https://github.com/antirez/redis/blob/4.0-rc1/00-RELEASENOTES
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| Major features | |
| --- | |
| * Redis modules system. Redis now allows developers to write modules that can extend the Redis functionalities and implement new data types. The module API implements a complete abstraction layer that separates the Redis core from the module implementation, allowing the same module to be loaded by different versions of Redis without modifications. See the modules documentation here for more info: https://github.com/antirez/redis/blob/unstable/src/modules/INTRO.md | |
| * Partial Replication (PSYNC) version 2. The replication protocol was enhanced (in a backward compatible way, so that 4.0 can still act as a slave of older instances) in order to be able to partially resynchronize slaves and masters in conditions where it was impossible in the past: after a master switch because of a failover and when a slave instance is restared. Even masters turned into slaves will usually be able to partially resynchronize with the new master, if the new master was a former slave of the old master instance. The way the replication work for chained slaves (sub-slaves) is now very different, and each slave receives the same replication stream generated by the top-level master. | |
| * Cache eviction improvements. Redis 4.0 implements LFU (Least Frequently Used) as a new eviction algorithm, and improves the functionality, performances and precision of the existing algorithms. This blog post contains info about the changes: http://antirez.com/news/109 | |
| * Lazy freeing of keys. Redis is now able to delete keys in the background in a different thread without blocking the server. The new `UNLINK` command is the same as `DEL` but working in a non blocking way. Similarly an `ASYNC` option was added to `FLUSHALL` and `FLUSHDB` in order to let the entire dataset or a single database to be freed asynchronously. | |
| * Mixed RDB-AOF format. If enabled the new format is used when rewriting the AOF file: the rewrite uses the more compact and faster to generate RDB format, and an AOF stream is appended to the file. This allows faster rewrites and reloads when using the AOF persistence. | |
| * A new MEMORY command, able to perform memory analysis of different kinds: troubleshooting of memory issues (with MEMORY DOCTOR, similar to LATENCY DOCTOR), reporting of the amount of memory used by a single key, more in-depth reporting of Redis memory usage compared to what the INFO command offers. | |
| * Redis Cluster support for NAT / Docker. There are new functionalities in order to force cluster instances to announce specific sets of IP address, client and bus ports, to the rest of the cluster, regardless of the auto detected IP. This required a bus protocol change that will force users to mass-restart all the nodes of a Redis 3.2 installation in order to upgrade to 4.0. | |
| * Redis uses now less memory in order to store the same amount of data. The gain depends a lot on the kind of dataset stored. | |
| Smaller features | |
| --- | |
| * Improvements to the RDB format to support 64 bit lengths, binary sorted set scores, and more. The RDB file check utility now uses the same code base of the one used by Redis itself in order to load the RDB file in memory. | |
| * SWAPDB command: ability to completely and immediately (no latency) replace two Redis databases. | |
| * Improvements to `dict.c`, the Redis hash table implementation. | |
| * Security improvements mapping POST and Host: commands to QUIT in order to prevent cross protocol scripting attacks. | |
| * RPUSHX and LPUSHX now accept a variable number of elements. | |
| * Reporting of additional memory used by copy on write in the INFO output. | |
| * Serious refactoring of many core parts of Redis. |