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Broker Balancers

Apache ActiveMQ Artemis broker balancers allow incoming client connections to be distributed across multiple target brokers. The target brokers are grouped in pools and the broker balancers use a target key to select a target broker from a pool of brokers according to a policy.

This feature is still EXPERIMENTAL and not meant to be run in production yet. Furthermore, its configuration can change until declared as officially stable.

Target Broker

Target broker is a broker that can accept incoming client connections and is local or remote. The local target is a special target that represents the same broker hosting the broker balancer. The remote target is another reachable broker.

Target Key

The broker balancer uses a target key to select a target broker. It is a string retrieved from an incoming client connection, the supported values are:

CLIENT_ID is the JMS client ID;
SNI_HOST is the hostname indicated by the client in the SNI extension of the TLS protocol;
SOURCE_IP is the source IP address of the client;
USER_NAME is the username indicated by the client.

Pools

The pool is a group of target brokers and checks periodically their state. It provides a list of ready target brokers to distribute incoming client connections only when it is active. A pool becomes active when the minimum number of ready target brokers defined by the quorum-size parameter is reached. When it is not active, it doesn't provide any target avoiding weird distribution at startup or after a restart. Including the local broker in the target pool allows broker hosting the balancer to accept incoming client connections as well. By default, a pool doesn't include the local broker, to include it as a target the local-target-enabled parameter must be true. There are two pool types: discovery pool and static pool.

Cluster Pool

The cluster pool uses a cluster connection to get the target brokers to add. Let's take a look at a cluster pool example from broker.xml that uses a cluster connection:

<pool>
  <cluster-connection>cluster1</cluster-connection>
</pool>

Discovery Pool

The discovery pool uses a discovery group to discover the target brokers to add. Let's take a look at a discovery pool example from broker.xml that uses a discovery group:

<pool>
    <discovery-group-ref discovery-group-name="dg1"/>
</pool>

Static Pool

The static pool uses a list of static connectors to define the target brokers to add. Let's take a look at a static pool example from broker.xml that uses a list of static connectors:

<pool>
    <static-connectors>
        <connector-ref>connector1</connector-ref>
        <connector-ref>connector2</connector-ref>
        <connector-ref>connector3</connector-ref>
    </static-connectors>
</pool>

Defining pools

A pool is defined by the pool element that includes the following items:

the username element defines the username to connect to the target broker;
the password element defines the password to connect to the target broker;
the check-period element defines how often to check the target broker, measured in milliseconds, default is 5000;
the quorum-size element defines the minimum number of ready targets to activate the pool, default is 1;
the quorum-timeout element defines the timeout to get the minimum number of ready targets, measured in milliseconds, default is 3000;
the local-target-enabled element defines whether the pool has to include a local target, default is false;
the cluster-connection element defines the cluster connection used by the cluster pool.
the static-connectors element defines a list of static connectors used by the static pool;
the discovery-group element defines the discovery group used by the discovery pool.

Let's take a look at a pool example from broker.xml:

<pool>
    <quorum-size>2</quorum-size>
    <check-period>1000</check-period>
    <local-target-enabled>true</local-target-enabled>
    <static-connectors>
        <connector-ref>connector1</connector-ref>
        <connector-ref>connector2</connector-ref>
        <connector-ref>connector3</connector-ref>
    </static-connectors>
</pool>

Policies

The policy define how to select a broker from a pool. The included policies are:

FIRST_ELEMENT to select the first target broker from the pool which is ready. It is useful to select the ready target brokers according to the priority defined with their sequence order, ie supposing there are 2 target brokers this policy selects the second target broker only when the first target broker isn't ready.
ROUND_ROBIN to select a target sequentially from a pool, this policy is useful to evenly distribute;
CONSISTENT_HASH to select a target by a key. This policy always selects the same target broker for the same key until it is removed from the pool.
LEAST_CONNECTIONS to select the targets with the fewest active connections. This policy helps you maintain an equal distribution of active connections with the target brokers.

A policy is defined by the policy element. Let's take a look at a policy example from broker.xml:

<policy name="FIRST_ELEMENT"/>

Cache

The broker balancer provides a cache with a timeout to improve the stickiness of the target broker selected, returning the same target broker for a target key as long as it is present in the cache and is ready. So a broker balancer with the cache enabled doesn't strictly follow the configured policy. By default, the cache is enabled, to disable the cache the cache-timeout parameter must be 0.

Defining broker balancers

A broker balancer is defined by broker-balancer element, it includes the following items:

the name attribute defines the name of the broker balancer;
the target-key element defines what key to select a target broker, the supported values are: CLIENT_ID, SNI_HOST, SOURCE_IP, USER_NAME, default is SOURCE_IP, see target key for further details;
the target-key-filter element defines a regular expression to filter the resolved keys;
the local-target-filter element defines a regular expression to match the keys that have to return a local target;
the cache-timeout element is the time period for a target broker to remain in the cache, measured in milliseconds, setting 0 will disable the cache, default is -1, meaning no expiration;
the pool element defines the pool to group the target brokers, see pools.
the policy element defines the policy used to select the target brokers, see policies;

Let's take a look at some broker balancer examples from broker.xml:

<broker-balancers>
    <broker-balancer name="simple-balancer">
        <policy name="FIRST_ELEMENT"/>
        <pool>
            <static-connectors>
                <connector-ref>connector1</connector-ref>
                <connector-ref>connector2</connector-ref>
                <connector-ref>connector3</connector-ref>
            </static-connectors>
        </pool>
    </broker-balancer>
    <broker-balancer name="consistent-hash-balancer">
        <target-key>USER_NAME</target-key>
        <local-target-filter>admin</local-target-filter>
        <policy name="CONSISTENT_HASH"/>
        <pool>
            <local-target-enabled>true</local-target-enabled>
            <discovery-group-ref discovery-group-name="dg1"/>
        </pool>
    <policy name="CONSISTENT_HASH"/>
    </broker-balancer>
    <broker-balancer name="evenly-balancer">
      <target-key>CLIENT_ID</target-key>
      <target-key-filter>^.{3}</target-key-filter>
      <policy name="LEAST_CONNECTIONS"/>
      <pool>
        <username>guest</username>
        <password>guest</password>
        <discovery-group-ref discovery-group-name="dg2"/>
      </pool>
    </broker-balancer>
</broker-balancers>

Broker Balancer Workflow

The broker balancer workflow include the following steps:

Retrieve the target key from the incoming connection;
Return the local target broker if the target key matches the local filter;
Return the cached target broker if it is ready;
Get ready target brokers from the pool;
Select one target broker using the policy;
Add the selected broker in the cache;
Return the selected broker.

Let's take a look at flowchart of the broker balancer workflow:

Redirection

Apache ActiveMQ Artemis provides a native redirection for supported clients and a new management API for other clients. The native redirection can be enabled per acceptor and is supported only for AMQP, CORE and OPENWIRE clients. The acceptor with the redirect-to url parameter will redirect the incoming connections. The redirect-to url parameter specifies the name of the broker balancer to use, ie the following acceptor will redirect the incoming CORE client connections using the broker balancer with the name simple-balancer:

<acceptor name="artemis">tcp://0.0.0.0:61616?redirect-to=simple-balancer;protocols=CORE</acceptor>

Native Redirect Sequence

The clients supporting the native redirection connect to the acceptor with the redirection enabled. The acceptor sends to the client the target broker to redirect if it is ready and closes the connection. The client connects to the target broker if it has received one before getting disconnected otherwise it connected again to the acceptor with the redirection enabled.

Management API Redirect Sequence

The clients not supporting the native redirection queries the management API of broker balancer to get the target broker to redirect. If the API returns a target broker the client connects to it otherwise the client queries again the API.

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