SWI-Prolog -- The ODBC layer

Set global properties for the environment. This must be called before calling any other ODBC predicates. Permitted properties currently include

connection_pooling(+bool): If true, then enable connection pooling for the entire process. Note that due to limitations of ODBC itself, it is not possible to turn pooling off once enabled.

2.2 Connection management

The ODBC interface deals with a single ODBC environment with multiple simultaneous connections. The predicates in this section deal with connection management.

odbc_connect(+DSN, -Connection, +Options)

Create a new ODBC connection to data-source DSN and return a handle to this connection in Connection. The connection handle is either an opaque structure or an atom of the alias option is used. In addition to the options below, options applicable to odbc_set_connection/2 may be provided.

user(User): Define the user-name for the connection. This option must be present if the database uses authorization.
password(Password): Provide a password for the connection. Normally used in combination with user(User).
alias(AliasName): Use AliasName as Connection identifier, making the connection available as a global resource. A good choice is to use the DSN as alias.
open(OpenMode): If OpenMode is once (default if an alias is provided), a second call to open the same DSN simply returns the existing connection. If multiple (default if there is no alias name), a second connection to the same data-source is opened.
mars(+Bool): If true, use Microsoft SQL server 2005 mars mode. This is support for multiple concurrent statements on a connection without requiring the dynamic cursor (which incurs an astounding 20-50x slowdown of query execution!!). MARS is a new feature in SQL2k5 apparently, and only works if you use the native driver. For the non-native driver, specifying that it is enabled will have absolutely no effect.
connection_pool_mode(+Bool): Determines how a connection is chosen from a connection pool if connection pooling is on. See odbc_set_option/1 for enabling pooling. Permitted values are’strict’(Only connections that exactly match the connection options in the call and the connection attributes set by the application are reused. This is the default) and’relaxed’(Connections with matching connection string keywords can be used. Keywords must match, but not all connection attributes must match.)
odbc_version(+Atom): Select the version of the ODBC connection. Default is '3.0'. The other supported value is '2.0'.

The following example connects to the WordNet^{1An
SQL version of WordNet is available from http://wordnet2sql.infocity.cjb.net/} [1] database, using the connection alias wordnet and opening the connection only once:

open_wordnet :-
        odbc_connect('WordNet', _,
                     [ user(jan),
                       password(xxx),
                       alias(wordnet),
                       open(once)
                     ]).

odbc_driver_connect(+DriverString, -Connection, +Options)

Connects to a database using SQLDriverConnect(). This API allows for driver-specific additional options. DriverString is passed without checking. Options should not include user and password.

Whenever possible, applications should use odbc_connect/3. If you need this predicate, please check the documentation for SQLDriverConnect() and the documentation of your driver.^{bugFacilities
to deal with prompted completion of the driver options are not yet
implemented.}

odbc_disconnect(+Connection)

Close the given Connection. This destroys the connection alias or, if there is no alias, makes further use of the Connection handle illegal.

odbc_current_connection(?Connection, ?DSN)

Enumerate the existing ODBC connections.

odbc_set_connection(+Connection, +Option)

Set options on an existing connection. All options defined here may also be specified with odbc_connect/2 in the option-list. Defined options are:

access_mode(Mode)

If read, tell the driver we only access the database in read mode. If update (default), tell the driver we may execute update commands.

auto_commit(bool)

If true (default), each update statement is committed immediately. If false, an update statement starts a transaction that can be committed or rolled-back. See section 2.4 for details on transaction management.

cursor_type(CursorType)

I haven't found a good description of what this does, but setting it to dynamic makes it possible to have multiple active statements on the same connection with Microsoft SQL server. Other values are static, forwards_only and keyset_driven.

encoding(+Encoding)

Define the encoding used to communicate to the driver. Defined values are given below. The default on MS-Windows is unicode while on other platforms it is utf8. Below, the *A() functions refer to the‘ansi’ODBC functions that exchange bytes and the *W() functions refer to the‘unicode’ODBC functions that exchange UCS-2 characters.

iso_latin_1: Communicate using the *A() functions and pass bytes untranslated.
locale: Communicate using the *A() functions and translated between Prolog Unicode characters and their (possibly) multibyte representation in the current locale.
utf8: Communicate using the *A() functions and translated between Prolog Unicode characters and their UTF-8 encoding.
unicode: Communicate using the *W() functions.

silent(Bool)

If true (default false), statements returning SQL_SUCCESS_WITH_INFO succeed without printing the info. See also section 2.8.1.

null(NullSpecifier)

Defines how the SQL constant NULL is represented. Without specification, the default is the atom $null$ . NullSpecifier is an arbitrary Prolog term, though the implementation is optimised for using an unbound variable, atom and functor with one unbound variable. The representation null(_) is a commonly used alternative.

The specified default holds for all statements executed on this connection. Changing the connection default does not affect already prepared or running statements. The null-value can also be specified at the statement level. See the option list of odbc_query/4.

wide_column_threshold(+Length)

If the width of a column exceeds Length, use the API SQLGetData() to get the value incrementally rather than using a (large) buffer allocated with the statement. The default is to use this alternate interface for columns larger than 1024 bytes. There are two cases for using this option. In time critical applications with wide columns it may provide better performance at the cost of a higher memory usage and to work around bugs in SQLGetData(). The latter applies to Microsoft SQL Server fetching the definition of a view.

odbc_get_connection(+Connection, ?Property)

Query for properties of the connection. Property is a term of the format Name(Value). If Property is unbound all defined properties are enumerated on backtracking. Currently the following properties are defined.

database_name(Atom): Name of the database associated to the connection.
dbms_name(Name): Name of the database engine. This constant can be used to identify the engine.
dbms_version(Atom): Version identifier from the database engine.
driver_name(Name): ODBC Dynamic Link Library providing the interface between ODBC and the database.
driver_odbc_version(Atom): ODBC version supported by the driver.
driver_version(Atom): The drivers version identifier.
active_statements(Integer): Maximum number of statements that can be active at the same time on this connection. Returns 0 (zero) if this is unlimited.^{2Microsoft
SQL server can have multiple active statements after setting the option cursor_type
to dynamic. See odbc_set_connection/2.}

odbc_data_source(?DSN, ?Description)

Query the defined data sources. It is not required to have any open connections before calling this predicate. DSN is the name of the data source as required by odbc_connect/3. Description is the name of the driver. The driver name may be used to tailor the SQL statements used on the database. Unfortunately this name depends on the local installing details and is therefore not universally useful.

2.3 Running SQL queries

ODBC distinguishes between direct execution of literal SQL strings and parameterized execution of SQL strings. The first is a simple practical solution for infrequent calls (such as creating a table), while parameterized execution allows the driver and database to precompile the query and store the optimized code, making it suitable for time-critical operations. In addition, it allows for passing parameters without going through SQL-syntax and thus avoiding the need for quoting.

2.3.1 One-time invocation

odbc_query(+Connection, +SQL, -RowOrAffected)

Same as odbc_query/4 using [] for Options.

odbc_query(+Connection, +SQL, -RowOrAffected, +Options)

Fire an SQL query on the database represented by Connection. SQL is any valid SQL statement. SQL statements can be specified as a plain atom, string or a term of the format Format-Arguments, which is converted using format/2.

If the statement is a SELECT statement the result-set is returned in RowOrAffected. By default rows are returned one-by-one on backtracking as terms of the functor row/Arity, where Arity denotes the number of columns in the result-set. The library pre-fetches the next value to be able to close the statement and return deterministic success when returning the last row of the result-set. Using the option findall/2 (see below) the result-set is returned as a list of user-specified terms. For other statements this argument returns affected(Rows), where Rows represents the number of rows affected by the statement. If you are not interested in the number of affected rows odbc_query/2 provides a simple interface for sending SQL-statements.

Below is a small example using the connection created from odbc_connect/3. Please note that the SQL-statement does not end in the‘;’character.

lemma(Lemma) :-
        odbc_query(wordnet,
                   'SELECT (lemma) FROM word',
                   row(Lemma)).

The following example adds a name to a table with parent-relations, returning the number of rows affected by the statement. Note that the SQL quote character is the ASCII single quote and, as this SQL quote is embedded in a single quoted Prolog atom, it must be written as \' or '' (two single quotes). We use the first alternative for better visibility.

insert_child(Child, Mother, Father, Affected) :-
        odbc_query(parents,
                   'INSERT INTO parents (name,mother,father) \
                      VALUES (\'mary\', \'christine\', \'bob\')',
                   affected(Affected)).

Options defines the following options.

types(ListOfTypes)

Determine the Prolog type used to report the column-values. When omitted, default conversion as described in section 2.7 is implied. A column may specify default to use default conversion for that column. The length of the type-list must match the number of columns in the result-set.

For example, in the table word the first column is defined with the SQL type DECIMAL(6). Using this SQL-type, “001” is distinct from “1” , but using Prolog integers is a valid representation for Wordnet wordno identifiers. The following query extracts rows using Prolog integers:

?- odbc_query(wordnet,
              'select * from word', X,
              [ types([integer,default])
              ]).

X = row(1, entity) ;
X = row(2, thing) ;
...

See also section 2.7 for notes on type-conversion.

null(NullSpecifier)

Specify SQL NULL representation. See odbc_set_connection/2 for details.

source(Bool)

If true (default false), include the source-column with each result-value. With this option, each result in the row/N-term is of the format below. TableName or ColumnName may be the empty atom if the information is not available.^{3This is one possible
interface to this information. In many cases it is more efficient and
convenient to provide this information separately as it is the same for
each result-row.}

column(TableName, ColumnName, Value)

findall(Template, row(Column, ...)

Instead of returning rows on backtracking this option makes odbc_query/3 return all rows in a list and close the statement. The option is named after the Prolog findall/3 predicate, as the it makes odbc_query/3 behave as the commonly used findall/3 construct below.

lemmas(Lemmas) :-
        findall(Lemma,
                odbc_query(wordnet,
                           'select (lemma) from word',
                           row(Lemma)),
                Lemmas).

Using the findall/2 option the above can be implemented as below. The number of argument of the row term must match the number of columns in the result-set.

lemmas(Lemmas) :-
        odbc_query(wordnet,
                   'select (lemma) from word',
                   Lemmas,
                   [ findall(Lemma, row(Lemma))
                   ]).

The current implementation is incomplete. It does not allow arguments of row(...) to be instantiated. Plain instantiation can always be avoided using a proper SELECT statement. Potentially useful however would be the translation of compound terms, especially to translate date/time/timestamp structures to a format for use by the application.

wide_column_threshold(+Length)

Specify threshold column width for using SQLGetData(). See odbc_set_connection/2 for details.

odbc_query(+Connection, +SQL)

As odbc_query/3, but used for SQL-statements that should not return result-rows (i.e. all statements except for SELECT). The predicate prints a diagnostic message if the query returns a result.

2.3.2 Parameterised queries

ODBC provides for‘parameterized queries’. These are SQL queries with a ?-sign at places where parameters appear. The ODBC interface and database driver may use this to precompile the SQL-statement, giving better performance on repeated queries. This is exactly what we want if we associate Prolog predicates to database tables. This interface is defined by the following predicates:

odbc_prepare(+Connection, +SQL, +Parameters, -Statement)

As odbc_prepare/5 using [] for Options.

odbc_prepare(+Connection, +SQL, +Parameters, -Statement, +Options)

Create a statement from the given SQL (which may be a format specification as described with odbc_query/3) statement that normally has one or more parameter-indicators (?) and unify Statement with a handle to the created statement. Parameters is a list of descriptions, one for each parameter. Each parameter description is one of the following:

default

Uses the ODBC function SQLDescribeParam() to obtain information about the parameter and apply default rules. See section 2.7 for details. If the interface fails to return a type or the type is unknown to the ODBC interface a message is printed and the interface handles the type as text, which implies the user must supply an atom. The message can be suppressed using the silent(true) option of odbc_set_connection/2. An alternative mapping can be selected using the > option of this predicate described below.

SqlType(Specifier, ...)

Declare the parameter to be of type SqlType with the given specifiers. Specifiers are required for char, varchar, etc. to specify the field-width. When calling odbc_execute/[2-3], the user must supply the parameter values in the default Prolog type for this SQL type. See section 2.7 for details.

PrologType > SqlType

As above, but supply values of the given PrologType, using the type-transformation defined by the database driver. For example, if the parameter is specified as

atom > date

The use must supply an atom of the format YYYY-MM-DD rather than a term date(Year,Month,Day). This construct enhances flexibility and allows for passing values that have no proper representation in Prolog.

Variable

Interpreted as default. It unifies Variable with the PrologType > SqlType as using the types derived.^{4 The current
version does not provide the field with in SqlType. Future
versions may improve on that.} This feature is first of all intended for debugging. Using odbc_debug(1), the library prints details on the derived types.

Options defines a list of options for executing the statement. See odbc_query/4 for details. In addition, the following option is provided:

fetch(FetchType): Determine the FetchType, which is one of auto (default) to extract the result-set on backtracking or fetch to prepare the result-set to be fetched using odbc_fetch/3.

odbc_execute(+Statement, +ParameterValues, -RowOrAffected)

Execute a statement prepared with odbc_prepare/4 with the given ParameterValues and return the rows or number of affected rows as odbc_query/4. This predicate may return type_error exceptions if the provided parameter values cannot be converted to the declared types.

ODBC doesn't appear to allow for multiple cursors on the same result-set.^{5Is this right?} This would imply there can only be one active odbc_execute/3 (i.e. with a choice-point) on a prepared statement. Suppose we have a table age (name char(25), age integer) bound to the predicate age/2 we cannot write the code below without special precautions. The ODBC interface therefore creates a clone of a statement if it discovers the statement is being executed, which is discarded after the statement is finished.^{6The
code is prepared to maintain a cache of statements. Practice should tell
us whether it is worthwhile activating this.}

same_age(X, Y) :-
        age(X, AgeX),
        age(Y, AgeY),
        AgeX = AgeY.

odbc_execute(+Statement, +ParameterValues)

Like odbc_query/2, this predicate is meant to execute simple SQL statements without interest in the result.

odbc_cancel_thread(+ThreadId)

If the thread ThreadId is currently blocked inside odbc_execute/3 then interrupt it. If ThreadId is not currently executing odbc_execute/4 then odbc_cancel_thread/1 succeeds but does nothing. If ThreadId is not a valid thread ID or alias, an exception is raised.

odbc_free_statement(+Statement)

Destroy a statement prepared with odbc_prepare/4. If the statement is currently executing (i.e. odbc_execute/3 left a choice-point), the destruction is delayed until the execution terminates.

2.3.3 Fetching rows explicitely

Normally SQL queries return a result-set that is enumerated on backtracking. Using this approach a result-set is similar to a predicate holding facts. There are some cases where fetching the rows one-by-one, much like read/1 reads terms from a file is more appropriate and there are cases where only part of the result-set is to be fetched. These cases can be dealt with using odbc_fetch/3, which provides an interface to SQLFetchScroll().

As a general rule of thumb, stay away from these functions if you do not really need them. Experiment before deciding on the strategy and often you'll discover the simply backtracking approach is much easier to deal with and about as fast.

odbc_fetch(+Statement, -Row, +Option)

Fetch a row from the result-set of Statement. Statement must be created with odbc_prepare/5 using the option fetch(fetch) and be executed using odbc_execute/2. Row is unified to the fetched row or the atom end_of_file^{7This
atom was selected to emphasise the similarity to read.} after the end of the data is reached. Calling odbc_fetch/2 after all data is retrieved causes a permission-error exception. Option is one of:

next: Fetch the next row.
prior: Fetch the result-set going backwards.
first: Fetch the first row.
last: Fetch the last row.
absolute(Offset): Fetch absolute numbered row. Rows count from one.
relative(Offset): Fetch relative to the current row. relative(1) is the same as next, except that the first row extracted is row 2.
bookmark(Offset): Reserved. Bookmarks are not yet supported in this interface.

In many cases, depending on the driver and RDBMS, the cursor-type must be changed using odbc_set_connection/2 for anything different from next to work.

Here is example code each time skipping a row from a table‘test’holding a single column of integers that represent the row-number. This test was executed using unixODBC and MySQL on SuSE Linux.

fetch(Options) :-
        odbc_set_connection(test, cursor_type(static)),
        odbc_prepare(test,
                     'select (testval) from test',
                     [],
                     Statement,
                     [ fetch(fetch)
                     ]),
        odbc_execute(Statement, []),
        fetch(Statement, Options).

fetch(Statement, Options) :-
        odbc_fetch(Statement, Row, Options),
        (   Row == end_of_file
        ->  true
        ;   writeln(Row),
            fetch(Statement, Options)
        ).

odbc_close_statement(+Statement)

Closes the given statement (without freeing it). This must be used if not the whole result-set is retrieved using odbc_fetch/3.

2.3.4 Fetching data from multiple result sets

Most SQL queries return only a single result set - a list of rows. However, some queries can return more than one result set. For example,’SELECT 1; SELECT 2’is a batch query that returns a single row (1) and then a single row(2). Queries involving stored procedures can easily generate such results.

To retrieve data from a subsequent result set, odbc_next_result_set/1 can be used, but only for prepared queries which were prepared with fetch(fetch) as the fetch style in the option list.

odbc_next_result_set(+Statement)

Succeeds if there is another result set, and positions the cursor at the first row of the new result set. If there are no more result sets, the predicate fails.

fetch(Options) :-
        odbc_prepare(test,
                     'select (testval) from test; select (anotherval)
                     from some_other_table',
                     [],
                     Statement,
                     [ fetch(fetch)
                     ]),
        odbc_execute(Statement, []),
        fetch(Statement, Options).

fetch(Statement, Options) :-
        odbc_fetch(Statement, Row, Options),
        (   Row == end_of_file
        ->  (   odbc_next_result_set(Statement)
            ->  writeln(next_result_set),
                fetch(Statement, Options)
            ;   true
            )
        ;   writeln(Row),
            fetch(Statement, Options)
        ).

2.4 Transaction management

ODBC can run in two modi. By default, all update actions are immediately committed on the server. Using odbc_set_connection/2 this behaviour can be switched off, after which each SQL statement that can be inside a transaction implicitly starts a new transaction. This transaction can be ended using odbc_end_transaction/2.

odbc_end_transaction(+Connection, +Action): End the currently open transaction if there is one. Using Action commit pending updates are made permanent, using rollback they are discarded.

The ODBC documentation has many comments on transaction management and its interaction with database cursors.

2.5 Accessing the database dictionary

With this interface we do not envision the use of Prolog as a database manager. Nevertheless, elementary access to the structure of a database is required, for example to validate a database satisfies the assumptions made by the application.

odbc_current_table(+Connection, -Table)

Return on backtracking the names of all tables in the database identified by the connection.

odbc_current_table(+Connection, ?Table, ?Facet)

Enumerate properties of the tables. Defines facets are:

qualifier(Qualifier)
owner(Owner)
comment(Comment): These facets are defined by SQLTables()
arity(Arity): This facet returns the number of columns in a table.

odbc_table_column(+Connection, ?Table, ?Column)

On backtracking, enumerate all columns in all tables.

odbc_table_column(+Connection, ?Table, ?Column, ?Facet)

Provides access to the properties of the table as defined by the ODBC call SQLColumns(). Defined facets are:

table_qualifier(Qualifier)
table_owner(Owner)
table_name(Table): See odbc_current_table/3.
data_type(DataType)
type_name(TypeName)
precision(Precision)
length(Length)
scale(Scale)
radix(Radix)
nullable(Nullable)
remarks(Remarks): These facets are defined by SQLColumns()
type(Type): More prolog-friendly representation of the type properties. See section 2.7.

odbc_type(+Connection, ?TypeSpec, ?Facet)

Query the types supported by the data source. TypeSpec is either an integer type-id, the name of an ODBC SQL type or the constant all_types to enumerate all known types. This predicate calls SQLGetTypeInfo() and its facet names are derived from the specification of this ODBC function:

name(Name): Name used by the data-source. Use this in CREATE statements
data_type(DataType): Numeric identifier of the type
precision(Precision): When available, maximum precision of the type.
literal_prefix(Prefix): When available, prefix for literal representation.
literal_suffix(Suffix): When available, suffix for literal representation.
create_params(CreateParams): When available, arguments needed to create the type.
nullable(Bool): Whether the type can be NULL. May be unknown
case_sensitive(Bool): Whether values for this type are case-sensitive.
searchable(Searchable): Whether the type can be searched. Values are false, true, like_only or all_except_like.
unsigned(Bool): When available, whether the value is signed. Please note that SWI-Prolog does not provide unsigned integral values.
money(Bool): Whether the type represents money.
auto_increment(Bool): When available, whether the type can be auto-incremented.
local_name(LocalName): Name of the type in local language.
minimum_scale(MinScale): Minimum scale of the type.
maximum_scale(MaxScale): Maximum scale of the type.

odbc_table_primary_key(+Connection, +Table, ?Column)

True when Column is a primary key in Table.

odbc_table_foreign_key(+Connection, ?PkTable, ?PkCol, ?FkTable, ?FkCol)

True when PkTable/PkCol FkTable/FkCol is a foreign keys column.

2.6 Getting more information

odbc_statistics(?Key)

Get statistical data on the ODBC interface. Currently defined keys are:

statements(Created, Freed): Number of SQL statements that have been Created and Freed over all connections. Statements executed with odbc_query/[2-3] increment Created as the query is created and Freed if the query is terminated due to deterministic success, failure, cut or exception. Statements created with odbc_prepare/[4-5] are freed by odbc_free_statement/1 or due to a fatal error with the statement.

odbc_debug(+Level)

Set the verbosity-level to Level. Default is 0. Higher levels make the system print debugging messages.

2.7 Representing SQL data in Prolog

Databases have a poorly standardized but rich set of datatypes. Some have natural Prolog counterparts, some not. A complete mapping requires us to define Prolog data-types for SQL types that have no standardized Prolog counterpart (such as timestamp), the definition of a default mapping and the possibility to define an alternative mapping for a specific column. For example, many variations of the SQL DECIMAL type cannot be mapped to a Prolog integer. Nevertheless, mapping to an integer may be the proper choice for a specific application.

The Prolog/ODBC interface defines the following Prolog result types with the indicated default transformation. Different result-types can be requested using the types(TypeList) option for the odbc_query/4 and odbc_prepare/5 interfaces.

atom: Used as default for the SQL types char, varchar, longvarchar, binary, varbinary, longvarbinary, decimal and numeric. Can be used for all types.
string: SWI-Prolog extended type string. Use the type for special cases where garbage atoms must be avoided. Can be used for all types.
codes: List of character codes. Use this type if the argument must be analysed or compatibility with Prolog systems that cannot handle infinite-length atoms is desired. Can be used for all types.
integer: Used as default for the SQL types bit, tinyint, smallint and integer. Please note that SWI-Prolog integers are signed 32-bit values, where SQL allows for unsigned values as well. Can be used for the integral, and decimal types as well as the types date and timestamp, which are represented as POSIX time-stamps (seconds after Jan 1, 1970).
float: Used as default for the SQL types real, float and double. Can be used for the integral and decimal types as well as the types date and timestamp, which are represented as POSIX time-stamps (seconds after Jan 1, 1970). Representing time this way is compatible to SWI-Prologs time-stamp handling.
date: A Prolog term of the form date(Year,Month,Day) used as default for the SQL type date.
time: A Prolog term of the form time(Hour,Minute,Second) used as default for the SQL type time.
timestamp: A Prolog term of the form timestamp(Year,Month,Day,Hour,Minute,Second,Fraction) used as default for the SQL type timestamp.

2.8 Errors and warnings

ODBC operations return success, error or‘success with info’. This section explains how results from the ODBC layer are reported to Prolog.

2.8.1 ODBC messages:‘Success with info’

If an ODBC operation returns‘with info’, the info is extracted from the interface and handled to the Prolog message dispatcher print_message/2. The level of the message is informational and the term is of the form:

odbc(State, Native, Message): Here, State is the SQL-state as defined in the ODBC API, Native is the (integer) error code of the underlying data source and Message is a human readable explanation of the message.

2.8.2 ODBC errors

If an ODBC operation signals an error, it throws the exception error(odbc(State, Native, Message), _). The arguments of the odbc/3 term are explained in section 2.8.1.

In addition, the Prolog layer performs the normal tests for proper arguments and state, signaling the conventional instantiation, type, domain and resource exceptions.

2.9 ODBC implementations

There is a wealth on ODBC implementations that are completely or almost compatible to this interface. In addition, a number of databases are delivered with an ODBC compatible interface. This implies you get the portability benefits of ODBC without paying the configuration and performance price. Currently this interface is, according to the PHP documentation on this subject, provided by Adabas D, IBM DB2, Solid, and Sybase SQL Anywhere.

Transaction management: This certainly requires a high-level interface. Possibly in combination with call_cleanup/3, providing automatic rollback on failure or exception and commit on success.
High-level interface: Attaching tables to predicates, partial DataLog implementation, etc.

2 The ODBC layer

2.1 Global configuration