OCaml deployment logging policy for caught and uncaught exceptions

Direct answer

Caught exceptions should not be assumed to produce a backtrace in deployment logs. The OCaml runtime prints a backtrace only for exceptions that escape to the top level, and only when the runtime is configured to do so. Exceptions caught by try ... with are handled by application code, so deployment code must explicitly log Printexc.to_string exc and, if a trace is wanted, Printexc.get_backtrace () immediately in the catch block.

Confirmed behavior

• Printexc.record_backtrace true must be called before the exception is raised; otherwise Printexc.get_backtrace () may return an empty trace.
• OCAMLRUNPARAM=b makes the runtime print a backtrace for uncaught exceptions that reach the top level. It does not affect caught exceptions.
• OCaml has no built-in redaction for Printexc backtraces; output can include absolute source paths, line numbers, module and function names, build directories, usernames, and hostnames.

Likely explanation for the policy gap

The boundary is not enforced by the runtime. A caught exception is handled, so the runtime has no reason to print it. Whether every deployment failure is logged with a trace is an application policy decision, not an OCaml default.

Steps to standardize

1. Call Printexc.record_backtrace true at process startup, before deployment work begins.
2. In each catch block, log the exception message and capture the trace immediately with Printexc.get_backtrace (). Do not defer capture, because the value refers to the most recent exception.
3. Run the trace through the application redaction layer before writing to centralized or third-party logs.
4. For uncaught exceptions, set OCAMLRUNPARAM=b in the deployment environment after confirming the target OCaml compiler and runtime version, because default uncaught-exception backtrace behavior differs across releases.

Redaction rules for source paths and package structure

• Strip or normalize absolute file paths and build-machine usernames.
• Replace project-root and package-install prefixes with a fixed token such as /app or [redacted].
• Keep module and function names only if they do not reveal internal package structure you treat as sensitive.
• Apply redaction in the application logging layer; Printexc does not redact automatically.

Missing diagnostic detail

What is the target OCaml compiler and runtime version? This changes whether OCAMLRUNPARAM=b must be added explicitly, since default uncaught-exception backtrace behavior is not identical across OCaml releases.

Verification

1. Compile and run a minimal program that enables Printexc.record_backtrace true, raises and catches an exception, then prints Printexc.get_backtrace () to confirm frames appear.
2. Run the same program with an uncaught exception under OCAMLRUNPARAM=b and without it on the target runtime to confirm whether uncaught backtraces print automatically.
3. Inspect printed backtraces for absolute paths, project-root segments, and package identifiers to define concrete redaction rules.

This is a conservative draft for human review. Confirm the target OCaml runtime version before finalizing the OCAMLRUNPARAM requirement.