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Redis is an open source (BSD licensed), in-memory data structure store, used as a database, cache and message broker. It supports data structures such as strings, hashes, lists, sets, sorted sets with range queries, bitmaps, hyperloglogs and geospatial indexes with radius queries. Redis has built-in replication, Lua scripting, LRU eviction, transactions and different levels of on-disk persistence, and provides high availability via Redis Sentinel and automatic partitioning with Redis Cluster.
During setup, you can choose to extend your master server storage using separate storage nodes. When you choose this option, Logstash on the master server outputs to redis. Storage nodes then consume from redis.
To see how many logs are in the redis queue:
redis-cli LLEN logstash:redis
If the queue is backed up and doesn't seem to be draining, try stopping Logstash on the master server:
sudo docker stop so-logstash
Then monitor the queue to see if it drains:
watch 'redis-cli llen logstash:redis'
If the Redis queue looks okay, but you are still having issues with logs getting indexed into Elasticsearch, you will want to check the Logstash statistics on the storage node(s).