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Module: clusters ./src/peak/running/

Utilities for dealing with clusters of loosely-coupled systems.

Many enterprise applications will be run, rather than on a single computer, in a loosely-coupled cluster of machines. A set of several web-servers, for example. We have found the clusterit tools to be very useful in such situations. Clusterit, inspired by the tools provided in IBM's PSSP, is simple to set up and use, if somewhat quirky in implementation.

PEAK supports the cluster definition file used by Clusterit-2.0, providing the information contained therein via the properties mechanism. Note that you don't have to have the Clusterit tools installed to make use of this, or for it to be useful. Nor will anything break if you make use of these properties in your application and they haven't been set up. In the abscence of a cluster definition file, PEAK will behave as if the local hostname was listed in the cluster file, without being part of a group. In other words, in the abscence of other information, the cluster consists of just the local machine.

Then environment variable CLUSTER (or, in PEAK's case, the property peak.running.cluster._filename, if it exists, else __main__.CLUSTER, which falls back to environ.CLUSTER) specifies a file defining the cluster. A cluster may be subdivided into groups. In PEAK it is allowable for groups to have overlapping membership. Let's look at a fairly complicated example:

PEAK treats this file as if it had seven groups. The existance and memberships of the first four groups, odd, even, prime, and qux, should be clear. A "lump" is a group defined in terms of other groups or lumps. In this case the group weird contains the union of the groups qux and prime. Note that is a member of both qux and prime. The group weird will contain it only once, however -- duplicates are removed automatically. Another group, __orphans__, contains any hosts that were listed at the beginning of the file before any GROUP: lines. Lastly the group __all__ contains all hosts listed in the file.

Each group is exported as a property peak.running.cluster.groups.<groupname> (for example, with its value a tuple of strings of the hostnames of the members.

The property peak.running.cluster.groups will contain a tuple of strings naming each group except the __orphans__ and __all__ groups (that is, it names all the explicitly-created groups).

The property peak.running.cluster.hosts.<hostname> will be a tuple of strings of the names of all groups the host belongs to (except the __all__ group). For example, in the example above, ''is ("odd", "prime", "qux", "wierd").

The property peak.running.cluster.hosts is a shortcut for peak.running.cluster.groups.__all__, listing all hosts in the cluster.

Finally, the property peak.running.cluster.hostname is a string with the local machine's network hostname (per socket.gethostname()), and peak.running.cluster.shortname is the the same, truncated after the first ".", if any.

Imported modules   
import os
from peak.api import *
from peak.util.Graph import Graph
from sets import Set
loadCluster (

parseCluster ( prefix,  file )

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