Tracing.markdown

layout	title
page	Tracing modules

#Tracing modules

In addition to the standard Protocol_{aux|clnt|srv} modules, orpc generates Protocol_trace with the following signature (see [ModulesInterfaces] (ModulesInterfaces.html) for the input file):

{% highlight ocaml %} module type Pp = sig val pp_foo : Format.formatter -> Protocol.foo -> unit;; val pp_baz'call : Format.formatter -> Protocol.foo -> unit;; val pp_baz'reply : Format.formatter -> int -> unit;; val pp_exn : Format.formatter -> exn -> unit;; val pp_exn'reply : Format.formatter -> exn -> unit;; end;; module Pp_pp (P : Pp) : Pp;; module Pp : Pp;; module Sync_pp (P : Pp) (T : Orpc.Trace) (A : Protocol.Sync) : Protocol. Sync;; module Sync (T : Orpc.Trace) (A : Protocol.Sync) : Protocol.Sync;; module Async_pp (P : Pp) (T : Orpc.Trace) (A : Protocol.Async) : Protocol. Async;; module Async (T : Orpc.Trace) (A : Protocol.Async) : Protocol.Async;; module Lwt_pp (P : Pp) (T : Orpc.Trace) (A : Protocol.Lwt) : Protocol.Lwt;; module Lwt (T : Orpc.Trace) (A : Protocol.Lwt) : Protocol.Lwt;; {% endhighlight %}

The pp_* functions print data types defined in the input to a Format.formatter, and the functors wrap a client or server to trace calls across the interface.

The idea with the Pp signature is as follows:

• module type Pp is a signature for pretty-printing types defined in the input and calls across the interface.
• module Pp is an implementation that defines default pretty-printing behavior.
• Pp_pp lets you override the default behavior. E.g., module Pp is defined as module rec Pp : Pp = struct include Pp_pp(Pp) end; for custom behavior, start with this then add overrides after the include.
• Sync et al. use the default pretty-printing behavior; Sync_pp et al. let you override it.