KDB

General methods to access the Key database. More...

Functions
KDB *	kdbOpen (Key *errorKey)
	Opens the session with the Key database.
int	kdbClose (KDB *handle, Key *errorKey)
int	kdbGet (KDB *handle, KeySet *ks, Key *parentKey)
	Retrieve keys in an atomic and universal way.
int	kdbSet (KDB *handle, KeySet *ks, Key *parentKey)
	Set keys in an atomic and universal way.

Detailed Description

General methods to access the Key database.

To use them:

#include <kdb.h>

The kdb*() methods are used to access the storage, to get and set KeySets .

They use some backend implementation to know the details about how to access the storage.

One backend consists of multiple plugins. See writing a new plugin for information about how to write a plugin.

Function Documentation

int kdbClose	(	KDB *	handle,
		Key *	errorKey
	)

Closes the session with the Key database.

This is the counterpart of kdbOpen().

You must call this method when you finished your affairs with the key database. You can manipulate Key and KeySet objects also after kdbClose(), but you must not use any kdb*() call afterwards. You can run kdbClose() in the atexit() handler.

The handle parameter will be finalized and all resources associated to it will be freed. After a kdbClose(), the handle can't be used anymore.

Precondition

The handle must be a valid handle as returned from kdbOpen()

errorKey must be a valid key, e.g. created with keyNew()

Parameters

handle	contains internal information of opened key database
errorKey	the key which holds error information

Return values

0	on success
-1	on NULL pointer

int kdbGet	(	KDB *	handle,
		KeySet *	ks,
		Key *	parentKey
	)

Retrieve keys in an atomic and universal way.

Precondition

kdbOpen() must be called before using this method.

The returned KeySet must be a valid KeySet, e.g. constructed with ksNew().

The returned KeySet may already contain some keys from previous kdbGet() calls. The new retrieved keys will be appended using ksAppendKey().

It will fully retrieve, at least, all keys under the parentKey folder, with all subfolders and their children.

Note

kdbGet() might retrieve more keys then requested (that are not below parentKey). These keys must be passed to calls of kdbSet(), otherwise they will be lost. This stems from the fact that the user has the only copy of the whole configuration and backends only write configuration that was passed to them. For example, if you kdbGet() "system/mountpoint/interest" you will not only get all keys below system/mountpoint/interest, but also all keys below system/mountpoint (if system/mountpoint is a mountpoint as the name suggests, but system/mountpoint/interest is not a mountpoint). Make sure to not touch or remove keys outside the keys of interest, because others may need them!

See Also

ksCut() for further remarks on that topic.

Example:

This example demonstrates the typical usecase within an application (without error handling).

KeySet *myConfig = ksNew(0,KS_END);
Key *pkey = keyNew("system/sw/MyApp",KEY_END);
KDB *handle = kdbOpen(pkey);

kdbGet(handle, myConfig, pkey); // get the config initially

Key *key = ksLookupByName(myConfig,"/sw/MyApp/key", 0);
// check if key is not 0 and work with it...

kdbGet(handle, myConfig, pkey); // get the config again

ksDel (myConfig); // delete the in-memory configuration

kdbClose(handle, pkey); // no more affairs with the key database.
keyDel(pkey);

Details:

If you pass NULL on any parameter kdbGet() will fail immediately without doing anything.

When a backend fails kdbGet() will return -1 with all error and warning information in the parentKey. The parameter returned will not be changed.

Updates:

In the first run of kdbGet all keys are retrieved. On subsequent calls only the keys are retrieved where something was changed inside the key database. The other keys stay unchanged in the keyset, even when they were manipulated.

It is your responsibility to save the original keyset if you need it afterwards.

If you must get the same keyset again, e.g. in another thread you need to open a second handle to the key database using kdbOpen().

Parameters

handle	contains internal information of opened key database
parentKey	parent key holds the information which keys should be get (it is possible that more are retrieved) - an invalid name gets all keys
ks	the (pre-initialized) KeySet returned with all keys found will not be changed on error or if no update is required

See Also

ksLookup(), ksLookupByName() for powerful lookups after the KeySet was retrieved

kdbSet() to save the configuration afterwards and kdbClose() to finish affairs with the Key database.

Return values

1	if the keys were retrieved successfully
0	if there was no update - no changes are made to the keyset then
-1	on failure - no changes are made to the keyset then

KDB* kdbOpen

(

Key *

errorKey

)

Opens the session with the Key database.

The method will bootstrap itself the following way. The first step is to open the default backend. With it system/elektra/mountpoints will be loaded and all needed libraries and mountpoints will be determined. These libraries for backends will be loaded and with it the KDB datastructure will be initialized.

You must always call this method before retrieving or committing any keys to the database. In the end of the program, after using the key database, you must not forget to kdbClose(). You can use the atexit () handler for it.

The pointer to the KDB structure returned will be initialized like described above, and it must be passed along on any kdb*() method your application calls.

Get a KDB handle for every thread using elektra. Don't share the handle across threads, and also not the pointer accessing it:

thread1
{
        KDB * h;
        h = kdbOpen(parent);
        // fetch keys and work with them
        kdbClose(h, parent);
}
thread2
{
        KDB * h;
        h = kdbOpen(parent);
        // fetch keys and work with them
        kdbClose(h, parent);
}

You don't need kdbOpen() if you only want to manipulate plain in-memory Key or KeySet objects.

Precondition

errorKey must be a valid key, e.g. created with keyNew()

Parameters

errorKey

the key which holds errors and warnings which were issued

See Also

kdbGet(), kdbClose() to end all affairs to the Key database.

Return values

handle	on success
NULL	on failure

int kdbSet	(	KDB *	handle,
		KeySet *	ks,
		Key *	parentKey
	)

Set keys in an atomic and universal way.

With parentKey you can only store a part of the given keyset. When other keys also belong to a backend, they will be used too, even when they are above parentKey.

Details:

If you pass a parentKey without a name the whole keyset will be set in an atomic way.

Each key is checked with keyNeedSync() before being actually committed. So only changed keys are updated. If no key of a backend needs to be synced any affairs to backends omitted and 0 is returned.

Errors:

If some error occurs, kdbSet() will stop. In this situation the KeySet internal cursor will be set on the key that generated the error.

None of the keys are actually commited.

You should present the error message to the user and let the user decide what to do. Possible solutions are:

• repeat the same kdbSet (for temporary errors)
• remove the key and set it again (for validation or type errors)
• change the value and try it again (for validation errors)
• do a kdbGet and then (for conflicts ...)
  • set the same keyset again (in favour of what was set by this user)
  • drop the old keyset (in favour of what was set elsewhere)
• export the configuration into a file (for unresolvable errors)

Example of how this method can be used:

int i;
KeySet *myConfig = ksNew(0,KS_END);
Key *parentKey = keyNew("system/sw/MyApp",KEY_END);
KDB *handle = kdbOpen(pkey);

kdbGet(handle, ks, parentKey); // kdbGet needs to be called first!
KeySet *base = ksDup(ks); // save a copy of original keyset

// change the keys within ks

KeySet *ours = ksDup(ks); // save a copy of our keyset
int ret=kdbSet(handle, ks, parentKey);
while (ret == -1) // as long as we have an error
{
        // We got an error. Warn user.
        Key *problemKey = ksCurrent(ks);
        // parentKey has the errorInformation
        // problemKey is the faulty key (may be null)
        int userInput = showElektraErrorDialog (parentKey, problemKey);
        switch (userInput)
        {
        case INPUT_USE_OURS:
                kdbGet(handle, ks, parentKey); // refresh key database
                ksDel(ks);
                ks = ours;
                break;
        case INPUT_DO_MERGE:
                kdbGet(handle, ks, parentKey); // refresh key database
                KeySet * res=doElektraMerge(ours, ks, base);
                ksDel(ks);
                ks = res;
                break;
        case INPUT_USE_THEIRS:
                // should always work, we just write what we got
                // but to be sure always give the user another way
                // to exit the loop
                kdbGet(handle, ks, parentKey); // refresh key database
                break;
        ...
        }
        ret=kdbSet(handle, ks, parentKey);
}

ksDel (ours);
ksDel (base);
ksDel (ks); // delete the in-memory configuration

kdbClose(handle, parentKey); // no more affairs with the key database.
keyDel(parentKey);

showElektraErrorDialog() and doElektraMerge() need to be implemented by the user. For doElektraMerge a 3-way merge algorithm exists in libelektra-tools.

Precondition

kdbGet() must be called before kdbSet():

• initially (after kdbOpen())
• after conflict errors in kdbSet().

Parameters

handle	contains internal information of opened key database
ks	a KeySet which should contain changed keys, otherwise nothing is done
parentKey	holds the information below which key keys should be set, see above

Return values

1	on success
0	if nothing had to be done
-1	on failure

See Also

keyNeedSync()

ksNext(), ksCurrent() for iteration over the KeySet

kdbOpen() and kdbGet() that must be called first

kdbClose() that must be called afterwards