/*! @page custom_data_sources Custom Data Sources

Applications can implement their own custom data sources underneath
WiredTiger using the WT_DATA_SOURCE interface.  Each data source should
support a set of methods for a different URI type (for example, in the
same way WiredTiger supports the built-in type "file:", an application
data source might support the type "dsrc:").

The WiredTiger distribution includes an example of a complete custom
data source implementation (based on Oracle's Berkeley DB database
engine), in the file test/format/kvs_bdb.c.  This example implementation is
public domain software, please feel free to use this code as a prototype
implementation of other custom data sources.

Applications register their WT_DATA_SOURCE interface implementations
with WiredTiger using the WT_CONNECTION::add_data_source method:

@snippet ex_data_source.c WT_DATA_SOURCE register

@section custom_ds_methods WT_DATA_SOURCE methods

Any of the WT_DATA_SOURCE methods may be initialized to NULL.  Calling
uninitialized WT_DATA_SOURCE methods through a WiredTiger API will
result in an "operation not supported" error.   For example, an
underlying data source that does not support a compaction operation
should set the \c compact field of their WT_DATA_SOURCE structure to
NULL.

@subsection custom_ds_create WT_DATA_SOURCE::create method

When implementing a custom data source, you may want to consider the 'r'
key and 't' value formats, and whether or not they should be implemented.

The 'r' key format indicates keys are \c uint64_t typed record numbers.
In this case, cursor methods will be passed and/or return a record
number in the \c recno field of the WT_CURSOR structure.

Cursor methods for data sources supporting fixed-length bit field
column-stores (that is, a store with an 'r' type key and 't' type value)
should accept and/or return a single byte in the value field of the
WT_CURSOR structure, where between 1 and 8 of the low-order bits of
that byte contain the bit-field's value.  For example, if the value
format is "3t", the key's value is the bottom 3 bits.

@section custom_ds_cursor_methods WT_CURSOR methods

The WT_DATA_SOURCE::open_cursor method is responsible for allocating and
returning a WT_CURSOR structure.  The only fields of the WT_CURSOR
structure that should be initialized by WT_DATA_SOURCE::open_cursor are
a subset of the underlying methods it supports.

The following methods of the WT_CURSOR structure must be initialized:
<ul>
<li>WT_CURSOR::close
<li>WT_CURSOR::next
<li>WT_CURSOR::prev
<li>WT_CURSOR::reset
<li>WT_CURSOR::search
<li>WT_CURSOR::search_near
<li>WT_CURSOR::insert
<li>WT_CURSOR::update
<li>WT_CURSOR::remove
</ul>

No other methods of the WT_CURSOR structure should be initialized, for
example initializing WT_CURSOR::get_key or WT_CURSOR::set_key will have
no effect.

Incorrectly configuring the WT_CURSOR methods will likely result in a core
dump.

The data source's WT_CURSOR::close method is responsible for discarding
the allocated WT_CURSOR structure returned by WT_DATA_SOURCE::open_cursor.

@subsection custom_ds_cursor_insert WT_CURSOR::insert method

Custom data sources supporting column-stores (that is, a store with an
'r' type key) should consider the \c append configuration string
optionally specified for the WT_DATA_SOURCE::open_cursor method.  The
\c append string configures WT_CURSOR::insert to allocate and return an
new record number key.

Custom data sources should consider the \c overwrite configuration string
optionally specified for the WT_DATA_SOURCE::open_cursor method.  If the
\c overwrite configuration is \c true, WT_CURSOR::insert, WT_CURSOR::remove
and WT_CURSOR::update should ignore the current state of the record, and
these methods will succeed regardless of whether or not the record previously
exists.

When an application configures \c overwrite to \c false, WT_CURSOR::insert
should fail with ::WT_DUPLICATE_KEY if the record previously exists, and
WT_CURSOR::update and WT_CURSOR::remove will fail with ::WT_NOTFOUND if the
record does not previously exist.

Custom data sources supporting fixed-length bit field column-stores (that
is, a store with an 'r' type key and 't' type value) may, but are not
required to, support the semantic that inserting a new record after the
current maximum record in a store implicitly creates the missing records
as records with a value of 0.

@section custom_ds_cursor_fields WT_CURSOR key/value fields

Custom data source cursor methods are expected to use the \c recno,
\c key and \c value fields of the WT_CURSOR handle.

The \c recno field is a uint64_t type and is the record number set or
retrieved using the cursor when the data source was configured for
record number keys.

The \c key and \c value fields are WT_ITEM structures.  The \c key.data,
\c key.size, \c value.data and \c value.size  fields are read by the
cursor methods storing items in the underlying data source and set by
the cursor methods retrieving items from the underlying data source.

@section custom_ds_error_handling Error handling

Some specific errors should be mapped to WiredTiger errors:
<ul>
<li>
attempts to insert an existing key should return WT_DUPLICATE_KEY
<li>
failure to find a key/value pair should return WT_NOTFOUND
<li>
fatal errors requiring the database restart should return WT_PANIC
</ul>

Otherwise, successful return from custom data source methods should be
indicated by a return value of zero; error returns may be any value
other than zero or an error in WiredTiger's @ref error_handling.  A simple
approach is to always return either a system error value (that is, \c
errno), or \c WT_ERROR.   Error messages can be displayed using the
WT_SESSION::msg_printf method.  For example:

@snippet ex_data_source.c WT_DATA_SOURCE error message

@section custom_ds_config Configuration strings

@subsection custom_ds_config_parsing Parsing configuration strings

Configuration information is provided to each WT_DATA_SOURCE method as an
argument.  This configuration information can be parsed using the
WT_EXTENSION_API::config method, and is returned in a WT_CONFIG_ITEM
structure.

For example, the \c append, \c overwrite \c key_format and \c value_format
configuration strings may be required for the WT_DATA_SOURCE::open_cursor
method to correctly configure itself.

A \c key_format value of "r" indicates the data source is being configured
to use record numbers as keys.  In this case initialized WT_CURSOR methods
must take their key value from the cursor's \c recno field, and not the
cursor's \c key field.  (It is not required that record numbers be supported
by a custom data source, the WT_DATA_SOURCE::open_cursor method can return
an error if an application attempts to configure a data source with a
\c key_format of "r".)

The WT_DATA_SOURCE::open_cursor method might retrieve the boolean or
integer value of a configuration string as follows:

@snippet ex_data_source.c WT_EXTENSION_CONFIG boolean
@snippet ex_data_source.c WT_EXTENSION_CONFIG integer

The WT_DATA_SOURCE::open_cursor method might retrieve the string value
of a configuration string as follows:

@snippet ex_data_source.c WT_EXTENSION config_get

@subsection custom_ds_config_add Creating data-specific configuration strings

Applications can add their own configuration strings to WiredTiger
methods using WT_CONNECTION::configure_method.

WT_CONNECTION::configure_method takes the following arguments:

- the method being extended, where the name is the concatenation of the handle
name, a period and the method name.  For example, \c "WT_SESSION.create"
would add new configuration strings to the WT_SESSION::create method,
and \c "WT_SESSION.open_cursor" would add the configuration string to the
WT_SESSION::open_cursor method.

- the object type of the data source being extended.  For example, \c "table:"
would extend the configuration arguments for table objects, and \c "my_data:"
could be used to extend the configuration arguments for a data source with
URIs beginning with the "my_data" prefix.  A NULL value for the object type
implies all object types.

- the additional configuration string, which consists of the name of the
configuration string and an optional default value.  The default value
is specified by appending an equals sign and a value.  For example, for
a new configuration string with the name \c "device", specifying either
\c "device=/path" or \c "device=35" would configure the default values.

- the type of the additional configuration, which must one of \c "boolean",
\c "int", \c "list" or \c "string".

- value checking information for the additional configuration, or NULL if
no checking information is provided.

For example, an application might add new boolean, integer, list or
string type configuration strings as follows:

@snippet ex_data_source.c WT_DATA_SOURCE configure boolean
@snippet ex_data_source.c WT_DATA_SOURCE configure integer
@snippet ex_data_source.c WT_DATA_SOURCE configure list
@snippet ex_data_source.c WT_DATA_SOURCE configure string

Once these additional configuration calls have returned, application calls to
the WT_SESSION::open_cursor method could then include configuration strings
such as \c my_boolean=false, or \c my_integer=37, or \c my_source=/home.

Additional checking information can be provided for \c int, \c list or
\c string type configuration strings.

For integers, either or both of a maximum or minimum value can be
provided, so an error will result if the application attempts to set the
value outside of the acceptable range:

@snippet ex_data_source.c WT_DATA_SOURCE configure integer with checking

For lists and strings, a set of valid choices can also be provided, so
an error will result if the application attempts to set the value to a
string not listed as a valid choice:

@snippet ex_data_source.c WT_DATA_SOURCE configure string with checking
@snippet ex_data_source.c WT_DATA_SOURCE configure list with checking

@section custom_ds_cursor_collator WT_COLLATOR

Custom data sources do not support custom ordering of records, and
attempting to create a custom data source with a collator configured
will fail.

@section custom_data_source_cursor_serialize Serialization

WiredTiger does not serialize calls to the WT_DATA_SOURCE methods or to
cursor methods configured by WT_DATA_SOURCE::open_cursor, and the
methods may be called from multiple threads concurrently.  It is the
responsibility of the implementation to protect any shared data.  For
example, object operations such as WT_DATA_SOURCE::drop might not be
permitted while there are open cursors for the WT_DATA_SOURCE object.

*/
