How to log request and response bodies in Spring WebFlux

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This is more or less similar to the situation in Spring MVC.

In Spring MVC, you can use a AbstractRequestLoggingFilter filter and ContentCachingRequestWrapper and/or ContentCachingResponseWrapper. Many tradeoffs here:

  • if you’d like to access servlet request attributes, you need to actually read and parse the request body
  • logging the request body means buffering the request body, which can use a significant amount of memory
  • if you’d like to access the response body, you need to wrap the response and buffer the response body as it’s being written, for later retrieval

ContentCaching*Wrapper classes don’t exist in WebFlux but you could create similar ones. But keep in mind other points here:

  • buffering data in memory somehow goes against the reactive stack, since we’re trying there to be very efficient with the available resources
  • you should not tamper with the actual flow of data and flush more/less often than expected, otherwise you’d risk breaking streaming uses cases
  • at that level, you only have access to DataBuffer instances, which are (roughly) memory-efficient byte arrays. Those belong to buffer pools and are recycled for other exchanges. If those aren’t properly retained/released, memory leaks are created (and buffering data for later consumption certainly fits that scenario)
  • again at that level, it’s only bytes and you don’t have access to any codec to parse the HTTP body. I’d forget about buffering the content if it’s not human-readable in the first place

Other answers to your question:

  • yes, the WebFilter is probably the best approach
  • no, you shouldn’t subscribe to the request body otherwise you’d consume data that the handler won’t be able to read; you can flatMap on the request and buffer data in doOn operators
  • wrapping the response should give you access to the response body as it’s being written; don’t forget about memory leaks, though

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