Simplifications and performance improvements (#200)

* Add pool_concurrency_test manual spec

Add MT connection count without Mutex

* Drop checkout_some, simpler pool_prepared statement

* Make pool statement a struct

* Drop StringKeyCache mutex

The StringKeyCache is now only used inside a connection. It's assumed that connections are not used concurrently with multiple queries.

* Drop do_close in pool statements

* Add specs and update comment

* Fix typo

spec/database_spec.cr

@@ -57,14 +57,6 @@ describe DB::Database do
     end
   end
 
-  it "should close pool statements when closing db" do
-    stmt = uninitialized DB::PoolStatement
-    with_dummy do |db|
-      stmt = db.build("query1")
-    end
-    stmt.closed?.should be_true
-  end
-
   it "should not reconnect if connection is lost and retry_attempts=0" do
     DummyDriver::DummyConnection.clear_connections
     DB.open "dummy://localhost:1027?initial_pool_size=1&max_pool_size=1&retry_attempts=0" do |db|
@@ -187,6 +179,25 @@ describe DB::Database do
     end
   end
 
+  it "should not checkout multiple connections if there is a statement error" do
+    with_dummy "dummy://localhost:1027?initial_pool_size=1&max_pool_size=10&retry_attempts=10" do |db|
+      expect_raises DB::Error do
+        db.exec("syntax error")
+      end
+      DummyDriver::DummyConnection.connections.size.should eq(1)
+    end
+  end
+
+  it "should attempt all retries if connection is lost" do
+    with_dummy "dummy://localhost:1027?initial_pool_size=1&max_pool_size=1&retry_attempts=10" do |db|
+      expect_raises DB::PoolRetryAttemptsExceeded do
+        db.exec("raise ConnectionLost")
+      end
+      # 1 initial + 10 retries
+      DummyDriver::DummyConnection.connections.size.should eq(11)
+    end
+  end
+
   describe "prepared_statements connection option" do
     it "defaults to true" do
       with_dummy "dummy://localhost:1027" do |db|
@@ -239,24 +250,6 @@ describe DB::Database do
     end
   end
 
-  describe "prepared_statements_cache connection option" do
-    it "should reuse prepared statements if true" do
-      with_dummy "dummy://localhost:1027?prepared_statements=true&prepared_statements_cache=true" do |db|
-        stmt1 = db.build("the query")
-        stmt2 = db.build("the query")
-        stmt1.object_id.should eq(stmt2.object_id)
-      end
-    end
-
-    it "should not reuse prepared statements if false" do
-      with_dummy "dummy://localhost:1027?prepared_statements=true&prepared_statements_cache=false" do |db|
-        stmt1 = db.build("the query")
-        stmt2 = db.build("the query")
-        stmt1.object_id.should_not eq(stmt2.object_id)
-      end
-    end
-  end
-
   describe "unprepared statements in pool" do
     it "creating statements should not create new connections" do
       with_dummy "dummy://localhost:1027?initial_pool_size=1" do |db|

spec/dummy_driver.cr

@@ -1,4 +1,3 @@
-require "spec"
 require "../src/db"
 
 class DummyDriver < DB::Driver
@@ -17,20 +16,37 @@ class DummyDriver < DB::Driver
   end
 
   class DummyConnection < DB::Connection
+    @@connections = [] of DummyConnection
+    @@connections_count = Atomic(Int32).new(0)
+
     def initialize(options : DB::Connection::Options)
       super(options)
       Fiber.yield
+      @@connections_count.add(1)
       @connected = true
-      @@connections ||= [] of DummyConnection
-      @@connections.not_nil! << self
+      {% unless flag?(:preview_mt) %}
+        # @@connections is only used in single-threaded mode in specs
+        # for benchmarks we want to avoid the overhead of synchronizing this array
+        @@connections << self
+      {% end %}
+    end
+
+    def self.connections_count
+      @@connections_count.get
     end
 
     def self.connections
-      @@connections.not_nil!
+      {% if flag?(:preview_mt) %}
+        raise "DummyConnection.connections is only available in single-threaded mode"
+      {% end %}
+      @@connections
     end
 
     def self.clear_connections
-      @@connections.try &.clear
+      {% if flag?(:preview_mt) %}
+        raise "DummyConnection.clear_connections is only available in single-threaded mode"
+      {% end %}
+      @@connections.clear
     end
 
     def build_prepared_statement(query) : DB::Statement
@@ -117,17 +133,31 @@ class DummyDriver < DB::Driver
   end
 
   class DummyStatement < DB::Statement
+    @@statements_count = Atomic(Int32).new(0)
+    @@statements_exec_count = Atomic(Int32).new(0)
     property params
 
     def initialize(connection, command : String, @prepared : Bool)
       @params = Hash(Int32 | String, DB::Any | Array(DB::Any)).new
       super(connection, command)
+      @@statements_count.add(1)
       raise DB::Error.new(command) if command == "syntax error"
+      raise DB::ConnectionLost.new(connection) if command == "raise ConnectionLost"
+    end
+
+    def self.statements_count
+      @@statements_count.get
+    end
+
+    def self.statements_exec_count
+      @@statements_exec_count.get
     end
 
     protected def perform_query(args : Enumerable) : DB::ResultSet
       assert_not_closed!
 
+      @@statements_exec_count.add(1)
+
       Fiber.yield
       @connection.as(DummyConnection).check
       set_params args
@@ -137,6 +167,8 @@ class DummyDriver < DB::Driver
     protected def perform_exec(args : Enumerable) : DB::ExecResult
       assert_not_closed!
 
+      @@statements_exec_count.add(1)
+
       @connection.as(DummyConnection).check
       set_params args
       raise DB::Error.new("forced exception due to query") if command == "raise"

spec/manual/pool_concurrency_test.cr

@@ -0,0 +1,68 @@
+# This file is to be executed as:
+#
+# % crystal run --release [-Dpreview_mt] ./spec/manual/pool_concurrency_test.cr -- --options="max_pool_size=5" --duration=30 --concurrency=4
+#
+#
+
+require "option_parser"
+require "../dummy_driver"
+require "../../src/db"
+
+options = ""
+duration = 3
+concurrency = 4
+
+OptionParser.parse do |parser|
+  parser.banner = "Usage: pool_concurrency_test [arguments]"
+  parser.on("-o", "--options=VALUE", "Connection string options") { |v| options = v }
+  parser.on("-d", "--duration=SECONDS", "Specifies the duration in seconds") { |v| duration = v.to_i }
+  parser.on("-c", "--concurrency=VALUE", "Specifies the concurrent requests to perform") { |v| concurrency = v.to_i }
+  parser.on("-h", "--help", "Show this help") do
+    puts parser
+    exit
+  end
+  parser.invalid_option do |flag|
+    STDERR.puts "ERROR: #{flag} is not a valid option."
+    STDERR.puts parser
+    exit(1)
+  end
+end
+
+multi_threaded = {% if flag?(:preview_mt) %} ENV["CRYSTAL_WORKERS"]?.try(&.to_i?) || 4 {% else %} false {% end %}
+release = {% if flag?(:release) %} true {% else %} false {% end %}
+
+if !release
+  puts "WARNING: This should be run in release mode."
+end
+
+db = DB.open "dummy://host?#{options}"
+
+start_time = Time.monotonic
+
+puts "Starting test for #{duration} seconds..."
+
+concurrency.times do
+  spawn do
+    loop do
+      db.scalar "1"
+      Fiber.yield
+    end
+  end
+end
+
+sleep duration.seconds
+
+end_time = Time.monotonic
+
+puts "          Options : #{options}"
+puts "   Duration (sec) : #{duration} (actual #{end_time - start_time})"
+puts "      Concurrency : #{concurrency}"
+puts "   Multi Threaded : #{multi_threaded ? "Yes (#{multi_threaded})" : "No"}"
+puts "Total Connections : #{DummyDriver::DummyConnection.connections_count}"
+puts " Total Statements : #{DummyDriver::DummyStatement.statements_count}"
+puts "    Total Queries : #{DummyDriver::DummyStatement.statements_exec_count}"
+puts " Throughput (q/s) : #{DummyDriver::DummyStatement.statements_exec_count / duration}"
+
+if !release
+  puts "WARNING: This should be run in release mode."
+end

src/db/database.cr

@@ -38,7 +38,6 @@ module DB
     @connection_options : Connection::Options
     @pool : Pool(Connection)
     @setup_connection : Connection -> Nil
-    @statements_cache = StringKeyCache(PoolPreparedStatement).new
 
     # Initialize a database with the specified options and connection factory.
     # This covers more advanced use cases that might not be supported by an URI connection string such as tunneling connection.
@@ -81,9 +80,6 @@ module DB
 
     # Closes all connection to the database.
     def close
-      @statements_cache.each_value &.close
-      @statements_cache.clear
-
       @pool.close
     end
 
@@ -99,15 +95,6 @@ module DB
 
     # :nodoc:
     def fetch_or_build_prepared_statement(query) : PoolStatement
-      if @connection_options.prepared_statements_cache
-        @statements_cache.fetch(query) { build_prepared_statement(query) }
-      else
-        build_prepared_statement(query)
-      end
-    end
-
-    # :nodoc:
-    def build_prepared_statement(query) : PoolStatement
       PoolPreparedStatement.new(self, query)
     end
 

src/db/pool.cr

@@ -158,27 +158,6 @@ module DB
       end
     end
 
-    # ```
-    # selected, is_candidate = pool.checkout_some(candidates)
-    # ```
-    # `selected` be a resource from the `candidates` list and `is_candidate` == `true`
-    # or `selected` will be a new resource and `is_candidate` == `false`
-    def checkout_some(candidates : Enumerable(WeakRef(T))) : {T, Bool}
-      sync do
-        candidates.each do |ref|
-          resource = ref.value
-          if resource && is_available?(resource)
-            @idle.delete resource
-            resource.before_checkout
-            return {resource, true}
-          end
-        end
-      end
-
-      resource = checkout
-      {resource, candidates.any? { |ref| ref.value == resource }}
-    end
-
     def release(resource : T) : Nil
       idle_pushed = false
 
@@ -227,8 +206,6 @@ module DB
           # if the connection is lost it will be closed by
           # the exception to release resources
           # we still need to remove it from the known pool.
-          # Closed connection will be evicted from statement cache
-          # in PoolPreparedStatement#clean_connections
           sync { delete(e.resource) }
         rescue e : PoolResourceRefused
           # a ConnectionRefused means a new connection

src/db/pool_prepared_statement.cr

@@ -4,75 +4,20 @@ module DB
   # The execution of the statement is retried according to the pool configuration.
   #
   # See `PoolStatement`
-  class PoolPreparedStatement < PoolStatement
-    # connections where the statement was prepared
-    @connections = Set(WeakRef(Connection)).new
-    @mutex = Mutex.new
-
+  struct PoolPreparedStatement < PoolStatement
     def initialize(db : Database, query : String)
       super
-      # Prepares a statement on some connection
-      # otherwise the preparation is delayed until the first execution.
-      # After the first initialization the connection must be released
-      # it will be checked out when executing it.
-
-      # This only happens if the db is configured to use prepared statements cache.
-      # Without that there is no reference to the already prepared statement we can
-      # take advantage of.
-      if db.prepared_statements_cache?
-        statement_with_retry &.release_connection
-      end
-
-      # TODO use a round-robin selection in the pool so multiple sequentially
-      #      initialized statements are assigned to different connections.
-    end
-
-    protected def do_close
-      @mutex.synchronize do
-        # TODO close all statements on all connections.
-        # currently statements are closed when the connection is closed.
-
-        # WHAT-IF the connection is busy? Should each statement be able to
-        # deallocate itself when the connection is free.
-        @connections.clear
-      end
     end
 
     # builds a statement over a real connection
-    # the connection is registered in `@connections`
     private def build_statement : Statement
-      clean_connections
-
-      conn, existing = @mutex.synchronize do
-        @db.checkout_some(@connections)
-      end
-
+      conn = @db.pool.checkout
       begin
-        stmt = conn.prepared.build(@query)
+        conn.prepared.build(@query)
       rescue ex
         conn.release
         raise ex
       end
-      if !existing && @db.prepared_statements_cache?
-        @mutex.synchronize do
-          @connections << WeakRef.new(conn)
-        end
-      end
-      stmt
-    end
-
-    private def clean_connections
-      return unless @db.prepared_statements_cache?
-
-      @mutex.synchronize do
-        # remove disposed or closed connections
-        @connections.each do |ref|
-          conn = ref.value
-          if !conn || conn.closed?
-            @connections.delete ref
-          end
-        end
-      end
     end
   end
 end

src/db/pool_statement.cr

@@ -3,7 +3,7 @@ module DB
   # a statement from the DB needs to be able to represent a statement in any
   # of the connections of the pool. Otherwise the user will need to deal with
   # actual connections in some point.
-  abstract class PoolStatement
+  abstract struct PoolStatement
     include StatementMethods
 
     def initialize(@db : Database, @query : String)

src/db/pool_unprepared_statement.cr

@@ -4,15 +4,11 @@ module DB
   # The execution of the statement is retried according to the pool configuration.
   #
   # See `PoolStatement`
-  class PoolUnpreparedStatement < PoolStatement
+  struct PoolUnpreparedStatement < PoolStatement
     def initialize(db : Database, query : String)
       super
     end
 
-    protected def do_close
-      # unprepared statements do not need to be release in each connection
-    end
-
     # builds a statement over a real connection
     private def build_statement : Statement
       conn = @db.pool.checkout