The primary server and backup server must share the same state. Server state includes three categories of information:
During a failover, the backup server re-reads all shared state information.
We recommend that you implement shared state using shared storage devices. The shared state must be accessible to both the primary and backup servers.
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 45.
![]() |
Always consult your shared storage vendor and your operating system vendor to ascertain that the storage solution you select satisfies all four criteria.
|
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.
|
Consider these examples of commonly-sold hardware options for shared storage:
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 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.
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:
Consider these examples of commonly-sold software options:
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.
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 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.
The tibemsd
server creates three files to store shared state.
For more information about the failsafe
destination property, see failsafe.
For more information about configuration files, see Chapter 7, Using the Configuration Files.33
Several configuration parameters apply to EMS storage files (even when fault-tolerant operation is not configured); see Storage Files.
TIBCO Enterprise Message Service™ User’s Guide Software Release 4.3, February 2006 Copyright © TIBCO Software Inc. All rights reserved www.tibco.com |