# Question about fd8: Fetch multiple messages from disk. Data isn't being prepended to response from log file correctly

**URL:** <https://forum.codecrafters.io/t/question-about-fd8-fetch-multiple-messages-from-disk-data-isnt-being-prepended-to-response-from-log-file-correctly/15691>\
**Category:** Challenges\
**Tags:** challenge:kafka\
**Created:** [December 25, 2025, 10:29pm UTC](https://forum.codecrafters.io/t/question-about-fd8-fetch-multiple-messages-from-disk-data-isnt-being-prepended-to-response-from-log-file-correctly/15691 "2025-12-25T22:29:50Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![whorst](https://yyz1.discourse-cdn.com/flex003/user_avatar/forum.codecrafters.io/whorst/32/19953_2.png) [@whorst](https://forum.codecrafters.io/u/whorst)\
**Post date:** [December 25, 2025, 10:29pm UTC](https://forum.codecrafters.io/t/question-about-fd8-fetch-multiple-messages-from-disk-data-isnt-being-prepended-to-response-from-log-file-correctly/15691/1 "2025-12-25T22:29:50Z")

</div>

Seems like I’m having trouble parsing the log partition file incorrectly. I’m receiving the following from the command line:

```auto
remote: [tester::#FD8] [client] Successfully connected to broker at localhost:9092
remote: [tester::#FD8] [Encoder] - FetchRequest
remote: [tester::#FD8] [Encoder] - Header
remote: [tester::#FD8] [Encoder] - APIKey (1)
remote: [tester::#FD8] [Encoder] - APIVersion (16)
remote: [tester::#FD8] [Encoder] - CorrelationID (330266902)
remote: [tester::#FD8] [Encoder] - ClientID (kafka-tester)
remote: [tester::#FD8] [Encoder] - Body
remote: [tester::#FD8] [Encoder] - MaxWaitMS (500)
remote: [tester::#FD8] [Encoder] - MinBytes (1)
remote: [tester::#FD8] [Encoder] - MaxBytes (2147483647)
remote: [tester::#FD8] [Encoder] - IsolationLevel (0)
remote: [tester::#FD8] [Encoder] - SessionID (0)
remote: [tester::#FD8] [Encoder] - SessionEpoch (0)
remote: [tester::#FD8] [Encoder] - Topics
remote: [tester::#FD8] [Encoder] - Length (2 (Array length(1) + 1))
remote: [tester::#FD8] [Encoder] - Topics[0]
remote: [tester::#FD8] [Encoder] - Topic
remote: [tester::#FD8] [Encoder] - UUID (71a59a51-8968-4f8b-937e-1b233c64eacb)
remote: [tester::#FD8] [Encoder] - Partitions
remote: [tester::#FD8] [Encoder] - Length (2 (Array length(1) + 1))
remote: [tester::#FD8] [Encoder] - Partitions[0]
remote: [tester::#FD8] [Encoder] - Partition
remote: [tester::#FD8] [Encoder] - ID (0)
remote: [tester::#FD8] [Encoder] - CurrentLeaderEpoch (-1)
remote: [tester::#FD8] [Encoder] - FetchOffset (0)
remote: [tester::#FD8] [Encoder] - LastFetchedOffset (-1)
remote: [tester::#FD8] [Encoder] - LogStartOffset (-1)
remote: [tester::#FD8] [Encoder] - PartitionMaxBytes (2147483647)
remote: [tester::#FD8] [Encoder] - ForgottenTopics
remote: [tester::#FD8] [Encoder] - Length (1 (Array length(0) + 1))
remote: [tester::#FD8] [Encoder] - RackID ("")
remote: [tester::#FD8] [client] Sending "Fetch" request
remote: [tester::#FD8] [client] Hexdump of sent "Fetch" request: 
remote: [tester::#FD8] [client] Idx | Hex | ASCII
remote: [tester::#FD8] [client] -----+-------------------------------------------------+-----------------
remote: [tester::#FD8] [client] 0000 | 00 00 00 63 00 01 00 10 13 af 79 16 00 0c 6b 61 | ...c......y...ka
remote: [tester::#FD8] [client] 0010 | 66 6b 61 2d 74 65 73 74 65 72 00 00 00 01 f4 00 | fka-tester......
remote: [tester::#FD8] [client] 0020 | 00 00 01 7f ff ff ff 00 00 00 00 00 00 00 00 00 | ................
remote: [tester::#FD8] [client] 0030 | 02 71 a5 9a 51 89 68 4f 8b 93 7e 1b 23 3c 64 ea | .q..Q.hO..~.#<d.
remote: [tester::#FD8] [client] 0040 | cb 02 00 00 00 00 ff ff ff ff 00 00 00 00 00 00 | ................
remote: [tester::#FD8] [client] 0050 | 00 00 ff ff ff ff ff ff ff ff ff ff ff ff 7f ff | ................
remote: [tester::#FD8] [client] 0060 | ff ff 00 00 01 01 00 | .......
remote: [tester::#FD8] [client] 
remote: [tester::#FD8] [client] Hexdump of received "Fetch" response: 
remote: [tester::#FD8] [client] Idx | Hex | ASCII
remote: [tester::#FD8] [client] -----+-------------------------------------------------+-----------------
remote: [tester::#FD8] [client] 0000 | 00 00 00 93 13 af 79 16 00 00 00 00 00 00 00 00 | ......y.........
remote: [tester::#FD8] [client] 0010 | 00 00 00 02 71 a5 9a 51 89 68 4f 8b 93 7e 1b 23 | ....q..Q.hO..~.#
remote: [tester::#FD8] [client] 0020 | 3c 64 ea cb 02 00 00 00 00 00 00 00 00 00 00 00 | <d..............
remote: [tester::#FD8] [client] 0030 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................
remote: [tester::#FD8] [client] 0040 | 00 00 00 01 00 00 00 00 4b 00 00 00 00 00 00 00 | ........K.......
remote: [tester::#FD8] [client] 0050 | 00 00 00 00 3e 00 00 00 00 02 07 c6 ce e2 00 00 | ....>...........
remote: [tester::#FD8] [client] 0060 | 00 00 00 00 00 00 01 91 e0 5b 6d 8b 00 00 01 91 | .........[m.....
remote: [tester::#FD8] [client] 0070 | e0 5b 6d 8b 00 00 00 00 00 00 00 00 00 00 00 00 | .[m.............
remote: [tester::#FD8] [client] 0080 | 00 00 00 00 00 01 18 00 00 00 01 0c 6f 72 61 6e | ............oran
remote: [tester::#FD8] [client] 0090 | 67 65 01 00 00 00 00 | ge.....
remote: [tester::#FD8] [client] 
remote: [tester::#FD8] [Decoder] - FetchResponse
remote: [tester::#FD8] [Decoder] - Header
remote: [tester::#FD8] [Decoder] - CorrelationID (330266902)
remote: [tester::#FD8] [Decoder] - Body
remote: [tester::#FD8] [Decoder] - ThrottleTimeMS (0)
remote: [tester::#FD8] [Decoder] - ErrorCode (0)
remote: [tester::#FD8] [Decoder] - SessionID (0)
remote: [tester::#FD8] [Decoder] - Topics
remote: [tester::#FD8] [Decoder] - Length (2 (Array length(1) + 1))
remote: [tester::#FD8] [Decoder] - Topics[0]
remote: [tester::#FD8] [Decoder] - UUID (71a59a51-8968-4f8b-937e-1b233c64eacb)
remote: [tester::#FD8] [Decoder] - Partitions
remote: [tester::#FD8] [Decoder] - Length (2 (Array length(1) + 1))
remote: [tester::#FD8] [Decoder] - Partitions[0]
remote: [tester::#FD8] [Decoder] - Id (0)
remote: [tester::#FD8] [Decoder] - ErrorCode (0)
remote: [tester::#FD8] [Decoder] - HighWaterMark (0)
remote: [tester::#FD8] [Decoder] - LastStableOffset (0)
remote: [tester::#FD8] [Decoder] - LogStartOffset (0)
remote: [tester::#FD8] [Decoder] - AbortedTransactions
remote: [tester::#FD8] [Decoder] - Length (1 (Array length(0) + 1))
remote: [tester::#FD8] [Decoder] - PreferredReadReplica (0)
remote: [tester::#FD8] [Decoder] - RecordBatches
remote: [tester::#FD8] [Decoder] - Size (75 (Record size(74) + 1))
remote: [tester::#FD8] [Decoder] - RecordBatches[0]
remote: [tester::#FD8] [Decoder] - Offset (0)
remote: [tester::#FD8] [Decoder] - Length (62)
remote: [tester::#FD8] [Decoder] - PartitionLeaderEpoch (0)
remote: [tester::#FD8] [Decoder] - MagicByte (2)
remote: [tester::#FD8] [Decoder] - CRC (130469602)
remote: [tester::#FD8] [Decoder] - Attributes (0)
remote: [tester::#FD8] [Decoder] - LastOffsetDelta (0)
remote: [tester::#FD8] [Decoder] - FirstTimeStamp (1726045973899)
remote: [tester::#FD8] [Decoder] - MaxTimeStamp (1726045973899)
remote: [tester::#FD8] [Decoder] - ProducerID (0)
remote: [tester::#FD8] [Decoder] - ProducerEpoch (0)
remote: [tester::#FD8] [Decoder] - BaseSequence (0)
remote: [tester::#FD8] [Decoder] - Records
remote: [tester::#FD8] [Decoder] - Length (1)
remote: [tester::#FD8] [Decoder] - Records[0]
remote: [tester::#FD8] [Decoder] - Record
remote: [tester::#FD8] [Decoder] - Length (12)
remote: [tester::#FD8] [Decoder] - Attributes (0)
remote: [tester::#FD8] [Decoder] - TimestampDelta (0)
remote: [tester::#FD8] [Decoder] - OffsetDelta (0)
remote: [tester::#FD8] [Decoder] - KeyLength (-1)
remote: [tester::#FD8] [Decoder] - ValueLength (6)
remote: [tester::#FD8] [Decoder] - Value ([111 114 97 110 103 101] -> UTF-8: (orange))
remote: [tester::#FD8] [Decoder] - HeadersLength (-1)
remote: [tester::#FD8] ✓ TopicResponses Length: 1
remote: [tester::#FD8] ✓ TopicResponse[0] TopicUUID: 71a59a51-8968-4f8b-937e-1b233c64eacb
remote: [tester::#FD8] ✓ PartitionResponses Length: 1
remote: [tester::#FD8] ✓ PartitionResponse[0] ErrorCode: 0 (NO_ERROR)
remote: [tester::#FD8] ✓ PartitionResponse[0] PartitionId: 0
remote: [tester::#FD8] 
remote: [tester::#FD8] Expected (bytes 24-124), hexadecimal: | ASCII:
remote: [tester::#FD8] 00 00 00 00 00 01 91 e0 5b 6d 8b 00 00 01 91 e0 5b 6d 8b 00 | ........[m......[m..
remote: [tester::#FD8] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 18 00 00 | ....................
remote: [tester::#FD8] 00 01 0c 6f 72 61 6e 67 65 01 | ...orange.
remote: [tester::#FD8] 
remote: [tester::#FD8] Actual (bytes 24-124), hexadecimal: | ASCII:
remote: [tester::#FD8] 00 00 00 00 00 01 91 e0 5b 6d 8b 00 00 01 91 e0 5b 6d 8b 00 | ........[m......[m..
remote: [tester::#FD8] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 18 00 00 | ....................
remote: [tester::#FD8] 00 01 0c 6f 72 61 6e 67 65 01 00 00 00 00 00 00 00 01 00 00 | ...orange...........
remote: [tester::#FD8] 00 3d 00 00 00 00 02 80 d7 c9 b3 00 00 00 00 00 00 00 00 01 | .=..................
remote: [tester::#FD8] 91 e0 5b 6d 8b 00 00 01 91 e0 5b 6d 8b 00 00 00 00 00 00 00 | ..[m......[m........
remote: [tester::#FD8] remote: [tester::#FD8] RecordBatch bytes do not match with the contents on disk
remote: [tester::#FD8] Test failed

```

I think simply appending the entire data file as I am [here](https://github.com/whorst/codecrafters-kafka/blob/master/infrastructure/adapters/repository/partition_repository/partition_file_repository.go#L43) (not much mutation) is correct. And [here](https://github.com/whorst/codecrafters-kafka/blob/master/infrastructure/adapters/parser/kafka_protocol_parser_fetch.go#L393) is where I add all the data together. What could be happening? Adding screenshots for readability

 ![Screenshot 2025-12-25 at 5.28.18 PM](https://canada1.discourse-cdn.com/flex003/uploads/codecrafters/original/3X/8/1/818a8e0b0e895f630a9d052b25f45ec841ea64e2.jpeg)

 ![Screenshot 2025-12-25 at 5.28.37 PM](https://canada1.discourse-cdn.com/flex003/uploads/codecrafters/original/3X/1/a/1ae53d0e237fedb4a690ad26426c8d1c2b6100a2.jpeg)

 ![Screenshot 2025-12-25 at 5.29.13 PM](https://canada1.discourse-cdn.com/flex003/uploads/codecrafters/original/3X/2/5/2588674d326c60dfcdd143939c39da14fc9c9665.jpeg)

---

<div class="post-metadata">

**Author:** ![andy1li](https://yyz1.discourse-cdn.com/flex003/user_avatar/forum.codecrafters.io/andy1li/32/10_2.png) [@andy1li](https://forum.codecrafters.io/u/andy1li)\
**Post date:** [December 25, 2025, 10:29pm UTC](https://forum.codecrafters.io/t/question-about-fd8-fetch-multiple-messages-from-disk-data-isnt-being-prepended-to-response-from-log-file-correctly/15691/2 "2025-12-25T22:29:50Z")

</div>

Hi, thanks for your post!

I’m currently out of the office for the holidays and will be back on January 5. I’ll get back to you as soon as possible once I return.

---

<div class="post-metadata">

**Author:** ![whorst](https://yyz1.discourse-cdn.com/flex003/user_avatar/forum.codecrafters.io/whorst/32/19953_2.png) [@whorst](https://forum.codecrafters.io/u/whorst)\
**Post date:** [December 25, 2025, 11:32pm UTC](https://forum.codecrafters.io/t/question-about-fd8-fetch-multiple-messages-from-disk-data-isnt-being-prepended-to-response-from-log-file-correctly/15691/4 "2025-12-25T23:32:28Z")

</div>

One other thing, before my program gives it’s output after an invocation, I’m seeing in the logs it’s sending back

What the program says is expected  
`00 00 01 91 e0 5b 6d 8b 00 00 01 91 e0 5b 6d 8b 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 1e 00 00 00 01 12 72 61 73 70 62 65 72 72 79 01`

What the program says I’m returning ( in the log `Actual (bytes 27-127), hexadecimal`)  
`00 00 01 91 e0 5b 6d 8b 00 00 01 91 e0 5b 6d 8b 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 1e 00 00 00 01 12 72 61 73 70 62 65 72 72 79 01 00 00 00 00 00 00 00 01 00 00 00 41 00 00 00 00 02 cf 35 bf 62 00 00 00 00 00 00 00 00 01 91 e0 5b 6d 8b 00 00 01 91 e0 5b 6d 8b 00 00 00 00 00 00 00`

Actual Return Value (My Logs)  
`00 00 00 96 0f ca b3 0e 00 00 00 00 00 00 00 00 00 00 00 02 71 a5 9a 51 89 68 4f 8b 93 7e c2 88 2a b8 39 a4 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 00 00 00 00 4e 00 00 00 00 00 00 00 00 00 00 00 41 00 00 00 00 02 b2 66 93 fc 00 00 00 00 00 00 00 00 01 91 e0 5b 6d 8b 00 00 01 91 e0 5b 6d 8b 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 1e 00 00 00 01 12 72 61 73 70 62 65 72 72 79 01 00 00 00 00`

---

<div class="post-metadata">

**Author:** ![whorst](https://yyz1.discourse-cdn.com/flex003/user_avatar/forum.codecrafters.io/whorst/32/19953_2.png) [@whorst](https://forum.codecrafters.io/u/whorst)\
**Post date:** [January 11, 2026, 9:21pm UTC](https://forum.codecrafters.io/t/question-about-fd8-fetch-multiple-messages-from-disk-data-isnt-being-prepended-to-response-from-log-file-correctly/15691/6 "2026-01-11T21:21:50Z")

</div>

@andy1li Any insights here?

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<div class="post-metadata">

**Author:** ![andy1li](https://yyz1.discourse-cdn.com/flex003/user_avatar/forum.codecrafters.io/andy1li/32/10_2.png) [@andy1li](https://forum.codecrafters.io/u/andy1li)\
**Post date:** [April 18, 2026, 1:41am UTC](https://forum.codecrafters.io/t/question-about-fd8-fetch-multiple-messages-from-disk-data-isnt-being-prepended-to-response-from-log-file-correctly/15691/7 "2026-04-18T01:41:54Z")

</div>

Hey @whorst, sorry for the delayed response!

There might be a few things going on here, but two issues stand out:

1. This condition seems unnecessary. You can simplify this by just going through the else branch and assigning the full data:

`partitionToFetch.TopicFetchResponse.Records = data`.

 ![image](https://canada1.discourse-cdn.com/flex003/uploads/codecrafters/original/3X/3/2/323e1783d3373d7315d01c8245ce496ac4c699cf.png)

1. The reversal logic here also looks unnecessary, since extracting the lower 7 bits is already correct:

 ![image](https://canada1.discourse-cdn.com/flex003/uploads/codecrafters/original/3X/2/c/2cef38de1d2aac2d23db8201c6c375df7d510f3b.png)

I’ve extended your membership by two weeks so you can have a bit more time to experiment. Let me know how it goes!

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<div class="post-metadata">

**Author:** ![system](https://canada1.discourse-cdn.com/flex003/uploads/codecrafters/original/3X/7/0/700657133935c15703e22c7c8870394f6e6dc27d.svg) [@system](https://forum.codecrafters.io/u/system)\
**Post date:** [April 23, 2026, 3:43am UTC](https://forum.codecrafters.io/t/question-about-fd8-fetch-multiple-messages-from-disk-data-isnt-being-prepended-to-response-from-log-file-correctly/15691/9 "2026-04-23T03:43:52Z")

</div>

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