AMPS uses the name of the client as a session identifier and as part of the identifier for messages originating from that client.

For this reason, when a transaction log is enabled in the AMPS instance (that is, when the instance is recording a sequence of publishes and attempting to eliminate duplicate publishes), an AMPS instance will only allow one application with a given client name to connect to the instance.

When a transaction log is present, AMPS requires the client name for a publisher to be:

• Unique within a set of replicated AMPS instances

• Consistent from invocation to invocation if the publisher will be publishing the same logical stream of messages

If publishers do not meet this contract (for example, if the publisher changes its name and publishes the same messages, or if a different publisher uses the same session name), message loss or duplication can happen.

60East recommends always using consistent, unique client names. For example, the client name could be formed by combining the application name, an identifier for the host system, and the ID of the user running the application. A strategy like this provides a name that will be different for different users or on different systems, but consistent for instances of the application that should be treated as equivalent to the AMPS system.

Likewise, if a publisher is sending a completely independent stream of messages (for example, a microservice that sends a different, unrelated sequence of messages each time it connects to AMPS), there is no need for a publisher to retain the same name each time it starts. However, if a publisher is resuming a stream of messages (as in the case when using a file-backed publish store), that publisher must use the same client name, since the publisher is resuming the session.

In this chapter, we will learn more about the structure and features of the AMPS C/C++ library, and build our first C/C++ program using AMPS.

Connecting to AMPS

Let’s begin by writing a simple program that connects to an AMPS server and sends a single message to a topic:

#include <amps/ampsplusplus.hpp>
#include <iostream>

int main(void)
{
    const char* uri = "tcp://127.0.0.1:9007/amps/json";

    // Construct a client with the name "examplePublisher".

    AMPS::Client ampsClient("examplePublisher");

    try
    {
        // connect to the server and log on
        ampsClient.connect(uri);
        ampsClient.logon();

        // publish a JSON message
        ampsClient.publish("messages",
                           R"({ "message" : "Hello, World!" ,)"
                           R"("client" : 1 })");

    }
    catch (const AMPS::AMPSException& e)
    {
        std::cerr << e.what() << std::endl;
        exit(1);
    }
    return 0;
}

In the example above, we show the entire program.

Future examples will isolate one or more specific portions of the code. The next section describes how to build and run the application and explains the code in further detail.

Build and run

To build the program that you've created:

• Create a new .cpp file and use your c compiler to build it, making sure the amps-c++-client/include directory is in your compiler’s include path

• Link to the libamps.a or amps.lib static libraries.

• Additionally, link to any operating system libraries required by AMPS; a full list may be found by examining the Makefile and project files in the samples directory.

If the message is published successfully, there is no output to the console. We will demonstrate how to create a subscriber to receive messages in Subscriptions section.

Examining the code:

Let us revisit the code we listed earlier:

/* These are the include files required for an AMPS C++ Client. The
 * first is <ampsplusplus.hpp>. This header includes everything needed to
 * compile C++ programs for AMPS. The next include is the Standard C++ Library
 * <iostream>, necessary due to use of std::cerr and std::endl.
 */
#include <amps/ampsplusplus.hpp>
#include <iostream>

int main()
{
    /* The URI to use to connect to AMPS. The URI consists of the transport,
     * the address, and the protocol to use for the AMPS connection. In this case,
     * the transport is tcp, the address is 127.0.0.1:9007, and the protocol is
     * amps. In this case, AMPS is configured to allow any message type on that
     * transport, so we specify json in the URI to let AMPS know which message
     * type this connection will use. Even though a transport that uses the
     * amps protocol can accept multiple message types, each connection must specify
     * the exact message type that connection will use. Check with the person who
     * manages the AMPS instance to get the connection string to use for your programs.
     */
    const char* uri = "tcp://127.0.0.1:9007/amps/json";


    /* This is where we first interact with AMPS by instantiating an AMPS::Client
     * object. Client is the class used to connect to and interact with an AMPS
     * server. We pass the string "exampleClient" as the clientName. This name
     * will be used to uniquely identify this client to the server. Errors relating
     * to this connection will be logged with reference to this name, and AMPS uses
     * this name to help detect duplicate messages. AMPS enforces uniqueness for
     * client names when a transaction log is configured, and it is good practice
     * to always use unique client names.
     */
    AMPS::Client ampsClient("exampleClient");

    /* Here we open a try block. AMPS C++ classes throw exceptions to indicate
     * errors. For the remainder of our interactions with AMPS, if an
     * error occurs, the exception thrown by AMPS will be caught and handled
     * in the exception handler below.
     */
    try
    {
        /* At this point, we establish a valid AMPS network connection and
         * can begin to use it to publish and subscribe to messages. In this
         * example, we use the URI specified earlier in the file. If any errors
         * occur while attempting to connect to AMPS, the connect() method will
         * throw an exception.
         */
        ampsClient.connect(uri);

        /* The AMPS logon() command connects to AMPS and creates a named
         * connection. This version of the logon() command uses a DefaultAuthenticator,
         * which uses the credentials provided in the URI. Without credentials, the
         * client logs on to AMPS anonymously. AMPS versions 5.0 and later
         * require a logon() command in the default configuration.
         *
         * If you need to use a different authentication scheme, implement an
         * Authenticator and pass that Authenticator to this command.
         */
        ampsClient.logon();

        /* publish a JSON message
         * Here, a single message is published to AMPS on the messages topic,
         * containing the data Hello world. This data is placed into an XML
         * message and sent to the server. Upon successful completion of this
         * function, the AMPS client has sent the message to the server, and
         * subscribers to the messages topic will receive this Hello world message.
         */
        ampsClient.publish("messages",
                           R"({ "message" : "Hello, World!" ,)"
                           R"( "client" : 1 })");
    }
    /* Error handling begins with the catch block. All exceptions thrown by
     * AMPS derive from AMPSException classes. More specific exceptions may
     * be caught to handle certain conditions, but catching
     * AMPSException& allows us to handle all AMPS errors in one
     * place. In this example, we print out the error to the console and exit
     * the program.
     */
    catch (const AMPS::AMPSException& e)
    {
        std::cerr << e.what() << std::endl;
        exit(1);
    }

    /* At this point we return from main() and our ampsClient object falls
     * out of scope.  When this happens AMPS automatically disconnects from the
     * server and frees all of the client resources associated with the
     * connection.  In the AMPS C++ client, objects are reference-counted,
     * meaning that you can safely copy a client, for example, and destroy copies
     * of client without worrying about premature closure of the server connection
     * or memory leaks.
     */
    return 0;
}

When a client logs on to AMPS, the client sends AMPS a username and password. The username is derived from the URI, using the standard syntax for providing a user name in a URI, for example, tcp://JohnDoe:@server:port/amps/messagetype to include the user name JohnDoe in the request.

For a given user name, the password is provided by an Authenticator. The AMPS client distribution includes a DefaultAuthenticator that simply returns the password, if any, provided in the URI. A logon() command that does not specify an Authenticator will use an instance of DefaultAuthenticator.

If your authentication system requires a different authentication token, you can implement an Authenticator that provides the appropriate token.

Providing Credentials in a Connection String

When using the DefaultAuthenticator, the AMPS clients support the standard format for including a username and password in a URI, as shown below:

tcp://user:password@host:port/protocol/message_type

When provided in this form, the default authenticator provides the username and password specified in the URI. If you have implemented another authenticator, that authenticator controls how passwords are provided to the AMPS server.

The AMPS clients use connection strings to determine the server, port, transport, and protocol to use to connect to AMPS. When the connection point in AMPS accepts multiple message types, the connection string also specifies the precise message type to use for this connection.

Connection strings have a number of elements:

As shown in the figure above, connection strings have the following elements:

• Transport - Defines the network used to send and receive messages from AMPS. In this case, the transport is tcp. For connections to transports that use the Secure Sockets Layer (SSL), use tcps. For connections to AMPS over a Unix domain socket, use unix.

• Host Address - Defines the destination on the network where the AMPS instance receives messages. The format of the address is dependent on the transport. For tcp and tcps, the address consists of a host name and port number. In this case, the host address is localhost:9007. For unix domain sockets, a value for hostname and port must be provided to form a valid URI, but the content of the hostname and port are ignored, and the file name provided in the path parameter is used instead (by convention, many connection strings use localhost:0 to indicate that this is a local connection that does not use TCP/IP).

• Protocol - Sets the format in which AMPS receives commands from the client. Most code uses the default amps protocol, which sends header information in JSON format. AMPS supports the ability to develop custom protocols as extension modules, and AMPS also supports legacy protocols for backward compatibility.

• MessageType - Specifies the message type that this connection uses. This component of the connection string is required if the protocol accepts multiple message types and the transport is configured to accept multiple message types. If the protocol does not accept multiple message types, this component of the connection string is optional, and defaults to the message type specified in the transport.

  Legacy protocols such as fix, nvfix and xml only accept a single message type, and therefore do not require or accept a message type in the connection string.

As an example, a connection string such as:

tcp://localhost:9007/amps/json

would work for programs connecting from the local host to a Transport configured as follows:

<AMPSConfig>
    ...

    <!-- This transport accepts any known message type for the instance: the
         client must specify the message type. -->
    <Transport>
        <Name>any-tcp</Name>
        <Type>tcp</Type>
        <InetAddr>9007</InetAddr>
        <Protocol>amps</Protocol>
    </Transport>

    ...
</AMPSConfig>

See the AMPS Configuration Guide for more information on configuring transports.

When specifying a URI for connection to an AMPS server, you may specify a number of transport-specific options in the parameters section of the URI connection parameters. Here is an example:

tcp://localhost:9007/amps/json?tcp_nodelay=true&tcp_sndbuf=100000

In this example, we have specified the AMPS instance on localhost, port 9007, connecting to a transport that uses the amps protocol and sending JSON messages. We have also set two parameters: tcp_nodelay, a Boolean (true/false) parameter, and tcp_sndbuf, an integer parameter. Multiple parameters may be combined to finely tune settings available on the transport. Normally, you'll want to stick with the defaults on your platform, but there may be some cases where experimentation and fine-tuning will yield higher or more efficient performance.

The AMPS client supports the value of tcp in the scheme component connection string for TCP/IP connections, and the value of tcps as the scheme for SSL encrypted connections.

For connections that use Unix domain sockets, the client supports the value of unix in the scheme, and requires an additional option, as described in the Unix Transports Parameters section below.

IPv6 Connections

Starting with version 5.3.3.0, the AMPS client supports creating connections over both IPv4 and IPv6 protocols if supported by the underlying Operating System.

By default, the AMPS client will prefer to resolve host names to IPv4 addresses, but this behavior can be adjusted by supplying the ip_protocol_prefer transport option, described in the table below.

TCP and SSL Transport Options

The following transport options are available for TCP connections:

Unix Transport Parameters

The unix transport type communicates over Unix domain sockets. This transport requires the following additional option:

Unix domain sockets always connect to the local system. When the scheme specified is unix, the host address is ignored in the connection string. For example, the connection string:

unix://localhost:0/amps/json?path=/sockets/the-amps-socket

and the connection string:

unix://unix:unix/amps/json?path=/sockets/the-amps-socket

are equivalent.

The other components of the connection string, including the protocol, message type, username, and authentication token are processed just as they would be for TCP/IP sockets.

AMPS Additional Logon Options

The connection string can also be used to pass logon parameters to AMPS. AMPS supports the following additional logon option: