# Vert.x AMQP Bridge

This component provides AMQP 1.0 producer and consumer support via a bridging layer implementing the Vert.x event bus MessageProducer and MessageConsumer APIs over the top of vertx-proton.

Warning | this module has the tech preview status, this means the API can change between versions.

## Using Vert.x AMQP Bridge

To use Vert.x AMQP Bridge, add the following dependency to the dependencies section of your build descriptor:

- Maven (in your `pom.xml`):

    <dependency>
      <groupId>io.vertx</groupId>
      <artifactId>vertx-amqp-bridge</artifactId>
      <version>3.4.1</version>
    </dependency>

- Gradle (in your `build.gradle` file):

    compile io.vertx:vertx-amqp-bridge:3.4.1

## Getting Started

### Sending a Message

Here is a simple example of creating a `MessageProducer` and sending a message with it. First, an `AmqpBridge` is created and started to establish the underlying AMQP connection, then when this is complete the producer is created and a message sent using it. You can optionally supply a username and password when starting the bridge, as well as supplying `AmqpBridgeOptions` in order to configure various options such as for using SSL connections.

    AmqpBridge bridge = AmqpBridge.create(vertx);
    // Start the bridge, then use the event loop thread to process things thereafter.
    bridge.start("localhost", 5672, res -> {
      // Set up a producer using the bridge, send a message with it.
      MessageProducer<JsonObject> producer = bridge.createProducer("myAmqpAddress");

      JsonObject amqpMsgPayload = new JsonObject();
      amqpMsgPayload.put("body", "myStringContent");

      producer.send(amqpMsgPayload);
    });

### Receiving a Message

Here is a simple example of creating a `MessageConsumer` and registering a handler with it. First, an `AmqpBridge` is created and started to establish the underlying AMQP connection, then when this is complete the consumer is created and a handler registered that prints the body of incoming AMQP messages.

    AmqpBridge bridge = AmqpBridge.create(vertx);
    // Start the bridge, then use the event loop thread to process things thereafter.
    bridge.start("localhost", 5672, res -> {
      // Set up a consumer using the bridge, register a handler for it.
      MessageConsumer<JsonObject> consumer = bridge.createConsumer("myAmqpAddress");
      consumer.handler(vertxMsg -> {
        JsonObject amqpMsgPayload = vertxMsg.body();
        Object amqpBody = amqpMsgPayload.getValue("body");

        System.out.println("Received a message with body: " + amqpBody);
      });
    });

Receipt of the AMQP message is accepted automatically as soon as the consumer’s handler returns upon delivering the message to the application.

## Message Payload

### Overview

The message payload is passed as a JsonObject with elements representing various sections of the AMQP message.

The top-level elements supported are:

- body: The content for the body section of the AMQP message.

- body_type: An optional String used to indicate whether the "body" element represents an AmqpValue (default), Data, or AmqpSequence section. The values used are "value", "data", and "sequence" respectively.

- header: An optional JsonObject representing the elements of the message Header section. Expanded below.

- properties: An optional JsonObject representing the elements of the message Properties section. Expanded below.

- application_properties: An optional JsonObject containing any application defined properties(/headers).

- message_annotations: An optional JsonObject representing any message annotations.

The elements of the optional "header" sub-element are:

- durable: optional boolean indicating whether the message is durable (default false).

- priority: optional short indicating the message priority (default 4).

- ttl: optional long indicating ttl in milliseconds (no default). See also 'properties' absolute expiry time.

- first_acquirer: boolean indicating if this is the first acquirer of the message (default false)

- delivery_count: long indicating the number of previous failed delivery attempts for message.

The elements of the optional "properties" sub-element are:

- to: optional string with address message is being sent to (no default).

- reply_to: optional string with address for replies (no default). Set automatically when sent with reply handler.

- message_id: optional string with message id (no default). Set automatically when sending with reply handler.

- correlation_id: optional string with correlation id (no default). Set automatically when implicit reply is sent.

- subject: optional string with message subject (no default).

- group_id: optional string with message group id (no default).

- group_sequence: optional long with message group sequence (no default).

- reply_to_group_id: optional string with message reply to group id (no default).

- content_type: optional string with message content type (no default). Only for use with Data body sections.

- content_encoding: optional string with message content encoding (no default).

- creation_time: optional long with message creation time in milliseconds since the unix epoch (no default).

- absolute_expiry_time: optional long with absolute expiry time as milliseconds since the unix epoch (no default).

- user_id: optional string with the id of the user sending the message (no default).

### Application Properties

To send a message with application properties, the "application_properties" element is added to the payload, containing a JsonObject whose contents represent the application property entries, which have string keys and a object representing a simple value such as String, Boolean, Integer, etc. For example, adding a property to a sent message could look something like:

    JsonObject applicationProperties = new JsonObject();
    applicationProperties.put("name", "value");

    JsonObject amqpMsgPayload = new JsonObject();
    amqpMsgPayload.put("application_properties", applicationProperties);

    producer.send(amqpMsgPayload);

When receiving a message with application properties, the "application_properties" element is added to the JsonObject payload returned, containing a JsonObject whose contents represent the application property entries. For example, retrieving an application-property from a received message might look like:

    JsonObject appProps = amqpMsgPayload.getJsonObject("application_properties");
    if(appProps != null) {
      Object propValue = appProps.getValue("propertyName");
    }

## Flow Control

Message transfer between peers, such as clients and servers, is governed by credit in AMQP 1.0, with receiving peers granting sending peers a number of credits to allow them to send messages. As each message is sent a unit of credit is used up, with the receiving peer needing to replenish the senders credit over time in order for message delivery to progress. This allows for recipients to flow control senders by governing the amount of outstanding credit available.

### Producers

While a MessageProducer will buffer outgoing messages if there are insufficient credits to send them all immediately, and then send them once credit is granted, it is typically more desirable for the application to work in tandem with the producer and attempt to send only what it knows can actually currently be sent.

This is possible by inspecting whether the producer write queue is full, i.e it currently has no credit to send:

    producer.writeQueueFull();

This check can be used in concert with a handler that can be registered to receive callbacks whenever the producer receives more credit and is able to send messages immediately rather than buffer them:

    producer.drainHandler(v -> {
      // ...do stuff and send...
    });

### Consumers

In the case of a MessageConsumer, the bridge automatically gives 1000 credits to the sending peer when the consumer handler is registered, and replenishes this credit automatically as messages are delivered to the handler. It is possible to adjust the amount of credit given initially (the value must be at least 1) by adjusting the maximum buffered message value before registering a handler, for example:

    consumer.setMaxBufferedMessages(5);
    consumer.handler(msg -> {
      // ...handle received messages...
    });

## Connecting using SSL

You can also optionally supply `AmqpBridgeOptions` when creating the bridge in order to configure various options, the most typically used of which are around behaviour for SSL connections.

The following is an example of using configuration to create a bridge connecting to a server using SSL, authenticating with a username and password, and supplying a PKCS12 based trust store to verify trust of the server certificate:

    AmqpBridgeOptions bridgeOptions = new AmqpBridgeOptions();
    bridgeOptions.setSsl(true);

    PfxOptions trustOptions = new PfxOptions().setPath("path/to/pkcs12.truststore")
                                              .setPassword("password");
    bridgeOptions.setPfxTrustOptions(trustOptions);

    AmqpBridge bridge = AmqpBridge.create(vertx, bridgeOptions);
    bridge.start("localhost", 5672, "username", "password", res -> {
      // ..do things with the bridge..
    });

The following is an example of using configuration to create a bridge connecting to a server requiring SSL Client Certificate Authentication, supplying both a PKCS12 based trust store to verify trust of the server certificate and also a PKCS12 based key store containing an SSL key and certificate the server can use to verify the client:

    AmqpBridgeOptions bridgeOptions = new AmqpBridgeOptions();
    bridgeOptions.setSsl(true);

    PfxOptions trustOptions = new PfxOptions().setPath("path/to/pkcs12.truststore")
                                              .setPassword("password");
    bridgeOptions.setPfxTrustOptions(trustOptions);

    PfxOptions keyCertOptions = new PfxOptions().setPath("path/to/pkcs12.keystore")
                                                .setPassword("password");
    bridgeOptions.setPfxKeyCertOptions(keyCertOptions);

    AmqpBridge bridge = AmqpBridge.create(vertx, bridgeOptions);
    bridge.start("localhost", 5672, res -> {
      // ..do things with the bridge..
    });

## Sending and Receiving replies.

Like many messaging protocols, AMQP includes support for a reply-to address to be set on each message sent so that recipients can be told where to send any responses required. The vert.x `Message` objects also support the concept of a reply address, though when using the Event Bus the sender doesn’t set it explicitly, and it is instead populated implicitly if a message is sent with a reply `Handler`. This section describes how the bridge handles sending and receiving AMQP messages with reply-to while using the Vert.x producer, consumer, and message APIs implemented by the bridge.

### Sent messages seeking a reply.

There are two options when sending messages to which responses are required:

- Populate the AMQP reply-to address of the outgoing message explicitly.

- Provide a reply handler when sending to populate it implicitly.

With the first option, you may explicitly populate the "reply_to" element of the message "properties" section, as outlined in the message payload overview. Here you would provide a string containing the name of the AMQP address on the server to which recipients should direct their responses, typically a named queue to which you have already established a consumer to receive the replies. This route may be necessary if you need to receive multiple replies to a given sent AMQP message.

With the second option a reply `Handler` may also be given in addition to the message payload when sending a message, to be registered such that it is invoked when a [single] response message is received for the message being sent.

To facilitate this, upon startup the bridge internally creates a consumer from a server-named dynamic address, the name of which it then uses as the reply-to address on any AMQP messages sent when a replyHandler was given. The bridge also populates the message-id of the outgoing AMQP message, and uses this value to keep track of the reply handler. Incoming messages on the internal 'reply consumer' have their correlation-id values inspected in order to match them to the reply handler originally given, requiring that reply senders populate the correlation-id field with the message-id of the original message.

The following shows the process for the second option:

1. The producer is used to send a message to an AMQP address, providing a reply handler.

2. The bridge send implementation populates the reply-to and message-id fields of the outgoing AMQP message, records the handler, and sends the message to the server.

3. The receiving application (perhaps also a Vert.x AMQP bridge) consumes the message and sends a reply to its reply-to address, setting its correlation-id field as the original messages message-id.

4. The server dispatches the reply message to the internal 'reply consumer' of the bridge.

5. The bridge processes the AMQP message, creating the Vert.x Message with JsonObject body, uses the correlation-id value to match it with the reply handler, and then invokes the handler with the reply message.

The following is a basic example of sending a message and providing a reply-handler to process the response:

    JsonObject amqpMsgPayload = new JsonObject();
    amqpMsgPayload.put("body", "myRequest");

    producer.<JsonObject> send(amqpMsgPayload, res -> {
      JsonObject amqpReplyMessagePayload = res.result().body();
      // ...do something with reply message...
    });

### Received messages seeking a reply.

When a message arrives, its replyAddress may be inspected. If the AMQP message had its reply-to field populated, then the address given will be returned from the Vert.x message replyAddress method. If no reply-to value was present on the message, the value returned will be null.

There are two options when receiving messages to which responses are required:

- Populate the AMQP reply-to address of an outgoing message sent explicitly using a producer.

- Send a reply using the Message reply method.

With the first option, you may explicitly populate the "reply_to" element of the message "properties" section, as outlined in the message payload overview, and send it explicitly using a producer established to the address using the bridge.

With the second option, a reply message may be sent by calling the reply method on the Vert.x message object. The reply method implementation ensures that the outgoing message correlation-id is populated appropriately using the message-id of the original message, such that the response can be matched in the case the original message was sent from a Vert.x AMQP bridge producer with a reply handler provided.

The following outlines the process for both routes, of receiving a message sent by an application (not shown), and sending a reply:

1. The server sends an AMQP message to the consumer, with a reply-to value set to another address.

2. The bridge processes the AMQP message, creating the Vert.x Message with JsonObject body. The Message replyAddress is set to the reply-to value from the AMQP message.

3. The Message is passed to the consumer Handler, which processes it, inspecting the replyAddress and preparing to send a response.

4. The handler chooses to either send a reply using an explicit producer, or call the reply method on the message object.

5. The reply message arrives at the response address on the server, ready to be sent to a reply consumer for the original sending application .

The following is a basic example of sending a reply using the message reply method:

    consumer.handler(msg -> {
      // ...do something with received message...then reply...
      String replyAddress = msg.replyAddress();
      if(replyAddress != null) {
        JsonObject amqpReplyMessagePayload = new JsonObject();
        amqpReplyMessagePayload.put("body", "myResponse");

        msg.reply(amqpReplyMessagePayload);
      }
    });