Development of a robust application, be it message publisher or message consumer, involves dealing with multiple kinds of failures: protocol exceptions, network failures, broker failures and so on. Correct error handling and recovery is not easy. This guide explains how the amqp gem helps you in dealing with issues like
as well as
This work is licensed under a Creative Commons Attribution 3.0 Unported License (including images and stylesheets). The source is available on Github.
This guide covers Ruby amqp gem 1.5.x.
There are several examples in the git repository dedicated to the topic of error handling and recovery. Feel free to contribute new examples.
When applications connect to the broker, they need to handle connection failures. Networks are not 100% reliable, even with modern system configuration tools like Chef or Puppet misconfigurations happen and the broker might also be down. Error detection should happen as early as possible. There are two ways of detecting TCP connection failure, the first one is to catch an exception:
begin
AMQP.start(connection_settings) do |connection, open_ok|
raise "This should not be reachable"
end
rescue AMQP::TCPConnectionFailed => e
puts "Caught AMQP::TCPConnectionFailed => TCP connection failed, as expected."
end
Full example:
#!/usr/bin/env ruby
# encoding: utf-8
require "rubygems"
require "amqp"
puts "=> TCP connection failure handling with a rescue statement"
puts
connection_settings = {
:port => 9689,
:vhost => "/amq_client_testbed",
:user => "amq_client_gem",
:password => "amq_client_gem_password",
:timeout => 0.3
}
begin
AMQP.start(connection_settings) do |connection, open_ok|
raise "This should not be reachable"
end
rescue AMQP::TCPConnectionFailed => e
puts "Caught AMQP::TCPConnectionFailed => TCP connection failed, as expected."
end
AMQP.connect (and AMQP.start) will raise
AMQP::TCPConnectionFailed if a connection fails. Code that catches
it can write to a log about the issue or use retry to execute the
begin block one more time. Because initial connection failures are due
to misconfiguration or network outage, reconnection to the same
endpoint (hostname, port, vhost combination) will result in the same
issue over and over. TBD: failover, connection to the cluster.
An alternative way of handling connection failure is with an errback (a callback for specific kind of error):
handler = Proc.new { |settings| puts "Failed to connect, as expected"; EventMachine.stop }
connection_settings = {
:port => 9689,
:vhost => "/amq_client_testbed",
:user => "amq_client_gem",
:password => "amq_client_gem_password",
:timeout => 0.3,
:on_tcp_connection_failure => handler
}
Full example:
require "rubygems"
require "amqp"
puts "=> TCP connection failure handling with a callback"
puts
handler = Proc.new { |settings| puts "Failed to connect, as expected"; EM.stop }
connection_settings = {
:port => 9689,
:vhost => "/amq_client_testbed",
:user => "amq_client_gem",
:password => "amq_client_gem_password",
:timeout => 0.3,
:on_tcp_connection_failure => handler
}
AMQP.start(connection_settings) do |connection, open_ok|
raise "This should not be reachable"
end
:on_tcp_connection_failure option accepts any object that responds to
#call.
If you connect to the broker from code in a class (as opposed to
top-level scope in a script), Object#method can be used to pass object
method as a handler instead of a Proc.
Another reason why a connection may fail is authentication failure. Handling authentication failure is very similar to handling initial TCP connection failure:
require "rubygems"
require "amqp"
puts "=> Authentication failure handling with a callback"
puts
handler = Proc.new { |settings| puts "Failed to connect, as expected"; EM.stop }
connection_settings = {
:port => 5672,
:vhost => "/amq_client_testbed",
:user => "amq_client_gem",
:password => "amq_client_gem_password_that_is_incorrect #{Time.now.to_i}",
:timeout => 0.3,
:on_tcp_connection_failure => handler,
:on_possible_authentication_failure => Proc.new { |settings|
puts "Authentication failed, as expected, settings are: #{settings.inspect}"
EM.stop
}
}
AMQP.start(connection_settings) do |connection, open_ok|
raise "This should not be reachable"
end
default handler raises AMQP::PossibleAuthenticationFailureError:
#!/usr/bin/env ruby
# encoding: utf-8
require "rubygems"
require "amqp"
puts "=> Authentication failure handling with a rescue block"
puts
handler = Proc.new { |settings| puts "Failed to connect, as expected"; EM.stop }
connection_settings = {
:port => 5672,
:vhost => "/amq_client_testbed",
:user => "amq_client_gem",
:password => "amq_client_gem_password_that_is_incorrect #{Time.now.to_i}",
:timeout => 0.3,
:on_tcp_connection_failure => handler
}
begin
AMQP.start(connection_settings) do |connection, open_ok|
raise "This should not be reachable"
end
rescue AMQP::PossibleAuthenticationFailureError => afe
puts "Authentication failed, as expected, caught #{afe.inspect}"
EventMachine.stop if EventMachine.reactor_running?
end
In case you are wondering why callback name has "possible” in it: AMQP 0.9.1 spec requires broker implementations to simply close TCP connection without sending any more data when an exception (such as authentication failure) occurs before AMQP connection is open. In practice, however, when broker closes TCP connection between successful TCP connection and before AMQP connection is open, it means that authentication has failed.
Network connectivity issues are a sad fact of life in modern software
systems. Even small products and projects these days consist of multiple
applications, often running on more than one machine. The Ruby amqp gem
detects TCP connection failures and lets you handle them by defining a
callback using
AMQP::Session#on_tcp_connection_loss.
That callback will be run when TCP connection fails, and will be passed
two parameters: connection object and settings of the last successful
connection.
connection.on_tcp_connection_loss do |connection, settings|
# reconnect in 10 seconds, without enforcement
connection.reconnect(false, 10)
end
Sometimes it is necessary for other entities in an application to react to network failures. amqp gem 0.8.0 and later provides a number of event handlers to make this task easier for developers. This set of features is known as the "shutdown protocol” (the word "protocol” here means "API interface” or "behavior”, not network protocol).
AMQP::Session, AMQP::Channel, AMQP::Exchange, AMQP::Queue and
AMQP::Consumer all implement shutdown protocol and thus
errorhandling API is consistent for all classes, with AMQP::Session
and AMQP::Channel have a few methods that other entities do not
have.
The Shutdown protocol revolves around two events:
In this section, we will concentrate on the former. When a network
connection fails, the underlying networking library detects it and runs
a piece of code on AMQP::Session to handle it. That, in
turn, propagates this event to channels, channels propagate it to
exchanges and queues, queues propagate it to their consumers (if any).
Each of these entities in the object graph can react to network
interruption by executing application-defined callbacks.
AMQP::Session#on_tcp_connection_lossAMQP::Session#on_connection_interruptionThe difference between these methods is that
AMQP::Session#on_tcp_connection_loss
is used to define a callback that will be executed once when TCP
connection fails. It is possible that reconnection attempts will not
succeed immediately, so there will be subsequent failures. To react to
those, AMQP::Session#on_connection_interruption method is used.
The first argument that both of these methods yield to the handler that your application defines is the connection itself. This is done to make sure that you can register Ruby objects as handlers, and they do not have to keep any state around (for example, connection instances):
connection.on_connection_interruption do |conn|
puts "Connection detected connection interruption"
end
# or
class ConnectionInterruptionHandler
#
# API
#
def handle(connection)
# handling logic
end
end
handler = ConnectionInterruptionHandler.new
connection.on_connection_interruption(&handler.method(:handle))
Note that AMQP::Session#on_connection_interruption
callback is called before this event is propagated to channels,
queues and so on.
Different applications handle connection failures differently. It is
very common to use
AMQP::Session#reconnect method to
schedule a reconnection to the same host, or use
AMQP::Session#reconnect_to to connect to a different
one.
For some applications it is OK to simply exit and wait to be restarted
at a later point in time, for example, by a process monitoring system
like Nagios or Monit.
AMQP::Channel provides only one method:
AMQP::Channel#on_connection_interruption,
that registers a callback similar to the one seen in the previous
section:
channel.on_connection_interruption do |ch|
puts "Channel #{ch.id} detected connection interruption"
end
Note that
AMQP::Channel#on_connection_interruption
callback is called after this event is propagated to exchanges,
queues and so on. Right after that channel state is reset, except for
error handling/recovery-related callbacks.
Many applications do not need per-channel network failure handling.
AMQP::Exchange provides only one method:
AMQP::Exchange#on_connection_interruption,
that registers a callback similar to the one seen in the previous
section:
exchange.on_connection_interruption do |ex|
puts "Exchange #{ex.name} detected connection interruption"
end
Many applications do not need per-exchange network failure handling.
AMQP::Queue provides only one method:
AMQP::Queue#on_connection_interruption,
that registers a callback similar tothe one seen in the previous
section:
queue.on_connection_interruption do |q|
puts "Queue #{q.name} detected connection interruption"
end
Note that AMQP::Queue#on_connection_interruption callback is called after this event is propagated to consumers.
Many applications do not need per-queue network failure handling.`
AMQP::Consumer provides only one method:
AMQP::Consumer#on_connection_interruption,
that registers a callback similar to the one seen in the previous
section:
consumer.on_connection_interruption do |c|
puts "Consumer with consumer tag #{c.consumer_tag} detected connection interruption"
end
Many applications do not need per-consumer network failure handling.`
Detecting network connections is nearly useless if an AMQP-based application cannot recover from them. Recovery is the hard part in "error handling and recovery”. Fortunately, the recovery process for many applications follows one simple scheme that the amqp gem can perform automatically for you.
The recovery process, both manual and automatic, always begins with re-opening an AMQP connection and then all the channels on that connection.`
Similarly to the Shutdown Protocol, the amqp gem entities implement a Recovery Protocol. The Recovery Protocol consists of three methods that connections, channels, queues, consumers and exchanges all implement:
AMQP::Session#before_recoveryAMQP::Session#auto_recoverAMQP::Session#after_recoveryAMQP::Session#before_recovery lets application developers register a
callback that will be executed after TCP connection is re-established
but before AMQP connection is reopened.
AMQP::Session#after_recovery is similar except that the callback is
run after AMQP connection is reopened.
AMQP::Channel, AMQP::Queue, AMQP::Consumer, and AMQP::Exchange
method's behavior is identical.
Recovery process for AMQP applications usually involves the following steps:
Many applications use the same recovery strategy that consists of the following steps:
The amqp gem provides a feature known as "automatic recovery” that is
opt-in (not opt-out, not used by default) and lets application
developers get the aforementioned recovery strategy by setting one
additional attribute on AMQP::Channel instance:
ch = AMQP::Channel.new(connection)
ch.auto_recovery = true
A more verbose way to do the same thing:
ch = AMQP::Channel.new(connection, AMQP::Channel.next_channel_id, :auto_recovery => true)
Note that if you do not want to pass any options, the second argument
can be left out as well, then it will default to
AMQP::Channel.next_channel_id.
To find out whether a channel uses automatic recovery mode or not, use
AMQP::Channel#auto_recovering?.
Auto recovery mode can be turned on and off any number of times during channel life cycle, although a very small percentage of applications actually do this. Typically you decide what channels should be using automatic recovery during the application design stage.
Full example (run it, then shut down AMQP broker running on localhost,
then bring it back up and use management tools such as rabbitmqctl to
see that queues, bindings and consumers have all recovered):
require "rubygems"
require "amqp" # requires version >= 0.8.0.RC14
puts "=> Example of automatic AMQP channel and queues recovery"
puts
AMQP.start(:host => "localhost") do |connection, open_ok|
connection.on_error do |ch, connection_close|
raise connection_close.reply_text
end
ch1 = AMQP::Channel.new(connection)
ch1.auto_recovery = true
ch1.on_error do |ch, channel_close|
raise channel_close.reply_text
end
if ch1.auto_recovering?
puts "Channel #{ch1.id} IS auto-recovering"
end
connection.on_tcp_connection_loss do |conn, settings|
puts "[network failure] Trying to reconnect..."
conn.reconnect(false, 2)
end
ch1.queue("amqpgem.examples.queue1", :auto_delete => true).bind("amq.fanout")
ch1.queue("amqpgem.examples.queue2", :auto_delete => true).bind("amq.fanout")
ch1.queue("amqpgem.examples.queue3", :auto_delete => true).bind("amq.fanout").subscribe do |metadata, payload|
end
show_stopper = Proc.new {
connection.disconnect { puts "Disconnected. Exiting…"; EventMachine.stop }
}
Signal.trap "TERM", show_stopper
Signal.trap "INT", show_stopper
EM.add_timer(30, show_stopper)
puts "Connected, authenticated. To really exercise this example, shut AMQP broker down for a few seconds. If you don't it will exit gracefully in 30 seconds."
end
Server-named queues, when recovered automatically, will get new server-generated names to guarantee there are no name collisions.
When in doubt, try using automatic recovery first. If it is not sufficient for your application, switch to manual recovery using events and callbacks introduced in the "Manual recovery” section.
AMQP applications see broker failure as TCP connection loss. There is no reliable way to know whether there is a network problem or a network peer is down.
Connection-level exceptions are rare and may indicate a serious issue
with a client library or in-flight data corruption. The AMQP 0.9.1
specification mandates that a connection that has errored cannot be used
any more and must be closed. In any case, your application should be
prepared to handle this kind of error. To define a handler, use
AMQP::Session#on_error method that
takes a callback and yields two arguments to it when a connection-level
exception happens:
connection.on_error do |conn, connection_close|
puts "Handling a connection-level exception."
puts
puts "AMQP class id : #{connection_close.class_id}"
puts "AMQP method id: #{connection_close.method_id}"
puts "Status code : #{connection_close.reply_code}"
puts "Error message : #{connection_close.reply_text}"
end
Status codes are similar to those of HTTP. For the full list of error codes and their meaning, consult AMQP 0.9.1 constants reference.
Only one connection-level exception handler can be defined per connection instance (the one added last replaces previously added ones).
Full example:
#!/usr/bin/env ruby
# encoding: utf-8
require "bundler"
Bundler.setup
$:.unshift(File.expand_path("../../../lib", __FILE__))
require 'amqp'
EventMachine.run do
AMQP.connect(:host => '127.0.0.1', :port => 5672) do |connection|
puts "Connected to AMQP broker. Running #{AMQP::VERSION} version of the gem..."
connection.on_error do |conn, connection_close|
puts <<-ERR
Handling a connection-level exception.
AMQP class id : #{connection_close.class_id},
AMQP method id: #{connection_close.method_id},
Status code : #{connection_close.reply_code}
Error message : #{connection_close.reply_text}
ERR
EventMachine.stop
end
# send_frame is NOT part of the public API, but it is public for entities like AMQ::Client::Channel
# and we use it here to trigger a connection-level exception. MK.
connection.send_frame(AMQ::Protocol::Connection::TuneOk.encode(1000, 1024 * 128 * 1024, 10))
end
end
When an AMQP broker is shut down, it properly closes connections first. To do so, it uses connection.close AMQP method. AMQP clients then need to check if the reply code is equal to 320 (CONNECTION_FORCED) to distinguish graceful shutdown. With RabbitMQ, when broker is stopped using
rabbitmqctl stop
reply_text will be set to
CONNECTION_FORCED - broker forced connection closure with reason
‘shutdown’
Each application chooses how to handle graceful broker shutdowns
individually, so amqp gem’s automatic reconnection does not cover
graceful broker shutdowns. Applications that want to reconnect when
broker is stopped can use
AMQP::Session#periodically_reconnect
like so:
connection.on_error do |conn, connection_close|
puts "[connection.close] Reply code = #{connection_close.reply_code}, reply text = #{connection_close.reply_text}"
if connection_close.reply_code == 320
puts "[connection.close] Setting up a periodic reconnection timer..."
# every 30 seconds
conn.periodically_reconnect(30)
end
end
Once AMQP connection is re-opened, channels in automatic recovery mode will recover just like they do after network outages.
Error handling can be easily integrated into object-oriented Ruby code (in fact, this is highly encouraged). A common technique is to combine Object#method and Method#to_proc and use object methods as error handlers:
class ConnectionManager
#
# API
#
def connect(*args, &block)
@connection = AMQP.connect(*args, &block)
# combines Object#method and Method#to_proc to use object
# method as a callback
@connection.on_error(&method(:on_error))
end # connect(*args, &block)
def on_error(connection, connection_close)
puts "Handling a connection-level exception."
puts
puts "AMQP class id : #{connection_close.class_id}"
puts "AMQP method id: #{connection_close.method_id}"
puts "Status code : #{connection_close.reply_code}"
puts "Error message : #{connection_close.reply_text}"
end # on_error(connection, connection_close)
end
Full example that uses objects:
#!/usr/bin/env ruby
# encoding: utf-8
require "bundler"
Bundler.setup
$:.unshift(File.expand_path("../../../lib", __FILE__))
require 'amqp'
class ConnectionManager
#
# API
#
def connect(*args, &block)
@connection = AMQP.connect(*args, &block)
# combines Object#method and Method#to_proc to use object
# method as a callback
@connection.on_error(&method(:on_error))
end # connect(*args, &block)
def on_error(connection, connection_close)
puts "Handling a connection-level exception."
puts
puts "AMQP class id : #{connection_close.class_id}"
puts "AMQP method id: #{connection_close.method_id}"
puts "Status code : #{connection_close.reply_code}"
puts "Error message : #{connection_close.reply_text}"
end # on_error(connection, connection_close)
end
EventMachine.run do
manager = ConnectionManager.new
manager.connect(:host => '127.0.0.1', :port => 5672) do |connection|
puts "Connected to AMQP broker. Running #{AMQP::VERSION} version of the gem..."
# send_frame is NOT part of the public API, but it is public for entities like AMQ::Client::Channel
# and we use it here to trigger a connection-level exception. MK.
connection.send_frame(AMQ::Protocol::Connection::TuneOk.encode(1000, 1024 * 128 * 1024, 10))
end
# shut down after 2 seconds
EventMachine.add_timer(2) { EventMachine.stop }
end
Channel-level exceptions are more common than connection-level ones.
They are handled in a similar manner, by defining a callback with
AMQP::Channel#on_error method that
takes a callback and yields two arguments to it when a channel-level
exception happens:
channel.on_error do |ch, channel_close|
puts "Handling a channel-level exception."
puts
puts "AMQP class id : #{channel_close.class_id}"
puts "AMQP method id: #{channel_close.method_id}"
puts "Status code : #{channel_close.reply_code}"
puts "Error message : #{channel_close.reply_text}"
end
Status codes are similar to those of HTTP. For the full list of error codes and their meaning, consult AMQP 0.9.1 constants reference.
Only one channel-level exception handler can be defined per channel instance (the one added last replaces previously added ones).`
Full example:
#!/usr/bin/env ruby
# encoding: utf-8
require "bundler"
Bundler.setup
$:.unshift(File.expand_path("../../../lib", __FILE__))
require 'amqp'
puts "=> Queue redeclaration with different attributes results in a channel exception that is handled"
puts
AMQP.start("amqp://guest:guest@dev.rabbitmq.com:5672") do |connection, open_ok|
AMQP::Channel.new do |channel, open_ok|
puts "Channel ##{channel.id} is now open!"
channel.on_error do |ch, channel_close|
puts <<-ERR
Handling a channel-level exception.
AMQP class id : #{channel_close.class_id},
AMQP method id: #{channel_close.method_id},
Status code : #{channel_close.reply_code}
Error message : #{channel_close.reply_text}
ERR
end
EventMachine.add_timer(0.4) do
# these two definitions result in a race condition. For sake of this example,
# however, it does not matter. Whatever definition succeeds first, 2nd one will
# cause a channel-level exception (because attributes are not identical)
AMQP::Queue.new(channel, "amqpgem.examples.channel_exception", :auto_delete => true, :durable => false) do |queue|
puts "#{queue.name} is ready to go"
end
AMQP::Queue.new(channel, "amqpgem.examples.channel_exception", :auto_delete => true, :durable => true) do |queue|
puts "#{queue.name} is ready to go"
end
end
end
show_stopper = Proc.new do
$stdout.puts "Stopping..."
connection.close { EventMachine.stop }
end
Signal.trap "INT", show_stopper
EM.add_timer(2, show_stopper)
end
Error handling can be easily integrated into object-oriented Ruby code (in fact, this is highly encouraged).A common technique is to combine Object#method and Method#to_proc and use object methods as error handlers. For example of this, see section on connection-level exceptions above.
Because channel-level exceptions may be raised because of multiple unrelated reasons and often indicate misconfigurations, how they are handled isvery specific to particular applications. A common strategy is to log an error and then open and use another channel.
A few channel-level exceptions are common and deserve more attention.
Distributed applications introduce a whole new class of failures developers need to be aware of. Many of them stem from unreliable networks. The famous Fallacies of Distributed Computing list common assumptions software engineers must not make:
Unfortunately, applications that use Ruby and AMQP are not immune to these problems and developers need to always keep that in mind. This list is just as relevant today as it was in the 90s.
Ruby amqp gem 0.8.x and later lets applications define handlers that handle connection-level exceptions, channel-level exceptions and TCP connection failures. Handling AMQP exceptions and network connection failures is relatively easy. Re-declaring AMQP instances that the application works with is where most of the complexity comes from. By using Ruby objects as error handlers, the declaration of AMQP entities can be done in one place, making code much easier to understand and maintain.
amqp gem error and interruption handling is not a copy of RabbitMQ Java client’s Shutdown Protocol, but they turn out to be similar with respect to network failures and connection-level exceptions.
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