Amazon SQS
Amazon Simple Queue Service (SQS) is a highly scalable distributed message queuing service provided by Amazon.com. AWS SQS offers two types of message queues:
Standard
- Nearly unlimited throughput
- Best-effort ordering
- At-least-once delivery
FIFO
- Limited number of transactions per second (TPS). See Amazon SQS FIFO developer guide for more information on limits.
- Order in which messages are sent/received is strictly preserved
- Exactly-once delivery
Broadway can work seamlessly with both, Standard and FIFO queues.
Getting Started
In order to use Broadway with SQS, we need to:
- Create a SQS queue (or use an existing one)
- Configure our Elixir project to use Broadway
- Define your pipeline configuration
- Implement Broadway callbacks
- Run the Broadway pipeline
- Tuning the configuration (Optional)
Create a SQS queue
Amazon provides a comprehensive Step-by-step Guide on creating SQS queues. In case you don't have an AWS account and want to test Broadway locally, use can easily install ElasticMQ, which is a message queue system that offers a SQS-compatible query interface.
Configure the project
In this guide we're going to use BroadwaySQS, which is a Broadway SQS Connector provided by Dashbit.
Starting a new project
If you plan to start a new project, just run:
$ mix new my_app --sup
The --sup
flag instructs Elixir to generate an application with a supervision tree.
Setting up dependencies
Add :broadway_sqs
to the list of dependencies in mix.exs
along the HTTP
client of your choice (defaults to :hackney
):
def deps do
[
...
{:broadway_sqs, "~> 0.7"},
{:hackney, "~> 1.9"},
]
end
Don't forget to check for the latest version of dependencies.
Define the pipeline configuration
Broadway is a process-based behaviour and to define a Broadway
pipeline, we need to define three functions: start_link/1
,
handle_message/3
and handle_batch/4
. We will cover start_link/1
in this section and the handle_
callbacks in the next one.
Similar to other process-based behaviour, start_link/1
simply
delegates to Broadway.start_link/2
, which should define the
producers, processors, and batchers in the Broadway pipeline.
Assuming we want to consume messages from a queue called
my_queue
, the minimal configuration would be:
defmodule MyBroadway do
use Broadway
alias Broadway.Message
def start_link(_opts) do
Broadway.start_link(__MODULE__,
name: __MODULE__,
producer: [
module: {BroadwaySQS.Producer,
queue_url: "https://us-east-2.queue.amazonaws.com/100000000001/my_queue"}
],
processors: [
default: []
],
batchers: [
default: [
batch_size: 10,
batch_timeout: 2000
]
]
)
end
...callbacks...
end
The above configuration also assumes that you have the AWS credentials
set up in your environment, for instance, by having the AWS_ACCESS_KEY_ID
and AWS_SECRET_ACCESS_KEY
environment variables set. If that's
not the case, you will need to pass that information to the client so it
can properly connect to the AWS servers. Here is how you can do it:
...
producer: [
module:
{BroadwaySQS.Producer,
queue_url: "https://us-east-2.queue.amazonaws.com/100000000001/my_queue",
config: [
access_key_id: "YOUR_AWS_ACCESS_KEY_ID",
secret_access_key: "YOUR_AWS_SECRET_ACCESS_KEY"
]}
]
...
For a full list of options for BroadwaySQS.Producer
, please see
BroadwaySQS documentation.
For general information about setting up Broadway, see Broadway
module docs as well as Broadway.start_link/2
.
Note: Even though batching is optional since Broadway v0.2, we recommend that all Amazon SQS pipelines have at least a default batcher. This lets you control the exact batch size and frequency that messages are acknowledged to Amazon SQS, often leading to pipelines that are more cost and time efficient.
Implement Broadway callbacks
In order to process incoming messages, we need to implement the required callbacks. For the sake of simplicity, we're considering that all messages received from the queue are just numbers:
defmodule MyBroadway do
use Broadway
alias Broadway.Message
...start_link...
@impl true
def handle_message(_, %Message{data: data} = message, _) do
message
|> Message.update_data(fn data -> data * data end)
end
@impl true
def handle_batch(_, messages, _, _) do
list = messages |> Enum.map(fn e -> e.data end)
IO.inspect(list, label: "Got batch of finished jobs from processors, sending ACKs to SQS as a batch.")
messages
end
end
We are not doing anything fancy here, but it should be enough for our
purpose. First we update the message's data individually inside
handle_message/3
and then we print each batch inside handle_batch/4
.
For more information, see Broadway.handle_message/3
and
Broadway.handle_batch/4
.
Run the Broadway pipeline
To run your Broadway
pipeline, you just need to add as a child in
a supervision tree. Most applications have a supervision tree defined
at lib/my_app/application.ex
. You can add Broadway as a child to a
supervisor as follows:
children = [
{MyBroadway, []}
]
Supervisor.start_link(children, strategy: :one_for_one)
Now the Broadway pipeline should be started when your application starts. Also, if your Broadway has any dependency (for example, it needs to talk to the database), make sure that Broadway is listed after its dependencies in the supervision tree.
Tuning the configuration
Some of the configuration options available for Broadway come already with a
"reasonable" default value. However those values might not suit your
requirements. Depending on the number of messages you get, how much processing
they need and how much IO work is going to take place, you might need completely
different values to optimize the flow of your pipeline. The concurrency
option
available for every set of producers, processors and batchers, among with
max_demand
, batch_size
, and batch_timeout
can give you a great deal
of flexibility.
The concurrency
option controls the concurrency level in each layer of
the pipeline.
See the notes on Producer concurrency
and Batcher concurrency
for details.
Here's an example on how you could tune them according to your needs.
defmodule MyBroadway do
use Broadway
def start_link(_opts) do
Broadway.start_link(__MODULE__,
name: __MODULE__,
producer: [
...
concurrency: 10,
],
processors: [
default: [
concurrency: 100,
max_demand: 1,
]
],
batchers: [
default: [
batch_size: 10,
concurrency: 10,
]
]
)
end
...callbacks...
end
In order to get a good set of configurations for your pipeline, it's important to respect the limitations of the servers you're running, as well as the limitations of the services you're providing/consuming data to/from. Broadway comes with telemetry, so you can measure your pipeline and help ensure your changes are effective.