1049 lines
40 KiB
Markdown
1049 lines
40 KiB
Markdown
# High Availability and Failover
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We define high availability as the *ability for the system to continue
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functioning after failure of one or more of the servers*.
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A part of high availability is *failover* which we define as the
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*ability for client connections to migrate from one server to another in
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event of server failure so client applications can continue to operate*.
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## Live - Backup Groups
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Apache ActiveMQ allows servers to be linked together as *live - backup* groups
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where each live server can have 1 or more backup servers. A backup
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server is owned by only one live server. Backup servers are not
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operational until failover occurs, however 1 chosen backup, which will
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be in passive mode, announces its status and waits to take over the live
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servers work
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Before failover, only the live server is serving the Apache ActiveMQ clients
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while the backup servers remain passive or awaiting to become a backup
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server. When a live server crashes or is brought down in the correct
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mode, the backup server currently in passive mode will become live and
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another backup server will become passive. If a live server restarts
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after a failover then it will have priority and be the next server to
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become live when the current live server goes down, if the current live
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server is configured to allow automatic failback then it will detect the
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live server coming back up and automatically stop.
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### HA Policies
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Apache ActiveMQ supports two different strategies for backing up a server
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*shared store* and *replication*. Which is configured via the
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`ha-policy` configuration element.
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<ha-policy>
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<replication/>
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</ha-policy>
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or
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<ha-policy>
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<shared-store/>
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</ha-policy>
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As well as these 2 strategies there is also a 3rd called `live-only`.
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This of course means there will be no Backup Strategy and is the default
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if none is provided, however this is used to configure `scale-down`
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which we will cover in a later chapter.
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> **Note**
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>
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> The `ha-policy` configurations replaces any current HA configuration
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> in the root of the `activemq-configuration.xml` configuration. All old
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> configuration is now deprecated altho best efforts will be made to
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> honour it if configured this way.
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> **Note**
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>
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> Only persistent message data will survive failover. Any non persistent
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> message data will not be available after failover.
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The `ha-policy` type configures which strategy a cluster should use to
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provide the backing up of a servers data. Within this configuration
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element is configured how a server should behave within the cluster,
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either as a master (live), slave (backup) or colocated (both live and
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backup). This would look something like:
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<ha-policy>
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<replication>
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<master/>
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</replication>
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</ha-policy>
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or
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<ha-policy>
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<shared-store/>
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<slave/>
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</shared-store/>
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</ha-policy>
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or
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<ha-policy>
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<replication>
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<colocated/>
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</replication>
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</ha-policy>
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### Data Replication
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Support for network-based data replication was added in version 2.3.
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When using replication, the live and the backup servers do not share the
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same data directories, all data synchronization is done over the
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network. Therefore all (persistent) data received by the live server
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will be duplicated to the backup.
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Notice that upon start-up the backup server will first need to
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synchronize all existing data from the live server before becoming
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capable of replacing the live server should it fail. So unlike when
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using shared storage, a replicating backup will not be a fully
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operational backup right after start-up, but only after it finishes
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synchronizing the data with its live server. The time it will take for
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this to happen will depend on the amount of data to be synchronized and
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the connection speed.
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> **Note**
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>
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> Synchronization occurs in parallel with current network traffic so
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> this won't cause any blocking on current clients.
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Replication will create a copy of the data at the backup. One issue to
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be aware of is: in case of a successful fail-over, the backup's data
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will be newer than the one at the live's storage. If you configure your
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live server to perform a failback to live server when restarted, it will synchronize its data
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with the backup's. If both servers are shutdown, the administrator will
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have to determine which one has the latest data.
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The replicating live and backup pair must be part of a cluster. The
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Cluster Connection also defines how backup servers will find the remote
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live servers to pair with. Refer to [Clusters](clusters.md) for details on how this is done,
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and how to configure a cluster connection. Notice that:
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- Both live and backup servers must be part of the same cluster.
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Notice that even a simple live/backup replicating pair will require
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a cluster configuration.
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- Their cluster user and password must match.
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Within a cluster, there are two ways that a backup server will locate a
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live server to replicate from, these are:
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- `specifying a node group`. You can specify a group of live servers
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that a backup server can connect to. This is done by configuring
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`group-name` in either the `master` or the `slave` element of the
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`activemq-configuration.xml`. A Backup server will only connect to a
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live server that shares the same node group name
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- `connecting to any live`. This will be the behaviour if `group-name`
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is not configured allowing a backup server to connect to any live
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server
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> **Note**
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>
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> A `group-name` example: suppose you have 5 live servers and 6 backup
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> servers:
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>
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> - `live1`, `live2`, `live3`: with `group-name=fish`
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>
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> - `live4`, `live5`: with `group-name=bird`
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>
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> - `backup1`, `backup2`, `backup3`, `backup4`: with `group-name=fish`
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>
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> - `backup5`, `backup6`: with `group-name=bird`
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>
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> After joining the cluster the backups with `group-name=fish` will
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> search for live servers with `group-name=fish` to pair with. Since
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> there is one backup too many, the `fish` will remain with one spare
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> backup.
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>
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> The 2 backups with `group-name=bird` (`backup5` and `backup6`) will
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> pair with live servers `live4` and `live5`.
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The backup will search for any live server that it is configured to
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connect to. It then tries to replicate with each live server in turn
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until it finds a live server that has no current backup configured. If
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no live server is available it will wait until the cluster topology
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changes and repeats the process.
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> **Note**
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>
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> This is an important distinction from a shared-store backup, if a
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> backup starts and does not find a live server, the server will just
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> activate and start to serve client requests. In the replication case,
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> the backup just keeps waiting for a live server to pair with. Note
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> that in replication the backup server does not know whether any data
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> it might have is up to date, so it really cannot decide to activate
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> automatically. To activate a replicating backup server using the data
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> it has, the administrator must change its configuration to make it a
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> live server by changing `slave` to `master`.
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Much like in the shared-store case, when the live server stops or
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crashes, its replicating backup will become active and take over its
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duties. Specifically, the backup will become active when it loses
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connection to its live server. This can be problematic because this can
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also happen because of a temporary network problem. In order to address
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this issue, the backup will try to determine whether it still can
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connect to the other servers in the cluster. If it can connect to more
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than half the servers, it will become active, if more than half the
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servers also disappeared with the live, the backup will wait and try
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reconnecting with the live. This avoids a split brain situation.
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#### Configuration
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To configure the live and backup servers to be a replicating pair,
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configure the live server in ' `activemq-configuration.xml` to have:
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<ha-policy>
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<replication>
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<master/>
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</replication>
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</ha-policy>
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.
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<cluster-connections>
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<cluster-connection name="my-cluster">
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...
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</cluster-connection>
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</cluster-connections>
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The backup server must be similarly configured but as a `slave`
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<ha-policy>
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<replication>
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<slave/>
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</replication>
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</ha-policy>
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#### All Replication Configuration
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The following table lists all the `ha-policy` configuration elements for
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HA strategy Replication for `master`:
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<table summary="HA Replication Master Policy" border="1">
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<colgroup>
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<col/>
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<col/>
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</colgroup>
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<thead>
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<tr>
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<th>Name</th>
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<th>Description</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>`check-for-live-server`</td>
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<td>Whether to check the cluster for a (live) server using our own server ID
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when starting up. This option is only necessary for performing 'fail-back'
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on replicating servers.</td>
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</tr>
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<tr>
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<td>`cluster-name`</td>
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<td>Name of the cluster configuration to use for replication. This setting is
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only necessary if you configure multiple cluster connections. If configured then
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the connector configuration of the cluster configuration with this name will be
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used when connecting to the cluster to discover if a live server is already running,
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see `check-for-live-server`. If unset then the default cluster connections configuration
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is used (the first one configured).</td>
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</tr>
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<tr>
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<td>`group-name`</td>
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<td>Whether to check the cluster for a (live) server using our own server ID when starting up. This option is only necessary for performing 'fail-back' on replicating servers.</td>
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</tr>
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<tr>
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<td>`check-for-live-server`</td>
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<td>If set, backup servers will only pair with live servers with matching group-name.</td>
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</tr>
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</tbody>
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</table>
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The following table lists all the `ha-policy` configuration elements for
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HA strategy Replication for `slave`:
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<table summary="HA Replication Slave Policy" border="1">
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<colgroup>
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<col/>
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<col/>
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</colgroup>
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<thead>
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<tr>
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<th>Name</th>
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<th>Description</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>`cluster-name`</td>
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<td>Name of the cluster configuration to use for replication.
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This setting is only necessary if you configure multiple cluster
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connections. If configured then the connector configuration of
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the cluster configuration with this name will be used when
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connecting to the cluster to discover if a live server is already
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running, see `check-for-live-server`. If unset then the default
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cluster connections configuration is used (the first one configured)</td>
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</tr>
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<tr>
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<td>`group-name`</td>
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<td>If set, backup servers will only pair with live servers with matching group-name</td>
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</tr>
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<tr>
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<td>`max-saved-replicated-journals-size`</td>
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<td>This specifies how many times a replicated backup server
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can restart after moving its files on start. Once there are
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this number of backup journal files the server will stop permanently
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after if fails back.</td>
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</tr>
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<tr>
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<td>`allow-failback`</td>
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<td>Whether a server will automatically stop when a another places a
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request to take over its place. The use case is when the backup has
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failed over</td>
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</tr>
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<tr>
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<td>`failback-delay`</td>
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<td>delay to wait before fail-back occurs on (failed over live's) restart</td>
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</tr>
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</tbody>
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</table>
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### Shared Store
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When using a shared store, both live and backup servers share the *same*
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entire data directory using a shared file system. This means the paging
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directory, journal directory, large messages and binding journal.
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When failover occurs and a backup server takes over, it will load the
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persistent storage from the shared file system and clients can connect
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to it.
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This style of high availability differs from data replication in that it
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requires a shared file system which is accessible by both the live and
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backup nodes. Typically this will be some kind of high performance
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Storage Area Network (SAN). We do not recommend you use Network Attached
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Storage (NAS), e.g. NFS mounts to store any shared journal (NFS is
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slow).
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The advantage of shared-store high availability is that no replication
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occurs between the live and backup nodes, this means it does not suffer
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any performance penalties due to the overhead of replication during
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normal operation.
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The disadvantage of shared store replication is that it requires a
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shared file system, and when the backup server activates it needs to
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load the journal from the shared store which can take some time
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depending on the amount of data in the store.
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If you require the highest performance during normal operation, have
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access to a fast SAN and live with a slightly slower failover (depending
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on amount of data).
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#### Configuration
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To configure the live and backup servers to share their store, configure
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id via the `ha-policy` configuration in `activemq-configuration.xml`:
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<ha-policy>
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<shared-store>
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<master/>
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</shared-store>
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</ha-policy>
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.
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<cluster-connections>
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<cluster-connection name="my-cluster">
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...
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</cluster-connection>
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</cluster-connections>
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The backup server must also be configured as a backup.
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<ha-policy>
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<shared-store>
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<slave/>
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</shared-store>
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</ha-policy>
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In order for live - backup groups to operate properly with a shared
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store, both servers must have configured the location of journal
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directory to point to the *same shared location* (as explained in [Configuring the message journal](persistence.md))
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> **Note**
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>
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> todo write something about GFS
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Also each node, live and backups, will need to have a cluster connection
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defined even if not part of a cluster. The Cluster Connection info
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defines how backup servers announce there presence to its live server or
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any other nodes in the cluster. Refer to [Clusters](clusters.md) for details on how this is
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done.
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### Failing Back to live Server
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After a live server has failed and a backup taken has taken over its
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duties, you may want to restart the live server and have clients fail
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back.
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In case of "shared disk", simply restart the original live server and
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kill the new live server by can do this by killing the process itself.
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Alternatively you can set `allow-fail-back` to `true` on the slave
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config which will force the backup that has become live to automatically
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stop. This configuration would look like:
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<ha-policy>
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<shared-store>
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<slave>
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<allow-failback>true</allow-failback>
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<failback-delay>5000</failback-delay>
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</slave>
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</shared-store>
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</ha-policy>
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The `failback-delay` configures how long the backup must wait after
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automatically stopping before it restarts. This is to gives the live
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server time to start and obtain its lock.
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In replication HA mode you need to set an extra property
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`check-for-live-server` to `true` in the `master` configuration. If set
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to true, during start-up a live server will first search the cluster for
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another server using its nodeID. If it finds one, it will contact this
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server and try to "fail-back". Since this is a remote replication
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scenario, the "starting live" will have to synchronize its data with the
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server running with its ID, once they are in sync, it will request the
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other server (which it assumes it is a back that has assumed its duties)
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to shutdown for it to take over. This is necessary because otherwise the
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live server has no means to know whether there was a fail-over or not,
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and if there was if the server that took its duties is still running or
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not. To configure this option at your `activemq-configuration.xml`
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configuration file as follows:
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<ha-policy>
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<replication>
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<master>
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<check-for-live-server>true</check-for-live-server>
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<master>
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</replication>
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</ha-policy>
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> **Warning**
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>
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> Be aware that if you restart a live server while after failover has
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> occurred then this value must be set to ``. If not the live server
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> will restart and server the same messages that the backup has already
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> handled causing duplicates.
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It is also possible, in the case of shared store, to cause failover to
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occur on normal server shutdown, to enable this set the following
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property to true in the `ha-policy` configuration on either the `master`
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or `slave` like so:
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<ha-policy>
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<shared-store>
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<master>
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<failover-on-shutdown>true</failover-on-shutdown>
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</master>
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</shared-store>
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</ha-policy>
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By default this is set to false, if by some chance you have set this to
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false but still want to stop the server normally and cause failover then
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you can do this by using the management API as explained at [Management](management.md)
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You can also force the running live server to shutdown when the old live
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server comes back up allowing the original live server to take over
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automatically by setting the following property in the
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`activemq-configuration.xml` configuration file as follows:
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<ha-policy>
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<shared-store>
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<slave>
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<allow-failback>true</allow-failback>
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</slave>
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</shared-store>
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</ha-policy>
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#### All Shared Store Configuration
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The following table lists all the `ha-policy` configuration elements for
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HA strategy shared store for `master`:
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<table summary="HA Shared Store Master Policy" border="1">
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<colgroup>
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<col/>
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<col/>
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</colgroup>
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<thead>
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<tr>
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<th>Name</th>
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<th>Description</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>`failback-delay`</td>
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<td>If a backup server is detected as being live,
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via the lock file, then the live server will wait
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announce itself as a backup and wait this amount
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of time (in ms) before starting as a live</td>
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</tr>
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<tr>
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<td>`failover-on-server-shutdown`</td>
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<td>If set to true then when this server is stopped
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normally the backup will become live assuming failover.
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If false then the backup server will remain passive.
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Note that if false you want failover to occur the you
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can use the the management API as explained at [Management](management.md)</td>
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</tr>
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</tbody>
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</table>
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The following table lists all the `ha-policy` configuration elements for
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HA strategy Shared Store for `slave`:
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|
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<table summary="HA Replication Slave Policy" border="1">
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<colgroup>
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<col/>
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<col/>
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</colgroup>
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<thead>
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<tr>
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<th>Name</th>
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<th>Description</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>`failover-on-server-shutdown`</td>
|
|
<td>In the case of a backup that has become live. then
|
|
when set to true then when this server is stopped normally
|
|
the backup will become liveassuming failover. If false then
|
|
the backup server will remain passive. Note that if false
|
|
you want failover to occur the you can use the the management
|
|
API as explained at [Management](management.md)</td>
|
|
</tr>
|
|
<tr>
|
|
<td>`allow-failback`</td>
|
|
<td>Whether a server will automatically stop when a another
|
|
places a request to take over its place. The use case is
|
|
when the backup has failed over.</td>
|
|
</tr>
|
|
<tr>
|
|
<td>`failback-delay`</td>
|
|
<td>After failover and the slave has become live, this is
|
|
set on the new live server. When starting If a backup server
|
|
is detected as being live, via the lock file, then the live
|
|
server will wait announce itself as a backup and wait this
|
|
amount of time (in ms) before starting as a live, however
|
|
this is unlikely since this backup has just stopped anyway.
|
|
It is also used as the delay after failback before this backup
|
|
will restart (if `allow-failback` is set to true.</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
|
|
#### Colocated Backup Servers
|
|
|
|
It is also possible when running standalone to colocate backup servers
|
|
in the same JVM as another live server. Live Servers can be configured
|
|
to request another live server in the cluster to start a backup server
|
|
in the same JVM either using shared store or replication. The new backup
|
|
server will inherit its configuration from the live server creating it
|
|
apart from its name, which will be set to `colocated_backup_n` where n
|
|
is the number of backups the server has created, and any directories and
|
|
its Connectors and Acceptors which are discussed later on in this
|
|
chapter. A live server can also be configured to allow requests from
|
|
backups and also how many backups a live server can start. this way you
|
|
can evenly distribute backups around the cluster. This is configured via
|
|
the `ha-policy` element in the `activemq-configuration.xml` file like
|
|
so:
|
|
|
|
<ha-policy>
|
|
<replication>
|
|
<colocated>
|
|
<request-backup>true</request-backup>
|
|
<max-backups>1</max-backups>
|
|
<backup-request-retries>-1</backup-request-retries>
|
|
<backup-request-retry-interval>5000</backup-request-retry-interval>
|
|
<master/>
|
|
<slave/>
|
|
</colocated>
|
|
<replication>
|
|
</ha-policy>
|
|
|
|
|
|
the above example is configured to use replication, in this case the
|
|
`master` and `slave` configurations must match those for normal
|
|
replication as in the previous chapter. `shared-store` is also supported
|
|
|
|
|
|
|
|
#### Configuring Connectors and Acceptors
|
|
|
|
If the HA Policy is colocated then connectors and acceptors will be
|
|
inherited from the live server creating it and offset depending on the
|
|
setting of `backup-port-offset` configuration element. If this is set to
|
|
say 100 (which is the default) and a connector is using port 61616 then
|
|
this will be set to 5545 for the first server created, 5645 for the
|
|
second and so on.
|
|
|
|
> **Note**
|
|
>
|
|
> for INVM connectors and Acceptors the id will have
|
|
> `colocated_backup_n` appended, where n is the backup server number.
|
|
|
|
#### Remote Connectors
|
|
|
|
It may be that some of the Connectors configured are for external
|
|
servers and hence should be excluded from the offset. for instance a
|
|
Connector used by the cluster connection to do quorum voting for a
|
|
replicated backup server, these can be omitted from being offset by
|
|
adding them to the `ha-policy` configuration like so:
|
|
|
|
<ha-policy>
|
|
<replication>
|
|
<colocated>
|
|
<excludes>
|
|
<connector-ref>remote-connector</connector-ref>
|
|
</excludes>
|
|
.........
|
|
</ha-policy>
|
|
|
|
|
|
#### Configuring Directories
|
|
|
|
Directories for the Journal, Large messages and Paging will be set
|
|
according to what the HA strategy is. If shared store the the requesting
|
|
server will notify the target server of which directories to use. If
|
|
replication is configured then directories will be inherited from the
|
|
creating server but have the new backups name appended.
|
|
|
|
The following table lists all the `ha-policy` configuration elements for colocated policy:
|
|
|
|
<table summary="HA Replication Colocation Policy" border="1">
|
|
<colgroup>
|
|
<col/>
|
|
<col/>
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th>Name</th>
|
|
<th>Description</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td>`request-backup`</td>
|
|
<td>If true then the server will request a backup on another node</td>
|
|
</tr>
|
|
<tr>
|
|
<td>`backup-request-retries`</td>
|
|
<td>How many times the live server will try to request a backup, -1 means for ever.</td>
|
|
</tr>
|
|
<tr>
|
|
<td>`backup-request-retry-interval`</td>
|
|
<td>How long to wait for retries between attempts to request a backup server.</td>
|
|
</tr>
|
|
<tr>
|
|
<td>`max-backups`</td>
|
|
<td>How many backups a live server can create</td>
|
|
</tr>
|
|
<tr>
|
|
<td>`backup-port-offset`</td>
|
|
<td>The offset to use for the Connectors and Acceptors when creating a new backup server.</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
|
|
### Scaling Down
|
|
|
|
An alternative to using Live/Backup groups is to configure scaledown.
|
|
when configured for scale down a server can copy all its messages and
|
|
transaction state to another live server. The advantage of this is that
|
|
you dont need full backups to provide some form of HA, however there are
|
|
disadvantages with this approach the first being that it only deals with
|
|
a server being stopped and not a server crash. The caveat here is if you
|
|
configure a backup to scale down.
|
|
|
|
Another disadvantage is that it is possible to lose message ordering.
|
|
This happens in the following scenario, say you have 2 live servers and
|
|
messages are distributed evenly between the servers from a single
|
|
producer, if one of the servers scales down then the messages sent back
|
|
to the other server will be in the queue after the ones already there,
|
|
so server 1 could have messages 1,3,5,7,9 and server 2 would have
|
|
2,4,6,8,10, if server 2 scales down the order in server 1 would be
|
|
1,3,5,7,9,2,4,6,8,10.
|
|
|
|
|
|
|
|
The configuration for a live server to scale down would be something
|
|
like:
|
|
|
|
<ha-policy>
|
|
<live-only>
|
|
<scale-down>
|
|
<connectors>
|
|
<connector-ref>server1-connector</connector-ref>
|
|
</connectors>
|
|
</scale-down>
|
|
</live-only>
|
|
</ha-policy>
|
|
|
|
|
|
In this instance the server is configured to use a specific connector to
|
|
scale down, if a connector is not specified then the first INVM
|
|
connector is chosen, this is to make scale down fromm a backup server
|
|
easy to configure. It is also possible to use discovery to scale down,
|
|
this would look like:
|
|
|
|
<ha-policy>
|
|
<live-only>
|
|
<scale-down>
|
|
<discovery-group-ref discovery-group-name="my-discovery-group"/>
|
|
</scale-down>
|
|
</live-only>
|
|
</ha-policy>
|
|
|
|
|
|
#### Scale Down with groups
|
|
|
|
It is also possible to configure servers to only scale down to servers
|
|
that belong in the same group. This is done by configuring the group
|
|
like so:
|
|
|
|
<ha-policy>
|
|
<live-only>
|
|
<scale-down>
|
|
...
|
|
<group-name>my-group</group-name>
|
|
</scale-down>
|
|
</live-only>
|
|
</ha-policy>
|
|
|
|
|
|
In this scenario only servers that belong to the group `my-group` will
|
|
be scaled down to
|
|
|
|
#### Scale Down and Backups
|
|
|
|
It is also possible to mix scale down with HA via backup servers. If a
|
|
slave is configured to scale down then after failover has occurred,
|
|
instead of starting fully the backup server will immediately scale down
|
|
to another live server. The most appropriate configuration for this is
|
|
using the `colocated` approach. it means as you bring up live server
|
|
they will automatically be backed up by server and as live servers are
|
|
shutdown, there messages are made available on another live server. A
|
|
typical configuration would look like:
|
|
|
|
<ha-policy>
|
|
<replication>
|
|
<colocated>
|
|
<backup-request-retries>44</backup-request-retries>
|
|
<backup-request-retry-interval>33</backup-request-retry-interval>
|
|
<max-backups>3</max-backups>
|
|
<request-backup>false</request-backup>
|
|
<backup-port-offset>33</backup-port-offset>
|
|
<master>
|
|
<group-name>purple</group-name>
|
|
<check-for-live-server>true</check-for-live-server>
|
|
<cluster-name>abcdefg</cluster-name>
|
|
</master>
|
|
<slave>
|
|
<group-name>tiddles</group-name>
|
|
<max-saved-replicated-journals-size>22</max-saved-replicated-journals-size>
|
|
<cluster-name>33rrrrr</cluster-name>
|
|
<restart-backup>false</restart-backup>
|
|
<scale-down>
|
|
<!--a grouping of servers that can be scaled down to-->
|
|
<group-name>boo!</group-name>
|
|
<!--either a discovery group-->
|
|
<discovery-group-ref discovery-group-name="wahey"/>
|
|
</scale-down>
|
|
</slave>
|
|
</colocated>
|
|
</replication>
|
|
</ha-policy>
|
|
|
|
|
|
#### Scale Down and Clients
|
|
|
|
When a server is stopping and preparing to scale down it will send a
|
|
message to all its clients informing them which server it is scaling
|
|
down to before disconnecting them. At this point the client will
|
|
reconnect however this will only succeed once the server has completed
|
|
scaledown. This is to ensure that any state such as queues or
|
|
transactions are there for the client when it reconnects. The normal
|
|
reconnect settings apply when the client is reconnecting so these should
|
|
be high enough to deal with the time needed to scale down.
|
|
|
|
## Failover Modes
|
|
|
|
Apache ActiveMQ defines two types of client failover:
|
|
|
|
- Automatic client failover
|
|
|
|
- Application-level client failover
|
|
|
|
Apache ActiveMQ also provides 100% transparent automatic reattachment of
|
|
connections to the same server (e.g. in case of transient network
|
|
problems). This is similar to failover, except it is reconnecting to the
|
|
same server and is discussed in [Client Reconnection and Session Reattachment](client-reconnection.md)
|
|
|
|
During failover, if the client has consumers on any non persistent or
|
|
temporary queues, those queues will be automatically recreated during
|
|
failover on the backup node, since the backup node will not have any
|
|
knowledge of non persistent queues.
|
|
|
|
### Automatic Client Failover
|
|
|
|
Apache ActiveMQ clients can be configured to receive knowledge of all live and
|
|
backup servers, so that in event of connection failure at the client -
|
|
live server connection, the client will detect this and reconnect to the
|
|
backup server. The backup server will then automatically recreate any
|
|
sessions and consumers that existed on each connection before failover,
|
|
thus saving the user from having to hand-code manual reconnection logic.
|
|
|
|
Apache ActiveMQ clients detect connection failure when it has not received
|
|
packets from the server within the time given by
|
|
`client-failure-check-period` as explained in section [Detecting Dead Connections](connection-ttl.md). If the client
|
|
does not receive data in good time, it will assume the connection has
|
|
failed and attempt failover. Also if the socket is closed by the OS,
|
|
usually if the server process is killed rather than the machine itself
|
|
crashing, then the client will failover straight away.
|
|
|
|
Apache ActiveMQ clients can be configured to discover the list of live-backup
|
|
server groups in a number of different ways. They can be configured
|
|
explicitly or probably the most common way of doing this is to use
|
|
*server discovery* for the client to automatically discover the list.
|
|
For full details on how to configure server discovery, please see [Clusters](clusters.md).
|
|
Alternatively, the clients can explicitly connect to a specific server
|
|
and download the current servers and backups see [Clusters](clusters.md).
|
|
|
|
To enable automatic client failover, the client must be configured to
|
|
allow non-zero reconnection attempts (as explained in [Client Reconnection and Session Reattachment](client-reconnection.md)).
|
|
|
|
By default failover will only occur after at least one connection has
|
|
been made to the live server. In other words, by default, failover will
|
|
not occur if the client fails to make an initial connection to the live
|
|
server - in this case it will simply retry connecting to the live server
|
|
according to the reconnect-attempts property and fail after this number
|
|
of attempts.
|
|
|
|
#### Failing over on the Initial Connection
|
|
|
|
Since the client does not learn about the full topology until after the
|
|
first connection is made there is a window where it does not know about
|
|
the backup. If a failure happens at this point the client can only try
|
|
reconnecting to the original live server. To configure how many attempts
|
|
the client will make you can set the property `initialConnectAttempts`
|
|
on the `ClientSessionFactoryImpl` or `ActiveMQConnectionFactory` or
|
|
`initial-connect-attempts` in xml. The default for this is `0`, that is
|
|
try only once. Once the number of attempts has been made an exception
|
|
will be thrown.
|
|
|
|
For examples of automatic failover with transacted and non-transacted
|
|
JMS sessions, please see ? and ?.
|
|
|
|
#### A Note on Server Replication
|
|
|
|
Apache ActiveMQ does not replicate full server state between live and backup
|
|
servers. When the new session is automatically recreated on the backup
|
|
it won't have any knowledge of messages already sent or acknowledged in
|
|
that session. Any in-flight sends or acknowledgements at the time of
|
|
failover might also be lost.
|
|
|
|
By replicating full server state, theoretically we could provide a 100%
|
|
transparent seamless failover, which would avoid any lost messages or
|
|
acknowledgements, however this comes at a great cost: replicating the
|
|
full server state (including the queues, session, etc.). This would
|
|
require replication of the entire server state machine; every operation
|
|
on the live server would have to replicated on the replica server(s) in
|
|
the exact same global order to ensure a consistent replica state. This
|
|
is extremely hard to do in a performant and scalable way, especially
|
|
when one considers that multiple threads are changing the live server
|
|
state concurrently.
|
|
|
|
It is possible to provide full state machine replication using
|
|
techniques such as *virtual synchrony*, but this does not scale well and
|
|
effectively serializes all operations to a single thread, dramatically
|
|
reducing concurrency.
|
|
|
|
Other techniques for multi-threaded active replication exist such as
|
|
replicating lock states or replicating thread scheduling but this is
|
|
very hard to achieve at a Java level.
|
|
|
|
Consequently it has decided it was not worth massively reducing
|
|
performance and concurrency for the sake of 100% transparent failover.
|
|
Even without 100% transparent failover, it is simple to guarantee *once
|
|
and only once* delivery, even in the case of failure, by using a
|
|
combination of duplicate detection and retrying of transactions. However
|
|
this is not 100% transparent to the client code.
|
|
|
|
#### Handling Blocking Calls During Failover
|
|
|
|
If the client code is in a blocking call to the server, waiting for a
|
|
response to continue its execution, when failover occurs, the new
|
|
session will not have any knowledge of the call that was in progress.
|
|
This call might otherwise hang for ever, waiting for a response that
|
|
will never come.
|
|
|
|
To prevent this, Apache ActiveMQ will unblock any blocking calls that were in
|
|
progress at the time of failover by making them throw a
|
|
`javax.jms.JMSException` (if using JMS), or a `ActiveMQException` with
|
|
error code `ActiveMQException.UNBLOCKED`. It is up to the client code to
|
|
catch this exception and retry any operations if desired.
|
|
|
|
If the method being unblocked is a call to commit(), or prepare(), then
|
|
the transaction will be automatically rolled back and Apache ActiveMQ will
|
|
throw a `javax.jms.TransactionRolledBackException` (if using JMS), or a
|
|
`ActiveMQException` with error code
|
|
`ActiveMQException.TRANSACTION_ROLLED_BACK` if using the core API.
|
|
|
|
#### Handling Failover With Transactions
|
|
|
|
If the session is transactional and messages have already been sent or
|
|
acknowledged in the current transaction, then the server cannot be sure
|
|
that messages sent or acknowledgements have not been lost during the
|
|
failover.
|
|
|
|
Consequently the transaction will be marked as rollback-only, and any
|
|
subsequent attempt to commit it will throw a
|
|
`javax.jms.TransactionRolledBackException` (if using JMS), or a
|
|
`ActiveMQException` with error code
|
|
`ActiveMQException.TRANSACTION_ROLLED_BACK` if using the core API.
|
|
|
|
> **Warning**
|
|
>
|
|
> The caveat to this rule is when XA is used either via JMS or through
|
|
> the core API. If 2 phase commit is used and prepare has already been
|
|
> called then rolling back could cause a `HeuristicMixedException`.
|
|
> Because of this the commit will throw a `XAException.XA_RETRY`
|
|
> exception. This informs the Transaction Manager that it should retry
|
|
> the commit at some later point in time, a side effect of this is that
|
|
> any non persistent messages will be lost. To avoid this use persistent
|
|
> messages when using XA. With acknowledgements this is not an issue
|
|
> since they are flushed to the server before prepare gets called.
|
|
|
|
It is up to the user to catch the exception, and perform any client side
|
|
local rollback code as necessary. There is no need to manually rollback
|
|
the session - it is already rolled back. The user can then just retry
|
|
the transactional operations again on the same session.
|
|
|
|
Apache ActiveMQ ships with a fully functioning example demonstrating how to do
|
|
this, please see ?
|
|
|
|
If failover occurs when a commit call is being executed, the server, as
|
|
previously described, will unblock the call to prevent a hang, since no
|
|
response will come back. In this case it is not easy for the client to
|
|
determine whether the transaction commit was actually processed on the
|
|
live server before failure occurred.
|
|
|
|
> **Note**
|
|
>
|
|
> If XA is being used either via JMS or through the core API then an
|
|
> `XAException.XA_RETRY` is thrown. This is to inform Transaction
|
|
> Managers that a retry should occur at some point. At some later point
|
|
> in time the Transaction Manager will retry the commit. If the original
|
|
> commit has not occurred then it will still exist and be committed, if
|
|
> it does not exist then it is assumed to have been committed although
|
|
> the transaction manager may log a warning.
|
|
|
|
To remedy this, the client can simply enable duplicate detection ([Duplicate Message Detection](duplicate-detection.md)) in
|
|
the transaction, and retry the transaction operations again after the
|
|
call is unblocked. If the transaction had indeed been committed on the
|
|
live server successfully before failover, then when the transaction is
|
|
retried, duplicate detection will ensure that any durable messages
|
|
resent in the transaction will be ignored on the server to prevent them
|
|
getting sent more than once.
|
|
|
|
> **Note**
|
|
>
|
|
> By catching the rollback exceptions and retrying, catching unblocked
|
|
> calls and enabling duplicate detection, once and only once delivery
|
|
> guarantees for messages can be provided in the case of failure,
|
|
> guaranteeing 100% no loss or duplication of messages.
|
|
|
|
#### Handling Failover With Non Transactional Sessions
|
|
|
|
If the session is non transactional, messages or acknowledgements can be
|
|
lost in the event of failover.
|
|
|
|
If you wish to provide *once and only once* delivery guarantees for non
|
|
transacted sessions too, enabled duplicate detection, and catch unblock
|
|
exceptions as described in [Handling Blocking Calls During Failover](ha.md)
|
|
|
|
### Getting Notified of Connection Failure
|
|
|
|
JMS provides a standard mechanism for getting notified asynchronously of
|
|
connection failure: `java.jms.ExceptionListener`. Please consult the JMS
|
|
javadoc or any good JMS tutorial for more information on how to use
|
|
this.
|
|
|
|
The Apache ActiveMQ core API also provides a similar feature in the form of the
|
|
class `org.apache.activemq.core.client.SessionFailureListener`
|
|
|
|
Any ExceptionListener or SessionFailureListener instance will always be
|
|
called by ActiveMQ on event of connection failure, **irrespective** of
|
|
whether the connection was successfully failed over, reconnected or
|
|
reattached, however you can find out if reconnect or reattach has
|
|
happened by either the `failedOver` flag passed in on the
|
|
`connectionFailed` on `SessionfailureListener` or by inspecting the
|
|
error code on the `javax.jms.JMSException` which will be one of the
|
|
following:
|
|
|
|
JMSException error codes
|
|
|
|
<table summary="HA Replication Colocation Policy" border="1">
|
|
<colgroup>
|
|
<col/>
|
|
<col/>
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th>Error code</th>
|
|
<th>Description</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td>FAILOVER</td>
|
|
<td>Failover has occurred and we have successfully reattached or reconnected.</td>
|
|
</tr>
|
|
<tr>
|
|
<td>DISCONNECT</td>
|
|
<td>No failover has occurred and we are disconnected.</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
|
|
### Application-Level Failover
|
|
|
|
In some cases you may not want automatic client failover, and prefer to
|
|
handle any connection failure yourself, and code your own manually
|
|
reconnection logic in your own failure handler. We define this as
|
|
*application-level* failover, since the failover is handled at the user
|
|
application level.
|
|
|
|
To implement application-level failover, if you're using JMS then you
|
|
need to set an `ExceptionListener` class on the JMS connection. The
|
|
`ExceptionListener` will be called by Apache ActiveMQ in the event that
|
|
connection failure is detected. In your `ExceptionListener`, you would
|
|
close your old JMS connections, potentially look up new connection
|
|
factory instances from JNDI and creating new connections.
|
|
|
|
For a working example of application-level failover, please see ?.
|
|
|
|
If you are using the core API, then the procedure is very similar: you
|
|
would set a `FailureListener` on the core `ClientSession` instances.
|