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add simpler example
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‎kitchen-sink/resource-to-sequence/resource-to-sequence.asciidoc‎

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@@ -34,89 +34,28 @@ The initial position depends on the context that this pattern is used in, it can
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----
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You can explore the usage of this recipe by going through a simple database paging example.
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To run the example you'll need to use leinigen and add the following dependencies to your project.clj file.
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1. [org.clojure/java.jdbc "0.3.0-alpha4"]
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2. [org.hsqldb/hsqldb "2.3.0"]
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Some helper functions and setup are required, paste the below code into your repl.
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You can explore the usage of this recipe by going through a simple file reading example.
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[source,clojure]
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----
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(use 'clojure.pprint)
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(require 'clojure.java.jdbc)
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(defn load-example []
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(def spec {:classname "org.hsqldb.jdbcDriver"
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:subprotocol "hsqldb"
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:subname "mem;sql.syntax_ora=true;create=true"
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:user "sa"
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:password ""
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})
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;create the database table
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(clojure.java.jdbc/with-connection spec
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(clojure.java.jdbc/transaction
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(try
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(do
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(clojure.java.jdbc/create-table
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"users"
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[:id "BIGINT" "NOT NULL" "PRIMARY KEY"]
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[:name "VARCHAR(20)" "NOT NULL"]
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[:age "BIGINT" "NOT NULL"])
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(clojure.java.jdbc/do-commands "CREATE INDEX users_index1 ON users(id,name)"))
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(catch Exception e (prn " ignore errors") ))))
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;truncate all previous data
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(clojure.java.jdbc/with-connection spec
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(clojure.java.jdbc/transaction (clojure.java.jdbc/do-commands "truncate table users")))
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;insert 1000 records
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(clojure.java.jdbc/with-connection spec
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(clojure.java.jdbc/transaction
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(dotimes [id 1000]
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(clojure.java.jdbc/insert-values
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:users
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[:id :name :age]
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[id (str id "-name") 20]))))
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;helper function that does a query
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(defn do-query [q max]
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(clojure.java.jdbc/with-connection spec
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(clojure.java.jdbc/transaction
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(clojure.java.jdbc/with-query-results rs [q]
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(vec (take max rs))))))
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;the db state with helper closures
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{:spec spec
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:query (fn [from max]
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(clojure.java.jdbc/with-connection spec
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(clojure.java.jdbc/transaction
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(clojure.java.jdbc/with-query-results rs [
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(str "select * from (select id,name,age, ROWNUM() rnum from (select id,name,age from users order by id asc) a where ROWNUM() <= "
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(+ from max)
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") WHERE rnum >= " from)]
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(vec (take max rs))
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))))
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})
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----
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The aim of this example is to show how you can use this recipe to select from the users table as if it was a single sequence without
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having to care about closing connections when pulling the data.
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[source,clojure]
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----
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(use 'clojure.pprint)
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"Methods that will build and read from a RandomAccessFile"
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(let [file "testfile.txt"
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get-bytes (fn [pos max]
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;reads the file from the line position pos
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(with-open [raf (java.io.RandomAccessFile. file "r")]
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(if (> pos 0) (.seek raf pos))
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(doall (take max (repeatedly #(.readByte raf))))))
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]
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(with-open [writer (clojure.java.io/writer file)]
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(doseq [x (range 1000) ] (.write writer x)))
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{:file file :bytes get-bytes} ))
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(defn buffered-select [f-select init-pos]
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"Creates a lazy sequence of messages for this datasource"
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"Creates a lazy sequence from f-select"
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(letfn [
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(m-seq [buff pos]
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(if-let [buff2 (if (empty? buff) (f-select pos) buff)]
@@ -129,49 +68,12 @@ having to care about closing connections when pulling the data.
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;normally you'd make the max argument much bigger for performance reasons,
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;here its set to 5 to show how connecting and quering is done.
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(defn select [pos] ((:query db) pos 5 ) )
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;the start pos is 1 because in sql the position 0 is 1.
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(def users (buffered-select select 1))
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(pprint (take 20 users))
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;; -> {:rnum 20, :age 20, :name "25-name", :id 25})
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;; -> ({:rnum 1, :age 20, :name "0-name", :id 0}
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;; -> {:rnum 2, :age 20, :name "1-name", :id 1}
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;; -> {:rnum 3, :age 20, :name "2-name", :id 2}
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;; -> {:rnum 4, :age 20, :name "3-name", :id 3}
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;; -> {:rnum 5, :age 20, :name "4-name", :id 4}
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;; -> {:rnum 6, :age 20, :name "5-name", :id 5}
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;; -> {:rnum 7, :age 20, :name "6-name", :id 6}
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;; -> {:rnum 8, :age 20, :name "7-name", :id 7}
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;; -> {:rnum 9, :age 20, :name "8-name", :id 8}
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;; -> {:rnum 10, :age 20, :name "9-name", :id 9}
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;; -> {:rnum 11, :age 20, :name "10-name", :id 10}
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;; -> {:rnum 12, :age 20, :name "11-name", :id 11}
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;; -> {:rnum 13, :age 20, :name "12-name", :id 12}
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;; -> {:rnum 14, :age 20, :name "13-name", :id 13}
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;; -> {:rnum 15, :age 20, :name "14-name", :id 14}
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;; -> {:rnum 16, :age 20, :name "15-name", :id 15}
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;; -> {:rnum 17, :age 20, :name "16-name", :id 16}
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;; -> {:rnum 18, :age 20, :name "17-name", :id 17}
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;; -> {:rnum 19, :age 20, :name "18-name", :id 18}
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;; -> {:rnum 20, :age 20, :name "19-name", :id 19})
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;print the name of the first 30 users
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(pprint (take 10 (map :name users)))
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;; -> ("0-name"
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;; -> "1-name"
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;; -> "2-name"
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;; -> "3-name"
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;; -> "4-name"
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;; -> "5-name"
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;; -> "6-name"
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;; -> "7-name"
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;; -> "8-name"
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;; -> "9-name")
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(defn select [pos] ((:bytes db) pos 5 ) )
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(def bts (buffered-select select 0))
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(pprint (take 20 bts))
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;; -> (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19)
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----
@@ -180,23 +82,12 @@ having to care about closing connections when pulling the data.
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===== Discussion
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The solution showed is a simple pattern (monad) with simple constructs, and its usage applies directly to database queries and network io,
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and more generally to all resources that are accessed using a connection.
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and more generally to all resources that are accessed using a connection or resource interface (as shown in the example).
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The select function connects, queries and closes a connection, then returns a sequence. The sequence can then be consumed by the using code
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and after the buffer has been consumed the select function is called again. To the user of the function it appears as if the sequence is
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The select function connects (opens a file), queries (sees to the position and reads max amount of bytes) and closes a connection (closes the file), then returns a sequence.
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The sequence can then be consumed by the using code and after the buffer has been consumed the select function is called again. To the user of the function it appears as if the sequence is
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one huge sequence over which higher order functions like map filter partition can be applied.
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The pattern also helps unroll the typical with-open pattern; where work must be done inside the scope of another function and cannot be pulled, to something more pull orientated and functional.
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The pattern also helps unroll the typical pattern:
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[source,clojure]
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----
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(with-open [resource get-resource] (do-work-on-resource resource) )
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----
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where work must be done inside the scope of another function and cannot be pulled, to something more pull orientated and functional:
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[source,clojure]
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----
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(do-work-unit (get-resource))
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----

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