Easy one?
Not even close

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As the included diagram portrays, in a common workflow, the worker instance will consume messages sent to a specified Amazon SQS from another service (e.g.: a web server or another worker). These messages will be received by the worker via POST requests. This eliminates the necessity of configuring a worker as an always-on service, as well as having to add code for reading and consuming messages from an AWS SQS queue. In other words, the worker is implemented as a standard RESTful API/Service that will react to a message sent to it at an specific endpoint via a POST request. This is an awesome approach by Amazon to microservices and reactive design.

The conversion of the SQS message to a POST request is executed by what AWS calls the "SQS Daemon" or "Sqsd". This is a simple daemon they pre-install in the worker tier instances that is constantly monitoring an specific AWS SQS queue (provided by configuration) for new messages. When new messages arrive, it constructs a POST request and sends it to a specific endpoint (also provided via configuration). If the endpoint consumes it without errors and returns a 2** HTTP Code in its response, the "Sqsd" deletes the message from the queue to signal that its consumption was successful.

However, even though this approach is extremely powerful, Amazon does not provide the code of this daemon as open source. Therefore, we have reproduced its behavior by creating our own version of the "Sqsd" free for everyone to use. Moreover, we have provided lots of customization and configuration properties so that it can be molded to your specific use cases.

spoiler alert: auto scaling is far to be magical

as usual with aws, everything is an object and for autoscaling u got several objects and several links between them
First you define a launch configuration : what type of machine you want to laucnh
Then an autoscaling group : it will use the launch configuration to create new EC2
In this autoscaling group, you have to define auto scaling policies ie what to do (remove or add x instances) and link them to a cloudwatch alert (cpu is high or network or whatever cloudwatch monitors)
the autoscaling group can also be linked to an ELB so when EC2 are added/removed, they also are registred/deregistered from ELB

via Doo

Cycle de vie des messages Amazon SQS

Les messages stockés dans Amazon SQS ont un cycle de vie facile à gérer, mais qui garantit le traitement de tous les messages.

Un système qui doit envoyer un message sélectionne une file d'attente Amazon SQS et utilise SendMessage pour lui envoyer un nouveau message.
Un autre système traitant des messages doit traiter plus de messages : il appelle donc ReceiveMessage, et ce message est renvoyé.
Dès lors qu'un message est renvoyé par ReceiveMessage, il ne sera pas renvoyé par une autre demande ReceiveMessage avant que le délai de visibilité ne soit expiré. Ainsi, plusieurs destinataires peuvent traiter le même message simultanément.
Si le système de traitement des messages termine avec succès le traitement de ce message, il appelle DeleteMessage, ce qui supprime le message de la file d'attente pour que personne d'autre ne le traite. Si ce système ne réussit pas à traiter le message, il sera alors lu par un autre appel ReceiveMessage dès que le délai de visibilité sera expiré.
Si vous avez associé une file d'attente de lettre morte à une file d'attente source, les messages seront déplacés vers la file d'attente de lettre morte lorsque le nombre de tentatives d'envoi que vous avez défini aura été atteint.