I've also enabled the "set-priority-class" option in the backend:
```
backend solr from base
    acl is_update path_end /update/
    http-check send meth GET uri /solr/area/admin/ping
    http-check expect string \"status\":\"OK\"
    http-request set-priority-class int(1) if is_update
    http-request set-priority-class int(2) if !is_update
    mode http
    option httpchk
    server solr-http 10.20.20.11:8983 check inter 500ms
```