Magento 2 503 Backend Fetch Failed
In Magento 2, this error is caused because the length of cache tags is more than 8192 characters. You can fix this error by increasing http_resp_hdr_len value in varnish configuration file.
I followed this tutorial https://magentip.com/magento-2-error-503-backend-fetch-failed-with-varnish-cache/ and successfully fixed that error.
Step 1: Depend on your OS, navigate to:
Ubuntu: /etc/default/varnish
CentOS 6.x: /etc/sysconfig/varnish
CentOS 7.x: /etc/varnish/varnish.params
Step 2:
Edit config file and add the following line:
-p http_resp_hdr_len=42000
For example:
DAEMON_OPTS="-a ${VARNISH_LISTEN_ADDRESS}:${VARNISH_LISTEN_PORT} \
-f ${VARNISH_VCL_CONF} \
-T ${VARNISH_ADMIN_LISTEN_ADDRESS}:${VARNISH_ADMIN_LISTEN_PORT} \
-t ${VARNISH_TTL} \
-p thread_pool_min=${VARNISH_MIN_THREADS} \
-p thread_pool_max=${VARNISH_MAX_THREADS} \
-p http_resp_hdr_len=42000 \
Save file and restart your server. The error should be gone.