property

Access to the data section of a message is provided via the Data property. The Data property will contain the unparsed data of the message. The Data property returns the data as a .Net string, which is suitable message formats that can be represented as Unicode text, such as JSON, XML, FIX, or NVFIX. For binary data, the AMPS C# client provides a getDataRaw() method to allow you to work with the underlying byte array in the message. See ` <#csharp-byte-buffers>`_ for details.

The AMPS C# client contains a collection of helper classes for working with message types that are specific to AMPS (for example, FIX, NVFIX, and AMPS composite message types). For message types that are widely used, such as JSON or XML, you can use whichever library you typically use in your environment.

Understanding Threading and Message Handlers

The first time a command causes an instance of the Client or HAClient to connect to AMPS (typically, the logon() command), the client creates a thread that runs in the background. This background thread is responsible for processing incoming messages from AMPS, which includes both messages that contain data and acknowledgments from the server.

When you call a command on the AMPS client, the command typically waits for an acknowledgment from the server and then returns. (The exception to this is publish. For performance, the publish command does not wait for an acknowledgment from the server before returning.)

In the simple case, using synchronous message processing, the client provides an internal handler function that populates the MessageStream. The MessageStream is used on the calling thread, so operations on the MessageStream do not block the background thread.

When using asynchronous message processing, AMPS calls the handler function from the background thread. Message handlers provided for asynchronous message processing must be aware of the following considerations.

The client creates one background thread per client object. A message handler that is only provided to a single client will only be called from a single thread. If your message handler will be used by multiple clients, then multiple threads will call your message handler. In this case, you should take care to protect any state that will be shared between threads.

For maximum performance, do as little work in the message handler as possible. For example, if you use the contents of the message to update an external database, a message handler that adds the relevant data to an update queue that is processed by a different thread will typically perform better than a message handler that does this update during the message handler.

While your message handler is running, the thread that calls your message handler is no longer receiving messages. This makes it easier to write a message handler, because you know that no other messages are arriving from the same subscription. However, this also means that you cannot use the same client that called the message handler to send commands to AMPS. Acknowledgments from AMPS cannot be processed, and your application will deadlock waiting for the acknowledgment. Instead, enqueue the command in a work queue to be processed by a separate thread, or use a different client object to submit the commands.

The AMPS client resets and reuses the Message provided to this function between calls. This improves performance in the client, but means that if your handler function needs to preserve information contained within the message, you must copy the information (either by making a copy of the entire message or copying the required fields) rather than just saving the message object. Otherwise, the AMPS client cannot guarantee the state of the object or the contents of the object when your program goes to use it.

Advanced Subscriptions

Client.subscribe() provides options for subscribing to topics even when you do not know their exact names, and for providing a filter that works on the server to limit the messages your application must process.

Regex topics

Regular Expression (Regex) Topics allow a regular expression to be supplied in the place of a topic name. When you supply a regular expression, it is as if a subscription is made to every topic that matches your expression, including topics that do not yet exist at the time of creating the subscription.

To use a regular expression, simply supply the regular expression in place of the topic name in the subscribe() call. For example:

Client c = ...;

foreach (Message msg in c.subscribe("client.*"))
{
   Console.WriteLine("{0}:{1}", msg.Topic, msg.Data);
}

Example 4.4: Regex topic subscription

In this example, messages on topics client and client1 would match the regular expression, and those messages would all be received by our subscription. As in the example, you can use the Topic property to determine the actual topic of the message sent to the lambda function.

Content Filtering

One of the most powerful features of AMPS is content filtering. With content filtering, filters based on message content are applied at the server, so that your application and the network are not utilized by messages that are uninteresting for your application. For example, if your application is only displaying messages from a particular user, you can send a content filter to the server so that only messages from that particular user are sent to the client.

To apply a content filter to a subscription, simply pass it into the Client.subscribe() call:

Client c = ...;

CommandId subscriptionId = c.subscribe((m) => Console.WriteLine(m), "messages",
    "/message/sender = 'mom' ", 5000); // Timeout

In this example, we have passed in a content filter /sender = 'mom'. This will cause the server to only send us messages from the messages topic that additionally have a sender field equal to mom.

For example, the AMPS server will send the following message, where /sender is mom:

{
    "sender" : "mom",
    "text" : "Happy Birthday!",
    "reminder" : "Call me Thursday!"
}

The AMPS server will not send a message with a different /sender value:

{
    "sender" : "henry dave",
    "text" : "Things do not change; we change."
}

Updating the Filter on a Subscribe

AMPS allows you to update the filter on a subscription. When you replace a filter on the the subscription, AMPS immediately begins sending only messages that match the updated filter. Notice that if the subscription was entered with a command that includes a SOW query, using the replace option can re-issue the SOW query (as described in the AMPS User Guide).

To update a the filter on a subscription, you create a subscribe command. You set the SubscriptionId provided on the Command to the identifier of the existing subscription, and include the replace option on the Command. When you send the Command, AMPS atomically replaces the filter and sends messages that match the updated filter from that point forward.

// Assumes client is connected and logged on to AMPS

// Enter subscription

Command subscribe_cmd = new Command("sow_and_subscribe")
    .setTopic("orders-sow")
    .setSubId("A42")   // Used later for replace
    .setFilter("/details/items/description LIKE 'puppy'");

MyHandler mh = new MyHandler();
client.executeAsync(subscribe_cmd, mh);

// ...

// replace filter elsewhere in the program

Command replace_cmd = new Command("sow_and_subscribe")
    .setTopic("orders-sow")
    .setSubId("A42")   // A42 is the ID of the subscription to replace
    .setFilter("/details/items/description LIKE 'kitten'")
    .setOptions("replace");

client.executeAsync(replace_cmd, mh);

Next Steps

At this point, you are able to build AMPS programs in C# that publish and subscribe to AMPS topics. For an AMPS application to be truly robust, it needs to be able to handle the errors and disconnections that occur in any distributed system. In the next chapter, we will take a closer look at error handling and recovery, and how you can use it to make your application ready for the realworld.