The paper describes a monitoring system specially designed for cloud infrastructures. We identify the features that are relevant for the task: scalability, that allows utilization in systems of thousands of nodes, flexibility, to be customized for a large number of applications, openness, to allow the coexistence of user and administration monitoring. We take as a starting point the Nagios monitoring system, that has been successfully used for Grid monitoring and is still used for clouds. We

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... lyze its shortcomings when applied to cloud monitoring, and propose a new monitoring system, that we call Rocmon, that sums up Nagios experience with a cloud perspective. Like Nagios, Rocmon is plugin-oriented to be flexible. To be fully inter-operable and long-living, it uses standard tools: the OGF OCCI for the configuration interface, the REST paradigm to take advantage of Web tools, and HTML5 WebSockets for data transfers. The design is checked with an open source Ruby implementation featuring the most relevant aspects.