Question 35 of 50
What is DLQ or Dead Letter Topic?
Question
"What is a DLQ (Dead Letter Queue/Topic)?"
What the interviewer wants to assess
Understanding of the DLQ's role as a failure-isolation mechanism — not just "where messages with errors end up."
Resposta rápida
DLQ (Dead Letter Topic, in Kafka's vocabulary) is a separate topic where messages that repeatedly failed are published, once retry attempts are exhausted — allowing manual investigation without blocking processing of the following messages.
Resposta nível Sênior
The DLQ's role is to isolate the poison pill effect: without it, a message that fails indefinitely would stall processing of everything after it in the same partition. By moving the problematic message to a separate topic once retry attempts run out, the main consumer keeps processing the following messages normally. The DLQ doesn't fix the problem — it only isolates it for later investigation, whether that's manual reprocessing after fixing the root cause, or simply discarding it, if the message is genuinely invalid.
In-depth explanation
See "DLQ in Kafka: it's not native" in Chapter 9.
Exemplo financeiro
A BoletoGerado event with a malformed tax ID fails all 3 configured retry attempts and is published to
boletos.gerados.DLT — meanwhile, the following invoices in the same partition keep being processed with
no delay.
"The DLQ fixes the problem automatically"
The DLQ only isolates the problematic message — it doesn't try to fix or reprocess it on its own. Someone (or some process) needs to investigate the failure's cause and decide what to do with the isolated message.
Pode vir a seguir
Likely follow-ups: "does Kafka have native DLQ?" and "what is a Poison Pill?".
Related chapters