# Using Core

Apache ActiveMQ Artemis core is a completely JMS-agnostic messaging system with its own
API. We call this the *core API*.

If you don't want to use JMS or other protocols you can use the core API directly. The core
API provides all the functionality of JMS but without much of the
complexity. It also provides features that are not available using JMS.

## Core Messaging Concepts

Some of the core messaging concepts are similar to JMS concepts, but
core messaging concepts differ in some ways. In general the core
messaging API is simpler than the JMS API, since we remove distinctions
between queues, topics and subscriptions. We'll discuss each of the
major core messaging concepts in turn, but to see the API in detail,
please consult the Javadoc.

### Message

-   A message is the unit of data which is sent between clients and
    servers.

-   A message has a body which is a buffer containing convenient methods
    for reading and writing data into it.

-   A message has a set of properties which are key-value pairs. Each
    property key is a string and property values can be of type integer,
    long, short, byte, byte[], String, double, float or boolean.

-   A message has an *address* it is being sent to. When the message
    arrives on the server it is routed to any queues that are bound to
    the address - if the queues are bound with any filter, the message
    will only be routed to that queue if the filter matches. An address
    may have many queues bound to it or even none. There may also be
    entities other than queues, like *diverts* bound to addresses.

-   Messages can be either durable or non durable. Durable messages in a
    durable queue will survive a server crash or restart. Non durable
    messages will never survive a server crash or restart.

-   Messages can be specified with a priority value between 0 and 9. 0
    represents the lowest priority and 9 represents the highest.
    Apache ActiveMQ Artemis will attempt to deliver higher priority messages before
    lower priority ones.

-   Messages can be specified with an optional expiry time. Apache ActiveMQ Artemis
    will not deliver messages after its expiry time has been exceeded.

-   Messages also have an optional timestamp which represents the time
    the message was sent.

-   Apache ActiveMQ Artemis also supports the sending/consuming of very large messages
    much larger than can fit in available RAM at any one time.

### Address

A server maintains a mapping between an address and a set of queues.
Zero or more queues can be bound to a single address. Each queue can be
bound with an optional message filter. When a message is routed, it is
routed to the set of queues bound to the message's address. If any of
the queues are bound with a filter expression, then the message will
only be routed to the subset of bound queues which match that filter
expression.

Other entities, such as *diverts* can also be bound to an address and
messages will also be routed there.

> **Note**
>
> In core, there is no concept of a Topic, Topic is a JMS only term.
> Instead, in core, we just deal with *addresses* and *queues*.
>
> For example, a JMS topic would be implemented by a single address to
> which many queues are bound. Each queue represents a subscription of
> the topic. A JMS Queue would be implemented as a single address to
> which one queue is bound - that queue represents the JMS queue.

### Queue

Queues can be durable, meaning the messages they contain survive a
server crash or restart, as long as the messages in them are durable.
Non durable queues do not survive a server restart or crash even if the
messages they contain are durable.

Queues can also be temporary, meaning they are automatically deleted
when the client connection is closed, if they are not explicitly deleted
before that.

Queues can be bound with an optional filter expression. If a filter
expression is supplied then the server will only route messages that
match that filter expression to any queues bound to the address.

Many queues can be bound to a single address. A particular queue is only
bound to a maximum of one address.

### ServerLocator

Clients use `ServerLocator` instances to create `ClientSessionFactory`
instances. `ServerLocator` instances are used to locate servers and
create connections to them.

In JMS terms think of a `ServerLocator` in the same way you would a JMS
Connection Factory.

`ServerLocator` instances are created using the `ActiveMQClient` factory
class.

### ClientSessionFactory

Clients use `ClientSessionFactory` instances to create `ClientSession`
instances. `ClientSessionFactory` instances are basically the connection
to a server

In JMS terms think of them as JMS Connections.

`ClientSessionFactory` instances are created using the `ServerLocator`
class.

### ClientSession

A client uses a ClientSession for consuming and producing messages and
for grouping them in transactions. ClientSession instances can support
both transactional and non transactional semantics and also provide an
`XAResource` interface so messaging operations can be performed as part
of a
[JTA](http://www.oracle.com/technetwork/java/javaee/tech/jta-138684.html)
transaction.

ClientSession instances group ClientConsumers and ClientProducers.

ClientSession instances can be registered with an optional
`SendAcknowledgementHandler`. This allows your client code to be
notified asynchronously when sent messages have successfully reached the
server. This unique Apache ActiveMQ Artemis feature, allows you to have full guarantees
that sent messages have reached the server without having to block on
each message sent until a response is received. Blocking on each
messages sent is costly since it requires a network round trip for each
message sent. By not blocking and receiving send acknowledgements
asynchronously you can create true end to end asynchronous systems which
is not possible using the standard JMS API. For more information on this
advanced feature please see the section [Guarantees of sends and commits](send-guarantees.md).

### ClientConsumer

Clients use `ClientConsumer` instances to consume messages from a queue.
Core Messaging supports both synchronous and asynchronous message
consumption semantics. `ClientConsumer` instances can be configured with
an optional filter expression and will only consume messages which match
that expression.

### ClientProducer

Clients create `ClientProducer` instances on `ClientSession` instances
so they can send messages. ClientProducer instances can specify an
address to which all sent messages are routed, or they can have no
specified address, and the address is specified at send time for the
message.

> **Warning**
>
> Please note that ClientSession, ClientProducer and ClientConsumer
> instances are *designed to be re-used*.
>
> It's an anti-pattern to create new ClientSession, ClientProducer and
> ClientConsumer instances for each message you produce or consume. If
> you do this, your application will perform very poorly. This is
> discussed further in the section on performance tuning [Performance Tuning](perf-tuning.md).

## A simple example of using Core

Here's a very simple program using the core messaging API to send and
receive a message. Logically it's comprised of two sections: firstly
setting up the producer to write a message to an *addresss*, and
secondly, creating a *queue* for the consumer, creating the consumer and
*starting* it.
``` java
ServerLocator locator = ActiveMQClient.createServerLocatorWithoutHA(new TransportConfiguration(
                                           InVMConnectorFactory.class.getName()));

// In this simple example, we just use one session for both producing and receiving

ClientSessionFactory factory =  locator.createClientSessionFactory();
ClientSession session = factory.createSession();

// A producer is associated with an address ...

ClientProducer producer = session.createProducer("example");
ClientMessage message = session.createMessage(true);
message.getBodyBuffer().writeString("Hello");

// We need a queue attached to the address ...

session.createQueue("example", "example", true);

// And a consumer attached to the queue ...

ClientConsumer consumer = session.createConsumer("example");

// Once we have a queue, we can send the message ...

producer.send(message);

// We need to start the session before we can -receive- messages ...

session.start();
ClientMessage msgReceived = consumer.receive();

System.out.println("message = " + msgReceived.getBodyBuffer().readString());

session.close();
```
