Performance Improvements for catalogs with 10s of thousands of records

Date	2010/03/06
Contact(s)	simonp
Last edited	Timestamp
Status	in progress, complete
Assigned to release	2.5
Resources	Not applicable

Overview

A number of performance issues have been found when GeoNetwork is applied to catalogs containing 10s of thousands of records:

• search speed: search speed appears to degrade surprisingly with the size of the result set, ie. searches with large result sets were taking a long time to return whereas a search on the same catalog that returns a small result set was very fast - this was surprising because in both cases the search only needed to return the first page of results
• indexing speed: importing 10s of thousands of records via batch import, harvesters, massive operations and CSW transactions was taking too long eg. local file system harvest of 20k records was taking 10-12 hours...

Proposal Type

• Type: Core Change
• App: GeoNetwork
• Module: Harvester, Kernel, Data Manager, Metadata Import, Lucene Index, Search results, Massive Operations and CSW Transaction support

Links

• Documents: Preparing a Postgres database to use PostGIS: ​http://www.source3.com/mgiswiki/tiki-index.php?page_ref_id=53, How to Make Lucene Indexing Faster: ​http://wiki.apache.org/lucene-java/ImproveIndexingSpeed

Voting History

• Not voted on yet.

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Motivations

Make GN perform better for catalogs with 10s of thousands of records.

Proposal

Addressing the search speed issue: Investigation found that the lucene component of searches in GeoNetwork is in fact very fast. The delay in returning search results came from processing of all results to gather the most frequently used keywords for the search summary. Since all hits were processed no matter how large the result set, large result sets could cause a big delay in the first page of search results getting back to the user. A simple fix is to limit the number of hits that are processed to build the keyword frequency info for the search summary. This parameter is specified on search services as maxRecordsInKeywordSummary and has been set to 1000, the LuceneSearcher.java code then limits the number of hits it examines to build the keyword frequency info for the search summary. 1000 is an arbitrary number that sites can change according to the number of keywords used in their metadata records and the time delay that is considered acceptable for search results to be returned to the user.

Addressing the indexing/loading issue: Investigation of this problem led to two issues:

• Speeding up Lucene indexing: GeoNetwork was opening and closing the Lucene IndexWriter every time it wrote a document to the search index. This is a very safe way to handle the Lucene Index Writer as only one Index Writer can be open. However the IndexWriter class in Lucene is now much more sophisticated than it was when GeoNetwork was first written. In particular, it is thread safe and can buffer documents in RAM before writing them out to the index file on disk with its own thread. To use the IndexWriter in this way without forcing major changes in the GeoNetwork code, resulted in an IndexWriter facade that allows:
  • code that intends to write a number of documents to the !Lucene Index to keep the IndexWriter open, and thus take advantage of the more sophisticated IndexWriter implementation
  • multiple threads to schedule documents that need to be written to the index without blocking
  • reduce the number of times GeoNetwork attempts to optimize the Lucene Index for search speed: optimizing the search index for maximum search speed is a very costly operation, particularly when the index has 10s of thousands of documents in it. The GeoNetwork approach is to optimize after a certain number of operations or a set period of time has passed (see lazyOptimize method in the new IndexWriter implementation). The advice from the Lucene web page is to optimize when no more documents are to be added to the index for a while. Operations that expect to write many documents to the Index will now only call optimize when they complete. The default number of operations and timeout period has also been increased so that optimizes occur less frequently (this is line with up to date advice from the Lucene IndexWriter javadoc).

• Speeding up spatial indexing using PostGIS: GeoNetwork uses a shapefile to hold spatial extents for searches that contain spatial queries eg. touch, intersect, contains, overlap etc. At present only the CSW service uses the spatial index for these queries, the web search interface uses boxes and ranges in Lucene. The spatial index needs to be maintained when records are added and deleted through import, harvesting, massive delete etc. Unfortunately the shapefile is not efficient for this purpose as the number of records in the catalog goes over 40,000 odd. In particular as the mechanism for deleting extents from the shapefile uses an attribute of the extent and these are not indexable. This means that there is a considerable cost for maintenance operations on the shapefile. To support fast maintenance and search of the spatial index for larger catalogs, it was decided to adopt the PostGIS implementation for the spatial index written for the geocat.ch sandbox by Jessie Eichar and to fall back to using a shapefile when the catalog is not using PostGIS for its database. An option has been added to GAST to allow the user to specify PostGIS as the database (the spatial index table will be built by the create-db-postgis.sql script when the Database->Setup option is used). When GeoTools 2.6.x is adopted, we will very likely be able to also allow the spatial index in Oracle for those who must use that.

The net result of these two fixes is much faster load, harvest, reindex and massive operations in GeoNetwork. For example, in one case doing a file system harvest of 20,000 records was taking 10-12 hours without these modifications. With the modifications described in this proposal, the same harvest now takes approx 30 minutes.

Backwards Compatibility Issues

• Single (or a few records?) transaction in CSW needs to be examined to make sure its not slower

Risks

• Lucene Index corruption if the IndexWriter thread safe implementation has bugs?

Participants

• Doug Nebert, Archie Warnock and team - testing and reporting
• Craig Jones and eMII team - testing and reporting
• Timo Proescholdt - provided some timing analysis for the search problem
• Jose Garcia - provided some timing of Lucene Index Writer speed ups, feedback and discussion
• geocat.ch developers provided changes to spatial index code necessary to support PostGIS