You can ensure that failure of a single component does not cause an entire directory service to fail by using redundancy. Redundancy involves providing redundant software components, hardware components, or both. Examples of this strategy include deploying multiple, replicated instances of Directory Server on separate hosts, or using redundant arrays of independent disks (RAID) for storage of Directory Server databases. Redundancy with replicated Directory Servers is the most efficient way to achieve high availability.
You can also use clustering to provide a highly available service. Clustering involves providing pre-packaged high availability hardware and software. An example of this strategy is deploying Sun Cluster hardware and software.
The more common approach to providing a highly available directory service is to use redundant server components and replication. Redundant solutions are usually less expensive and easier to implement than clustering solutions. These solutions are also generally easier to manage. Note that replication, as part of a redundant solution, has numerous functions other than availability. While the main advantage of replication is the ability to split the read load across multiple servers, this advantage causes additional overhead in terms of server management. Replication also offers scalability on read operations and, with proper design, scalability on write operations, within certain limits. For an overview of replication concepts, see Chapter 4, Directory Server Replication, in Sun Java System Directory Server Enterprise Edition 6.1 Reference.
During a failure, a redundant system might provide poorer availability than a clustering solution. Imagine, for example, an environment in which the load is shared between two redundant server components. The failure of one server component might put an excessive load on the other server, making this server respond more slowly to client requests. A slow response might be considered a failure for clients that rely on quick response times. In other words, the availability of the service, even though the service is operational, might not meet the availability requirements of the client.
The main advantage of a clustered solution is automatic recovery from failure, that is, recovery without user intervention. Disadvantages of clustering are complexity and inability to recover from database corruption.
In a clustered environment, the cluster uses the same IP address for Directory Server and Directory Proxy Server, regardless of which cluster node is actually running the service. That is, the IP address is transparent to the client application. In a replicated environment, each machine in the topology has its own IP address. In this case, Directory Proxy Server can be used to provide a single point of access to the directory topology. The replication topology is therefore effectively hidden from client applications. To increase this transparency, Directory Proxy Server can be configured to follow referrals and search references automatically. Directory Proxy Server also provides load balancing and the ability to switch to another machine when one fails.
In terms of the SPOFs that are described at the beginning of this chapter, redundancy and clustering handle failure in the following ways:
Single hardware failure. In a clustered environment, this kind of failure has no impact on the directory service. Only multiple hardware failures impact the service in a cluster.
A single hardware failure is fatal to a machine that is not in a clustered environment. Therefore, even if you have redundant hardware, manual intervention is required to repair the failure.
Directory Server or Directory Proxy Server failure. In a clustered environment, the server is automatically restarted. Software failure must occur multiple times in quick succession to trigger the service group to switch to another node in the cluster. This handling of a software failure is also true in a redundant environment.
Database corruption. A cluster cannot survive this kind of failure. Depending on the architecture, a redundant solution should be able to survive database corruption.
This section provides basic information about hardware redundancy. Many publications provide comprehensive information about using hardware redundancy for high availability. In particular, see “Blueprints for High Availability” published by John Wiley & Sons, Inc.
Hardware SPOFs can be broadly categorized as follows:
Failure of the physical servers on which Directory Server or Directory Proxy Server are running
Load balancer failures
Storage subsystem failures
Power supply failures
Failure at the network level can be mitigated by having redundant network components. When designing your deployment, consider having redundant components for the following:
Network interface card
Gateways and routers
You can mitigate the load balancer as an SPOF by including a redundant load balancer in your architecture.
In the event of database corruption, you must have a database failover strategy to ensure availability. You can mitigate against SPOFs in the storage subsystem by using redundant server controllers. You can also use redundant cabling between controllers and storage subsystems, redundant storage subsystem controllers, or redundant arrays of independent disks.
If you have only one power supply, loss of this supply could make your entire service unavailable. To prevent this situation, consider providing redundant power supplies for hardware, where possible, and diversifying power sources. Additional methods of mitigating SPOFs in the power supply include using surge protectors, multiple power providers, and local battery backups, and generating power locally.
Failure of an entire data center can occur if, for example, a natural disaster strikes a particular geographic region. In this instance, a well-designed multiple data center replication topology can prevent an entire distributed directory service from becoming unavailable. For more information, see Using Replication and Redundancy for High Availability.
Excessive response time
Maximized file descriptors
Maximized file system
Poor storage configuration
Too many indexes
Replication propagation delay
Large wildcard searches
These SPOFs can be mitigated by having redundant instances of Directory Server and Directory Proxy Server. Redundancy at the software level involves the use of replication. Replication ensures that the redundant servers remain synchronized, and that requests can be rerouted with no downtime. For more information, see Using Replication and Redundancy for High Availability.