private void addRequestProperty(final HttpUriRequestBase request,
                                    final String key,
                                    final String value) {
        request.addHeader(key, value);

        // It's not possible to inspect the connection to see what req props have been set
        // as that implicitly opens the connection, so store them in our own map for logging later
        if (LOGGER.isDebugEnabled()) {
            if (key != null) {
                requestProperties.computeIfAbsent(key, k -> new HashSet<>())
                        .add(value);
            } else {
                LOGGER.debug("Null key with value '{}'", value);
            }
        }
    }
Handler class that forwards the request to a URL.
stroom-pipeline/src/main/java/stroom/pipeline/writer/HTTPAppender.java