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Central Services<br />

• The Central Services run on one host and form a separate Java <strong>in</strong>stance. They<br />

consist of <strong>the</strong> message service and <strong>the</strong> enqueue service.<br />

• The Central Services provide <strong>the</strong> basis for communication and synchronization<br />

for <strong>the</strong> Java cluster:<br />

– The message service adm<strong>in</strong>isters a list of <strong>the</strong> dispatchers and <strong>the</strong> server<br />

processes of <strong>the</strong> Java cluster. It represents <strong>the</strong> <strong>in</strong>frastructure of data<br />

exchange (for small quantities of data) between <strong>the</strong> nodes <strong>in</strong>volved. In <strong>the</strong><br />

case of load balanc<strong>in</strong>g between a large number of Java <strong>in</strong>stances, it also<br />

provides <strong>the</strong> load balanc<strong>in</strong>g <strong>in</strong>formation for <strong>the</strong> SAP Web Dispatcher.<br />

– The enqueue service adm<strong>in</strong>isters logical locks that are set <strong>in</strong> a server<br />

process by <strong>the</strong> executed application program. It is also used for cluster-wide<br />

synchronization.<br />

• The Central Services are essentially required when a Java cluster is <strong>in</strong>stalled.<br />

They are started on a host with a separate system number and <strong>the</strong> system ID<br />

(SID) of <strong>the</strong> entire system. If <strong>the</strong> Central Services are runn<strong>in</strong>g, o<strong>the</strong>r Java<br />

<strong>in</strong>stances are started with <strong>the</strong> program JControl (see <strong>the</strong> unit Start<strong>in</strong>g and<br />

Stopp<strong>in</strong>g a SAP NetWeaver AS Java).<br />

18 October 2010

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