# Virtual methods for providers

Database providers usually (that is except for virtual providers explained later) subclass the GdaServerProvider class and implement at least the mandatory virtual methods.

Virtual providers are database providers when the database engine accessed does not support SQL (or supports it poorly), such as Berkeley databases, MDB, or LDAP. These provider's implementation's design is to create GdaDataModel data model objects and make each of them appear as a named table once the connection is opened. For example the MDB provider creates a read-only data model for each table in an MDB file and make it appear using the original table name used in MS Access (the tables cannot be modified but it is possible to use all SQLite's SQL to make SELECT queries).

Virtual providers inherit the GdaVproviderDataModel class and not the GdaServerProvider as "normal" providers do, and the number of virtual methods to implement is very limited: only the get_name(), get_version(), get_server_version(), and open_connection() should be implemented, optionnally the close_connection() can also be implemented.

## Synchronous / asynchronous mode

Since version 6, all the provider's code is executed in a GdaWorker's worker thread and will not block the user thread (refer to gda_connection_set_main_context()'s documentation). Normally each connection is associated with such a worker thread and any provider code for that connection will be executed in the context of that thread.

The database provider can however decide, by the way the create_worker() is implemented, if each connection can have its own GdaWorker object, or if all the connections share the same object (in which case any provider code will only be called within the context of that GdaWorker's working thread).

## Multi threaded environment

Since version 6, the connections are fully thread safe (i.e. can be created and used by any thread) for any kind of connection.

FIXME: document object which implement the GdaLockable and objects which don't (and are not thread safe).

Each time a GdaConnection is passed as an argument of any virtual function implemented by the provider, the connection will have been locked, so no other thread can modify it.

Note that any of the provider specific code will only ever be executed within the worker thread of a GdaWorker object created by create_worker(), except, of course, for that method's code which will be executed from within any thread. Also note that no other code than the provider's code will ever be executed from within the worker thread (and some code from the GdaServerProvider object itself to do the binding).

### create_worker() - mandatory

Regarding thread safety, each database API has one of the following behaviours:

1. the API is enough thread safe to allow the API's objects related to a connection to be used by a thread while the API's objects related to another connection can be used by another thread. In this case, each connection is used from its own "private" thread (and thus a GdaWorker object per connection)

2. the API is not enough thread safe to be in the 1st cathegory. However, it supports being used from several threads providing that at any time, all the API's objects are used from only one thread. In this case it is possible for example at one point to use the API from a thread A, and at a later time from thread B (in case thread A has finished for example). In this case all the connections are used from a single thread (and thus a single GdaWorker object).

3. the API is not thread safe, and the thread which first uses (or initializes) the API must be the one from which all the API objects are manipulated. In this case, the thread which first uses the API must be the only one which ever uses the API and thus only a single GdaWorker object must ever exist for the database provider.

This method creates GdaWorker objects when the common database provider implementation needs to have a GdaWorker object.

If this function is not implemented, the database provider considers that the API is thread safe enough to have a GdaWorker for each connection (1st cathegory above).

## Methods - provider's information

### get_name() - mandatory

This method returns the name of the provider; it should only contain alphanumeric characters.

### get_version() - mandatory

This method returns the version of the provider (providers built as part of &LIBGDA; have the same version as &LIBGDA;).

### get_server_version() - mandatory

This method returns the version of the database server to which a connection is opened. The connection argument will always be non NULL and in an opened state.

### supports_feature() - mandatory

This method allows one to ask the provider if some specific features are available. The connection argument may be NULL or not NULL but in an opened state.

## Methods - connection management

### open_connection() - mandatory

This method actually opens a connection to the database.

Any data specific to the connection (such as a database handle created by the database API when opening the connection) must be stored in a private structure (which should be defined in the `gda-<providername>.h` file as a `<Providername>ConnectionData`); this private structure's adress should be "attached" to the GdaConnection object using the gda_connection_internal_set_provider_data().

### close_connection() - mandatory

This method actually closes a connection previously opened by the open_connection() virtual method. This method is not mandatory for virtual providers.

The connection specific data for the database API can be retrieved using the gda_connection_internal_get_provider_data_error() method.

## Methods - DDL queries

This group of virtual methods is composed of virtual methods which make it possible to handle DDL (data definition) queries to be executed using descriptions of the data required for each kind of DDL query, rather than using SQL directly (because DDL SQL is usually very database specific and must thus be adapted when migrating to another database type). For more information, see the global introduction to DDL queries.

Implementing these methods is not mandatory, and virtual provider's implementation should not implement them.

### supports_operation() - optional

This virtual method tells if a particular DDL query type is supported (DDL query types are identified as GdaServerOperationType enums). The connection argument may be NULL or not NULL but in an opened state.

### create_operation() - optional

This virtual method is used to create and initialize a GdaServerOperation object. The connection argument may be NULL or not NULL but in an opened state.

### render_operation() - optional

This virtual method uses the information stored in a GdaServerOperation object to create the SQL statement(s) which would be executed if the operation was performed. The connection argument may be NULL or not NULL but in an opened state.

Note: more than one SQL statement may be returned by this method

Note: some operations don't return any SQL at all, if the operation can only be done using an API (and not through SQL)

### perform_operation() - optional

This virtual method "performs" (execute) a GdaServerOperation object. The connection argument may be NULL or not NULL but in an opened state. If this method is not defined, but the create_operation() one is defined, then the default rendering and executing is done (with the information in the GdaServerOperation object)

## Methods - transactions management

When a connection has been opened and no transaction has been started, then the provider should execute queries in auto commit mode.

### begin_transaction()

This method starts a new transaction.

### commit_transaction()

This method commits the current transaction.

### rollback_transaction()

This method cancels the current transaction.

### add_savepoint()

This method adds a save point to the current transaction, which allows for partial rollbacks within transactions.

### rollback_savepoint()

This method rolls back to a previously defined save point within the current transaction.

### delete_savepoint()

This method deletes a previously defined save point within the current transaction, it does not affect the transaction but it simply won't be possible to roll back to the savepoint afterwards.

## Methods - DML queries

This group of virtual methods is related to executing SQL queries of any kind. &LIBGDA; imposes that SQL statements be converted to one or more GdaStatement object(s), each GdaStatement object containing exactly one SQL statement (usually terminated by a semi colon). Each GdaStatement can then be prepared and/or executed individually.

Creating a GdaStatement object from some SQL is the job of a GdaSqlParser object.

### create_parser()

This method instantiates a GdaSqlParser object which is adapted to the database's specific SQL dialect. If the provider does not implement its own parser, then this method should not be implemented.

### statement_to_sql()

This method converts a GdaStatement object to its SQL representation. It should be implemented only if the default rendering is incorrect (to support some SQL specific dialect). The rendering process is decomposed into several rendering methods, and it is possible to override only some of the methods (see the PostgreSQL's implementation as an example).

### statement_prepare()

This method requests that a GdaStatement object be prepared for a future execution. It is called when gda_connection_statement_prepare() is called.

### statement_execute() - mandatory

This method actually executes a query represented by a GdaStatement object. See gda_connection_statement_execute() for more information, note that this method is also always the one called when any of the gda_connection_statement_execute*() methods are called.

A non NULL `exec_cb` parameter specifies that the user requests an asynchronous execution: the function has to return the NULL value immediately (it must not be blocking), and the `task_id` parameter must be set to contain a provider specific task ID. The `exec_cb` parameter points to a callback function (specified by the GdaConnection) which the provider has to call once the statement has been executed, using the same task ID which was defined when the statement_execute() function was called, and the `cb_data` specified when the statement_execute() function was called.

Note that if an asynchronous execution is requested, then the `stmt`, `params`, `col_types`, `last_inserted_row`, and `cb_data` parameters are supposed to exist and not be altered during the time the statement is executed (the GdaConnection ensures this) which means it's not necessary to make copies of them during the execution.

### handle_async()

This method is called by gda_connection_async_fetch_result().

This method, if implemented, is called to give the database provider a chance to execute some code in case an asynchronous statement's execution has been requested. Often providers will send some data over the network to the database server when the statement_execute() is called, and implement this virtual function as a way to get some data from the database server to see if the execution is terminated.

This function must return FALSE in case an error occurs, and TRUE if no error occurred (even if nothing was to be done).

## Methods - data representation

This group of virtual methods is related to converting some data between the three representations which are the SQL representation (to be directly included in a SQL statement), the human readable representation, and the GValue representation. For each type of data (identified as a GType type) which the provider can handle, a GdaDataHandler object is responsible of the conversions.

### get_data_handler() - optional

This method returns a GdaDataHandler for the requested data type. It should only instantiate each GdaDataHandler once and reuse it if the same request happens (the returned object will not be modified). The connection argument may be NULL or not NULL but in an opened state.

If the GType is G_TYPE_INVALID, then this method should consider the dbms_type argument; otherwise it should ignore it.

This method is called by both gda_server_provider_get_data_handler_g_type() and gda_server_provider_get_data_handler_dbms() methods.

### get_def_dbms_type()

This method returns the name of the most common (database specific) data type which has the requested type.

### escape_string()

This method adds escape characters to a string, to be able to safely add it into an SQL statement.

### unescape_string()

This method does the reverse of the escape_string() method.

## Methods - metadata

The GdaServerProviderMeta structure defines all the methods which database providers must implement for the meta data extraction feature to work. Each method is used internally by the GdaConnection object when one calls the gda_connection_update_meta_store() method (where the connection object itself computes the list and order of methods to call). Each method must update the contents of one table in the connection's associated metadata (in its associated GdaMetaStore object), each has the same "base" arguments and some have extra arguments further specifying what needs to be updated.

For each table to update, there are two methods, which differ in their name only by the fact that one is starting with an underscore '_'. The method starting with an underscore must update the whole contents of the meta data table, and the other one must accept some more parameters to refine what gets updated. There are exception to this rule (such as the "tables_views()" method which must update both the "_tables" and "_views" tables, or the for the tables which have no foreign key (no dependency) to any other table).

Also, by convetion, the arguments can't be NULL unless the argument name has the "_n" suffix. For example the signature of the "tables_views()" method has the following signature (as defined in the `gda-server-provider.h` file)

    gboolean (*tables_views) (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                              GdaMetaContext *, GError **,
                              const GValue *table_catalog, const GValue *table_schema,
                              const GValue *table_name_n);

which means that the `table_catalog` and `table_schema` arguments can't be NULL, whereas the `table_name` can be NULL (and in this case the "tables_views()" method must update the "_tables" and "_views" tables regarding all the tables which are in the specified catalog and schema.

Make sure you read the information about the meta data's database structure in the Database structure, and specifically the data types section and the SQL identifiers section.

### Important note about SQL identifiers

As mentioned in the SQL identifiers section, any SQL identifier in the meta store is represented either:

- between double quotes if the SQL identifier is case sensitive

- all in lower case if the SQL identifier is not case sensitive

For database engines which internally store case insensitive SQL identifiers in lower case (such as PostgreSQL), the meta data reported by the database engine can be used almost AS IS, but for database engines which internally store case insensitive SQL identifiers in upper case (such as Oracle), the upper case needs to be converted to lower case. Also case sensitive SQL identifiers also need to be double quoted.

To minimize the work required to implement a database provider, &LIBGDA; allows the database provider to specifiy how case insensitive SQL identifiers are represented using gda_meta_store_set_identifiers_style() and the GdaMetaStore object will perform the work itself (the default being GDA_SQL_IDENTIFIERS_LOWER_CASE in GdaSqlIdentifierStyle.

Also note that the extra arguments for each virtual method listed below, when they are present and when they represent an SQL identifier, will be represented:

- for case insensitive SQL identifiers: using all lower or all upper case (depending on the setting set using gda_meta_store_set_identifiers_style()

- for case sensitive SQL identifiers: without the double quotes, but possibily with mixed or not lower and upper characters

### Reserved SQL keywords

Every database engine reserves some keywords for its own usage or because they are part of the SQL language. Reserved keywords can be used as SQL identifiers if they are put between double quotes.

As Each database engine has its own set of reserved keywords, the database provider has to tell the GdaMetaStore object what its keywords are, which is done using gda_meta_store_set_reserved_keywords_func() and passing a function which determines if a specific string is a reserved keyword. The usage of this function is similar to the usage of the gda_meta_store_set_identifiers_style() mentioned above.

Writing a function which tests if a string is a reserved keyword is a non complicated but error prone and not optimized, in the same way as writing a parser/lexer directly, so &LIBGDA; has a tool which generates a static hash table from a list of reserved keywords, which is in the `keywords.list` (several keywords can appear on the same line, separated by spaces or commas but the last line must remain empty).

### _info()

    gboolean (*_info) (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                       GdaMetaContext *, GError **);

This method must update the contents of the "_information_schema_catalog_name" table, which must contain exactly one row describing the catalog name for the connection.

### _btypes()

    gboolean (*_btypes) (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                         GdaMetaContext *, GError **);

This method must update the contents of the "_builtin_data_types" table which lists all the database's built in data types. There is no specific parameter.

### schemata() and _schemata()

    gboolean (*_schemata) (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                           GdaMetaContext *, GError **);
    gboolean (*schemata)  (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                           GdaMetaContext *, GError **,
                           const GValue *catalog_name, const GValue *schema_name_n);

This method must update the contents of the "_schemata" table, which lists all the schemas (namespaces).

### tables_views() and _tables_views()

    gboolean (*_tables_views) (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                               GdaMetaContext *, GError **);
    gboolean (*tables_views)  (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                               GdaMetaContext *, GError **,
                               const GValue *table_catalog, const GValue *table_schema,
                               const GValue *table_name_n);

This method must update the contents of the "_tables" and "_views" tables which list all the tables and views.

### columns() and _columns()

    gboolean (*_columns) (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                          GdaMetaContext *, GError **);
    gboolean (*columns)  (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                          GdaMetaContext *, GError **,
                          const GValue *table_catalog, const GValue *table_schema,
                          const GValue *table_name);

This method must update the contents of the "_columns" table which lists all the columns of all the tables and views.

### constraints_tab() and _constraints_tab()

    gboolean (*_constraints_tab) (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                                  GdaMetaContext *, GError **);
    gboolean (*constraints_tab)  (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                                  GdaMetaContext *, GError **,
                                  const GValue *table_catalog, const GValue *table_schema,
                                  const GValue *table_name, const GValue *constraint_name_n);

This method must update the contents of the "_table_constraints" table which lists all the constraints (primary key, foreign key, unique or check constraints) for each table.

### constraints_ref() and _constraints_ref()

    gboolean (*_constraints_ref) (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                                  GdaMetaContext *, GError **);
    gboolean (*constraints_ref)  (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                                  GdaMetaContext *, GError **,
                                  const GValue *table_catalog, const GValue *table_schema,
                                  const GValue *table_name, const GValue *constraint_name);

This method must update the contents of the "_referential_constraints" table which lists all the referential constraints (which are also listed in the "_table_constraints" table).

### key_columns() and _key_columns()

    gboolean (*_key_columns) (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                              GdaMetaContext *, GError **);
    gboolean (*key_columns)  (GdaServerProvider *, GdaConnection *, GdaMetaStore *,
                              GdaMetaContext *, GError **,
                              const GValue *table_catalog, const GValue *table_schema,
                              const GValue *table_name, const GValue *constraint_name);

This method must update the contents of the "_key_column_usage" table which lists all the columns involved in each table constraint (as listed in the "_table_constraints" table).

## Methods - misc.

### create_connection()

Reserved for internal implementation.