DODB: basic usage and basic indexes.
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@ -14,6 +14,13 @@
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.QE
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..
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.
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.de COMMAND
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.I \\$*
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..
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.de DIRECTORY
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.I \\$*
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..
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.
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. \" The document starts here.
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.
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.TITLE Document Oriented DataBase (DODB)
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@ -154,6 +161,8 @@ DODB is a hash table.
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The key of the hash is an auto-incremented number and the value is the stored data.
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The following section will explain how to use DODB for basic cases including the few added mechanisms to speed-up searches.
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Also, the file-system representation of the data will be presented since it enables easy off-application searches.
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.
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.
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.SS Before starting: the example database
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First things first, the following code is the structure used in the rest of the document to present the different aspects of DODB.
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This is a simple object
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@ -166,6 +175,8 @@ class Car
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property keywords : Array(String)
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end
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.SOURCE
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.
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.
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.SS DODB basic usage
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Let's create a DODB database for our cars.
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.SOURCE Ruby ps=10
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@ -210,36 +221,138 @@ The file "0000000000" contains the following:
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]
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}
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.SOURCE
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The car is serialized as expected in the file
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.I 0000000000 .
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.QE
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.de FUNCTION_CALL
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.I \\$*
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..
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.
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.
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Next step, to retrieve, to modify or to delete a value, its key will be required.
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.
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.QP
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.SOURCE Ruby ps=10
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# Get a value based on its key.
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db[key]
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# Update a value based on its key.
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db[key] = new_value
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# Delete a value based on its key.
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db.delete 0
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.SOURCE
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.QE
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.
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The function
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.FUNCTION_CALL each_with_index
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lists the entries with their keys.
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.
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.QP
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.SOURCE Ruby ps=10
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db.each_with_index do |value, key|
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puts "#{key}: #{value}"
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end
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.SOURCE
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.QE
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Of course, browsing the entire database to find a value (or its key) is a waste of resources and isn't practical for any non-trivial database.
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That is when indexes come into play.
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.
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.
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.SS Indexes
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Database entries can be
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Entries can be
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.I indexed
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based on their attributes.
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There are currently three main ways to search a value by its attributes: basic indexes, partitions and tags.
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.SSS Basic indexes (1 to 1 relation)
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There are currently three main ways to search for a value by its attributes: basic indexes, partitions and tags.
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.
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.SSS Basic indexes (1 to 1 relations)
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Basic indexes represent one-to-one relations, such as an index in SQL.
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For example, in a database of
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.I cars ,
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each car can have a dedicted (unique) name.
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In the Car database, each car has a dedicated (unique) name.
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This
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.I name
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attribute can be used to speed-up searches.
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On the file-system, this will be translated as this:
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.QP
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.SOURCE Ruby ps=10
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# Create an index based on the "name" attribute of the cars.
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cars_by_name = cars.new_index "name", do |car|
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car.name
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end
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.SOURCE
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Once the index has been created, every added or modified entry in the database will be indexed.
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.QE
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.
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Adding an index (basic index, partition or tag) provides an object used to manipulate the database based on this index.
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.QP
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.SOURCE Ruby ps=10
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# Retrieve the car named "Corvet".
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corvet = cars_by_name.get? "Corvet"
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# Modify the car named "Corvet".
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new_car = Car.new "Corvet", "green", ["eco-friendly"]
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cars_by_name.update "Corvet", new_car
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# In case the index hasn't changed (the name attribute in this example),
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# the update can be even simpler.
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new_car = Car.new "Corvet", "green", ["eco-friendly"]
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cars_by_name.update new_car
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# Delete the car named "Corvet".
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cars_by_name.delete "Corvet"
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.SOURCE
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A car can now be searched, modified or deleted based on its name.
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.QE
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.
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.
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On the file-system, indexes are represented as symbolic links.
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.TREE1
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storage
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+-- data
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| `-- 0000000000
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| `-- 0000000000 <- the car named "Corvet"
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`-- indexes
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`-- by_name
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`-- Ford C-MAX -> ../../data/0000000000
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`-- Corvet -> ../../data/0000000000
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.TREE2
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As shown, the file "Ford C-MAX" is a symbolic link to a data file.
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.QP
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As shown, the file "Corvet" is a symbolic link to a data file.
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The name of the symlink file has been extracted from the value itself, enabling to list all the cars and their names with a simple
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.UL ls
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.COMMAND ls
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in the
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.I storage/indexes/by_name/
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.DIRECTORY storage/indexes/by_name/
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directory.
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.TBD
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.QE
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.
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The basic indexes as shown in this section already give a taste of what is possible to do with DODB.
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The following indexes will cover some other usual cases.
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.
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.SSS Partitions (1 to n relations)
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An attribute can have a value that is shared by other entries in the database, such as the
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.I color
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attribute in our cars.
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.
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.TREE1
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db-cars
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+-- data
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| +-- 0000000000 <- this car is blue
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| `-- 0000000001 <- this car is red
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| ...
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`-- partitions
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`-- by_color
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+-- blue
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`-- 0000000000 -> 0000000000
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`-- red
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`-- 0000000001 -> 0000000001
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.TREE2
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.QP
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Listing all the blue cars is simple as a
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.COMMAND ls
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in the
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.DIRECTORY db-cars/partitions/by_color/blue
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directory!
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.QE
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.
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.SSS Tags (n to n relations)
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Tags are basically partitions but the attribute can have multiple values.
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.
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.SECTION A few more options
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.TBD
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.SECTION Limits of DODB
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@ -292,6 +405,7 @@ class Car
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end
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.SOURCE
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.
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.
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.SS Basic indexes (1 to 1 relations)
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.LP
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An index enables to match a single value based on a small string.
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