The Replica Location Service (RLS) is one component of data management services for Grid environments. RLS is a tool that provides the ability keep track of one or more copies, or replicas, of files in a Grid environment. This tool, which is included in the Globus Toolkit, is especially helpful for users or applications that need to find where existing files are located in the Grid.

1. Replica Location Service (RLS)

RLS is a simple registry that keeps track of where replicas exist on physical storage systems. Users or services register files in RLS when the files are created. Later, users query RLS servers to find these replicas.

RLS is a distributed registry, meaning that it may consist of multiple servers at different sites. By distributing the RLS registry, we are able to increase the overall scale of the system and store more mappings than would be possible in a single, centralized catalog. We also avoid creating a single point of failure in the Grid data management system. If desired, RLS can also be deployed as a single, centralized server.

Before explaining RLS in detail, we need to define a few terms.

A logical file name is a unique identifier for the contents of a file.
A physical file name is the location of a copy of the file on a storage system.

These terms are illustrated in Figure 1 (below). The job of RLS is to maintain associations, or mappings, between logical file names and one or more physical file names of replicas. A user can provide a logical file name to an RLS server and ask for all the registered physical file names of replicas. The user can also query an RLS server to find the logical file name associated with a particular physical file location.

In addition, RLS allows users to associate attributes or descriptive information (such as size or checksum) with logical or physical file names that are registered in the catalog. Users can also query RLS based on these attributes.

Figure 1. Example of the associations between a logical file name and three replicas on different storage sites (RLS).

2. Using RLS: An Example

One example of a system that uses RLS as part of its data management infrastructure is the Laser Interferometer Gravitational Wave Observatory (LIGO) project. LIGO scientists have instruments at two sites that are designed to detect the existence of gravitational waves. During a run of scientific experiments each LIGO instrument site produces millions of data files. Scientists at eight other sites want to copy these large data sets to their local storage systems so that they can run scientific analysis on the data. Therefore, each LIGO data file may be replicated at up to ten physical locations in the Grid. LIGO deploys RLS servers at each site to register local mappings and to collect information about mappings at other LIGO sites. To find a copy of a data file a scientist requests the file from LIGO’s data management system, called the Lightweight Data Replicator (LDR). LDR queries the Replica Location Service to find out whether there is a local copy of the file; if not, RLS tells the data management system where the file exists in the Grid. Then the LDR system generates a request to copy the file to the local storage system and registers the new copy in the local RLS server.

LIGO currently uses the Replica Location Service in its production data management environment. The system registers mappings between more than 3 million logical file names and 30 million physical file locations.

3. For more information

For more detailed information about RLS, click here.

For more information about RLS, see the documentation.

Prev		Next
Data movement	Home	Higher level data services