Caché System Administration Guide - InterSystems Documentation

Using Caché on UNIX, Linux, and Mac OS X
• Internal Caché system group ID — a special Caché internal effective group ID, which also has
all privileges to all files and executables in the installation. For maximum security, no actual users
should belong to this group (cacheusr is the default).
• Internal Caché user ID — the effective user ID for processes started by the superserver and Job
servers. Again, for maximum security, no actual users should have this user ID (cacheusr is the
default).
6.2 Startup on UNIX
There are three (3) resources a Caché instance uses to control starting, stopping, and creating new
processes:
1. The cache.ids file in the install-dir\mgr directory.
2. The key identifier information for the shared memory allocated for the running instance.
3. The shared memory itself.
This release of Caché uses additional resources to manage these basic resources, as well as to prevent
instances from running concurrently on different nodes, or starting concurrently on a single node.
6.2.1 Shared Memory Object
The shared memory identifiers are kept in a memory-mapped shared object (an operating system
internal data structure that may or may not be backed by a file in the file system). This object is not
generally visible to users of the system (HP/UX is the one exception). It is referred to as the ids shared
memory object. The purpose of this object is to hide the resource that contains the shared memory
identifiers so that it is not as susceptible to accidental corruption or deletion.
6.2.2 Daemon Resource Locks
In addition, Caché uses advisory file locking to prevent multiple startups of the same instance on different
machines. With advisory file locking, a single lock file (in this case, the file clock in the
install-dir\bin directory) may be used to exclusively lock multiple resources. The Control Process,
Write daemon, and Journal daemon each lock a separate section of the lock file. If this section of the
clock file is already locked, startup terminates. The locks held by the different daemons are called
Daemon Resource Locks.
A file lock is held by a process until the process terminates. Thus if any lock is held, it indicates that
some daemon process on some node is running. It does not indicate, however, whether or not the
instance is healthy and running normally.
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