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The preferred and supported method of web services in openmrs is through the Webservices.rest Module. See that page for how to install and configure the module. You can also read about the REST Web Services API, and the provided REST Web Service Resources in OpenMRS 1.8.
(There is also an older Unsupported REST Module that provides simple access to patients, obs, and users. However, this will no longer be supported in favor of the previously mentioned module)
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