Cluster clients must be aware of key positions in commands so commands can go to matching instances, but Redis commands vary between accepting one key, multiple keys, or even multiple keys separated by other data.
You can use
COMMAND to cache a mapping between commands and key positions for
each command to enable exact routing of commands to cluster instances.
Nested Result Array
Each top-level result contains six nested results. Each nested result is:
- command name
- command arity specification
- nested @array-reply of command flags
- position of first key in argument list
- position of last key in argument list
- step count for locating repeating keys
Command name is the command returned as a lowercase string.
Command arity follows a simple pattern:
- positive if command has fixed number of required arguments.
- negative if command has minimum number of required arguments, but may have more.
Command arity includes counting the command name itself.
GETarity is 2 since the command only accepts one argument and always has the format
MGETarity is -2 since the command accepts at a minimum one argument, but up to an unlimited number:
MGET _key1_ [key2] [key3] ....
Also note with
MGET, the -1 value for “last key position” means the list
of keys may have unlimited length.
Command flags is @array-reply containing one or more status replies:
- write - command may result in modifications
- readonly - command will never modify keys
- denyoom - reject command if currently OOM
- admin - server admin command
- pubsub - pubsub-related command
- noscript - deny this command from scripts
- random - command has random results, dangerous for scripts
- sort_for_script - if called from script, sort output
- loading - allow command while database is loading
- stale - allow command while replica has stale data
- skip_monitor - do not show this command in MONITOR
- asking - cluster related - accept even if importing
- fast - command operates in constant or log(N) time. Used for latency monitoring.
- movablekeys - keys have no pre-determined position. You must discover keys yourself.
1) 1) "sort"
2) (integer) -2
3) 1) write
4) (integer) 1
5) (integer) 1
6) (integer) 1
Some Redis commands have no predetermined key locations. For those commands,
movablekeys is added to the command flags @array-reply. Your Redis
Cluster client needs to parse commands marked
movabkeleys to locate all relevant key positions.
Complete list of commands currently requiring key location parsing:
ZUNIONSTORE- keys stop when
ZINTERSTORE- keys stop when
EVAL- keys stop after
EVALSHA- keys stop after
COMMAND GETKEYS for getting your Redis server tell you where keys
are in any given command.
First Key in Argument List
For most commands the first key is position 1. Position 0 is always the command name itself.
Last Key in Argument List
Redis commands usually accept one key, two keys, or an unlimited number of keys.
If a command accepts one key, the first key and last key positions is 1.
If a command accepts two keys (e.g.
RENAME, …) then the
last key position is the location of the last key in the argument list.
If a command accepts an unlimited number of keys, the last key position is -1.
Key step count allows us to find key positions in commands
MSET where the format is
MSET _key1_ _val1_ [key2] [val2] [key3] [val3]....
In the case of
MSET, keys are every other position so the step value is 2. Compare
MGET above where the step value is just 1.
@array-reply: nested list of command details. Commands are returned in random order.