class Sequel::ShardedTimedQueueConnectionPool

A connection pool allowing multi-threaded access to a sharded pool of connections, using a timed queue (only available in Ruby 3.2+).

Attributes

max_size[R]

The maximum number of connections this pool will create per shard.

Public Class Methods

new(db, opts = OPTS) click to toggle source

The following additional options are respected:

:max_connections

The maximum number of connections the connection pool will open (default 4)

:pool_timeout

The amount of seconds to wait to acquire a connection before raising a PoolTimeout (default 5)

:servers

A hash of servers to use. Keys should be symbols. If not present, will use a single :default server.

:servers_hash

The base hash to use for the servers. By default, Sequel uses Hash.new(:default). You can use a hash with a default proc that raises an error if you want to catch all cases where a nonexistent server is used.

Calls superclass method Sequel::ConnectionPool.new
# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 23
def initialize(db, opts = OPTS)
  super

  @max_size = Integer(opts[:max_connections] || 4)
  raise(Sequel::Error, ':max_connections must be positive') if @max_size < 1
  @mutex = Mutex.new  
  @timeout = Float(opts[:pool_timeout] || 5)

  @allocated = {}
  @sizes = {}
  @queues = {}
  @servers = opts.fetch(:servers_hash, Hash.new(:default))

  add_servers([:default])
  add_servers(opts[:servers].keys) if opts[:servers]
end

Public Instance Methods

add_servers(servers) click to toggle source

Adds new servers to the connection pool. Allows for dynamic expansion of the potential replicas/shards at runtime. servers argument should be an array of symbols.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 42
def add_servers(servers)
  sync do
    servers.each do |server|
      next if @servers.has_key?(server)

      @servers[server] = server
      @sizes[server] = 0
      @queues[server] = Queue.new
      (@allocated[server] = {}).compare_by_identity
    end
  end
  nil
end
all_connections() { |conn| ... } click to toggle source

Yield all of the available connections, and the one currently allocated to this thread (if one is allocated). This will not yield connections currently allocated to other threads, as it is not safe to operate on them.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 59
def all_connections
  thread = Sequel.current
  sync{@queues.to_a}.each do |server, queue|
    if conn = owned_connection(thread, server)
      yield conn
    end

    # Use a hash to record all connections already seen.  As soon as we
    # come across a connection we've already seen, we stop the loop.
    conns = {}
    conns.compare_by_identity
    while true
      conn = nil
      begin
        break unless (conn = queue.pop(timeout: 0)) && !conns[conn]
        conns[conn] = true
        yield conn
      ensure
        queue.push(conn) if conn
      end
    end
  end

  nil
end
disconnect(opts=OPTS) click to toggle source

Removes all connections currently in the pool's queue. This method has the effect of disconnecting from the database, assuming that no connections are currently being used.

Once a connection is requested using hold, the connection pool creates new connections to the database.

If the :server option is provided, it should be a symbol or array of symbols, and then the method will only disconnect connectsion from those specified shards.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 94
def disconnect(opts=OPTS)
  (opts[:server] ? Array(opts[:server]) : sync{@servers.keys}).each do |server|
    raise Sequel::Error, "invalid server" unless queue = sync{@queues[server]}
    while conn = queue.pop(timeout: 0)
      disconnect_pool_connection(conn, server)
    end
    fill_queue(server)
  end
  nil
end
hold(server=:default) { |conn| ... } click to toggle source

Chooses the first available connection for the given server, or if none are available, creates a new connection. Passes the connection to the supplied block:

pool.hold(:server1) {|conn| conn.execute('DROP TABLE posts')}

Pool#hold is re-entrant, meaning it can be called recursively in the same thread without blocking.

If no connection is immediately available and the pool is already using the maximum number of connections, Pool#hold will block until a connection is available or the timeout expires. If the timeout expires before a connection can be acquired, a Sequel::PoolTimeout is raised.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 118
def hold(server=:default)
  server = pick_server(server)
  t = Sequel.current
  if conn = owned_connection(t, server)
    return yield(conn)
  end

  begin
    conn = acquire(t, server)
    yield conn
  rescue Sequel::DatabaseDisconnectError, *@error_classes => e
    if disconnect_error?(e)
      oconn = conn
      conn = nil
      disconnect_pool_connection(oconn, server) if oconn
      sync{@allocated[server].delete(t)}
      fill_queue(server)
    end
    raise
  ensure
    release(t, conn, server) if conn
  end
end
pool_type() click to toggle source
# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 189
def pool_type
  :sharded_timed_queue
end
remove_servers(servers) click to toggle source

Remove servers from the connection pool. Similar to disconnecting from all given servers, except that after it is used, future requests for the servers will use the :default server instead.

Note that an error will be raised if there are any connections currently checked out for the given servers.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 153
def remove_servers(servers)
  conns = []
  raise(Sequel::Error, "cannot remove default server") if servers.include?(:default)

  sync do
    servers.each do |server|
      next unless @servers.has_key?(server)

      queue = @queues[server]

      while conn = queue.pop(timeout: 0)
        @sizes[server] -= 1
        conns << conn
      end

      unless @sizes[server] == 0
        raise Sequel::Error, "cannot remove server #{server} as it has allocated connections"
      end

      @servers.delete(server)
      @sizes.delete(server)
      @queues.delete(server)
      @allocated.delete(server)
    end
  end

  nil
ensure
  disconnect_connections(conns)
end
servers() click to toggle source

Return an array of symbols for servers in the connection pool.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 185
def servers
  sync{@servers.keys}
end
size(server=:default) click to toggle source

The total number of connections in the pool. Using a non-existant server will return nil.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 143
def size(server=:default)
  sync{@sizes[server]}
end

Private Instance Methods

acquire(thread, server) click to toggle source

Assigns a connection to the supplied thread, if one is available.

This should return a connection if one is available within the timeout, or raise PoolTimeout if a connection could not be acquired within the timeout.

Calling code should not have the mutex when calling this.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 305
def acquire(thread, server)
  queue = sync{@queues[server]}
  if conn = queue.pop(timeout: 0) || try_make_new(server) || queue.pop(timeout: @timeout)
    sync{@allocated[server][thread] = conn}
  else
    name = db.opts[:name]
    raise ::Sequel::PoolTimeout, "timeout: #{@timeout}, server: #{server}#{", database name: #{name}" if name}"
  end
end
can_make_new?(server, current_size) click to toggle source

Whether the given size is less than the maximum size of the pool. In that case, the pool's current size is incremented. If this method returns true, space in the pool for the connection is preallocated, and #preallocated_make_new should be called to create the connection.

Calling code should have the mutex when calling this.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 258
def can_make_new?(server, current_size)
  if @max_size > current_size
    @sizes[server] += 1
  end
end
checkin_connection(conn, server) click to toggle source

Adds a connection to the queue of available connections, returns the connection.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 362
def checkin_connection(conn, server)
  sync{@queues[server]}.push(conn)
  conn
end
disconnect_connections(conns) click to toggle source

Disconnect all available connections immediately, and schedule currently allocated connections for disconnection as soon as they are returned to the pool. The calling code should NOT have the mutex before calling this.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 217
def disconnect_connections(conns)
  conns.each{|conn| disconnect_connection(conn)}
end
disconnect_pool_connection(conn, server) click to toggle source

Decrement the current size of the pool for the server when disconnecting connections.

Calling code should not have the mutex when calling this.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 224
def disconnect_pool_connection(conn, server)
  sync{@sizes[server] -= 1}
  disconnect_connection(conn)
end
fill_queue(server) click to toggle source

If there are any threads waiting on the queue, try to create new connections in a separate thread if the pool is not yet at the maximum size.

The reason for this method is to handle cases where acquire could not retrieve a connection immediately, and the pool was already at the maximum size. In that case, the acquire will wait on the queue until the timeout. This method is called after disconnecting to potentially add new connections to the pool, so the threads that are currently waiting for connections do not timeout after the pool is no longer full.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 240
def fill_queue(server)
  queue = sync{@queues[server]}
  if queue.num_waiting > 0
    Thread.new do
      while queue.num_waiting > 0 && (conn = try_make_new(server))
        queue.push(conn)
      end
    end
  end
end
owned_connection(thread, server) click to toggle source

Returns the connection owned by the supplied thread for the given server, if any. The calling code should NOT already have the mutex before calling this.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 317
def owned_connection(thread, server)
  sync{@allocated[server][thread]}
end
pick_server(server) click to toggle source

If the server given is in the hash, return it, otherwise, return the default server.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 322
def pick_server(server)
  sync{@servers[server]}
end
preallocated_make_new(server) click to toggle source

Create a new connection, after the pool's current size has already been updated to account for the new connection. If there is an exception when creating the connection, decrement the current size.

This should only be called after can_make_new?. If there is an exception between when can_make_new? is called and when #preallocated_make_new is called, it has the effect of reducing the maximum size of the connection pool by 1, since the current size of the pool will show a higher number than the number of connections allocated or in the queue.

Calling code should not have the mutex when calling this.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 207
def preallocated_make_new(server)
  make_new(server)
rescue Exception
  sync{@sizes[server] -= 1}
  raise
end
preconnect(concurrent = false) click to toggle source

Create the maximum number of connections immediately. This should not be called with a true argument unless no code is currently operating on the database.

Calling code should not have the mutex when calling this.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 330
def preconnect(concurrent = false)
  conn_servers = sync{@servers.keys}.map!{|s| Array.new(@max_size - @sizes[s], s)}.flatten!

  if concurrent
    conn_servers.map! do |server|
      queue = sync{@queues[server]}
      Thread.new do 
        if conn = try_make_new(server)
          queue.push(conn)
        end
      end
    end.each(&:value)
  else
    conn_servers.each do |server|
      if conn = try_make_new(server)
        sync{@queues[server]}.push(conn)
      end
    end
  end

  nil
end
release(thread, _, server) click to toggle source

Releases the connection assigned to the supplied thread back to the pool.

Calling code should not have the mutex when calling this.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 356
def release(thread, _, server)
  checkin_connection(sync{@allocated[server].delete(thread)}, server)
  nil
end
sync() { || ... } click to toggle source

Yield to the block while inside the mutex.

Calling code should not have the mutex when calling this.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 370
def sync
  @mutex.synchronize{yield}
end
try_make_new(server) click to toggle source

Try to make a new connection if there is space in the pool. If the pool is already full, look for dead threads/fibers and disconnect the related connections.

Calling code should not have the mutex when calling this.

# File lib/sequel/connection_pool/sharded_timed_queue.rb, line 269
def try_make_new(server)
  return preallocated_make_new(server) if sync{can_make_new?(server, @sizes[server])}

  to_disconnect = nil
  do_make_new = false

  sync do
    current_size = @sizes[server]
    alloc = @allocated[server]
    alloc.keys.each do |t|
      unless t.alive?
        (to_disconnect ||= []) << alloc.delete(t)
        current_size -= 1
      end
    end
  
    do_make_new = true if can_make_new?(server, current_size)
  end

  begin
    preallocated_make_new(server) if do_make_new
  ensure
    if to_disconnect
      to_disconnect.each{|conn| disconnect_pool_connection(conn, server)}
      fill_queue(server)
    end
  end
end