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<h1 STYLE="margin-top: 0.17in; margin-bottom: 0.08in; page-break-after: avoid">
<font face="helvetica, sans-serif">Qmaster<a NAME="sdfootnote1anc" CLASS="sdfootnoteanc" HREF="#sdfootnote1sym"></a></font><sup><a href="#sdfootnote1sym" CLASS="sdfootnoteanc" NAME="sdfootnote1anc">1</a></sup></h1>
<p><br>Qmaster is the central application within a Grid Engine installation.
A running instance of this application controls the overall behavior of
a whole Grid Engine cluster.
<p>This process holds the most current data about states of jobs, queues
and other necessary objects. Data persistence is secured by the qmaster
daemon. Objects are spooled to a filesystem before the user gets a confirmation
about requested changes. Every daemon and client application which is interested
in information about a specific object has to ask the qmaster daemon. Communication
with qmaster is possible via the Grid Engine Database Interface (<a href="../../libs/gdi/gdi.html">GDI</a>
)
<p>Qmaster interoperates with the scheduler daemon <a href="../schedd/schedd.html">schedd</a><font color="#000000">
for the task of assigning suitable resources (hosts, queues) to jobs waiting
for execution.</font>
<br>Schedd gets all its information from the qmaster via an event driven
update protocol utilizing the GDI. Once schedd has determined an assignment
of jobs to queues, qmaster will get notified via a list of so called "orders",
which is again a GDI-based communication.
<p>Such a job execution order will result in the action, that qmaster sends
the corresponding job to the execution daemon <a href="../execd/execd.html">execd
</a>of the selected host. This execd will start with the execution of the
job. The current status of a job or a host again will be reported to qmaster
by execd using the GDI.
<h2>
Interoperating with Clients - Implementing the Grid Engine Database Interface</h2>
GDI stands for Grid Engine Database Interface. All functions which are
needed for a client application or a daemon to use this interface are packaged
together in the library libgdi.a. Documentation for this library can be
found <a href="../../libs/gdi/gdi.html">here.</a><i><font color="#CC0000"></font></i><i><font color="#CC0000"></font></i>
<p>One of the core tasks of qmaster is to implement the framework which
handles incoming GDI requests. The most important part of this framework
is the array gdi_object[]. Its elements contain the same constants used
to identify GDI requests, and function pointers describing which functions
serve a particular request.
<p>When a client request arrives at qmaster it will pass through several
steps. Information from the gdi_object[] array will be used as necessary.
The following explains the steps being processed in the example of a modify
request of a Checkpointing object. Please find all function definitions
and constants mentioned below in source/libs/gdi/sge_gdi.h and source/daemons/qmaster/sge_c_gdi.c.
<ol>sge_c_gdi() will be called with a new request structure. The function
recognizes that it got a SGE_GDI_MOD-request and calls sge_c_gdi_mod().
<p>sge_c_gdi_mod() identifies the type of the object which should be modified
(SGE_CKPT_LIST). It calls sge_gdi_add_mod_generic().
<p STYLE="margin-bottom: 0in">sge_gdi_add_mod_generic() looks up the following
three function pointer entries in gdi_object[] and executes them in the
right order:
<br>
<ol>A function which is capable of changing a copy of an object according
to the information received from the client,
<p>a function which will spool the changed object to a corresponding file,
and
<p>a function which will be called after the object was successfully modified.</ol>
<p><br>During the previous steps, lists will be filled with answer messages
sent to the client before the request is completely finished.</ol>
<h2>
Interoperating with the Scheduler - Events and Orders</h2>
The communication and interoperation with schedd is another core task of
qmaster. The data exchange between the both is performed via an event and
order protocol being based upon the GDI. An introduction concerning this
protocol can be found <a href="../schedd/schedd.html">here</a>.<i><font color="#CC0000"></font></i>
<h2>
Interoperating with the Execution Daemon - Job and Load Protocols</h2>
Jobs get dispatched to execd by qmaster and execd reports job, host status
and load information back to qmaster. A protocol between qmaster and execd
handles these and similar tasks. More information about it can be found
<a href="../execd/execd.html">here</a>.
<h2>
Process Flow</h2>
When Qmaster starts up successfully following actions are taken:
<blockquote>
<div STYLE="margin-left: 0.79in">Start a Commd process when none is running.</div>
<div STYLE="margin-left: 0.79in">Connect to Commd.</div>
<div STYLE="margin-left: 0.79in">Prevent that other Qmaster processes can
start up successfully.</div>
<div STYLE="margin-left: 0.79in">Read Qmasters configuration</div>
<div STYLE="margin-left: 0.79in">Read all spooled GDI objects from the
filesystem</div>
</blockquote>
After startup, Qmaster enters its main loop where it
<blockquote>
<div STYLE="margin-left: 0.79in">accepts GDI requests</div>
<div STYLE="margin-left: 0.79in">gets reports from Execds</div>
<div STYLE="margin-left: 0.79in">answers acknowledge requests</div>
</blockquote>
You can find the implementation of the main loop in the main() function
in source/daemons/qmaster/qmaster.c
<br>
<br>
<div ID="sdfootnote1">
<center><a NAME="sdfootnote1sym" CLASS="sdfootnotesym" HREF="#sdfootnote1anc"></a><a href="#sdfootnote1anc" CLASS="sdfootnotesym" NAME="sdfootnote1sym">1</a>Copyright
2001 Sun Microsystems, Inc. All rights reserved.</center>
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