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Aucb.564 fa.editor-p utcsrgv!utzoo!decvax!ucbvax!C70:editor-people Mon Mar 1 15:17:11 1982 Re: [Arthur Keller <ARK SU-AI AT>: Concurrent File Access ] >From DODDS@BBNA Mon Mar 1 15:08:38 1982 The problem mentioned by Arthur Keller certainly is the classic one of access conflicts in shared databases. We in the HERMES group at BBN have been there before.

TENEX and its descendant TOPS-20 have never had a proper way of locking a file for the use of one process without opening it and keeping it open. In HERMES we beat the problem to death by using the companion file to the given message file (where parse computations are stored for use in later sessions) as the repository for interlock information. We keep IT open, in order to keep the message file itself closed as much as possible, to receive mail. The price of this has been periodic clobbered companion files, due to system crashes with the file opened and mapped in user space.

The HERMES scheme, worked out several years ago by Austin Henderson and me, contains a number of elaborate features directed at fine-grained shared access to a message file (database). The editor problem is a considerably simpler subset of the database problem; in particular, one wants not to share access to a file, safely, with other processes, but rather just to lock other processes out of access until the first one is done with its work. Still, I think a similar consideration of the premises and constraints of the problem is in order before attempting to implement a scheme for an existing editor.

In particular, I want to emphasize the melancholy conclusion that I had to accept in considering this business: due to a basic design deficiency of the TENEX/TOPS-20 file system, there is no complete way to interlock access to a file. Any scheme implemented in user software, no matter how careful, can only be an incomplete approximation of the missing system ability.

If anybody is interested, I will be happy to write up an overview of the considerations involved in the HERMES interlock design.

Doug Dodds <DODDS BBNA AT>

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