NAME

       elektra-data-structures - data structures

       For an introduction, please read first about elektra classes elektra-classes.md. You might
       want to read about architecture first elektra-architecture.md.

Introduction

       Data  structures  define  the  common  layer  in  Elektra.  They  are  used  to   transfer
       configuration between Elektra and applications, but also between plugins.

   ADT
       Both the KeySet and the interface to metadata within a Key are actually ADT (abstract data
       types). The API is designed so that different implementations of the data  structures  can
       be used internally.

       A  hash data structure presents a good candidate as alternative data structure, especially
       for the metadata interface. It is believed to be much faster on lookup,  but  considerably
       slower on sorted enumeration.

       A  AVL  tree also serve as a competitor. AVL trees are expected to be faster for inserting
       keys at any place, but may be slower for appending because of the needed  reorganisations.
       Their disadvantage is that they need to allocate a large number of small pieces of memory.
       Further investigations, namely implementation and benchmarks, are required to decide.

       A trie could also be used for lookup of key names. It performs well for lookup, but  needs
       more memory allocations.

       Currently  the  KeySet  is  implemented  as  a  sorted  array. It is fast on appending and
       iterating, and has nearly no size-overhead. To improve the lookup-time, an additional hash
       will be used.

   ABI compatibility
       Application  binary  interface,  or  ABI,  is  the  interface to all data structures of an
       application or library directly allocated or accessed by the user.

       Special care has been taken in Elektra to support all changes within the  data  structures
       without  any  ABI  changes. ABI changes would entail the recompilation of applications and
       plugins using Elektra. The functions keyNew(), ksNew() and  kdbOpen()  allocate  the  data
       structures  for  the applications. The user only gets pointers to them. It is not possible
       for the user to allocate or access these data structures  directly  when  only  using  the
       public  header  file  <kdb.h>.  The  functions  keyDel(),  ksDel() and kdbClose() free the
       resources after use. Using the C++ binding deallocation is done automatically.

Meta data

       Read here elektra-meta-data.md.

KeySet

       This subsection describes what has changed between 0.7 and 0.8 and deals with some general
       implementation issues.

   Operations
       KeySet  resembles  the  classical mathematical set. Operations like union, intersection or
       difference are well defined. In mathematics typically every operation yields  a  new  set.
       Instead, we try to reuse sets in the following ways:

       •   A   completely  new  and  independent  KeySet  as  return  value  would  resemble  the
           mathematical ideal closely. This operation would be expensive. Every Key needs  to  be
           duplicated and inserted into a new KeySet.

           Such a deep duplication was only needed in kdbSet().

       •   The  resulting  KeySet  is created during the operation, but only a flat copy is made.
           This means that the keys in it are actually not duplicated, but only  their  reference
           counter  is increased. This method is similar to the mathematical model. Compared with
           a deep copy it can achieve good performance. But all changes to the values of keys  in
           the resulting KeySet affect the original KeySet, too.

           ksDup(const  KeySet *source) produces a new KeySet that way. The source is not changed
           as shown by the const modifier.

       •   The result of the operation is applied to the KeySet passed as argument directly. This
           is  actually  quite  common,  but for this situation other names of the operations are
           more suitable.

           For example, a union which changes the KeySet is called ksAppend().

       •   A new KeySet is created, but the KeySet passed as parameter is  reduced  by  the  keys
           needed  for the new KeySet. This is useful in situations where many operations have to
           be applied in a sequence reducing the given KeySet until no more keys are  left.  None
           of the reference pointers changes in this situation.

           ksCut(KeySet *ks, const Key *cutpoint) works that way. All keys below the cutpoint are
           moved from ks to the returned key set.

   Consistency
       There are several ways to define consistency relations on key sets. For strict consistency
       every  parent  key  must exist before the user can append a key to a key set. For example,
       the key set with the keys

           system
           system/elektra
           system/elektra/mountpoints

       would  allow  the  key  system/elektra/mountpoints/tcl  to  be  added,  but  not  the  key
       system/apps/abc  because  system/apps  is  missing.  File  systems  enforce  this  kind of
       consistency.

       These  semantics  are  however  not  useful  for  configurations.  Especially   for   user
       configurations  often only some keys need to be overwritten. It is not a good idea to copy
       all parent keys to the  users  configuration.  For  this  reason  we  use  a  less  strict
       definition of consistency supporting such holes.

       We  also  evaluated  a form of weak consistency. It avoids adding some unnecessary keys. A
       constraint is that a key can be added only if it has a parent key. But the constraint does
       not  apply  if no other key exists above the key about to be inserted. From that moment it
       will serve as parent key for other  keys.  With  the  current  implementation  of  KeySet,
       however,  it  is  not  possible  to  decide  this constraint in constant time. Instead its
       worst-case complexity would be $log(n) * x$ where $n$ is the number of keys  currently  in
       the key set and $x$ is the depth of the key. The depth is the number of / in the key name.
       The worst-case of the complexity applies when the inserting works without  a  parent  key.
       For example, with the keys

           user/sw/apps/abc/current/bindings
           user/sw/apps/abc/current/bindings/key1
           user/sw/apps/abc/current/bindings/key2

       the  weak consistency would allow inserting user/sw/apps/abc/current/bindings/key3 because
       it is directly below an existing key. It would also allow adding  user/sw/apps/xyz/current
       because    it    does    not   have   any   parent   key.   But   it   would   not   allow
       user/sw/apps/abc/current/bindings/dir/key1 to add. The worst-case complexity was found  to
       be too expensive, and hence KeySet has no consistency check at all.

       This  means  any  key  with  a  valid  key name can be inserted into KeySet. The KeySet is
       changed so that it is now impossible to append keys without a  name.  ksAppendKey(ks,  Key
       *toAppend)  takes  ownership of the key toAppend and will delete the key in that case. The
       caller does not have to free toAppend: either it is in the key set or it is deleted.

       Binary search determines the position where to insert a  key.  The  C  version  of  binary
       search  bsearch() cannot tell where to insert a key when it is not found. So the algorithm
       has to be reimplemented. Java´s binary search binarySearch() uses a trick to both indicate
       where  a  key  is  found and where to insert it with the same return code by returning the
       negative value ((-insertion point) - 1) indicating where the new value should be  inserted
       when the key is not found. Elektra now also uses this trick internally.

   Internal Cursor
       KeySet  supports  an  external  iterator  with  the  two functions ksRewind() to go to the
       beginning and ksNext() to advance the internal cursor to the next key. This side effect is
       used  to  indicate a position for operations on a KeySet without any additional parameter.
       This technique is comfortable to see which key has caused an error after  an  unsuccessful
       key database operation.

       Elektra  only  has  some  functions to change the cursor of a key set. But these allow the
       user to compose powerful functions. Plugins do that extensively as we will  see  later  in
       ksLookupRE().  The  user  can  additionally  write  more such functions for his or her own
       purposes. To change the internal cursor, it is sufficient to iterate over the  KeySet  and
       stop at the wanted key. With this technique, we can, for example, realise lookup by value,
       by specific metadata and by parts of the name. Without an  additional  index,  it  is  not
       possible  that  such operations perform more efficiently than by a linear iteration key by
       key. For that reason, Elektra´s  core  does  not  provide  such  functions.  The  function
       ksLookupByName(),  however, uses the more efficient binary search because the array inside
       the KeySet is ordered by name.

   External Cursor
       External cursor is an alternative to the approach  explained  above.  Elektra  provides  a
       limited  external  cursor through the interface ksGetCursor() and ksSetCursor(). It is not
       possible to advance this cursor. The  difference  to  the  internal  cursor  is  that  the
       position is not stored within KeySet.

       We  considered providing an external cursor for performance reasons. But we found out that
       the speed of iteration is mainly  limited  because  of  safety  checks.  The  investigated
       methods become slower proportional to the ease of use and safety. When using null pointers
       and range checks, the function is noticeably slower than without. With the same amount  of
       checks,  using  an  external  cursor is not much faster than the ksNext(). External cursor
       with checks is in a benchmark about 10\% faster.

       But an external cursor directly accessing the array can be much faster. Using an unchecked
       external  cursor can be about 50% faster than using the internal cursor with ksNext(). For
       this endeavour, Elektra´s private header files need  to  be  included.  Including  private
       header  files,  however,  should not be done with levity because ABI compatibility will be
       gone on any changes of the data structures. This fact  means  the  application  or  plugin
       needs  to  be  recompiled  when  any of the internal structures of Elektra are changed. We
       strongly discourage including these header files.

       Nevertheless, the overall performance impact for  iteration  is  minimal  and  should  not
       matter too much. Even if only a single keySetMeta() is used inside the iteration loop, the
       iteration costs are insignificant. Only for  trivial  actions  such  as  just  changing  a
       variable,  counter or marker for every key the iteration costs become the lion´s share. In
       such situations an internal iterator  yields  better  results.  For  example,  ksForEach()
       applies  a  user defined function for every key in a KeySet without having null pointer or
       out of range problems.

Trie vs. Split

       Up to now, we have discussed external data structures visible to the user of the  library.
       The  application  and  plugin programmer needs them to access configuration. Last, but not
       least, we will show two  internal  data  structures.  The  user  will  not  see  them.  To
       understand the algorithm, however, the user needs to understand them as well.

   Trie
       Trie  or  prefix  tree  is  an  ordered  tree  data structure. In Elektra, it provides the
       information to decide in  which  backend  a  key  resides.  The  algorithm,  presented  in
       algorithm  elektra-algorithm.md,  also  needs a list of all backends. The initial approach
       was to iterate over the Trie to get a list of all backends. But the  transformation  of  a
       Trie  to a list of backends, contained many bugs caused by corner cases in connection with
       the default backend and cascading mount points.

   Split
       So, instead of transforming the trie to a list of  backends,  we  introduced  a  new  data
       structure  \intro[Split@\lstinline{Split}]{Split}. The name Split comes from the fact that
       an initial key set is split into many key sets. These key sets are  stored  in  the  Split
       object. Split advanced to the central data structure for the algorithm:

           typedef struct _Split   Split;

           struct _Split {
               size_t size;
               size_t alloc;
               KeySet **keysets;
               Backend **handles;
               Key **parents;
               int *syncbits;
           };

       The data structure Split contains the following fields:

       •   size: contains the number of key sets currently in Split.

       •   alloc: allows us to allocate more items than currently in use.

       •   keysets  represents  a  list of key sets. The keys in one of the key sets are known to
           belong to a specific backend.

       •   handles: contains a list of handles to backends.

       •   parents: represents a list of keys. Each parentKey contains the root key of a backend.
           No  key  of  the  respective key set is above the parentKey. The key name of parentKey
           contains the mount point of a backend. The resolver writes  the  file  name  into  the
           value of the parentKey.

       •   syncbits:  are  some  bits  that  can be set for every backend. The algorithm uses the
           syncbits to decide if the key set needs to be synchronised.

       Continue reading with the error handling elektra-error-handling.md.

                                            July 2017                  ELEKTRA-DATA-STRUCTURES(7)