Flexible replication: revert failed over forests

flip-forests.xqy

xquery version "1.0-ml";
 
(: This script looks for forests configured as master on this host that
   are currently acting as a replica.  This implies that a failover has
   occurred and a replica for the forest has taken over as master, and
   also that the master is now back online but is in the role of replica.
  
   Restarting the acting master will cause the configured master to
   resume its role as master.  That will allow the server node that
   owns the master forest to resume handling query load for it.
  
   This module is intended to be installed in the modules database and
   run by a periodic scheduled task about once a day.
:)
 
declare namespace fs = "http://marklogic.com/xdmp/status/forest";
import module namespace admin = "http://marklogic.com/xdmp/admin" at "/MarkLogic/admin.xqy";
 
let $replicas :=
  let $me := xdmp:host()
  for $db in xdmp:databases()
  for $f in xdmp:database-forests($db)
  let $status := xdmp:forest-status($f)
  where $status/fs:state = "sync replicating"
  return
    for $replica in $status/fs:replica-forests/fs:replica-forest
    let $replica-status := xdmp:forest-status($replica)
    where ($replica-status/fs:state = ("open","open replica")) and ($replica-status/fs:host-id = $me)
    return $replica
let $replica-count := fn:count ($replicas) let $msg := fn:concat (
  "Restarting replica forest",
   if ($replica-count gt 1) then "s" else "",
   " to re-target master: xdmp:forest-restart((",
   fn:string-join($replicas/xdmp:forest-name(.), ", "),
   "))")
 
return
if ($replica-count gt 0)
then ( xdmp:log ($msg), xdmp:forest-restart ($replicas) ) else xdmp:log ("No acting-master replica forests found that need restarting", "info")