Integration / Apache NiFi Interview Questions

How does NiFi achieve guaranteed delivery and what are its durability guarantees?

NiFi's architecture is specifically designed to provide guaranteed delivery — once data enters NiFi, it will not be silently lost due to hardware failure, software crash, or network issues. Several design decisions work together to achieve this.

Persistent connection queues: FlowFiles in connection queues are tracked in the FlowFile Repository's Write-Ahead Log. On restart after a crash, NiFi replays the WAL to restore every FlowFile to its exact queue position before the crash.

Immutable content repository: FlowFile content is written to disk before the FlowFile is considered active. Content is never deleted until all references are removed. Even if NiFi crashes mid-write, the old content version is preserved.

Transactional processor sessions: Each processor invocation runs within a ProcessSession. All changes within a session are committed atomically. If the processor throws an exception before committing, the session is rolled back: all FlowFiles return to their input queues as if nothing happened.

Site-to-Site acknowledgment: Data transferred via S2S is acknowledged by the receiver before the sender removes FlowFiles from its queues. If the receiver crashes before acknowledgment, the sender retains the data and retries.

The practical implication: NiFi provides at-least-once delivery by default. Under failure and retry scenarios, a FlowFile may be processed more than once. Achieving exactly-once requires idempotent downstream systems or explicit deduplication logic in the flow.

What delivery guarantee does NiFi provide by default, and what scenario can violate exactly-once semantics?At-least-once — a processor crash after writing but before session commit causes the FlowFile to be retried, potentially duplicating the write

✓ Well done — NiFi's session rollback on failure re-queues the FlowFile for retry, which can cause duplicate writes to non-idempotent sinks.

Exactly-once — the transactional session prevents any duplicate processing

✗ Try again — NiFi sessions are transactional within NiFi, but external writes may succeed before the session commits, causing duplicates on retry.

Best-effort — NiFi may silently drop FlowFiles under high load

✗ Try again — NiFi's persistent WAL and back-pressure prevent silent data loss; it is at-least-once, not best-effort.

What happens to a ProcessSession's changes if the processor throws an uncaught exception before calling session.commit()?The session is rolled back and all affected FlowFiles are returned to their input queues

✓ Well done — NiFi automatically rolls back uncommitted sessions on exception, restoring FlowFiles for retry without data loss.

The session is partially committed up to the point of failure

✗ Try again — NiFi sessions are all-or-nothing; partial commits are not possible.

The FlowFiles are routed to the processor's failure relationship automatically

✗ Try again — on unhandled exceptions, the session rolls back; routing to failure requires explicit session.transfer() calls within a try-catch.

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More Related questions...

What is Apache NiFi and what problem does it solve? What is a FlowFile in Apache NiFi? What are the three NiFi repositories and what does each store? What is a Processor in Apache NiFi and what are the main processor categories? What is a Connection in NiFi and how does back-pressure work? What is NiFi Expression Language and where can it be used? What is data provenance in Apache NiFi and how do you access it? What is a Process Group in NiFi and why is it used? What is NiFi Registry and how does it integrate with NiFi? How does NiFi clustering work and what is the role of ZooKeeper? What is a Controller Service in NiFi and how is it different from a Processor? What is the GenerateTableFetch and QueryDatabaseTable pattern for incremental database ingestion? What is the Record-based processing model in NiFi and why is it preferred? What is State Management in NiFi and what types of state scope exist? What is NiFi Site-to-Site (S2S) and when do you use it? What is NiFi and how does it relate to Apache NiFi? What is NiFi Parameter Context and how does it differ from Variables? How does NiFi handle security — TLS, authentication, and authorization? What is the NiFi NAR (NiFi Archive) classloading model? What are Reporting Tasks in NiFi and what are common use cases? How do you handle errors and failures in a NiFi flow? What is the SplitText processor and how do you control split behavior? What is the MergeContent processor and how is it used? What is the InvokeHTTP processor and what are key configuration considerations? What is the PublishKafka and ConsumeKafka processor pair and what are key configuration options? What is the ExecuteScript processor and what scripting languages does it support? What is the JoltTransformJSON processor and how do you use it? What is the PutDatabaseRecord processor and how does it differ from ExecuteSQL? What is the ListSFTP and FetchSFTP processor pattern and how does it work? What is the LookupRecord processor used for? What is the PartitionRecord processor and what is a common use case? What is the ConvertRecord processor and how is it used for format conversion? What are the NiFi processor scheduling strategies? What is the difference between EvaluateJsonPath and FlattenJson processors? How does NiFi integrate with Apache Hadoop and HDFS? What is the UpdateAttribute processor and how is its Advanced Mode used? How do you implement deduplication in a NiFi flow? What is the HandleHttpRequest and HandleHttpResponse processor pair used for? How does NiFi achieve guaranteed delivery and what are its durability guarantees? What is the Funnel component in NiFi and when do you use it? What is the difference between GetFile and ListFile + FetchFile processors? How does NiFi support schema evolution in data pipelines? What is the RouteText processor and how does it differ from RouteOnContent? What performance tuning options are available in NiFi and what are common bottleneck patterns? How does NiFi integrate with cloud storage services like Amazon S3?

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Integration / Apache NiFi Interview Questions

How does NiFi achieve guaranteed delivery and what are its durability guarantees?

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