README.md in pwwka-0.8.0 vs README.md in pwwka-0.9.0.RC1
- old
+ new
@@ -6,36 +6,57 @@
---
[](https://travis-ci.org/stitchfix/pwwka)
-This gem connects to a topic exchange on a RabbitMQ server. It gives any app using it the ability to do two things:
+Provides the means to both send and handle messages on an exchange of a RabbitMQ server. In a sense, this provides the RabbitMQ equivalent
+of `Resque.enqueue` and `SomeResqueJob.perform`.
-* Transmit messages to the exchange
-* Receive messages from the exchange and tell the exchange whether or not the message has been acknowledged.
+## Set Up
-Any app can do one or both of these things.
+In your `Gemfile`:
-The basic principle of the Pwwka Message Bus is this:
+```ruby
+gem 'pwwka'
+```
-> The transmitter should send messages to inform anyone who listens that an event has occurred. It's up to the receiver to interpret the message.
+(of course, you can always run `gem install pwwka` to install it without Bundler)
-As an example:
+To run applications locally, you will need Rabbit installed. The [installation guide](https://www.rabbitmq.com/download.html) is a great
+place to start. This repo includes a `docker-compose.yml` file which will run Rabbit inside a Docker container. It's used by the tests,
+but you can use that, too.
-* public-app sends a message that a new client has signed up
-* admin-app receives that message and updates its client index
-* email-app receives that message and sends a welcome email to the client
+### Configuration
-## Persistence
+Somewhere in your app, run the following code (in Rails, this would be `config/initializers/pwwka.rb`):
-All transmitters and receivers share the same exchange. This means that all receivers can read all messages that any transmitter sends. To ensure that all messages are received by eveyone who wants them the Pwwka message bus is configured as follows:
+```ruby
+require 'pwwka'
+Pwwka.configure do |config|
+ config.rabbit_mq_host = ENV['RABBITMQ_URL']
+ config.topic_exchange_name = "mycompany-topics-#{Rails.env}"
+ config.delayed_exchange_name = "mycompany-topics-#{Rails.env}"
+ config.options = {allow_delayed: true}
+ config.requeue_on_error = true
+end
+```
-* The exchange is named and durable. If the service goes down and restarts the named exchange will return with the same settings so everyone can reconnect.
-* The receiver queues are all named and durable. If the service goes down and restarts the named queue will return with the same settings so everyone can reconnect, and with any unacknowledged messages waiting to be received.
-* All messages are sent as persistent and require acknowledgement. They will stick around and wait to be received and acknowledged by every queue that wants them, regardless of service interruptions.
+Note that the absence of `RABBITMQ_URL` in your environment will cause the underlying RabbitMQ library to use the defaults. If you aren't
+using the defaults, set that environment variable to something like this:
+```
+amqp://«user»:«password»@«host»:«port»/«vhost»
+```
+The defaults should be `amqp://guest:guest@localhost:5672/`, i.e.:
+
+* user: guest
+* password: guest
+* host: localhost
+* port: 5672
+* vhost: `/`
+
## Setting it up
### Install RabbitMQ locally
```
@@ -50,64 +71,48 @@
```ruby
gem 'pwwka'
```
+## Using Pwwka
-### Set up your pwwka configration
+Pwwka provides the ability to send a message into Rabbit as well a the ability to receive/handle a message. Your app can do both of these
+things if it needs to.
-Connect to your RabbitMQ instance using the url and choose a name for your
-topic exchange.
-In `config/initializers/pwwka`:
+### Sending a message
-```ruby
-require 'pwwka'
-Pwwka.configure do |config|
- config.rabbit_mq_host = ENV['RABBITMQ_URL']
- config.topic_exchange_name = "mycompany-topics-#{Rails.env}"
- config.options = {allow_delayed: true}
-end
-```
-
-## Sending a message
-
You can send any kind of message using `Pwwka::Transmitter.send_message!`:
```ruby
payload = {client_id: '13452564'}
routing_key = 'sf.clients.client.created'
Pwwka::Transmitter.send_message!(payload, routing_key)
```
-The payload should be a simple hash containing primitives. Don't send objects because the payload will be converted to JSON for sending. This will blow up if an exception is raised. If you want the exception to be rescued and logged, use this instead:
-```ruby
-Pwwka::Transmitter.send_message_safely(payload, routing_key)
-```
+The payload should be a simple hash containing primitives. Don't send objects because the payload will be converted to JSON for sending.
-You can also use the two convenience methods for sending a message. To include these methods
-in your class use:
-```ruby
-include Pwwka::Handling
-```
+#### Error Handling
-Then you can call:
+`Pwwka::Transmitter.send_message!` accepts several strategies for handling errors, passed in using the `on_error` parameter:
+ * `:raise` - Log the error and raise the exception received from Bunny. (default strategy)
+ * `:ignore` - Log the error and return false.
+ * `:resque` - Log the error and return false. Also, enqueue a job with Resque to send the message. See `send_message_async` below. **Note, this doesn't guarantee the message will actually be sent—it just guarantees an attempt is made to queue a Resque job [which could fail]**
+
+Example usage:
+
```ruby
-send_message!(payload, routing_key)
+payload = {client_id: '13452564'}
+routing_key = 'sf.clients.client.created'
+Pwwka::Transmitter.send_message!(payload, routing_key, on_error: :ignore)
```
-### Error Handling
-This method accepts several strategies for handling errors, pass in using the `on_error` parameter:
+#### Delayed Messages
- * `:raise`: Log the error and raise the exception received from Bunny. (default strategy)
- * `:ignore`: Log the error and return false.
- * `:resque`: Log the error and return false. Also, enqueue a job with Resque to send the message. See `send_message_async` below. **Note, this doesn't guarantee the message will actually be sent—it just guarantees an attempt is made to queue a Resque job [which could fail]**
-
-### Delayed Messages
You might want to delay sending a message (for example, if you have just created a database
record and a race condition keeps catching you out). In that case you can use delayed message
options:
```ruby
@@ -119,29 +124,29 @@
`delay_by` is an integer of milliseconds to delay the message. The default (if no value is set) is 5000 (5 seconds).
These extra arguments work for all message sending methods - the safe ones, the handling, and the `message_queuer` methods (see below).
-### Sending message Async with Resque
+#### Sending message Async with Resque
To enqueue a message in a background Resque job, use `Transmitter.send_message_async`
```ruby
Pwwka::Transmitter.send_message_async(payload, routing_key, delay_by_ms: 5000) # default delay is 0
```
If `Resque::Plugins::ExponentialBackoff` is available, the job will use it.
Customize the backoff intervals using the configuration `send_message_resque_backoff_strategy`.
The default backoff will retry quickly in case of an intermittent glitch, and then every ten
minutes for half an hour.
-
+
The name of the queue created is `pwwka_send_message_async`.
+#### Message Queuer
-### Message Queuer
You can queue up messages and send them in a batch. This is most useful when multiple messages
need to sent from within a transaction block.
-
+
For example:
```ruby
# instantiate a message_queuer object
message_queuer = MessageQueuerService.new
@@ -154,41 +159,43 @@
end
# send the queued messages if we make it out of the transaction alive
message_queuer.send_messages_safely
```
-## Receiving messages
+### Receiving messages
-The message-handler comes with a rake task you can use in your Procfile to start up your message handler worker:
+The message-handler comes with a rake task you can use (e.g. in your Procfile) to start up your message handler worker:
```ruby
message_handler: rake message_handler:receive HANDLER_KLASS=ClientIndexMessageHandler QUEUE_NAME=adminapp_style_index ROUTING_KEY='client.#.updated'
```
-* `HANDLER_KLASS` (required) refers to the class you have to write in you app (equivalent to a `job` in Resque)
+It requires some environment variables to work:
+
+* `HANDLER_KLASS` (required) refers to the class you have to write in your app (equivalent to a `job` in Resque)
* `QUEUE_NAME` (required) we must use named queues - see below
* `ROUTING_KEY` (optional) defaults to `#.#` (all messages)
You'll also need to bring the Rake task into your app. For Rails, you'll need to edit the top-level `Rakefile`:
```ruby
require 'pwwka/tasks'
```
-### Queues - what messages will your queue receive
+#### Queues - what messages will your queue receive
It depends on your `routing_key`. If you set your routing key to `#.#` (the default) it will receive all the messages. The `#` is a wildcard so if you set it to `client.#` it will receive any message with `client.` at the beginning. The exchange registers the queue's name and routing key so it knows what messages the queue is supposed to receive. A named queue will receive each message it expects to get once and only once.
The available wildcards are as follows (and are not intuitive):
* `*` (star) can substitute for **exactly one word**.
* `#` (hash) can substitute for zero or more words.
-__A note on re-queuing:__ At the moment messages that raise an error on receipt are marked 'not acknowledged, don't resend', and the failure message is logged. All unacknowledged messages will be resent when the worker is restarted. The next iteration of this gem will allow for a specified number of resends without requiring a restart.
+__A note on re-queuing:__ At the moment messages that raise an error on receipt are marked 'not acknowledged, don't resend', and the failure message is logged. You can configure a single retry by setting the configuration option `requeue_on_error`. Note that all unacknowledged messages will be resent when the worker is restarted.
__Spinning up some more handlers to handle the load:__ Since each named queue will receive each message only once you can spin up multiple process using the *same named queue* and they will share the messages between them. If you spin up three processes each will receive roughly one third of the messages, but each message will still only be received once.
-### Handlers
+#### Handlers - The class that handles received messages
Handlers are simple classes that must respond to `self.handle!`. The receiver will send the handler three arguments:
* `delivery_info` - [a bunch of stuff](http://rubybunny.info/articles/queues.html#accessing_message_delivery_information)
* `properties` - [a bunch of other stuff](http://rubybunny.info/articles/queues.html#accessing_message_properties_metadata)
@@ -197,59 +204,52 @@
Here is an example:
```ruby
class ClientIndexMessageHandler
- def initialize(payload)
- @payload = payload
- end
-
def self.handle!(delivery_info, properties, payload)
- # for this handler we only care about the payload
- handler = new(payload)
- handler.do_a_thing
+ handler.do_a_thing(payload)
end
- def do_a_thing
+private
+
+ def self.do_a_thing(payload)
###
# some stuff that is being done
###
end
end
```
-#### Handling Errors
-By default, if your handler raises an uncaught exception, the message will be Nacked, **but not requeued**. This means
-it's dropped on the floor and likely won't have been completely processed.
+#### Errors From Your Handler
-You can configure `requeue_on_error` in the configuration to change this behavior:
+By default, all unhandled errors will crash your handler. This is good, because it allows you to recover from most intermittent things. Just be aware of this when configuring your handler so that it gets
+restarted after a crash.
-```ruby
-require 'pwwka'
-Pwwka.configure do |config|
+What happens to the message you received during the error depends:
- # ...
+* If the error is not a `StandardError` or a subclass, the message will not be ack'ed and will be waiting on the queue for you when you next fetch a message
+* If the errors *is* a `StandardError` or a subclass, the message will be ack'ed and removed from the queue.
+ - By default, the message is not re-queued and is essentially dropped on the floor. Its payload is logged, so you can recover that way.
+ - If you set `requeue_on_error = true` in your Pwwka configuration, a message gets requeued **exactly once** on failure. If the message involved in the failure has been redelivered before, it's dropped on the floor. This behavior allows you to recover from most intermittent failures, like so:
+ 1. You receive message for the first time.
+ 1. Intermittent failure (e.g. network problem) happens, and an exception is raised.
+ 1. Pwwka catches this exception and requeues the message.
+ 1. Pwwka then crashes your handler.
+ 1. Your handler restarts.
+ 1. The message is in the queue, waiting for you.
+ 1. You handle it. (*if you error here, the message is not requeued*)
- config.requeue_on_error = true
-end
-```
+The reason we don't always requeue on error is that a hard failure would result in an infinite loop. The reason we don't use the dead letter exchange is that there is no way in the Rabbit console to deal with
+these messages. Some day Pwwka might have code to allow that. Today is not that day.
-This will requeue the message **exactly once**. It uses the headers to check if the message has been retried. If it
-hasn't, and your handler raises an exception, it will be placed back on the queue. The second time your handler
-processes it, there is a header indicating it's been retried, so if a failure happens again, the message **is not
-requeued**.
+**You should configure `requeue_on_error`**. It's not the default for backwards compatibility.
-Because requeuing puts the message at the head of the queue, a hard failure will result in an infinite loop, which will
-lead to filling up your queue. Nevertheless, this should address intermittent failures.
-
-**It is recommended that you set this option**. It's off for backwards compatibility.
-
#### Handling Messages with Resque
-If you use [Resque][resque], and you wish to handle messages in a resque job, you can use `Pwwka::QueueResqueJobHandler`, which is an adapter between the
-standard `handle!` method provided by pwwka and your Resque job.
+If you use [Resque][resque], and you wish to handle messages in a resque job, you can use `Pwwka::QueueResqueJobHandler`, which is an adapter between the standard `handle!` method provided by pwwka and your Resque job.
1. First, modify your `Gemfile` or otherwise arrange to include `pwwka/queue_resque_job_handler`:
```ruby
gem 'pwwka', require: [ 'pwwka', 'pwwka/queue_resque_job_handler' ]
@@ -260,11 +260,11 @@
```ruby
require 'pwwka/queue_resque_job_handler'
```
2. Now, configure your handler. For a `Procfile` setup:
-
+
```
my_handler: rake message_handler:receive HANDLER_KLASS=Pwwka::QueueResqueJobHandler JOB_KLASS=MyResqueJob QUEUE_NAME=my_queue ROUTING_KEY="my.key.completed"
```
Note the use of the environment variable `JOB_KLASS`. This tells `QueueResqueJobHandler` which class to queue.
@@ -286,47 +286,12 @@
do.
3. Profit!
[resque]: https://github.com/resque/resque/tree/1-x-stable
+### Testing
-## Monitoring
-
-RabbitMQ has a good API that should make it easy to set up some simple monitoring. In the meantime there is logging and manual monitoring.
-
-### Logging
-
-The receiver logs details of any exception raised in message handling:
-
-```ruby
-error "Error Processing Message on #{queue_name} -> #{payload}, #{delivery_info.routing_key}: #{e}"
-```
-
-The transmitter will likewise log an error if you use the `_safely` methods:
-
-```ruby
-error "Error Transmitting Message on #{routing_key} -> #{payload}: #{e}"
-```
-
-If your payloads are large, you may not want to log them 2-3 times per message. In that case, you can adjust `payload_logging` in the configuration:
-
-```ruby
-Pwwka.configuration.payload_logging = :info # The default - payloads appear at INFO and above log levels
-Pwwka.configuration.payload_logging = :error # Only log payloads for ERROR or FATAL messages
-Pwwka.configuration.payload_logging = :fatal # Only log payloads for FATAL messages
-```
-
-### Manual monitoring
-
-RabbitMQ has a web interface for checking out the health of connections, channels, exchanges and queues. Access it via the Heroku add-ons page for Enigma.
-
-
-
-
-
-## Testing
-
This gem has test coverage of interacting with RabbitMQ, so for unit tests, your best
strategy is to simply mock calls to `Pwwka::Transmitter`.
For integration tests, however, you can examine the actual message bus by setting up
the provided `Pwwka::TestHandler` like so:
@@ -364,14 +329,65 @@
expect(payload).to eq({ item: { id: 42, size: "L" } })
end
end
```
-The pwwka gem has tests for all its functionality, so testing your app is best done with mocks on this gem.
+[See CONTRIBUTING.md for details on testing this gem](CONTRIBUTING.md#testing)
-However, if you want to test the message bus end-to-end in your app you can use some helpers in `lib/pwwka/test_handler.rb`. See this gem's specs for examples of how to use them.
-[See CONTRIBUTING.md for details on testing this gem](CONTRIBUTING.md#testing)
+## Better Know a Message Bus
+
+If you aren't familiar with Rabbit or Message Busses, the idea is that messages can be sent “into the ether” with no particular
+destination. Subscribers can listen for those messages and choose to respond.
+
+For example, suppose a customer purchases an order. The app serving our public website sends a message that this has happened. Another
+app that sends emails will hear that message, and use it to trigger a receipt email to the customer. A yet other app that does financial
+reporting might hear this same message and record the sale to the company's ledger. The app serving our public website doesn't know about
+any of these things.
+
+### How Pwwka Uses Rabbit
+
+All transmitters and receivers share the same exchange. This means that all receivers can read all messages that any transmitter sends. To ensure that all messages are received by eveyone who wants them the Pwwka configures everything as follows:
+
+* The exchange is named and durable. If the service goes down and restarts the named exchange will return with the same settings so everyone can reconnect.
+* The receiver queues are all named and durable. If the service goes down and restarts the named queue will return with the same settings so everyone can reconnect, and with any unacknowledged messages waiting to be received.
+* All messages are sent as persistent and require acknowledgement. They will stick around and wait to be received and acknowledged by every queue that wants them, regardless of service interruptions.
+
+
+### Monitoring
+
+RabbitMQ has a good API that should make it easy to set up some simple monitoring. In the meantime there is logging and manual monitoring.
+
+#### Logging
+
+The receiver logs details of any exception raised in message handling:
+
+```ruby
+error "Error Processing Message on #{queue_name} -> #{payload}, #{delivery_info.routing_key}: #{e}"
+```
+
+The transmitter will likewise log an error if you use the `_safely` methods:
+
+```ruby
+error "Error Transmitting Message on #{routing_key} -> #{payload}: #{e}"
+```
+
+If your payloads are large, you may not want to log them 2-3 times per message. In that case, you can adjust `payload_logging` in the configuration:
+
+```ruby
+Pwwka.configuration.payload_logging = :info # The default - payloads appear at INFO and above log levels
+Pwwka.configuration.payload_logging = :error # Only log payloads for ERROR or FATAL messages
+Pwwka.configuration.payload_logging = :fatal # Only log payloads for FATAL messages
+```
+
+#### Manual monitoring
+
+RabbitMQ has a web interface for checking out the health of connections, channels, exchanges and queues. Your RabbitMQ provider should
+provide a link. If you are running Rabbit locally, the management interface is on port 15672 by default (or port 10002 if using the included `docker-compose.yml`). The user is "guest" and the password is "guest".
+
+
+
+
## Contributing
We're actively using Pwwka in production here at [Stitch Fix](http://technology.stitchfix.com/) and look forward to seeing Pwwka grow and improve with your help. Contributions are warmly welcomed.