Introduction

EOSCTA is CERN's next-generation software solution for the archival and retrieval of data stored on tape. The CERN Tape Archive (CTA) is the tape back-end to the EOS disk system, and together EOS+CTA forms the complete data archival system:

1. The EOS disk system manages the namespace for data stored on tape; provides a storage buffer for files during archival and retrieval operations; and provides the interface between the external client and CTA.
2. CTA manages the tape file catalogue; provides queueing and scheduling functions; and controls data transfers to and from the tape hardware.
3. The third element in the system is the client software, which communicates with EOSCTA using the XRootD protocol. The client is responsible for transferring files into and out of EOS, and managing failures and retries. The most commonly-used client is CERN's File Transfer System (FTS) but in principle any client which can communicate using the XRootD protocol can be used.

This document describes the EOSCTA workflows (archival, retrieval and deletion of files), and the interfaces and protocols used between the different system components.