The reason is:

1. Normal heartbeat packets carry the full configuration of a node, that can be replaced in an idempotent way with the old in order to update an old config. This means they contain the slots configuration for a node, in raw form, that uses 2k of space with16k slots, but would use a prohibitive 8k of space using 65k slots.
2. At the same time it is unlikely that Redis Cluster would scale to more than 1000 mater nodes because of other design tradeoffs.

So 16k was in the right range to ensure enough slots per master with a max of 1000 maters, but a small enough number to propagate the slot configuration as a raw bitmap easily. Note that in small clusters the bitmap would be hard to compress because when N is small the bitmap would have slots/N bits set that is a large percentage of bits set.

Hey,i have two questions about you answer:

1. why 8K of space is prohibitive using 65 slots?
2. why the bitmap should be compressed?

@Medusar

1. not 65, that's 65k = 8 * 8 (8 bit/byte) * 1024(1k) = 8K bitmap size
2. Compress bitmap to reduce network traffic

@foojolt thank you very much

why:
16k was in the right range to ensure enough slots per master with a max of 1000 maters.
what is the logic in the background？

thx. The balance of bitmap's size and number of master is the key.

what is other design tradeoffs? @foojolt

8K bitmap size means that bitmap size is 8 bits? @foojolt

How to understand the sentence not 65, that's 65K = 8 * 8 (8 bit/byte) * 1024 (1k) = 8K bitmap size?@foojolt

How to understand the sentence not 65, that's 65K = 8 * 8 (8 bit/byte) * 1024 (1k) = 8K bitmap size?@foojolt

crc16() can have 2^16 -1=65535, which means the bitmap has 65535 bits. so the size of the bitmap can be calculated by 65535 / 8 (8bit/byte)/1024(1k)=7.99 Kbytes.

Thanks for answers

How to understand the sentence not 65, that's 65K = 8 * 8 (8 bit/byte) * 1024 (1k) = 8K bitmap size?@foojolt

crc16() can have 2^16 -1=65535, which means the bitmap has 65535 bits. so the size of the bitmap can be calculated by 65535 / 8 (8bit/byte)/1024(1k)=7.99 Kbytes.

Hi, I didn't find where to compress the slots bitmap in source code, did I miss something?

How to understand the sentence not 65, that's 65K = 8 * 8 (8 bit/byte) * 1024 (1k) = 8K bitmap size?@foojolt

crc16() can have 2^16 -1=65535, which means the bitmap has 65535 bits. so the size of the bitmap can be calculated by 65535 / 8 (8bit/byte)/1024(1k)=7.99 Kbytes.

Hi, I didn't find where to compress the slots bitmap in source code, did I miss something?

No compression, just 1 bit for 1 slot.
See the code: https://github.com/antirez/redis/blob/ac441c741379dd4002f664c81047e8412cb793d0/src/cluster.h#L117

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Why isn't slot 8192？

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This is issue is quite old, yet I was asked this same question for so many times even after @antirez had already given his answer. Maybe it's the language gap. I think @antirez has already made it clear in English, and I don't want to be superfluous. Following is my understanding in Chinese: 总结： 1、消息大小考虑：尽管crc16能得到65535个值，但redis选择16384个slot，是因为16384的消息只占用了2k，而65535则需要8k。 2、集群规模设计考虑：集群设计最多支持1000个分片，16384是相对比较好的选择，需要保证在最大集群规模下，slot均匀分布场景下，每个分片平均分到的slot不至于太小。

需要注意2个问题： 1、为什么要传全量的slot状态？ 因为分布式场景，基于状态的设计更合理，状态的传播具有幂等性 2、为什么不考虑压缩？ 集群规模较小的场景下，每个分片负责大量的slot，很难压缩。

wow,subject representative!