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1. enable_read_while_writer turns the feature on. It's off by default (as of 4.0.x, we might want to modify this for 5.x)
2. The background fill feature should be allowed to kick in for every possible request. This is necessary, in case the writer ("first client session") goes away, someone needs to take over the session. The original client's request can go away after background_fill_active_timeout seconds, and the object will continue fetching in the background. The object then can start being served to another request after background_fill_completed_threshold % of the object has been fetched from originHence, you should set the background fill timeouts and threshold to zero; this assures they never times out and always is allowed to kick in.
3. The proxy.config.cache.max_doc_size must be unlimited (set to 0), since we potentially don't know the size of the object, we can't allow it to disconnect due to going over this limit.

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