SYNOPSIS

int af_errno

DESCRIPTION

AtFS is an extension to the UNIX file system interface that allows the storage of files as complex of content data and an arbitrary number of associated attributes. Attributes are either standard attributes or user defined attributes (also called application defined attributes). A data complex consisting of contents data and attributes, is called Attributed Software Object (ASO). In the following they are also referred to as object versions or simply versions. AtFS has a built-in version control system that manages object histories. A object history consists of an optional busy version and a number of saved versions. The busy version is represented by an ordinary alterable UNIX file. It can be accessed via AtFS and UNIX file system operations. Saved versions come into being by making a copy of the current state of the busy version. They can be stored as source versions or as derived versions.

Source versions are typically composed manually (e.g. by use of a text editor). AtFS stores source versions as immutable objects. Once saved, they cannot be modified any longer. Saved versions are stored in archive files, residing in a subdirectory named AtFS. AtFS maintains two archive files for each history of source versions - one holding the attributes and the other holding the data. To save disk space, the versions in an archive are stored as deltas, that means, only differences between two successive versions are stored.

Derived versions are typically derived automatically (e.g. by a compiler) from a source version and thus be reproducible at any time. They are kept in a derived object cache a data store of limited size that is administered in a cache fashion. When space runs short, old versions are cleaned out of the cache according to a defined policy. Check af_cache(3) for details.

AtFS makes no assumptions whether a copy of a busy object shall be stored as source object or as derived object. The application has to decide that by calling the appropriate function (af_saverev - manual page af_version(3) or af_savecache - manual page af_cache(3)).

The main data types for AtFS applications are:

• The object key that uniquely identifies an ASO (version). The structure of this type can be different in different implementations of AtFS. Consequently, application programs should handle this type as opaque type and should not access single fields.
• A set descriptor represents a set of object keys. A set descriptor contains information about the size of the set and a list of object keys in the set.
• The user identification represents a user. As AtFS realizes a simple network user concept, it does not identify users by their UNIX user id, but rather by the user name and the domain where this name is valid. See af_afuser (manual page af_misc(3)) for details.
• An attribute buffer is capable to hold all attributes of a software object (standard attributes and user defined attributes). Attribute buffers have two different purposes. First, they can hold a retrieve pattern, i.e. they may be (partially) filled with desired attribute values and then be passed as argument to a retrieve operation (af_find and af_cachefind - manual page af_retrieve(3)). Second, an attribute buffer is used to return all attributes of an identified ASO on demand.

There are several ways for an AtFS application to get an object key pointing to a specific object version. The most important is the function af_getkey (manual page af_retrieve(3)), that returns a key for an explicitly named version. After having retrieved a key, the data for that object version remains cached in memory as long as the application does not explicitly give the key back. The function af_dropkey (manual page af_retrieve(3)) gives a key back and releases the object version. A retrieved set of keys has also to be given back by use of af_dropset (manual page af_retrieve(3)). af_dropall (manual page af_retrieve(3)) sets all reference counters for cached object versions to zero, that means, it gives all formerly retrieved keys and sets back. In the case, that any attribute, or the contents data of a cached object version is modified on disk by another application, the data in the cache is automatically updated. In that case, a warning is written to the error log file.

For handling sets of keys, AtFS provides a full set algebra with functions for adding and deleting single keys, sorting, building subsets, intersections, unions and differences of sets.

The built-in version control functionality features a status mode for versions. Each object version has a certain state, one of busy, saved, proposed, published, accessed and frozen. The already known busy version always has the state busy, while when previously referencing a saved versions we meant a version that can have any state from saved to frozen. AtFS does not enforce any specific semantics with the version state. It is rather a help for applications to implement a classification for versions. Another part of the version control functionality is a locking facility. Adding a new version to an existing object history always requires a lock on the most recent version or the generation to which the version shall be added. Locks can be used for implementing a synchronization mechanism for concurrent updates to one object history.

A user defined attribute (or application defined attribute) has the general form name[=value [value [...]]]. It has a name and a (possibly empty) list of values. The name may consist of any characters (even non-alphanumeric) but an equal sign (=). The equal sign is the delimiter between name and value. The attribute value may not contain Ctrl-A (\001) characters. Although AtFS promises the storage of an arbitrary number of user defined attributes, the current implementation limits their number to 255.

Most of the AtFS calls have one or more error returns. An error is indicated by a return value that is either -1 or a nil pointer, depending on the return type of the function. If one of the functions returns with an error, the variable af_errno is set to indicate the appropriate error number. af_errno is not cleared upon successful calls.

LIST OF FUNCTIONS

af_abort      af_transact.3 abort transaction
af_access     af_history.3  test existence of history
af_afname     af_misc.3     get ASO name from UNIX path
af_afpath     af_misc.3     get ASO syspath from UNIX path
af_aftype     af_misc.3     get ASO type from UNIX path
af_afuser     af_misc.3     get AtFS user from UNIX uid
af_allattrs   af_attrs.3    get all attributes of ASO
af_cachefind  af_retrieve.3 find derived objects by attributes
af_cachenames af_history.3  return list of names in derived object cache
af_cachesize  af_cache.3    define object cache size policy
af_chauthor   af_protect.3  change author of ASO
af_chmod      af_protect.3  change protection of ASO
af_chowner    af_protect.3  change owner of ASO
af_cleanup    af_error.3    do cleanup after error
af_close      af_files.3    close ASO contents
af_commit     af_transact.3 commit transaction
af_copyset    af_sets.3     copy sets
af_crkey      af_files.3    create object key for UNIX-file
af_diff       af_sets.3     build difference between two sets of object keys
af_dropall    af_retrieve.3 drop all accessed object keys
af_dropkey    af_retrieve.3 drop object key
af_dropset    af_retrieve.3 drop set of object keys
af_errmsg     af_error.3    return AtFS- or system error message
af_errno      af_error.3    global error code variable
af_establish  af_files.3    establish ASO as UNIX file
af_find       af_retrieve.3 find ASOs by attributes
af_freeattr   af_attrs.3    free memory associated with attribute value
af_freeattrbuf              af_attrs.3free memory associated with attribute buffer
af_getkey     af_retrieve.3 get key by unique attributes
af_histories  af_history.3  return list of history names matching pattern
af_initattrs  af_retrieve.3 initialize attribute buffer
af_initset    af_sets.3     initialize set
af_intersect  af_sets.3     build intersection between two sets of object keys
af_isstdval   af_attrs.3    check if attribute is from a standard attribute
af_lock       af_lock.3     set lock on ASO 
af_newgen     af_version.3  increase generation number of ASO
af_nrofkeys   af_sets.3     return number of keys in set of object keys
af_open       af_files.3    open ASO contents
af_perror     af_error.3    report AtFS- or system error
af_predsucc   af_version.3  get successor or predecessor of version
af_restore    af_files.3    restore derived ASO
af_retattr    af_attrs.3    return value of attribute as string
af_retnumattr af_attrs.3    return value of numeric attribute
af_rettimeattr              af_attrs.3return value of time attribute
af_retuserattr              af_attrs.3return value of user attribute
af_rm         af_files.3    remove ASO
af_rnote      af_note.3     return note attribute
af_savecache  af_cache.3    save derived object to cache
af_saverev    af_version.3  save busy version of source object
af_setaddkey  af_sets.3     add key to set of object keys
af_setarchpath              af_misc.3set path for location of archives
af_setattr    af_attrs.3    set user defined attribute
af_setbusy    af_version.3  set version busy
af_setgkey    af_sets.3     get key from set of object keys
af_setposrmkey              af_sets.3remove key (id. by position) from set of object keys
af_setrmkey   af_sets.3     remove key from set of object keys
af_snote      af_note.3     modify note attribute
af_sortset    af_sets.3     sort set of object keys
af_sstate     af_version.3  set version state
af_subset     af_sets.3     build subset of set of object keys
af_svnum      af_version.3  set version number
af_testlock   af_lock.3     see if ASO is locked
af_transaction              af_transact.3begin transaction
af_union      af_sets.3     build union of two sets of object keys
af_unlock     af_lock.3     remove lock from ASO
af_version    af_misc.3     return identification of current AtFS version

FILES

AUTHOR

 Technische Universitaet Berlin

Sekr. FR 5-6

Franklinstr. 28/29

D-10587 Berlin, Germany