Is there any way MQXR can be considered scalable?
If the subscribers don't know who the publishers are - how is it possible to ensure load-balanced publishers can publish to load balanced subscribers?
If my terminology is all wrong I'm happy to be corrected.
For any given message there is only ever a single publisher, so I'm not sure how you would get "load balanced publishers"
Load balancing messages across subscribers (to handle more messages than a single subscriber could process or to provide redundancy if a processing subscriber fails) can be handled by using Shared Subscriptions where the broker will ensure that each message is only sent to a single subscribe as part of a group.
Related
Let's say on twitter, a celebrity update her status, and it's pushed to all of her followers.
If we set up queue, where the publisher is service that fetches the celebrity's status, and the consumer are the individual followers. But with a million followers, whoever receives that message first will see the update, while others will not ? What is a common pattern to use here so that every one of her followers will see the update and not 'compete' with each other to consume the message first?
I guess you are thinking of queueing system as only having the ability to make point-to-point communication, ie, producer to queue to consumer. This is partly correct. Most queueing systems have atleast two patterns:
Producer-consumer: In this scenario, a message is delivered to just one consumer, ie, if there are multiple consumers, they are competing against each other to get the message from the "queue".
Publish-subscribe: Here, publishers push the message on a "topic" and consumers subscribe to the topic to get messages. The consumers are not competing - each consumer gets all the messages once they have subscribed.
In your example, it's publish-subscribe pattern in action. Underlying implementation may be different, but the basic pattern is that of publish-subscribe.
Refer: https://stackoverflow.com/a/42477769/6886283
I'm building a system which sends commands to external systems (all identical but with different locations and IDs).
The commands are sent to a FIFO SQS queue and the external systems read and delete from that queue.
Currently the plan is to create one queue for each external system, so I'd just have a Lambda that updates the list of queues when the DB table of systems is changed.
But I can see that the SQS FIFO supports message group IDs so I wonder if I should just have one single queue, where all systems only read from their own message group ID.
I like the simplicity of this solution - however, I cannot see a way to limit access for reading and deleting messages for a specific message group, which means that if one external system is compromised, its credentials can be used to hijack the shared queue for all external systems and therefore, take down everything.
Is there a workaround for this, so I can set some permissions for a specific queue and message group ID, in any way?
I am also concerned about the missing option of purging only one group of messages, not the entire queue.
You can't read "from" a specific message group in a FIFO Queue, and there are no related permissions.
Message groups are opaque labels that tell the FIFO queue whether any two messages must be delivered to consumers in strict FIFO order relative to each other. If two messages share the same message group, they must be strictly ordered, but two messages with different message-group-ids do not need to be strictly ordered.
This capability allows faster overall processing of messages when there are parallel identical consumers, because without this feature, only one consumer could be handling messages at any one time, and the overall throughput of the queue would be limited to how quickly a single consumer could handle a message (since no messages would be delivered to another consumer as long as a single message was in flight).
Message Group ID
The tag that specifies that a message belongs to a specific message group. Messages that belong to the same message group are always processed one by one, in a strict order relative to the message group (however, messages that belong to different message groups might be processed out of order).
[...]
If you require a single group of ordered messages, provide the same message group ID for messages sent to the FIFO queue.
https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/FIFO-queues.html
Note also that while the ReceiveMessage API allows you to ask that the message-group-id be returned with each message, it has no provision for specifying which message-group-id you want to receive messages from, because that isn't the purpose of this feature.
I have a multiple publishers publishing events for a shipment entity on an SQS queue and I have multiple listeners on it for parallel processing. But I want events for a particular shipment (having some identifier) to be processed sequentially in order. Is there any in-built feature to support this?
ActiveMQ has a similar concept of Exclusive Consumer which is not exactly what I need but could be adapted
Yes, there is; they are called FIFO (First-In-First-Out) queues
FIFO (First-In-First-Out) queues are designed to enhance messaging between applications when the order of operations and events is critical, or where duplicates can't be tolerated.
You will need to ensure that the messages you want processed in the correct order belong to the same Message Group ID:-
The tag that specifies that a message belongs to a specific message group. Messages that belong to the same message group are always processed one by one, in a strict order relative to the message group (however, messages that belong to different message groups might be processed out of order).
Hope that helps!
I'm using an Amazon SQS queue to send notifications to an external system.
If the HTTP request fails when using SQS' SendMessage, I don't know whether the message has been queued or not. My default policy would be to retry posting the message to the queue, but there's a risk to post the message twice, which might not be acceptable depending on the use case.
Is there a way to have SQS refuse the message if there is a duplicate on the message body (or some kind of message metadata, such as a unique ID we could provide) so that we could retry until the message is accepted, and be confident that there won't be a duplicate if the first request had been already queued, but the response had been lost?
No, there's no such mechanism in SQS. Going further, it is also possible that a message will be delivered twice or more (at-least-once delivery semantics). So even if such a mechanism existed, you wouldn't be able to guarantee that the message isn't delivered multiple times.
See: http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/DistributedQueues.html
For exactly-once deliveries, you need some form of transactions (and HTTP isn't a transactional protocol) both on the sending and receiving end.
AFAIK, right now SQS does support what was asked!
Please see the "What's new" post entitled Amazon SQS Introduces FIFO Queues with Exactly-Once Processing and Lower Prices for Standard Queues
According to SQS FAQ:
FIFO queues provide exactly-once processing, which means that each message is delivered once and remains available until a consumer processes it and deletes it. Duplicates are not introduced into the queue.
There's also an AWS Blog post with a bit more insight on the subject:
These queues are designed to guarantee that messages are processed exactly once, in the order that they are sent, and without duplicates.
......
Exactly-once processing applies to both single-consumer and multiple-consumer scenarios. If you use FIFO queues in a multiple-consumer environment, you can configure your queue to make messages visible to other consumers only after the current message has been deleted or the visibility timeout expires. In this scenario, at most one consumer will actively process messages; the other consumers will be waiting until the first consumer finishes or fails.
Duplicate messages can sometimes occur when a networking issue outside of SQS prevents the message sender from learning the status of an action and causes the sender to retry the call. FIFO queues use multiple strategies to detect and eliminate duplicate messages. In addition to content-based deduplication, you can include a MessageDeduplicationId when you call SendMessage for a FIFO queue. The ID can be up to 128 characters long, and, if present, takes higher precedence than content-based deduplication.
According to the documentation:
Q: How many times will I receive each message?
Amazon SQS is
engineered to provide “at least once” delivery of all messages in its
queues. Although most of the time each message will be delivered to
your application exactly once, you should design your system so that
processing a message more than once does not create any errors or
inconsistencies.
Is there any good practice to achieve the exactly-once delivery?
I was thinking about using the DynamoDB “Conditional Writes” as distributed locking mechanism but... any better idea?
Some reference to this topic:
At-least-once delivery (Service Behavior)
Exactly-once delivery (Service Behavior)
FIFO queues are now available and provide ordered, exactly once out of the box.
https://aws.amazon.com/sqs/faqs/#fifo-queues
Check your region for availability.
The best solution really depends on exactly how critical it is that you not perform the action suggested in the message more than once. For some actions such as deleting a file or resizing an image it doesn't really matter if it happens twice, so it is fine to do nothing. When it is more critical to not do the work a second time I use an identifier for each message (generated by the sender) and the receiver tracks dups by marking the ids as seen in memchachd. Fine for many things, but probably not if life or money depends on it, especially if there a multiple consumers.
Conditional writes sound like a clever solution, but it has me wondering if perhaps AWS isn't such a great solution for your problem if you need a bullet proof exactly-once solution.
Another alternative for distributed locking is Redis cluster, which can also be provisioned with AWS ElasticCache. Redis supports transactions which guarantee that concurrent calls will get executed in sequence.
One of the advantages of using cache is that you can set expiration timeouts, so if your message processing fails the lock will get timed release.
In this blog post the usage of a low-latency control database like Amazon DynamoDB is also recommended:
https://aws.amazon.com/blogs/compute/new-for-aws-lambda-sqs-fifo-as-an-event-source/
Amazon SQS FIFO queues ensure that the order of processing follows the
message order within a message group. However, it does not guarantee
only once delivery when used as a Lambda trigger. If only once
delivery is important in your serverless application, it’s recommended
to make your function idempotent. You could achieve this by tracking a
unique attribute of the message using a scalable, low-latency control
database like Amazon DynamoDB.
In short - we can put item or update item in dynamodb table with condition expretion attribute_not_exists(for put) or if_not_exists(for update), please check example here
https://stackoverflow.com/a/55110463/9783262
If we get an exception during put/update operations, we have to return from a lambda without further processing, if not get it then process the message (https://aws.amazon.com/premiumsupport/knowledge-center/lambda-function-idempotent/)
The following resources were helpful for me too:
https://ably.com/blog/sqs-fifo-queues-message-ordering-and-exactly-once-processing-guaranteed
https://aws.amazon.com/blogs/aws/introducing-amazon-sns-fifo-first-in-first-out-pub-sub-messaging/
https://youtu.be/8zysQqxgj0I