HA-OSCAR: the Birth of Highly Available OSCAR
We already described the HA-OSCAR hardware architecture. A key enhancement is the addition of dual head nodes that provide a backup head node for a failover in case of a primary head node outage. However, a hardware redundancy solution alone is not sufficient to archive HA unless detection and recovery mechanisms are incorporated.
Few existing solutions provide outage detection and failover. We have evaluated and selected a failover LVS. The solution includes LVS, Linux Director Dæmon (ldirectord), Heartbeat and Coda. Linux Virtual Server is a software tool that directs network connections to multiple servers that share a workload. ldirectord is a standalone dæmon to monitor services. Heartbeat provides a primary node outage detection and failover mechanism through serial line and UDP connectivity. Coda is a fault-tolerant distributed filesystem. This solution not only provides HA capability, but load balancing as well. However, additional LVS services must be enhanced in HA-OSCAR, including SIP, PBS and Web services. An external Heartbeat (eHB) mechanism to a fault management system also has been added. eHB is a precaution in case of a total outage (for example, double head node failures) from which the fault management detects, raises an alarm and sends a page to a system administrator.
OSCAR currently is installed on clusters deployed mostly on private networks, where security is not a major concern. That is because these clusters are not connected to any networks outside the lab boundaries. However, when HA-OSCAR is deployed on clusters connected to the Internet, security is vital. Security is a major concern for both OSCAR and HA-OSCAR not only because a hacker might access the cluster and the data sitting on it, but also because a malicious hacker also might disrupt the normal workings of the system and its availability.
Many security solutions exist, ranging from external solutions (firewalls) to internal solutions (integrity-checking software). Unfortunately, all of them are based on a single node approach and lack a homogeneous view of the cluster. Most of the time, administrators end up installing, patching, integrating and managing several security solutions. The increased management difficulty soon leads to decreased security, as interoperability issues increase with updates of the heterogeneous pieces.
Consequently, the Distributed Security Infrastructure (DSI) was initiated as an open-source project to provide an adequate security solution for carrier-grade clustered servers. DSI is a security framework that provides applications running on clustered systems with distributed mechanisms for access control, authentication, confidentiality and integrity of communications. It also provides auditing services with process-level granularity.
Therefore, HA-OSCAR can be more successful with telecom and other mission-critical sectors if it supports advanced security features. For this reason, HA-OSCAR adopted DSI from Ericsson.
HA-OSCAR supports a mechanism that allows users to add and remove nodes from the cluster dynamically, in a transparent fashion, without affecting either the end-user experience or the running applications. Two open-source projects provide similar functionalities: Eddie, an Ericsson open-source initiative, and LVS. We currently are investigating the best mechanism and will implement it in HA-OSCAR. Our goal is to ensure that adding nodes to accommodate higher traffic or removing nodes for service purposes is a seamless operation and does not affect service availability.
The subject of which kernel to adopt came in addition to the decision about whether to patch the HA-OSCAR kernel ourselves or try to have our patches accepted by the mainstream kernel tree. We decided to use the latest stable 2.4 kernel and submit the patches we create to the kernel mailing list. We are trying to provide a simplified kernel building tool for these HA-OSCAR users. Users can recompile based on their local configurations.
A network/distributed filesystem is an essential component for building clusters. A number of open-source projects aim to provide network filesystems for Linux clusters. Based on our previous research and lab testing, we ascertained that a different networked filesystem may be required, depending on the type of applications being run on the cluster. For instance, using the parallel virtual file system (PVFS) offers the advantage of high I/O performance for large files on a streaming video and audio server. On the other hand, sharing configuration files among cluster nodes can be achieved using the NFS without the need for high I/O. If it is desirable to maintain high availability and support storage area networks (SANs), OpenGFS, with its journaling capability, can handle such a task. Therefore, HA-OSCAR is working to support all the possible network filesystems that can be used in target environments.
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