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FAQ: lcg-ce to cream-ce

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The FAQ provides the questions and answers those may arise for the user communities during the transition from lcg-ce to cream-ce. It does not replace the CREAM User Guide (see the references section), but it provides a list of pointers to relevant documentation.

Q1: What is cream-ce?

The CREAM (Computing Resource Execution And Management) Service is a simple, lightweight service that implements all the operations at the Computing Element (CE) level; its well-defined Web Service-based interface and its implementation as an extension of the Java-Axis servlet (running inside the Apache Tomcat container) provide interoperability with clients written in any programming language and running on any computer platform.

The CREAM interface is well-defined using the Web Service Description Language (WSDL); anyone can generate his/her CREAM client by simply filling in the stub code generated by WSDL parser (gSOAP for C/C++, Axis for Java, Perl module for perl).

The CREAM architecture is illustrated here: Diagram

The interface exposes a set of operations which can be classified in three groups.

  • Lease management: allows users to define and manage leases associated with jobs. The lease mechanism ensure that all jobs get eventually properly managed, even if the CREAM loses connection with the client application.
  • Job management:operations are provided to create and subsequently manage jobs.
  • Service Management: operations that deal with the whole CREAM service.

Another service part of CREAM CE isCEMonitor, it is a generic framework for information gathering, it is widely used to manage compute resources. Coupled with CREAM CEMonitor notifies users when job status changes occur.

Q2: What batch systems are supported by CREAM?

LSF, Torque
CREAM supports different batch systems, through the concept of LRMS connectors. Currently CREAM supports all the batch system supported by BLAHPD (Batch Local Ascii Helper)

Q3: How does proxy credential delegation and proxy renewal work in CREAM?

Detailed documentation about security-related aspects of CREAM are available at the CREAM User Guide.

Q4: how are jobs submitted to CREAM?

CREAM has ability to accept either direct Jobs from gLite-UI, SOAP clients or any other developed UIs based on web-services or Jobs sent through WMS. Jobs can be controlled through operations such as suspend, resume and delete. CREAM supports the execution of normal/serial batch jobs, and parallel (MPI) jobs.

In case of submission through the Workload Management System (WMS), the ICE (Interface to CREAM Environment) component of the WMS is used. To select the CREAM instances to which the job can be submitted, submission criteria can be defined by means of the Job Description Language - JDL (see the CREAM JDL Guide).

Intructions for the management of input and output files are available here.

Q5: is the performance of CREAM comparable to the one of the lcg-ce?

CREAM has been intentionally designed to deliver higher performance than lcg-ce.

Q6: What client errors are returned by CREAM?

These are documented here: Cream Client Error Messages

Q7: Where can I find more information about the known issues of CREAM?

The known issues of current CREAM software in production release are documented here:

Q8: When is the end-of-support of lcg-CE scheduled?

The exact schedule is under discussion, but lcg-ce will likely reqch end of support in Summer 2011. More information will be made available in this FAQ as soon as possible.

Q9: How can I get support about CREAM?

You are recommended to submit a ticket to the EGI Helpdesk system (GGUS).

Q10: Where can I find more documentation about CREAM?