A method of determining an operational attribute of a virtual server executed on a first execution platform and providing a service, the method comprising: - performing a measurement indicative of an operational attribute of the server, wherein said operational attribute is indicative of an operational performance of the server with respect to the provision of the service and wherein the measurement is performed by a platform observer system executed on said first execution platform; wherein the platform observer system comprises a first observer module comprised in the server, and a second observer module separate from the server, wherein the method further comprises: - performing said measurement indicative of the operational attribute of the server by the first observer module included in the server; - communicating a result of said measurement by the first observer module to said second observer module executed on said first execution platform, wherein the communicating comprises integrity protecting the communicated result; - communicating the communicated result of said measurement to an external observer system, wherein the communicating comprises protecting secrecy of the communicated result; - verifying, by the external observer system, that the received measurement result is indicative of a measurement performed on said server, and wherein the second observer module performs the following steps: - determining, by performing a measurement, an operational attribute of at least one of the server and the first execution platform; - verifying that the determined operational attribute is consistent with the measurement result received by the second observer module; wherein communicating the communicated result of said measurement to the external observer system is responsive to verification that the determined operational attribute is consistent with the measurement result received by the second observer module. - performing a measurement indicative of an operational attribute of the server, wherein said operational attribute is indicative of an operational performance of the server with respect to the provision of the service and wherein the measurement is performed by a platform observer system executed on said first execution platform; - performing said measurement indicative of the operational attribute of the server by the first observer module included in the server; - communicating a result of said measurement by the first observer module to said second observer module executed on said first execution platform, wherein the communicating comprises integrity protecting the communicated result; - communicating the communicated result of said measurement to an external observer system, wherein the communicating comprises protecting secrecy of the communicated result; - verifying, by the external observer system, that the received measurement result is indicative of a measurement performed on said server, - determining, by performing a measurement, an operational attribute of at least one of the server and the first execution platform; - verifying that the determined operational attribute is consistent with the measurement result received by the second observer module;