@NotNull
    private AttributeMap cloneFromStreamMeta() {

        final AttributeMap streamMetaAttributeMap = metaDataHolder.getMetaData();
        LOGGER.debug(() -> LogUtil.message(
                "streamMetaAttributeMap:\n{}\nstreamMetaDataAllowList: {}\nstreamMetaDataDenyList: {}",
                attributeMapToLines(streamMetaAttributeMap, "  "),
                httpHeadersStreamMetaDataAllowList, httpHeadersStreamMetaDataDenyList));

        streamMetaAttributeMap.computeIfAbsent(StandardHeaderArguments.GUID, k -> UUID.randomUUID().toString());
        final AttributeMap clonedAttributeMap = AttributeMapUtil.cloneAllowable(streamMetaAttributeMap);
        final AttributeMap effectiveAttributeMap;

        // Allow trumps deny
        if (NullSafe.isNonBlankString(httpHeadersStreamMetaDataAllowList)) {
            effectiveAttributeMap = new AttributeMap();
            final Set<String> allowSet = keysToSet(httpHeadersStreamMetaDataAllowList);
            clonedAttributeMap.forEach((key, value) -> {
                if (allowSet.contains(key.toLowerCase())) {
                    effectiveAttributeMap.put(key, value);
                }
            });
        } else {
            effectiveAttributeMap = clonedAttributeMap;
            if (NullSafe.isNonBlankString(httpHeadersStreamMetaDataDenyList)) {
                final Set<String> denySet = keysToSet(httpHeadersStreamMetaDataDenyList);
                effectiveAttributeMap.removeAll(denySet);
            }
        }

        LOGGER.debug(() -> LogUtil.message("effectiveAttributeMap:\n{}",
                attributeMapToLines(effectiveAttributeMap, "  ")));
        return effectiveAttributeMap;
    }
Handler class that forwards the request to a URL.
stroom-pipeline/src/main/java/stroom/pipeline/writer/HTTPAppender.java