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> Shared State
> For the most robust failover protection, the primary server and backup server must share the same state. Server state includes three categories of information:

> • | persistent message data (for queues and topics)

> • | client connections of the primary server

> • | metadata about message delivery

> During a failover, the backup server re-reads all shared state information.
> Implementing Shared State
> We recommend that you implement shared state using shared storage devices. The shared state must be accessible to both the primary and backup servers.
> Support Criteria
> Several options are available for implementing shared storage using a combination of hardware and software. EMS requires that your storage solution guarantees all four criteria in Table 71.

> | Always consult your shared storage vendor and your operating system vendor to ascertain that the storage solution you select satisfies all four criteria.

> Table 71 Shared Storage Criteria for Fault Tolerance
> Criterion | Description
> Write Order | The storage solution must write data blocks to shared storage in the same order as they occur in the data buffer. (Solutions that write data blocks in any other order (for example, to enhance disk efficiency) do not satisfy this requirement.)
> Synchronous Write Persistence | Upon return from a synchronous write call, the storage solution guarantees that all the data have been written to durable, persistent storage.
> Distributed File Locking | The EMS servers must be able to request and obtain an exclusive lock on the shared storage. The storage solution must not assign the locks to two servers simultaneously. (See Software Options.) EMS servers use this lock to determine the primary server.
> Unique Write Ownership | The EMS server process that has the file lock must be the only server process that can write to the file. Once the system transfers the lock to another server, pending writes queued by the previous owner must fail.

> Hardware Options
> Consider these examples of commonly-sold hardware options for shared storage:

> • | Dual-Port SCSI device

> • | Storage Area Network (SAN)

> • | Network Attached Storage (NAS)

> SCSI and SAN
> Dual-port SCSI and SAN solutions generally satisfy the Write Order and Synchronous Write Persistence criteria. (The clustering software must satisfy the remaining two criteria.) As always, you must confirm all four requirements with your vendors.
> NAS
> NAS solutions require a CS (rather than a CFS) to satisfy the Distributed File Locking criterion (see below).
> Some NAS solutions satisfy the criteria, and some do not; you must confirm all four requirements with your vendors.
> NAS with NFS
> When NAS hardware uses NFS as its file system, it is particularly difficult to determine whether the solution meets the criteria. Our research indicates the following conclusions:

> • | NFS v2 and NFS v3 definitely do not satisfy the criteria.

> • | NFS v4 with TCP might satisfy the criteria. Consult with the NAS vendor to verify that the NFS server (in the NAS) satisfies the criteria. Consult with the operating system vendor to verify that the NFS client (in the OS on the server host computer) satisfies the criteria. When both vendors certify that their components cooperate to guarantee the criteria, then the shared storage solution supports EMS.

> For more information on how the EMS locks shared store files, see How EMS Manages Access to Shared Store Files.
> Software Options
> Consider these examples of commonly-sold software options:

> • | Cluster Server (CS)

> A cluster server monitors the EMS server processes and their host computers, and ensures that exactly one server process is running at all times. If the primary server fails, the CS restarts it; if it fails to restart the primary, it starts the backup server instead.

> • | Clustered File System (CFS)

> A clustered file system lets the two EMS server processes run simultaneously. It even lets both servers mount the shared file system simultaneously. However, the CFS assigns the lock to only one server process at a time. The CFS also manages operating system caching of file data, so the backup server has an up-to-date view of the file system (instead of a stale cache).
> With dual-port SCSI or SAN hardware, either a CS or a CFS might satisfy the Distributed File Locking criterion. With NAS hardware, only a CS can satisfy this criterion (CFS software generally does not). Of course, you must confirm all four requirements with your vendors.
> Messages Stored in Shared State
> Messages with PERSISTENT delivery mode are stored, and are available in the event of primary server failure. Messages with NON_PERSISTENT delivery mode are not available if the primary server fails.
> For more information about recovery of messages during failover, see Message Redelivery.
> Storage Files
> By default, the tibemsd server creates three file-based stores to store shared state:

> • | $sys.failsafe—This store holds persistent messages using synchronous I/O calls.

> • | $sys.nonfailsafe—This file stores messages using asynchronous I/O calls.

> • | $sys.meta—This store holds state information about durable subscribers, fault-tolerant connections, and other metadata.

> These stores are fully customizable through parameters in the stores configuration file. More information about these files and the default configuration settings are fully described in stores.conf on page 242.
> To prevent two servers from using the same store file, each server restricts access to its store file for the duration of the server process. For more information on how the EMS manages shared store files, see How EMS Manages Access to Shared Store Files.

> | These default files can be changed or modified. See Default Store Files for more information.

> Storage Parameters
> Several configuration parameters apply to EMS storage files (even when fault-tolerant operation is not configured); see Storage File Parameters.

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