192 lines
10 KiB
Markdown
192 lines
10 KiB
Markdown
# Broker Balancers
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Apache ActiveMQ Artemis broker balancers allow incoming client connections to be distributed across multiple [target brokers](target-brokers).
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The target brokers are grouped in [pools](#pools) and the broker balancers use a [target key](#target-key)
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to select a target broker from a pool of brokers according to a [policy](#policies).
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### This feature is still **EXPERIMENTAL** and not meant to be run in production yet. Furthermore, its configuration can change until declared as **officially stable**.
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## Target Broker
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Target broker is a broker that can accept incoming client connections and is local or remote.
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The local target is a special target that represents the same broker hosting the broker balancer.
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The remote target is another reachable broker.
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## Target Key
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The broker balancer uses a target key to select a target broker.
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It is a string retrieved from an incoming client connection, the supported values are:
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* `CLIENT_ID` is the JMS client ID;
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* `SNI_HOST` is the hostname indicated by the client in the SNI extension of the TLS protocol;
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* `SOURCE_IP` is the source IP address of the client;
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* `USER_NAME` is the username indicated by the client.
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## Pools
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The pool is a group of target brokers and checks periodically their state.
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It provides a list of ready target brokers to distribute incoming client connections only when it is active.
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A pool becomes active when the minimum number of ready target brokers defined by the `quorum-size` parameter is reached.
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When it is not active, it doesn't provide any target avoiding weird distribution at startup or after a restart.
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Including the local broker in the target pool allows broker hosting the balancer to accept incoming client connections as well.
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By default, a pool doesn't include the local broker, to include it as a target the `local-target-enabled` parameter must be `true`.
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There are three pool types: [cluster pool](#cluster-pool), [discovery pool](#discovery-pool) and [static pool](#static-pool).
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### Cluster Pool
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The cluster pool uses a [cluster connection](clusters.md#configuring-cluster-connections) to get the target brokers to add.
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Let's take a look at a cluster pool example from broker.xml that uses a cluster connection:
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```xml
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<pool>
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<cluster-connection>cluster1</cluster-connection>
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</pool>
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```
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### Discovery Pool
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The discovery pool uses a [discovery group](clusters.md#discovery-groups) to discover the target brokers to add.
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Let's take a look at a discovery pool example from broker.xml that uses a discovery group:
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```xml
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<pool>
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<discovery-group-ref discovery-group-name="dg1"/>
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</pool>
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```
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### Static Pool
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The static pool uses a list of static connectors to define the target brokers to add.
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Let's take a look at a static pool example from broker.xml that uses a list of static connectors:
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```xml
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<pool>
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<static-connectors>
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<connector-ref>connector1</connector-ref>
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<connector-ref>connector2</connector-ref>
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<connector-ref>connector3</connector-ref>
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</static-connectors>
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</pool>
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```
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### Defining pools
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A pool is defined by the `pool` element that includes the following items:
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* the `username` element defines the username to connect to the target broker;
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* the `password` element defines the password to connect to the target broker;
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* the `check-period` element defines how often to check the target broker, measured in milliseconds, default is `5000`;
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* the `quorum-size` element defines the minimum number of ready targets to activate the pool, default is `1`;
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* the `quorum-timeout` element defines the timeout to get the minimum number of ready targets, measured in milliseconds, default is `3000`;
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* the `local-target-enabled` element defines whether the pool has to include a local target, default is `false`;
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* the `cluster-connection` element defines the [cluster connection](clusters.md#configuring-cluster-connections) used by the [cluster pool](#cluster-pool).
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* the `static-connectors` element defines a list of static connectors used by the [static pool](#static-pool);
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* the `discovery-group` element defines the [discovery group](clusters.md#discovery-groups) used by the [discovery pool](#discovery-pool).
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Let's take a look at a pool example from broker.xml:
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```xml
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<pool>
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<quorum-size>2</quorum-size>
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<check-period>1000</check-period>
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<local-target-enabled>true</local-target-enabled>
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<static-connectors>
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<connector-ref>connector1</connector-ref>
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<connector-ref>connector2</connector-ref>
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<connector-ref>connector3</connector-ref>
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</static-connectors>
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</pool>
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```
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## Policies
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The policy define how to select a broker from a pool. The included policies are:
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* `FIRST_ELEMENT` to select the first target broker from the pool which is ready. It is useful to select the ready target brokers
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according to the priority defined with their sequence order, ie supposing there are 2 target brokers
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this policy selects the second target broker only when the first target broker isn't ready.
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* `ROUND_ROBIN` to select a target sequentially from a pool, this policy is useful to evenly distribute;
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* `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.
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* `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.
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A policy is defined by the `policy` element. Let's take a look at a policy example from broker.xml:
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```xml
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<policy name="FIRST_ELEMENT"/>
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```
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## Cache
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The broker balancer provides a cache with a timeout to improve the stickiness of the target broker selected,
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returning the same target broker for a target key as long as it is present in the cache and is ready.
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So a broker balancer with the cache enabled doesn't strictly follow the configured policy.
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By default, the cache is enabled, to disable the cache the `cache-timeout` parameter must be `0`.
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## Defining broker balancers
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A broker balancer is defined by `broker-balancer` element, it includes the following items:
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* the `name` attribute defines the name of the broker balancer;
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* 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](#target-key) for further details;
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* the `target-key-filter` element defines a regular expression to filter the resolved keys;
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* the `local-target-filter` element defines a regular expression to match the keys that have to return a local target;
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* 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;
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* the `pool` element defines the pool to group the target brokers, see [pools](#pools).
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* the `policy` element defines the policy used to select the target brokers, see [policies](#policies);
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Let's take a look at some broker balancer examples from broker.xml:
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```xml
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<broker-balancers>
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<broker-balancer name="simple-balancer">
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<policy name="FIRST_ELEMENT"/>
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<pool>
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<static-connectors>
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<connector-ref>connector1</connector-ref>
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<connector-ref>connector2</connector-ref>
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<connector-ref>connector3</connector-ref>
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</static-connectors>
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</pool>
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</broker-balancer>
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<broker-balancer name="consistent-hash-balancer">
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<target-key>USER_NAME</target-key>
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<local-target-filter>admin</local-target-filter>
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<policy name="CONSISTENT_HASH"/>
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<pool>
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<local-target-enabled>true</local-target-enabled>
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<discovery-group-ref discovery-group-name="dg1"/>
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</pool>
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<policy name="CONSISTENT_HASH"/>
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</broker-balancer>
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<broker-balancer name="evenly-balancer">
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<target-key>CLIENT_ID</target-key>
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<target-key-filter>^.{3}</target-key-filter>
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<policy name="LEAST_CONNECTIONS"/>
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<pool>
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<username>guest</username>
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<password>guest</password>
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<discovery-group-ref discovery-group-name="dg2"/>
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</pool>
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</broker-balancer>
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</broker-balancers>
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```
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## Broker Balancer Workflow
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The broker balancer workflow include the following steps:
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* Retrieve the target key from the incoming connection;
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* Return the local target broker if the target key matches the local filter;
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* Return the cached target broker if it is ready;
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* Get ready target brokers from the pool;
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* Select one target broker using the policy;
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* Add the selected broker in the cache;
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* Return the selected broker.
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Let's take a look at flowchart of the broker balancer workflow:
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## Redirection
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Apache ActiveMQ Artemis provides a native redirection for supported clients and a new management API for other clients.
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The native redirection can be enabled per acceptor and is supported only for AMQP, CORE and OPENWIRE clients.
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The acceptor with the `redirect-to` url parameter will redirect the incoming connections.
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The `redirect-to` url parameter specifies the name of the broker balancer to use,
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ie the following acceptor will redirect the incoming CORE client connections using the broker balancer with the name `simple-balancer`:
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```xml
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<acceptor name="artemis">tcp://0.0.0.0:61616?redirect-to=simple-balancer;protocols=CORE</acceptor>
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```
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### Native Redirect Sequence
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The clients supporting the native redirection connect to the acceptor with the redirection enabled.
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The acceptor sends to the client the target broker to redirect if it is ready and closes the connection.
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The client connects to the target broker if it has received one before getting disconnected
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otherwise it connected again to the acceptor with the redirection enabled.
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### Management API Redirect Sequence
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The clients not supporting the native redirection queries the management API of broker balancer
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to get the target broker to redirect. If the API returns a target broker the client connects to it
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otherwise the client queries again the API.
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