Transfer Large Files Asynchronously In Flask

Flask is a synchronous framework, you can try flask+gevent and streaming responses like explained here: http://flask.pocoo.org/docs/0.12/patterns/streaming/.

Anyway, if you want to upload properly very large files, I suggest you to use a different approach. Instead of trying to do asynchronous networking with a synchronous framework try delegating the transfer with Nginx upload_module, like explained here: http://blog.thisisfeifan.com/2013/03/nginx-upload-module-vs-flask.html

Nginx is faster and won't load up the files in memory, a thing that regular frameworks like Flask or Django even in Asynchronous mode will do. Remember to configure flask to receive after upload POST with directive upload_pass. The only caveat is that you'll have to learn how to compile a full fledge Nginx from source, here an example of working Dockerfile:

FROM buildpack-deps:jessie##### NGINX ###### Base StuffRUN apt-get update && apt-get install -y -qq \    libssl-dev# Nginx with upload_module and upload_progress_module# "Stable version".ENV ZLIB_VERSION 1.2.11ENV PCRE_VERSION 8.39ENV NGX_UPLOAD_MODULE_VERSION 2.2ENV NGX_UPLOAD_PROGRESS_VERSION 0.9.1ENV NGX_HEADERS_MORE_VERSION 0.32ENV NGX_SPPEDPAGE_VERSION 1.11.33.4ENV NGINX_VERSION 1.11.8RUN cd /tmp \    && wget http://nginx.org/download/nginx-${NGINX_VERSION}.tar.gz \    && tar xvf nginx-${NGINX_VERSION}.tar.gz \    && wget https://github.com/openresty/headers-more-nginx-module/archive/v${NGX_HEADERS_MORE_VERSION}.tar.gz \    && tar -xzvf v${NGX_HEADERS_MORE_VERSION}.tar.gz \    && wget https://github.com/pagespeed/ngx_pagespeed/archive/latest-stable.tar.gz \    && tar -xzvf latest-stable.tar.gz \    && wget https://dl.google.com/dl/page-speed/psol/${NGX_SPPEDPAGE_VERSION}.tar.gz \    && tar -xzvf ${NGX_SPPEDPAGE_VERSION}.tar.gz \    && mv psol ngx_pagespeed-latest-stable/ \    && git clone -b ${NGX_UPLOAD_MODULE_VERSION} https://github.com/Austinb/nginx-upload-module \    && wget http://zlib.net/zlib-${ZLIB_VERSION}.tar.gz \    && tar xvf zlib-${ZLIB_VERSION}.tar.gz \    && wget ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/pcre-${PCRE_VERSION}.tar.bz2 \    && tar -xjf pcre-${PCRE_VERSION}.tar.bz2 \    && wget https://github.com/masterzen/nginx-upload-progress-module/archive/v${NGX_UPLOAD_PROGRESS_VERSION}.tar.gz \    && tar xvf v${NGX_UPLOAD_PROGRESS_VERSION}.tar.gz \    && cd nginx-${NGINX_VERSION} \    && ./configure \    --with-pcre=../pcre-${PCRE_VERSION}/ \    --with-zlib=../zlib-${ZLIB_VERSION}/ \    --add-module=../nginx-upload-module \    --add-module=../nginx-upload-progress-module-${NGX_UPLOAD_PROGRESS_VERSION} \    --add-module=../ngx_pagespeed-latest-stable \    --add-module=../headers-more-nginx-module-${NGX_HEADERS_MORE_VERSION} \    --with-select_module \    --with-poll_module \    --with-file-aio \    --with-http_ssl_module \    --with-ipv6 \    --with-pcre-jit \    --with-http_gzip_static_module \    --with-http_ssl_module \    --with-http_v2_module \    --with-http_realip_module \    --user=nginx --group=nginx --sbin-path=/usr/sbin/nginx --conf-path=/etc/nginx/nginx.conf --error-log-path=/var/log/nginx/error.log --http-log-path=/var/log/nginx/access.log --pid-path=/var/run/nginx.pid --with-cpu-opt=CPU --    with-ld-opt="-Wl,-E" \    && make \   && make installEXPOSE 80 443CMD ["nginx", "-g", "daemon off;"] 

NOTE: Please in this image is lacking the provision of nginx.conf and default.conf.