[FSSDK-11902] Fix concurrency bug in cmab service #375
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Mat001
requested changes
Oct 13, 2025
lib/optimizely/cmab/cmab_service.rb
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| # Create a hash of user_id + rule_id for consistent lock selection | ||
| hash_input = "#{user_id}#{rule_id}" | ||
| # Use Ruby's built-in hash method and ensure positive result | ||
| hash_value = hash_input.hash.abs |
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String#hash is not deterministic across Ruby processes. It uses a random seed that changes per process/restart to prevent hash collision attacks. This means:
- Same input → Different hash values in different Ruby processes
- Cache keys will map to different locks in different processes
- If using multi-process deployment (Unicorn, Puma workers), this defeats the purpose
Hmm, we shld use a use a deterministic hash function.
is there ruby gem for murmurhash?
If so the you could use:
require 'murmurhash3'
hash_value = MurmurHash3::V32.str_hash(hash_input)
hash_value % NUM_LOCK_STRIPES
if not, then could use:
require 'digest'
hash_value = Digest::MD5.hexdigest(hash_input).hex % (2**32)
hash_value % NUM_LOCK_STRIPES
…k index calculation
FarhanAnjum-opti
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farhan/FSSDK-11902-fix-cmab-concurrency-bug
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Summary
Fixes concurrency bug in cmab service by introducing locking in order to ensure consistent result.
Test plan
Introduced unit tests
Issues
FSSDK-11902