Chapter 18. Configuring a multi-site, fault-tolerant messaging system

Red Hat Ceph Storage is a clustered object storage system. Red Hat Ceph Storage uses data sharding of objects and policy-based replication to guarantee data integrity and system availability.

Red Hat Ceph Storage uses an algorithm called CRUSH (Controlled Replication Under Scalable Hashing) to determine how to store and retrieve data by automatically computing data storage locations. You configure Ceph items called CRUSH maps, which detail cluster topography and specify how data is replicated across storage clusters.

CRUSH maps contain lists of Object Storage Devices (OSDs), a list of ‘buckets’ for aggregating the devices into a failure domain hierarchy, and rules that tell CRUSH how it should replicate data in a Ceph cluster’s pools.

By reflecting the underlying physical organization of the installation, CRUSH maps can model — and thereby address — potential sources of correlated device failures, such as physical proximity, shared power sources, and shared networks. By encoding this information into the cluster map, CRUSH can separate object replicas across different failure domains (for example, data centers) while still maintaining a pseudo-random distribution of data across the storage cluster. This helps to prevent data loss and enables the cluster to operate in a degraded state.

Red Hat Ceph Storage clusters require a number of nodes (physical or virtual) to operate. Clusters must include the following types of nodes:

AMQ Broker multi-site, fault-tolerant architectures use Red Hat Ceph Storage 3. By replicating data across data centers, a Red Hat Ceph Storage cluster effectively creates a shared store available to brokers in separate data centers. You configure your brokers to use the shared store high-availability (HA) policy and store messaging data in the Red Hat Ceph Storage cluster.

Red Hat Ceph Storage clusters intended for production use should have a minimum of:

The procedures linked-to from this section show you how to install a Red Hat Ceph Storage 3 cluster that includes MON, MGR, OSD, and MDS nodes.

This example procedure shows how to configure your Red Hat Ceph storage cluster for fault tolerance. You create CRUSH buckets to aggregate your Object Storage Device (OSD) nodes into data centers that reflect your real-life, physical installation. In addition, you create a rule that tells CRUSH how to replicate data in your storage pools. These steps update the default CRUSH map that was created by your Ceph installation.

The following figure shows the Red Hat Ceph Storage cluster created by the preceding example procedure. The storage cluster has OSDs organized into CRUSH buckets corresponding to data centers.

The following figure shows a possible layout of the first data center, including your broker servers. Specifically, the data center hosts:

The following figure shows a complete example topology. To ensure fault tolerance in your storage cluster, the MON, MGR, and MDS nodes are distributed across three separate data centers.

The figure below illustrates replication of data between the two data centers that have broker servers.

Before you can configure brokers in your messaging system to store messaging data in your Red Hat Ceph Storage cluster, you first need to mount a Ceph File System (CephFS).

The procedure linked-to from this section shows you how to mount the CephFS on your broker servers.

To configure your brokers as part of a multi-site, fault-tolerant messaging system, you need to:

18.5.1. Adding backup brokers

Within each of your data centers, you need to add idle backup brokers that can take over from live master-slave broker groups that shut down in the event of a data center outage. You should replicate the configuration of live master brokers in your idle backup brokers. You also need to configure your backup brokers to accept client connections in the same way as your existing brokers.

In a later procedure, you see how to configure an idle backup broker to join an existing master-slave broker group. You must locate the idle backup broker in a separate data center to that of the live master-slave broker group. It is also recommended that you manually start the idle backup broker only in the event of a data center failure.

The following figure shows an example topology.

Additional resources

• To learn how to create additional broker instances, see Creating a standalone broker.
• For information about configuring broker network connections, see Network Connections: Acceptors and Connectors.

An internal client application is one that is running on a machine located in the same data center as the broker server. The following figure shows this topology.

An external client application is one running on a machine located outside the broker data center. The following figure shows this topology.

The following sub-sections describe show examples of configuring your internal and external client applications to connect to a backup broker in another data center in the event of a data center outage.

<ConnectionFactory connectionFactory = new ActiveMQConnectionFactory(“(tcp://host1:port,tcp://host2:port)?ha=true&retryInterval=100&retryIntervalMultiplier=1.0&reconnectAttempts=-1”);

<ConnectionFactory connectionFactory = new ActiveMQConnectionFactory(“(tcp://host3:port)?ha=true&retryInterval=100&retryIntervalMultiplier=1.0&reconnectAttempts=-1”);

When you have configured your Red Hat Ceph Storage cluster for fault tolerance, the cluster continues to run in a degraded state without losing data, even when one of your data centers fails.

This procedure shows how to verify the status of your cluster while it runs in a degraded state.

When you run any of the preceding commands, you see output indicating that the storage cluster is still running, but in a degraded state. Specifically, you should see a warning that resembles the following:

health: HEALTH_WARN
        2 osds down
        Degraded data redundancy: 42/84 objects degraded (50.0%), 16 pgs unclean, 16 pgs degraded

The following procedure shows you how to keep brokers and associated messaging data available to clients during a data center outage. Specifically, when a data center fails, you need to:

When a previously failed data center is back online, follow these steps to restore the original state of your messaging system:

The following sub-sections show to perform these steps.