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Task Server Scripts for Forest Rebalancing

NOTE: MarkLogic 7 includes a built-in rebalancer, which makes this code largely obsolete. The project will stick around as sample code, but I strongly recommend upgrading to MarkLogic 7 or later.

Often a MarkLogic Server database will have several forests. If the forests are present from the time when the database was created, they will each have approximately the same number of documents. But if the administrator adds more forests later on, the newer forests will tend to have fewer documents. In cases like this, we can rebalance the forests.

Or perhaps you want to replace the existing forests entirely, without losing any documents? That would make sense if the replacements are on faster or larger storage, for example. Set the old forests to updates-allowed=delete-only and this tool can evacuate those forests. Once they are empty, detach them from the database and then delete them.

Before using this tool, consider that rebalancing is generally slower than clearing the database and reloading. That is because many documents must be updated, and updates are more expensive than inserts. So rebalancing the forests may not be the best way to solve the problem. If you have the luxury of clearing the database and reloading everything, do it.

Due to limitations in MarkLogic, this tool does not rebalance directory fragments or standalone properties. If you see extra fragments left behind after balancing away from delete-only forests, this may be the explanation. If you need to move fragments of this kind, you may be better off reloading.

Finally, note that following this procedure may result in forests containing many deleted fragments. To recover disk space, you may wish to force some forests to merge. This code does not monitor free disk space: that is your job.


This project contains several XQuery modules, intended for use with the built-in Task Server. There are four entry point modules:

  • scheduled-rebalancer.xqy: entry point for use as a scheduled task.
  • forests.xqy: entry point for xdmp:invoke, HTTP request, or scheduled task.
  • disable.xqy: spawn this task to keep rebalancer tasks from running.
  • enable.xqy: spawn this task to re-enable rebalancer tasks.

Those four modules work with two task modules:

  • forest-uris.xqy: spawned by forests.xqy to rebalance one forest.
  • rebalance.xqy: spawned by forest-uris.xqy to move one document.

The database that you wish to rebalance must have the URI lexicon enabled. If that lexicon is not enabled, you must enable it and reindex. In that case you may be better off reloading all your content (see above).

This code uses xdmp:host-forests, which requires MarkLogic 5.0 or later. If you are interested in support for 4.2 or earlier releases, feel free to open an issue (or create a patch and a pull request).


If you want to remove all documents from one or more forests, set updates-allowed for those forests to delete-only. Make sure you have at least some forests with updates-allowed=all, so the documents have somewhere to land.

To start the rebalancer manually, use this XQuery expression:

  (xs:QName('LIMIT'), 0,
   xs:QName('MODULE'), 'forest-uris.xqy',
   xs:QName('RESPAWN'), true()),
  <options xmlns="xdmp:eval">
    <database>{ xdmp:database('DATABASE-NAME') }</database>

This assumes the task-rebalancer code is on the local fileystem. If you have copied it into a modules database, replace the modules option accordingly.

Be careful not to invoke forests.xqy multiple times, especially with RESPAWN set. Doing so should not damage your system, but may waste resources and cause extra work.

To run the rebalancer as a scheduled task, use scheduled-rebalancer.xqy.

  • Take care to set the scheduled task database correctly.
  • Set the module location, and module root so that the scheduled task runs in the task-rebalancer directory.
  • Do not run this task at short intervals, since the forest-uris tasks can conflict with one another. The period should be at least one hour.

Note that forest-uris.xqy and rebalance.xqy will run on the Task Server. The forests.xqy task will only spawn forest-uris.xqy for forests local to the host. This is done so that you can invoke forests.xqy on each host in your cluster. This improves concurrency in clustered environments.

The LIMIT option caps the number of URIs that will be checked for rebalancing. This should usually be 0, except when debugging or when you wish to throttle the rebalancer. The Task Server queue size limit will have a similar effect, since large numbers of URIs will quickly fill up the Task Server queue.

The RESPAWN option controls whether or not the forest-uris tasks will respawn automatically after filling up the Task Server queue. Set this true() when running the rebalancer manually. The scheduled-rebalancer.xqy task sets this option to false(), to avoid conflicting forest-uris tasks.

By default, the Task Server thread pool size is 4. If your host has more than 4 CPU cores, you may wish to increase the thread pool size to match. You must have more than one thread per local forest, or the task-rebalancer will refuse to run. So for 8 forests you must configure at least 9 threads. This requirement helps avoid deadlocks.

Beyond that minimum, you can use the Task Server thread pool size to throttle the impact of the rebalance tasks on other users of the system. When using the scheduled task, you can also use the task interval and the LIMIT option as throttles.


Because most of the work happens on the Task Server, most error messages will only appear in the ErrorLog.txt file. You may wish to increase the log-level to Debug to aid any troubleshooting.

The error code XDMP-URILXCNNOTFOUND means that the selected database does not have a URI lexicon enabled. If you recently enabled the uri lexicon, make sure the database has been allowed to reindex.

Note that this tool does not attempt to balance on-disk size. Instead, it uses xdmp:document-assign to ensure that each document is in the correct forest for its document URI.

Detaching and reattaching forests may cause the order of the forests from xdmp:document-forests to change. If this happens, xdmp:document-assign may want to reassign documents to different forests. For predicatable results, try to ensure that the forests as listed in the admin UI Database > Forests are in some determinate order: for example, keep the forests alphabetized.

If this tool runs out of control, or if multiple copies run at the same time, use the following expression to halt it:

  <options xmlns="xdmp:eval">
    <database>{ xdmp:database() }</database>

Watch the Task Server status page. Once all the tasks have finished, use this expression to renable the rebalancer.

  <options xmlns="xdmp:eval">
    <database>{ xdmp:database() }</database>

You can also try to stop it by moving the task-rebalancer directory aside. Either of these techniques may take some time: if you are in a hurry, it may be faster to restart MarkLogic on the affected host.

As mentioned above, these code samples assume that the task-rebalancer code is on the local fileystem. If you have copied it into a modules database, replace the modules option accordingly.


Copyright (c) 2011-2013 Michael Blakeley. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

The use of the Apache License does not indicate that this project is affiliated with the Apache Software Foundation.