---
title: Metakit
---
Metakit Extension for Jim Tcl
=============================
OVERVIEW
--------
The mk extension provides an interface to the Metakit small-footprint
embeddable database library (<http://equi4.com/metakit/>). The underlying
library is efficient at manipulating not-so-large amounts of data and takes a
different approach to composing database operations than common SQL-based
relational databases.
Both the Metakit core library and the mk package can be linked either
statically or dynamically and loaded using
package require mk
CREATING A DATABASE
-------------------
A database (called a "storage" in Metakit terms) may either reside totally in
memory or be backed by a file. To open or create a database, call the
`storage` command with an optional filename parameter:
set db [storage test.mk]
The returned handle can be used as a command name to access the database. When
you are done, execute the `close` method, that is, run
$db close
A lost handle won't be found by GC but will be closed when the interpreter
exits. Note that by default Metakit will only record changes to the database
when you close the handle. Use the `commit` method to record the current
state of the database to disk.
CREATING VIEWS
--------------
*Views* in Metakit are what is called "tables" in conventional databases. A view
may several typed *properties*, or columns, and contains homogeneous *rows*, or
records. New properties may be added to a view as needed; however, new properties
are not stored in the database file by default. The structure method specifies
the stored properties of a view, creating a new view or restructuring an old one
as needed:
$db structure viewName description
The view description must be a list of form `{propName type propName type ...}`.
The supported property types include:
`string`
: A NULL-terminated string, stored as an array of bytes (without any encoding
assumptions).
`binary`
: **Not yet supported by the `mk` extension.**
Blob of binary data that may contain embedded NULLs (zero bytes). Stored
as-is. This is more efficient than `string` when storing large blocks of
data (e.g. images) and will adjust the storage strategy as needed.
`integer`
: An signed integer value occupying a maximum of 32 bits. If all values
stored in a column can fit in a smaller range (16, 8, or even 4 or 2 bits),
they are packed automatically.
`long`
: Like `integer`, but is required to fit into 64 bits.
`float` and `double`
: 32-bit and 64-bit IEEE floating-point values respectively.
`subview`
: This type is not usually specified directly; instead, a structure
description of a nested view is given. `subview` properties store complete
views as their value, creating hierarchical data structures. When retrieved
from a view, a value of a subview property is a normal view handle.
Without a `description` parameter, the `structure` method returns the current
structure of the named view; without any parameters, it returns a dictionary
containing structure descriptions of all views stored in the database.
After specifying the properties you expect to see in the view, call
[$db view $viewName] as viewHandle
to obtain a view handle. These handles are also commands, but are
garbage-collected and also destroy themselves after a single method call; the
`as viewHandle` call assigns the view handle to the specified variable and also
tells the view not to destroy itself until all the references to it are gone.
View handles may also be made permanent by giving them a global command name,
e.g.
rename [$db view data] .db.data
However, such view handles are not managed automatically at all and must be
destroyed using the `destroy` method, or by renaming them to `""`.
MANIPULATING DATA
-----------------
The value of a particular property is obtained using
cursor get $cur propName
where `$cur` is a string of form `viewHandle!index`. Row indices are zero-based
and may also be specified relative to the last row of the view using the
`end[+-]integer` notation.
A dictionary containing all property name and value pairs can be retrieved by
omitting the `propName` argument:
cursor get $cur
Setting property values is also performed either individually, using
cursor set $cur propName value ?propName value ...?
or via a dictionary with
cursor set $cur dictValue
In the first form of the command, property names may also be preceded by a
-_typeName_ option. In this case, a new property of the specified type will be
created if it doesn't already exist; note that this will cause *all* the rows
in the view to have the property (but see **A NOTE ON NULL** below).
If the row index points after the end of the view, an appropriate number of
fresh rows will be inserted first. So, for example, you can use `end+1`
to append a new row. (Note that you then have to set it all at once, though.)
The total number of rows can be obtained using
$viewHandle size
and set manually with
$viewHandle resize newSize
For example, you can use `$viewHandle resize 0` to clear a view.
INSERT AND REMOVE
-----------------
New rows may also be inserted at an arbitrary position in a view with
cursor insert $cur ?count?
This will insert _count_ fresh rows into the view so that _$cur_ points to
the first one. The inverse of this operation is
cursor remove $cur ?count?
COMPOSING VIEWS
---------------
The real power of Metakit lies in the way existing views are combined to create
new ones to obtain a particular perspective on the stored data. A single
operation takes one or more views and possibly additional options and produces a
new view, usually tracking notifications to the underlying views and sometimes
even supporting modification.
Binary operations are left-biased when there are conflicting property values;
that is, they always prefer the values from the left view.
### Unary operations ###
*view* `unique`
: Derived view with duplicate rows removed.
*view* `sort` *crit ?crit ...?*
: Derived view sorted on the specified criteria, in order. A single _crit_
is either a property name or a property name preceded by a dash; the latter
specifies that the sorting is to be performed in reverse order.
### Binary operations ###
The operations taking _set_ arguments require that the given views have no
duplicate rows. The `unique` method can be used to ensure this.
*view1* `concat` *view2*
: Vertical concatenation; that is, all the rows of _view1_ and then all rows
of _view2_.
*view1* `pair` *view2*
: Pairing, or horizontal concatenation: every row in _view1_ is matched with
a row with the same index in _view2_; the result has all the properties of
_view1_ and all the properties of _view2_.
*view1* `product` *view2*
: Cartesian product: each row in _view1_ horizontally concatenated with every
row in _view2_.
*set1* `union` *set2*
: Set union. Unlike `concat`, this operation removes duplicates from the
result. A row is in the result if it is in _set1_ **or** in _set2_.
*set1* `intersect` *set2*
: Set intersection. A row is in the result if it is in _set1_ **and** in
_set2_.
*set1* `different` *set2*
: Symmetric difference. A row is in the result if it is in _set1_ **xor** in
_set2_, that is, in _set1_ or in _set2_, but not in both.
*set1* `minus` *set2*
: Set minus. A row is in the result if it is in _set1_ **and not** in _set2_.
### Relational operations ###
*view1* `join` *view2* ?`-outer`? *prop ?prop ...?*
: Relational join on the specified properties: the rows from _view1_ and
_view2_ with all the specified properties equal are concatenated to form a
new row. If the `-outer` option is specified, the rows from _view1_ that do
not have a corresponding one in _view2_ are also left in the view, with the
properties existing only in _view2_ filled with default values.
*view* `group` *subviewName prop ?prop ...?*
: Groups the rows with all the specified properties equal; moves all the
remaining properties into a newly created subview property called
_subviewName_.
*view* `flatten` *subviewProp*
: The inverse of `group`.
### Projections and selections ###
*view* `project` *prop ?prop ...?*
: Projection: a derived view with only the specified properties left.
*view* `without` *prop ?prop ...?*
: The opposite of `project`: a derived view with the specified properties
removed.
*view* `range` *start end ?step?*
A slice or a segment of _view_: rows at _start_, _start+step_, and so on,
until the row number becomes larger than _end_. The usual `end[+-]integer`
notation is supported, but the indices don't change if the underlying view
is resized.
**(!) select etc. should go here**
### Search and storage optimization ###
*view* `blocked`
: Invokes an optimization designed for storing large amounts of data. _view_
must have a single subview property called `_B` with the desired structure
inside. This additional level of indirection is used by `blocked` to create
a view that looks like a usual one, but can store much more data
efficiently. As a result, indexing into the view becomes a bit slower. Once
this method is invoked, all access to _view_ must go through the returned
view.
*view* `ordered` *prop ?prop ...?*
: Does not transform the structure of the view in any way, but signals that
the view should be considered ordered on a unique key consisting of the
specified properties, enabling some optimizations. Note that duplicate keys
are not allowed in an ordered view.
**(!) TODO: hash, indexed(?) -- these make no sense until searches are implemented**
### Pipelines ###
Because constructs like `[[view op1 ...] op2 ...] op3 ...` tend to be common in
programs using Metakit, a shorthand syntax is introduced: such expressions may
also be written as `view op1 ... | op2 ... | op3 ...`.
Note though that this syntax is not in any way magically wired into the
interpreter: it is understood only by the view handles and the two commands that
can possibly return a view: `$db view` and `cursor get`. If you want to support
this syntax in Tcl procedures, you'll need to do this yourself, or you may want
to create a custom view method and have the view handle work out the syntax for
you (see **USER-DEFINED METHODS** below).
OTHER VIEW METHODS
------------------
*view* `copy`
: Creates a copy of view with the same data.
*view* `clone`
: Creates a view with the same structure, but no data.
*view* `pin`
: Specifies that the view should not be destroyed after a single method call.
Returns _view_.
*view* `as` *varName*
: In addition to the actions performed by `pin`, assigns the view handle to
the variable named varName in the caller's scope.
*view* `properties`
: Returns the names of all properties in the view.
*view* `type` *prop*
: Returns the type of the specified property.
A NOTE ON NULL
--------------
Note that Metakit does not have a special `NULL` value like conventional
relational databases do. Instead, it defines _default_ property values: `""` for
`string` and `binary` types, `0` for all numeric types and a view with no rows
for subviews. These defaults are used when a fresh row is inserted and when
a new property is added to the view to fill in the missing values.
USER-DEFINED METHODS
--------------------
The storage and view handles support custom methods defined in Tcl: to define
_methodName_ on every storage or view handle, create a procedure called
{`mk.storage` *methodName*} or {`mk.view` *methodName*} respectively. These
procedures will receive the handle as the first argument and all the remaining
arguments. Remember to `pin` the view handle in view methods if you call more
than one method of it!
Custom `cursor` subcommands may also be defined by creating a procedure called
{`cursor` *methodName*}. These receive all the arguments without any
modifications.