Add docs about situations where messages require a DLQ

docs/configuration/source-config.md

@@ -78,7 +78,10 @@ EOF
 
 - Quickwit does not automatically delete the source files after a successful ingestion. You can use [S3 object expiration](https://docs.aws.amazon.com/AmazonS3/latest/userguide/lifecycle-expire-general-considerations.html) to configure how long they should be retained in the bucket.
 - Configure the notification to only forward events of type `s3:ObjectCreated:*`. Other events are acknowledged by the source without further processing and an warning is logged.
-- We strongly recommend using a [dead letter queue](https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-dead-letter-queues.html) to receive all messages that couldn't be processed by the file source. A `maxReceiveCount` of 5 is a good default value.
+- We strongly recommend using a [dead letter queue](https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-dead-letter-queues.html) to receive all messages that couldn't be processed by the file source. A `maxReceiveCount` of 5 is a good default value. Here are some common situations where the notification message ends up in the dead letter queue:
+  - the notification message could not be parsed (e.g it is not a valid S3 notification)
+  - the file was not found
+  - the file is corrupted (e.g unexpected compression)
 
 :::
 

quickwit/quickwit-indexing/src/source/queue_sources/coordinator.rs

@@ -176,7 +176,7 @@ impl QueueCoordinator {
                 limit_per_min = 10,
                 count = format_errors.len(),
                 last_err = ?format_errors.last().unwrap(),
-                "invalid message(s) not processed, use a dead letter queue to limit retries"
+                "invalid messages not processed, use a dead letter queue to limit retries"
             );
         }
         if preprocessed_messages.is_empty() {
@@ -268,7 +268,7 @@ impl QueueCoordinator {
                     rate_limited_error!(
                         limit_per_min = 5,
                         err = ?err,
-                        "failed to start processing message"
+                        "failed to start message processing"
                     );
                 }
             }