next up previous contents
Next: Objects stored on UNIX Up: Objects Previous: Objects

 

Attributes

Valid attributes include ACCESS-METHOD, CLOSURE, DESCRIPTION, FORWARDING-POINTER, KEYWORDS, LAST-REFERENCED, LAST-WRITER, LOCKS, OWNER, REPLICAS, SIZE, STORAGE-LOCATION, TTL, TTL-EXPIRES. VERSION-NUMBER, VIRT-SYS, WELL-KNOWN-NAMES, and WRITE-DATE.

Prospero maintains the following system defined attributes for each Prospero object:

BASE-TYPE
See section gif for a description of this attribute.

ACCESS-METHOD

This is a tuple of at least 5 elements. The first element is the name of the access method. Currently supported values are AFTP, GOPHER, AFS, NFS, FTP, WAIS, and LOCAL. The next 2 elements are the host-type and host-name of the host to which the access method should connect in order to retrieve the object. A port number may be included as part of the hostname, if relevant. If they are zero-length tokens (''), then they default to the host-type and host-name specified in the link. The next 2 elements are the hsoname-type and hsoname which the access method will use to retrieve the object. If they are zero-length tokens, then they default to the hsoname-type and hsoname specified in the link. This constraint on format applies to all access methods. If a particular type of access method doesn't need one of these fields, then it still must be specified, but the access method is free to ignore it. The whole point of the '' shorthand is merely to save bytes in the protocol messages. It is always appropriate to send them fully expanded, and not use the '' shorthand.

The sixth and subsequent elements are dependent upon the particular access method. For AFTP and FTP, the sixth element will be BINARY or TEXT. For NFS, the sixth element will be the name of the filesystem on the remote host. LOCAL, and AFS have only five-element names.

The GOPHER access method may have five or six elements in the name. The actual protocol used to retrieve a document or object through Gopher varies depending on its Gopher item-type. (See Internet RFC 1436 for details on the interpretation of the Gopher item-type characters.). This item type is usually the 1st character of a Gopher document or object's selector string (the hsoname element of the access method). However, the protocol does not require that this be the case. If a sixth token is present in a GOPHER access method, it will be treated as the Gopher item-type character; otherwise, the item-type will be taken from the 1st character of the hsoname element.

Some examples:

ATTRIBUTE FIELD ACCESS-METHOD GOPHER INTERNET-D MERMAID.MICRO.UMN.EDU(150) ASCII '1/Fun Stuff/Pyrotechnics/PyroGuide 1'gif

This access method could also be displayed in the six-token format as:

ATTRIBUTE FIELD ACCESS-METHOD GOPHER INTERNET-D MERMAID.MICRO.UMN.EDU(150) ASCII '1/Fun Stuff/Pyrotechnics/PyroGuide 1' 1gif

Andrew File System names are the same irrespective of what host one is using them from. Therefore, the host-name token in the access method is irrelevant and is ignored by the client.

ATTRIBUTE FIELD ACCESS-METHOD AFS DUMMY DUMMY ASCII /grand.central.org/doc/afs/dce/usenix90/README

This file can be retrieved by NFS mounting the /auto/gum/gum filesystem on the host PROSPERO.ISI.EDU. Note that the server is responsible for knowing which client machines it will allow to NFS mount that filesystem.

ATTRIBUTE FIELD ACCESS-METHOD NFS INTERNET-D PROSPERO.ISI.EDU ASCII ftp/pub/prospero/README-prospero-documents /auto/gum/gum

hsoname tokens in the FTP access method are full local hostname paths that one would give when using full FTP to the host.

ATTRIBUTE FIELD ACCESS-METHOD FTP INTERNET-D PROSPERO.ISI.EDU ASCII /ftp/pub/prospero/README-prospero-documents TEXT

hsoname tokens in the AFTP access method are pathnames relative to the root of the anonymous FTP area.

ATTRIBUTE FIELD ACCESS-METHOD AFTP INTERNET-D PROSPERO.ISI.EDU ASCII /pub/prospero/README-prospero-documents TEXT

If one is querying from a host which has a file accessible via the local filesystem, and if the server has knowledge of this fact, a LOCAL access method will also be returned for a file. For the LOCAL access method, the host-name token in the access method is ignored.

ATTRIBUTE FIELD ACCESS-METHOD LOCAL '' '' ASCII /auto/gum/gum/ftp/pub/prospero/README-prospero-documents

CLOSURE
The attribute-type of thie attribute is LINK. It is a link to the virtual system to be used when resolving names embedded in the object. The link's name-component will be ignored, but by convention it is the ugly-name of the virtual system.

OWNING-VIRTUAL-SYSTEM
The attribute-type of this attribute is LINK. It is a link to the description of the virtual system which is considered to own the object. (Every virtual system, by convention, has a link to its description named /VS-DESCRIPTION. For directories, if you are logged into the directory's OWNING-VIRTUAL-SYSTEM, then if you try to change the directory, your client will assume that you really want to try to change the directory. If you are logged into a different virtual system from the directory's OWNING-VIRTUAL-SYSTEM, then if you try to change the directory, your client will assume that you really want to customize your own virtual system, but not affect the master directory. Your intentions are completely distinct from whether you actually have write permission on the directory. If you try to change a directory that you can't write to (if its OWNING-VIRTUAL-SYSTEM is the same as the virtual system you're logged into, but you don't have permission to write to it), then the current clients will all give you a ``permission denied'' error message, and the nice ones will suggest that you might want to change virtual systems and customize instead. The link's name-component will be ignored, but by convention it is the ugly-name of the virtual system.

VERSION-NUMBER
The attribute-type of this attribute is SEQUENCE. It is represented in the protocol as a decimal number. It is currently always zero; see section gif for further details.

OWNER
The principal responsible for the object. The attribute-type of this attribute is SEQUENCE. It is a tuple of three or more elements. This is the same as the PRINCIPAL attribute on the owner's VS-DESCRIPTION directory.

Although the OWNER field may be referred to with the OWNER ACL type, we don't really encourage people to use it as a shorthand for granting access rights. Its primary purpose is informational.

FORWARDING-POINTER
This attribute has type LINK. The link's target is FP. It is set for objects that have migrated to another host. The target of its value is the new location of the object. If an object has moved to a new host, its FORWARDING-POINTER will be returned in a FORWARD protocol message in response to any attempts to access it.

Last writer, write date, etc.

Version info
(optional). Number of versions to keep, version number, etc.

Attributes or keywords
(optional). User specified.

Short description
(optional).

Locks
(optional).

List of replicas
(optional).

Replication type
(optional).

Other replication information
(optional).

Time to live
. The lifetime for newly created or refreshed links to the object.

TTL-EXPIRES
This is the TTL plus the time that a link to the object was last created or refreshed.

Back links
. A possibly incomplete list of directories with links to the object.

Note that the object's name is not one of its attributes. The object's name is the concatenation of the name components starting from the active virtual system. An object may have different names in different virtual systems, or even multiple names within a single virtual systemgif.


next up previous contents
Next: Objects stored on UNIX Up: Objects Previous: Objects

Padma Indraganti
Thu Jun 20 13:02:20 PDT 1996