Can a single service client be used from multiple threads?

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If you are not using a persistent service, it does not matter if you create a new instance per call since non-persistent services always first query the core for the the service server, connect to it and drop the connection after the service call finishes. Even if you don't create a new service client, according to the source code, a new link is established for each call which should be thread safe.

With persistent services, the same socket is used for multiple calls. According to the roscpp source code, it seems like service calls are synchronized, i.e. using the same service server in multiple threads is safe, but all calls will be processed sequentially. For maximal performance, you probably should create something like a service client pool and your server should run multiple spin-threads (at least as many as you have service clients in your client pool).