class Sequel::ShardedThreadedConnectionPool

The slowest and most advanced connection pool, dealing with both multi-threaded access and configurations with multiple shards/servers.

In addition, this pool subclass also handles scheduling in-use connections to be removed from the pool when they are returned to it.

Public Class Methods

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

The following additional options are respected:

: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::ThreadedConnectionPool::new
   # File lib/sequel/connection_pool/sharded_threaded.rb
18 def initialize(db, opts = OPTS)
19   super
20   @available_connections = {}
21   @connections_to_remove = []
22   @connections_to_disconnect = []
23   @servers = opts.fetch(:servers_hash, Hash.new(:default))
24   remove_instance_variable(:@waiter)
25   remove_instance_variable(:@allocated)
26   @allocated = {}
27   @waiters = {}
28 
29   add_servers([:default])
30   add_servers(opts[:servers].keys) if opts[:servers]
31 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_threaded.rb
35 def add_servers(servers)
36   sync do
37     servers.each do |server|
38       unless @servers.has_key?(server)
39         @servers[server] = server
40         @available_connections[server] = []
41         allocated = {}
42         allocated.compare_by_identity
43         @allocated[server] = allocated
44         @waiters[server] = ConditionVariable.new
45       end
46     end
47   end
48 end
all_connections() { |conn| ... } click to toggle source

Yield all of the available connections, and the ones currently allocated to this thread. This will not yield connections currently allocated to other threads, as it is not safe to operate on them. This holds the mutex while it is yielding all of the connections, which means that until the method’s block returns, the pool is locked.

   # File lib/sequel/connection_pool/sharded_threaded.rb
63 def all_connections
64   t = Sequel.current
65   sync do
66     @allocated.values.each do |threads|
67       threads.each do |thread, conn|
68         yield conn if t == thread
69       end
70     end
71     @available_connections.values.each{|v| v.each{|c| yield c}}
72   end
73 end
allocated(server=:default) click to toggle source

A hash of connections currently being used for the given server, key is the Thread, value is the connection. Nonexistent servers will return nil. Treat this as read only, do not modify the resulting object. The calling code should already have the mutex before calling this.

   # File lib/sequel/connection_pool/sharded_threaded.rb
54 def allocated(server=:default)
55   @allocated[server]
56 end
available_connections(server=:default) click to toggle source

An array of connections opened but not currently used, for the given server. Nonexistent servers will return nil. Treat this as read only, do not modify the resulting object. The calling code should already have the mutex before calling this.

   # File lib/sequel/connection_pool/sharded_threaded.rb
79 def available_connections(server=:default)
80   @available_connections[server]
81 end
disconnect(opts=OPTS) click to toggle source

Removes all connections currently available on all servers, optionally yielding each connection to the given block. This method has the effect of disconnecting from the database, assuming that no connections are currently being used. If connections are being used, they are scheduled to be disconnected as soon as they are returned to the pool.

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

:server

Should be a symbol specifing the server to disconnect from, or an array of symbols to specify multiple servers.

    # File lib/sequel/connection_pool/sharded_threaded.rb
100 def disconnect(opts=OPTS)
101   (opts[:server] ? Array(opts[:server]) : sync{@servers.keys}).each do |s|
102     disconnect_connections(sync{disconnect_server_connections(s)})
103   end
104 end
freeze() click to toggle source
Calls superclass method
    # File lib/sequel/connection_pool/sharded_threaded.rb
106 def freeze
107   @servers.freeze
108   super
109 end
hold(server=:default) { |conn| ... } click to toggle source

Chooses the first available connection to 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_threaded.rb
124 def hold(server=:default)
125   server = pick_server(server)
126   t = Sequel.current
127   if conn = owned_connection(t, server)
128     return yield(conn)
129   end
130   begin
131     conn = acquire(t, server)
132     yield conn
133   rescue Sequel::DatabaseDisconnectError, *@error_classes => e
134     sync{@connections_to_remove << conn} if conn && disconnect_error?(e)
135     raise
136   ensure
137     sync{release(t, conn, server)} if conn
138     while dconn = sync{@connections_to_disconnect.shift}
139       disconnect_connection(dconn)
140     end
141   end
142 end
pool_type() click to toggle source
    # File lib/sequel/connection_pool/sharded_threaded.rb
173 def pool_type
174   :sharded_threaded
175 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 server will use the :default server instead.

    # File lib/sequel/connection_pool/sharded_threaded.rb
147 def remove_servers(servers)
148   conns = []
149   raise(Sequel::Error, "cannot remove default server") if servers.include?(:default)
150 
151   sync do
152     servers.each do |server|
153       if @servers.include?(server)
154         conns.concat(disconnect_server_connections(server))
155         @waiters.delete(server)
156         @available_connections.delete(server)
157         @allocated.delete(server)
158         @servers.delete(server)
159       end
160     end
161   end
162 
163   nil
164 ensure
165   disconnect_connections(conns)
166 end
servers() click to toggle source

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

    # File lib/sequel/connection_pool/sharded_threaded.rb
169 def servers
170   sync{@servers.keys}
171 end
size(server=:default) click to toggle source

The total number of connections opened for the given server. Nonexistent servers will return the created count of the default server. The calling code should NOT have the mutex before calling this.

   # File lib/sequel/connection_pool/sharded_threaded.rb
86 def size(server=:default)
87   @mutex.synchronize{_size(server)}
88 end

Private Instance Methods

_size(server) click to toggle source

The total number of connections opened for the given server. The calling code should already have the mutex before calling this.

    # File lib/sequel/connection_pool/sharded_threaded.rb
181 def _size(server)
182   server = @servers[server]
183   @allocated[server].length + @available_connections[server].length
184 end
acquire(thread, server) click to toggle source

Assigns a connection to the supplied thread, if one is available. The calling code should NOT already have the mutex when calling this.

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

    # File lib/sequel/connection_pool/sharded_threaded.rb
192 def acquire(thread, server)
193   if conn = assign_connection(thread, server)
194     return conn
195   end
196 
197   timeout = @timeout
198   timer = Sequel.start_timer
199 
200   if conn = acquire_available(thread, server, timeout)
201     return conn
202   end
203 
204   until conn = assign_connection(thread, server)
205     elapsed = Sequel.elapsed_seconds_since(timer)
206     # :nocov:
207     raise_pool_timeout(elapsed, server) if elapsed > timeout
208 
209     # It's difficult to get to this point, it can only happen if there is a race condition
210     # where a connection cannot be acquired even after the thread is signalled by the condition variable
211     if conn = acquire_available(thread, server, timeout - elapsed)
212       return conn
213     end
214     # :nocov:
215   end
216 
217   conn
218 end
acquire_available(thread, server, timeout) click to toggle source

Acquire a connection if one is already available, or waiting until it becomes available.

    # File lib/sequel/connection_pool/sharded_threaded.rb
221 def acquire_available(thread, server, timeout)
222   sync do
223     # Check if connection was checked in between when assign_connection failed and now.
224     # This is very unlikely, but necessary to prevent a situation where the waiter
225     # will wait for a connection even though one has already been checked in.
226     # :nocov:
227     if conn = next_available(server)
228       return(allocated(server)[thread] = conn)
229     end
230     # :nocov:
231 
232     @waiters[server].wait(@mutex, timeout)
233 
234     # Connection still not available, could be because a connection was disconnected,
235     # may have to retry assign_connection to see if a new connection can be made.
236     if conn = next_available(server)
237       return(allocated(server)[thread] = conn)
238     end
239   end
240 end
assign_connection(thread, server) click to toggle source

Assign a connection to the thread, or return nil if one cannot be assigned. The caller should NOT have the mutex before calling this.

    # File lib/sequel/connection_pool/sharded_threaded.rb
244 def assign_connection(thread, server)
245   alloc = nil
246 
247   do_make_new = false
248   sync do
249     alloc = allocated(server)
250     if conn = next_available(server)
251       alloc[thread] = conn
252       return conn
253     end
254 
255     if (n = _size(server)) >= (max = @max_size)
256       alloc.to_a.each do |t,c|
257         unless t.alive?
258           remove(t, c, server)
259         end
260       end
261       n = nil
262     end
263 
264     if (n || _size(server)) < max
265       do_make_new = alloc[thread] = true
266     end
267   end
268 
269   # Connect to the database outside of the connection pool mutex,
270   # as that can take a long time and the connection pool mutex
271   # shouldn't be locked while the connection takes place.
272   if do_make_new
273     begin
274       conn = make_new(server)
275       sync{alloc[thread] = conn}
276     ensure
277       unless conn
278         sync{alloc.delete(thread)}
279       end
280     end
281   end
282 
283   conn
284 end
checkin_connection(server, conn) click to toggle source

Return a connection to the pool of available connections for the server, returns the connection. The calling code should already have the mutex before calling this.

    # File lib/sequel/connection_pool/sharded_threaded.rb
289 def checkin_connection(server, conn)
290   available_connections(server) << conn
291   @waiters[server].signal
292   conn
293 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_threaded.rb
315 def disconnect_connections(conns)
316   conns.each{|conn| disconnect_connection(conn)}
317 end
disconnect_server_connections(server) click to toggle source

Clear the array of available connections for the server, returning an array of previous available connections that should be disconnected (or nil if none should be). Mark any allocated connections to be removed when they are checked back in. The calling code should already have the mutex before calling this.

    # File lib/sequel/connection_pool/sharded_threaded.rb
299 def disconnect_server_connections(server)
300   remove_conns = allocated(server)
301   dis_conns = available_connections(server)
302   raise Sequel::Error, "invalid server: #{server}" unless remove_conns && dis_conns
303 
304   @connections_to_remove.concat(remove_conns.values)
305 
306   conns = dis_conns.dup
307   dis_conns.clear
308   @waiters[server].signal
309   conns
310 end
next_available(server) click to toggle source

Return the next available connection in the pool for the given server, or nil if there is not currently an available connection for the server. The calling code should already have the mutex before calling this.

    # File lib/sequel/connection_pool/sharded_threaded.rb
322 def next_available(server)
323   case @connection_handling
324   when :stack
325     available_connections(server).pop
326   else
327     available_connections(server).shift
328   end
329 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_threaded.rb
333 def owned_connection(thread, server)
334   sync{@allocated[server][thread]}
335 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_threaded.rb
338 def pick_server(server)
339   sync{@servers[server]}
340 end
preconnect(concurrent = false) click to toggle source

Create the maximum number of connections immediately. The calling code should NOT have the mutex before calling this.

    # File lib/sequel/connection_pool/sharded_threaded.rb
344 def preconnect(concurrent = false)
345   conn_servers = sync{@servers.keys}.map!{|s| Array.new(max_size - _size(s), s)}.flatten!
346 
347   if concurrent
348     conn_servers.map!{|s| Thread.new{[s, make_new(s)]}}.map!(&:value)
349   else
350     conn_servers.map!{|s| [s, make_new(s)]}
351   end
352 
353   sync{conn_servers.each{|s, conn| checkin_connection(s, conn)}}
354 end
raise_pool_timeout(elapsed, server) click to toggle source

Raise a PoolTimeout error showing the current timeout, the elapsed time, the server the connection attempt was made to, and the database’s name (if any).

    # File lib/sequel/connection_pool/sharded_threaded.rb
358 def raise_pool_timeout(elapsed, server)
359   name = db.opts[:name]
360   raise ::Sequel::PoolTimeout, "timeout: #{@timeout}, elapsed: #{elapsed}, server: #{server}#{", database name: #{name}" if name}"
361 end
release(thread, conn, server) click to toggle source

Releases the connection assigned to the supplied thread and server. If the server or connection given is scheduled for disconnection, remove the connection instead of releasing it back to the pool. The calling code should already have the mutex before calling this.

    # File lib/sequel/connection_pool/sharded_threaded.rb
367 def release(thread, conn, server)
368   if @connections_to_remove.include?(conn)
369     remove(thread, conn, server)
370   else
371     conn = allocated(server).delete(thread)
372 
373     if @connection_handling == :disconnect
374       @connections_to_disconnect << conn
375     else
376       checkin_connection(server, conn)
377     end
378   end
379 
380   if waiter = @waiters[server]
381     waiter.signal
382   end
383 end
remove(thread, conn, server) click to toggle source

Removes the currently allocated connection from the connection pool. The calling code should already have the mutex before calling this.

    # File lib/sequel/connection_pool/sharded_threaded.rb
387 def remove(thread, conn, server)
388   @connections_to_remove.delete(conn)
389   allocated(server).delete(thread) if @servers.include?(server)
390   @connections_to_disconnect << conn
391 end