1. Experiences with Data Indexing services supported by the NorduGrid ARC middleware
Oxana Smirnova, Jakob Nielsen (Lund University/CERN)
for the NorduGrid collaboration
CHEP 2004, Interlaken,

2. NorduGrid and ARC
NorduGrid is a research collaboration established by universities in Denmark, Estonia, Finland, Norway and Sweden
Focuses on providing production-quality Grid middleware for academic researchers
ARC (Advanced Resource Connector) is the Grid middleware developed by the NorduGrid
Based on Globus libraries and API
Original architectural solutions, services and implementations
Supports one of the largest Grid production systems
10 countries, 40+ sites, ~4000 CPUs, ~30 TB storage

3. ARC features
ARC is based on Globus Toolkit with core services replaced
Currently uses Globus Toolkit 2
Alternative/extended Grid services:
Grid Manager that
Checks user credentials and authorization
Handles jobs locally on clusters (interfaces to LRMS)
Does stage-in and stage-out of files
Lightweight User Interface with built-in resource broker
Information System based on MDS with a NorduGrid schema
xRSL job description language (extended Globus RSL)
Grid Monitor
Simple, stable and non-invasive

4. ARC Indexing Services
NorduGrid has standard Globus indexing services deployed:
Globus Replica Catalog (RC)
Deployed and used since spring 2002
Used as the primary indexing service in ATLAS Data Challenge 1
Globus Replica Location Service (RLS)
Deployed and used since spring 2004
Used in ATLAS Data Challenge 2
Both are internally supported by the ARC middleware
But both has been unsatisfactory in many ways

5. ARC support for RC and RLS
If a user refers to a logical filename formed as an RC (rc://…) or RLS (rls://… ) pseudo-URL, the ARC UI will
Query the indexing service for the size and location of the corresponding physical file
Use this information in the resource brokering
Furthermore the ARC Grid Manager on the chosen cluster will
Contact the corresponding indexing service
Obtain the physical filename (URL)
Download the file to the cluster
If the file exists in the local cluster cache, a symbolic link will be created instead
At the end of a job, the ARC Grid Manager can automatically
Upload the physical file to a Storage Element
Obtains Storage Elements list by querying the RC/RLS
Register the file in the given indexing service with the logical filename specified

6. Command Line Interface Support
ARC provides a set of command-line file management tools:
ngls lists physical files and their attributes; uses either physical file names (PFN) or logical file names (LFN)
ngcopy and ngrequest copy and replicate files registered in indexing services; uses either PFN or LFN
Multithreaded and 3rd party transfer supported
ngremove deletes physical files and logical records

7. Globus Replica Catalog
Stores mappings between LFN and PFN
Supports file-collections and file-attributes (i.e. filesize, checksum)
By default runs with an LDAP-backend
Problems:
Random crashes
Automatic restart script necessary
LFN has to be identical with PFN
Files could only belong to one collection at a time
In default installation only clear-text authentication
Was patched by NorduGrid to allow GSI-authentication instead
Clumsy command-line tool
Not supported anymore by Globus
Snapshot of the Replica Catalog Browser from ATLAS Data Challenge 1.

8. Globus Replica Location Service
Stores mappings between LFNs and PFNs
Supports arbitrary file attributes and searching on them
Runs with a SQL server backend
Supported are MySQL, PostGreSQL and Oracle
Has GSI authentication
Reasonably logical and useful API
Problems:
Until recently there were random freeze-outs
Automatic restart script necessary
Now patched – stable for last 3+ weeks
Does not internally support collections
Clumsy command-line tool
No fine-grained access control
Either no write-access or full write-access
Users can overwrite/delete each other's records

9. Indexing Services
Requirements for an ideal indexing service supporting multiple users and sub-projects:
GSI Authentication
Mappings between logical and physical file names
Support for collections
Files should be allowed to be listed in several collections
Empty and recursive collections must be supported
Support for arbitrary file attributes
Fine-grained access control
Possibility to give read-write access to collections, different for different users
Distinguish between write-access (e.g. Production managers) and read-access (e.g. regular users) to collections
The access restrictions to the physical files should be reflected in the indexing service

10. Summary
NorduGrid has deployed the standard Globus Indexing Services: RC and RLS
Stress-tested during the ATLAS Data Challenges
Support for both RC and RLS is implemented in the ARC Grid Manager, User Interface and the command line tools
Neither are satisfactory solutions:
Unstable (until recently at least)
No decent support for collections
Clumsy command-line tools
No fine-grained access control
Access restrictions of the corresponding physical files are not stored/propagated
An indexing service meeting the requirements of the future LHC experiments is being designed at the moment