spring-boot-demo/redis/docs/03-string-template-and-two-keyspaces.md

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3. StringRedisTemplate, and why two templates do not share keys

StringRedisTemplate is the template Boot registers for text. Keys, values, hash keys and hash values all go through StringRedisSerializer, which is UTF-8 in and out (04-string-template.txt):

key serializer:   StringRedisSerializer
value serializer: StringRedisSerializer
hash key serializer:   StringRedisSerializer
hash value serializer: StringRedisSerializer

Everything is then readable from redis-cli: strings, counters (INCR works and can be read back), hashes (opsForHash), lists, sets and sorted sets. The transcript shows each one.

Two templates, two keyspaces

The important thing about a serializer is that it is part of the key. Write user:1 with the StringRedisTemplate and read it with the default RedisTemplate, and you get null, because the default template looks for a different byte sequence (05-two-templates-two-keyspaces.txt):

stringTemplate.opsForValue().get("user:1")  = from-string-template
redisTemplate.opsForValue().get("user:1")   = null

Write it through both and Redis holds two entries (DBSIZE is 2), only one of which redis-cli GET user:1 can see. This is one way to end up with "my cache works in one service and misses in another": two services, two templates, one Redis, and keys that only look the same in the logs.

The fix is a rule, not a class: pick one key serializer per Redis and put it in one shared configuration. StringRedisSerializer for keys is what you want unless you have a reason, and every template in JsonRedisConfig does that.

When StringRedisTemplate is enough

Counters, flags, locks, session ids, anything you would be happy to GET by hand. If you find yourself calling objectMapper.writeValueAsString before every set, you are doing by hand what a JSON value serializer does (chapter 4).

Going deeper

• StringTemplateTest
• Spring Data Redis: RedisTemplate and StringRedisTemplate

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