When writing data into the dCache, and possibly later on into an HSM, checksums may be calculated at different points within this chain.

The graph below sketches the different schemes for dCap and FTP with and without client checksum calculation:

As far as the server data mover is concerned, only the Client Checksum and the Transfer Checksum are of interrest. While the client checksum is just delivered to the server mover as part of the protocol (e.g. close operation for dCap), the transfer checksum has to be calcalated by the server mover on the fly. In order to communicate the different checksums to the embedding pool, the server mover has to implement the ChecksumMover interface in addition to the MoverProtocol Interface. A mover, not implementing the MoverProtocol is assumed not to handle checksums at all. The Disk File Checksum is calculated independedly of the mover within the pool itself.

public interface ChecksumMover {

        public void     setDigest( Checksum transferChecksum ) ;
        public Checksum getClientChecksum() ;
        public Checksum getTransferChecksum() ;

}

The pool will or will not call the setDigest method to advise the mover which checksum algorithm to use. If setDigest is not called, the mover is not assumed to calculate the Transfer Checksum.

java.security.MessageDigest transferDigest = transferChecksum.getMessageDigest() ;

                ***

        while( ... ){

                rc = read( buffer , 0 , buffer.length ) ;

                ***

                transferDigest.update( buffer , 0 , rc ) ;
        }

getClientChecksum and getTransferChecksum are called by the pool after the MoverProtocols runIO method has been successfully processed. These routines should return null if the corresponding checksum could not be determined for whatever reason.

public void  setDigest( Checksum transferChecksum ){

        this.transferChecksum = transferChecksum ;

        }
        public Checksum getClientChecksum(){
                return clientChecksumString == null ?
                        null :
                        Checksum( clientChecksumString ) ;
        }
        public Checksum getTransferChecksum(){ return transferChecksum ; }

The DCapProtocol_3_nio mover implements the ChecksumMover interface and is able to report the Client Checksum and the Transfer Checksum to the pool. To enable the DCapProtocol_3_nio Mover to calculate the Transfer Checksum, either the cell context dCap3-calculate-transfer-crc or the cell batch line option calculate-transfer-crc must be set to true. The latter may as well be set in the *.poolist file. DCapProtocol_3_nio disables checksum calculation as soon as the mover receives a client command except ’write’ (e.g. read, seek or seek_and_write).

The checksum module (as part of the Pool) and its command subset (csm ...) determines the behavious of the checksum calculation.