# Active-Passive Messaging Clusters

## Overview

The High Availability (HA) module provides active-passive, hot-standby messaging clusters to provide fault tolerant message delivery.

In an active-passive cluster only one broker, known as the primary, is active and serving clients at a time. The other brokers are standing by as backups. Changes on the primary are replicated to all the backups so they are always up-to-date or "hot". Backup brokers reject client connection attempts, to enforce the requirement that clients only connect to the primary.

If the primary fails, one of the backups is promoted to take over as the new primary. Clients fail-over to the new primary automatically. If there are multiple backups, the other backups also fail-over to become backups of the new primary.

This approach relies on an external cluster resource manager to detect failures, choose the new primary and handle network partitions. Rgmanager is supported initially, but others may be supported in the future.

### Avoiding message loss

In order to avoid message loss, the primary broker delays acknowledgment of messages received from clients until the message has been replicated and acknowledged by all of the back-up brokers, or has been consumed from the primary queue.

This ensures that all acknowledged messages are safe: they have either been consumed or backed up to all backup brokers. Messages that are consumed before they are replicated do not need to be replicated. This reduces the work load when replicating a queue with active consumers.

Clients keep unacknowledged messages in a buffer

You can control the maximum number of messages in the buffer by setting the client's `capacity`. For details of how to set the capacity in client code see "Using the Qpid Messaging API" in Programming in Apache Qpid.

until they are acknowledged by the primary. If the primary fails, clients will fail-over to the new primary and re-send all their unacknowledged messages.

Clients must use "at-least-once" reliability to enable re-send of unacknowledged messages. This is the default behavior, no options need be set to enable it. For details of client addressing options see "Using the Qpid Messaging API" in Programming in Apache Qpid.

If the primary crashes, all the acknowledged messages will be available on the backup that takes over as the new primary. The unacknowledged messages will be re-sent by the clients. Thus no messages are lost.

Note that this means it is possible for messages to be duplicated. In the event of a failure it is possible for a message to received by the backup that becomes the new primary and re-sent by the client. The application must take steps to identify and eliminate duplicates.

When a new primary is promoted after a fail-over it is initially in "recovering" mode. In this mode, it delays acknowledgment of messages on behalf of all the backups that were connected to the previous primary. This protects those messages against a failure of the new primary until the backups have a chance to connect and catch up.

Not all messages need to be replicated to the back-up brokers. If a message is consumed and acknowledged by a regular client before it has been replicated to a backup, then it doesn't need to be replicated.

Status of a HA broker

Joining

  Initial status of a new broker that has not yet connected to the primary.

Catch-up

  A backup broker that is connected to the primary and catching up on queues and messages.

Ready

  A backup broker that is fully caught-up and ready to take over as primary.

Recovering

  The newly-promoted primary, waiting for backups to connect and catch up.

Active

  The active primary broker with all backups connected and caught-up.

### Limitations

There are a some known limitations in the current implementation. These will be fixed in future versions.

- Transactional changes to queue state are not replicated atomically. If the primary crashes during a transaction, it is possible that the backup could contain only part of the changes introduced by a transaction.

- Configuration changes (creating or deleting queues, exchanges and bindings) are replicated asynchronously. Management tools used to make changes will consider the change complete when it is complete on the primary, it may not yet be replicated to all the backups.

- Federation links to the primary will fail over correctly. Federated links from the primary will be lost in fail over, they will not be re-connected to the new primary. It is possible to work around this by replacing the `qpidd-primary` start up script with a script that re-creates federation links when the primary is promoted.

## Virtual IP Addresses

Some resource managers (including `rgmanager`) support virtual IP addresses. A virtual IP address is an IP address that can be relocated to any of the nodes in a cluster. The resource manager associates this address with the primary node in the cluster, and relocates it to the new primary when there is a failure. This simplifies configuration as you can publish a single IP address rather than a list.

A virtual IP address can be used by clients to connect to the primary. The following sections will explain how to configure virtual IP addresses for clients or brokers.

## Configuring the Brokers

The broker must load the `ha` module, it is loaded by default. The following broker options are available for the HA module.

Broker Options for High Availability Messaging Cluster

Options for High Availability Messaging Cluster
`ha-cluster yes|no` | Set to "yes" to have the broker join a cluster.
`ha-queue-replication yes|no` | Enable replication of specific queues without joining a cluster, see .
`ha-brokers-url URL` | The URL The full format of the URL is given by this grammar:
    url = ["amqp:"][ user ["/" password] "@" ] addr ("," addr)*
    addr = tcp_addr / rmda_addr / ssl_addr / ...
    tcp_addr = ["tcp:"] host [":" port]
    rdma_addr = "rdma:" host [":" port]
    ssl_addr = "ssl:" host [":" port]'
used by cluster brokers to connect to each other. The URL should contain a comma separated list of the broker addresses, rather than a virtual IP address.
`ha-public-url URL` | The URL is advertised to clients as the "known-hosts" for fail-over. It can be a list or a single virtual IP address. A virtual IP address is recommended. Using this option you can put client and broker traffic on separate networks, which is recommended. Note: When HA clustering is enabled the broker option `known-hosts-url` is ignored and over-ridden by the `ha-public-url` setting.
`ha-replicate` VALUE | Specifies whether queues and exchanges are replicated by default. VALUE is one of: `none`, `configuration`, `all`. For details see .
`ha-username USER` `ha-password PASS` `ha-mechanism MECHANISM` | Authentication settings used by HA brokers to connect to each other. If you are using authorization () then this user must have all permissions.
`ha-backup-timeout SECONDS` | Maximum time that a recovering primary will wait for an expected backup to connect and become ready.
`link-maintenance-interval SECONDS` | Interval for the broker to check link health and re-connect links if need be. If you want brokers to fail over quickly you can set this to a fraction of a second, for example: 0.1.
`link-heartbeat-interval SECONDS` | Heartbeat interval for replication links. The link will be assumed broken if there is no heartbeat for twice the interval.

To configure a HA cluster you must set at least `ha-cluster` and `ha-brokers-url`.

## The Cluster Resource Manager

Broker fail-over is managed by a cluster resource manager. An integration with rgmanager is provided, but it is possible to integrate with other resource managers.

The resource manager is responsible for starting the `qpidd` broker on each node in the cluster. The resource manager then promotes one of the brokers to be the primary. The other brokers connect to the primary as backups, using the URL provided in the `ha-brokers-url` configuration option.

Once connected, the backup brokers synchronize their state with the primary. When a backup is synchronized, or "hot", it is ready to take over if the primary fails. Backup brokers continually receive updates from the primary in order to stay synchronized.

If the primary fails, backup brokers go into fail-over mode. The resource manager must detect the failure and promote one of the backups to be the new primary. The other backups connect to the new primary and synchronize their state with it.

The resource manager is also responsible for protecting the cluster from split-brain conditions resulting from a network partition. A network partition divide a cluster into two sub-groups which cannot see each other. Usually a quorum voting algorithm is used that disables nodes in the inquorate sub-group.

## Configuring `rgmanager` as resource manager

This section assumes that you are already familiar with setting up and configuring clustered services using `cman` and `rgmanager`. It will show you how to configure an active-passive, hot-standby `qpidd` HA cluster with `rgmanager`.

You must provide a `cluster.conf` file to configure `cman` and `rgmanager`. Here is an example `cluster.conf` file for a cluster of 3 nodes named node1, node2 and node3. We will go through the configuration step-by-step.

    <?xml version="1.0"?>
    <!--
    This is an example of a cluster.conf file to run qpidd HA under rgmanager.
    This example assumes a 3 node cluster, with nodes named node1, node2 and node3.

    NOTE: fencing is not shown, you must configure fencing appropriately for your cluster.
    -->

    <cluster name="qpid-test" config_version="18">
      <!-- The cluster has 3 nodes. Each has a unique nodid and one vote
           for quorum. -->
      <clusternodes>
        <clusternode name="node1.example.com" nodeid="1"/>
        <clusternode name="node2.example.com" nodeid="2"/>
        <clusternode name="node3.example.com" nodeid="3"/>
      </clusternodes>
      <!-- Resouce Manager configuration. -->
      <rm>
        <!--
    	There is a failoverdomain for each node containing just that node.
    	This lets us stipulate that the qpidd service should always run on each node.
        -->
        <failoverdomains>
          <failoverdomain name="node1-domain" restricted="1">
    	<failoverdomainnode name="node1.example.com"/>
          </failoverdomain>
          <failoverdomain name="node2-domain" restricted="1">
    	<failoverdomainnode name="node2.example.com"/>
          </failoverdomain>
          <failoverdomain name="node3-domain" restricted="1">
    	<failoverdomainnode name="node3.example.com"/>
          </failoverdomain>
        </failoverdomains>

        <resources>
          <!-- This script starts a qpidd broker acting as a backup. -->
          <script file="/etc/init.d/qpidd" name="qpidd"/>

          <!-- This script promotes the qpidd broker on this node to primary. -->
          <script file="/etc/init.d/qpidd-primary" name="qpidd-primary"/>

          <!-- This is a virtual IP address for client traffic. -->
          <ip address="20.0.20.200" monitor_link="1"/>
        </resources>

        <!-- There is a qpidd service on each node, it should be restarted if it fails. -->
        <service name="node1-qpidd-service" domain="node1-domain" recovery="restart">
          <script ref="qpidd"/>
        </service>
        <service name="node2-qpidd-service" domain="node2-domain" recovery="restart">
          <script ref="qpidd"/>
        </service>
        <service name="node3-qpidd-service" domain="node3-domain"  recovery="restart">
          <script ref="qpidd"/>
        </service>

        <!-- There should always be a single qpidd-primary service, it can run on any node. -->
        <service name="qpidd-primary-service" autostart="1" exclusive="0" recovery="relocate">
          <script ref="qpidd-primary"/>
          <!-- The primary has the IP addresses for brokers and clients to connect. -->
          <ip ref="20.0.20.200"/>
        </service>
      </rm>
    </cluster>

There is a `failoverdomain` for each node containing just that one node. This lets us stipulate that the qpidd service should always run on all nodes.

The `resources` section defines the `qpidd` script used to start the `qpidd` service. It also defines the `qpid-primary` script which does not actually start a new service, rather it promotes the existing `qpidd` broker to primary status.

The `resources` section also defines a pair of virtual IP addresses on different sub-nets. One will be used for broker-to-broker communication, the other for client-to-broker.

To take advantage of the virtual IP addresses, `qpidd.conf` should contain these lines:

          ha-cluster=yes
          ha-public-url=20.0.10.200
          ha-brokers-url=20.0.20.1,20.0.20.2,20.0.20.3

This configuration allows clients to connect to a single address: 20.0.10.200. The brokers connect to each other directly via the addresses listed in `ha-brokers-url`. Note the client and broker addresses are on separate sub-nets, this is recommended but not required.

The `service` section defines 3 `qpidd` services, one for each node. Each service is in a restricted fail-over domain containing just that node, and has the `restart` recovery policy. The effect of this is that rgmanager will run `qpidd` on each node, restarting if it fails.

There is a single `qpidd-primary-service` using the `qpidd-primary` script which is not restricted to a domain and has the `relocate` recovery policy. This means rgmanager will start `qpidd-primary` on one of the nodes when the cluster starts and will relocate it to another node if the original node fails. Running the `qpidd-primary` script does not start a new broker process, it promotes the existing broker to become the primary.

## Broker Administration Tools

Normally, clients are not allowed to connect to a backup broker. However management tools are allowed to connect to a backup brokers. If you use these tools you must not add or remove messages from replicated queues, nor create or delete replicated queues or exchanges as this will disrupt the replication process and may cause message loss.

`qpid-ha` allows you to view and change HA configuration settings.

The tools `qpid-config`, `qpid-route` and `qpid-stat` will connect to a backup if you pass the flag `ha-admin` on the command line.

## Controlling replication of queues and exchanges

By default, queues and exchanges are not replicated automatically. You can change the default behavior by setting the `ha-replicate` configuration option. It has one of the following values:

- all: Replicate everything automatically: queues, exchanges, bindings and messages.

- configuration: Replicate the existence of queues, exchange and bindings but don't replicate messages.

- none: Don't replicate anything, this is the default.

You can over-ride the default for a particular queue or exchange by passing the argument `qpid.replicate` when creating the queue or exchange. It takes the same values as `ha-replicate`

Bindings are automatically replicated if the queue and exchange being bound both have replication `all` or `configuration`, they are not replicated otherwise.

You can create replicated queues and exchanges with the `qpid-config` management tool like this:

          qpid-config add queue myqueue --replicate all

To create replicated queues and exchanges via the client API, add a `node` entry to the address like this:

          "myqueue;{create:always,node:{x-declare:{arguments:{'qpid.replicate':all}}}}"

There are some built-in exchanges created automatically by the broker, these exchanges are never replicated. The built-in exchanges are the default (nameless) exchange, the AMQP standard exchanges (`amq.direct, amq.topic, amq.fanout` and `amq.match`) and the management exchanges (`qpid.management, qmf.default.direct` and `qmf.default.topic`)

Note that if you bind a replicated queue to one of these exchanges, the binding will not be replicated, so the queue will not have the binding after a fail-over.

## Client Connection and Fail-over

Clients can only connect to the primary broker. Backup brokers reject any connection attempt by a client. Clients rejected by a backup broker will automatically fail-over until they connect to the primary. if `ha-public-url` contains multiple addresses, the client will them all in rotation. If it is a virtual IP address the clients will retry on the same address until it is reconnected.

Clients are configured with the URL for the cluster (details below for each type of client). There are two possibilities

- The URL contains multiple addresses, one for each broker in the cluster.

- The URL contains a single virtual IP address that is assigned to the primary broker by the resource manager. This is the recommended configuration.

In the first case, clients will repeatedly re-try each address in the URL until they successfully connect to the primary. In the second case the resource manager will assign the virtual IP address to the primary broker, so clients only need to re-try on a single address.

When the primary broker fails, clients re-try all known cluster addresses until they connect to the new primary. The client re-sends any messages that were previously sent but not acknowledged by the broker at the time of the failure. Similarly messages that have been sent by the broker, but not acknowledged by the client, are re-queued.

TCP can be slow to detect connection failures. A client can configure a connection to use a heartbeat to detect connection failure, and can specify a time interval for the heartbeat. If heartbeats are in use, failures will be detected no later than twice the heartbeat interval. The following sections explain how to enable heartbeat in each client.

Note: the following sections explain how to configure clients with multiple dresses, but if you are using a virtual IP address you only need to configure that one address for clients, you don't need to list all the addresses.

Suppose your cluster has 3 nodes: `node1`, `node2` and `node3` all using the default AMQP port, and you are not using a virtual IP address. To connect a client you need to specify the address(es) and set the `reconnect` property to `true`. The following sub-sections show how to connect each type of client.

### C++ clients

With the C++ client, you specify multiple cluster addresses in a single URL

The full grammar for the URL is:

    	    url = ["amqp:"][ user ["/" password] "@" ] addr ("," addr)*
    	    addr = tcp_addr / rmda_addr / ssl_addr / ...
    	    tcp_addr = ["tcp:"] host [":" port]
    	    rdma_addr = "rdma:" host [":" port]
    	    ssl_addr = "ssl:" host [":" port]'

You also need to specify the connection option `reconnect` to be true. For example:

    	qpid::messaging::Connection c("node1,node2,node3","{reconnect:true}");

Heartbeats are disabled by default. You can enable them by specifying a heartbeat interval (in seconds) for the connection via the `heartbeat` option. For example:

    	  qpid::messaging::Connection c("node1,node2,node3","{reconnect:true,heartbeat:10}");

### Python clients

With the python client, you specify `reconnect=True` and a list of host:port addresses as `reconnect_urls` when calling `Connection.establish` or `Connection.open`

    	connection = qpid.messaging.Connection.establish("node1", reconnect=True, reconnect_urls=["node1", "node2", "node3"])

Heartbeats are disabled by default. You can enable them by specifying a heartbeat interval (in seconds) for the connection via the 'heartbeat' option. For example:

    	connection = qpid.messaging.Connection.establish("node1", reconnect=True, reconnect_urls=["node1", "node2", "node3"], heartbeat=10)

### Java JMS Clients

In Java JMS clients, client fail-over is handled automatically if it is enabled in the connection. You can configure a connection to use fail-over using the `failover` property:

    	connectionfactory.qpidConnectionfactory = amqp://guest:guest@clientid/test?brokerlist='tcp://localhost:5672'&failover='failover_exchange'

This property can take three values:

Fail-over Modes

failover_exchange

  If the connection fails, fail over to any other broker in the cluster.

roundrobin

  If the connection fails, fail over to one of the brokers specified in the `brokerlist`.

singlebroker

  Fail-over is not supported; the connection is to a single broker only.

In a Connection URL, heartbeat is set using the `idle_timeout` property, which is an integer corresponding to the heartbeat period in seconds. For instance, the following line from a JNDI properties file sets the heartbeat time out to 3 seconds:

    	connectionfactory.qpidConnectionfactory = amqp://guest:guest@clientid/test?brokerlist='tcp://localhost:5672',idle_timeout=3

## Security.

You can secure your cluster using the authentication and authorization features described in .

Backup brokers connect to the primary broker and subscribe for management events and queue contents. You can specify the identity used to connect to the primary with the following options:

Security options for High Availability Messaging Cluster

Security options for High Availability Messaging Cluster
`ha-username USER` `ha-password PASS` `ha-mechanism MECH` | Authentication settings used by HA brokers to connect to each other. If you are using authorization () then this user must have all permissions.

This identity is also used to authorize actions taken on the backup broker to replicate from the primary, for example to create queues or exchanges.

## Integrating with other Cluster Resource Managers

To integrate with a different resource manager you must configure it to:

- Start a qpidd process on each node of the cluster.

- Restart qpidd if it crashes.

- Promote exactly one of the brokers to primary.

- Detect a failure and promote a new primary.

The `qpid-ha` command allows you to check if a broker is primary, and to promote a backup to primary.

To test if a broker is the primary:

    	qpid-ha -b broker-address status --expect=primary

This command will return 0 if the broker at broker-address is the primary, non-0 otherwise.

To promote a broker to primary:

    	qpid-ha -b broker-address promote

`qpid-ha --help` gives information on other commands and options available. You can also use `qpid-ha` to manually examine and promote brokers. This can be useful for testing failover scenarios without having to set up a full resource manager, or to simulate a cluster on a single node. For deployment, a resource manager is required.

## Replicating specific queues

In addition to the automatic replication performed in a cluster, you can set up replication for specific queues between arbitrary brokers, even if the brokers are not members of a cluster. The command:

          qpid-ha replicate QUEUE REMOTE-BROKER

sets up replication of QUEUE on REMOTE-BROKER to QUEUE on the current broker.

Set the configuration option `ha-queue-replication=yes` on both brokers to enable this feature on non-cluster brokers. It is automatically enabled for brokers that are part of a cluster.

Note that this feature does not provide automatic fail-over, for that you need to run a cluster.

## Using a message store in a cluster

If you use a persistent store for your messages then each broker in a cluster will have its own store. If the entire cluster fails and is restarted, the *first* broker that becomes primary will recover from its store. All the other brokers will clear their stores and get an update from the primary to ensure consistency.