ContributorsLast modified on: Mar 24, 2016
farhan687
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Returns @array-reply of details about all Redis commands.

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

Command name is the command returned as a lowercase string.

Command Arity

1) 1) "get"
   2) (integer) 2
   3) 1) readonly
   4) (integer) 1
   5) (integer) 1
   6) (integer) 1

1) 1) "mget"
   2) (integer) -2
   3) 1) readonly
   4) (integer) 1
   5) (integer) -1
   6) (integer) 1

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.

Examples:

  • GET arity is 2 since the command only accepts one argument and always has the format GET _key_.
  • MGET arity 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.

Flags

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.

Movable Keys

1) 1) "sort"
   2) (integer) -2
   3) 1) write
      2) denyoom
      3) movablekeys
   4) (integer) 1
   5) (integer) 1
   6) (integer) 1

Some Redis commands have no predetermined key locations. For those commands, flag 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:

  • SORT - optional STORE key, optional BY weights, optional GET keys
  • ZUNIONSTORE - keys stop when WEIGHT or AGGREGATE starts
  • ZINTERSTORE - keys stop when WEIGHT or AGGREGATE starts
  • EVAL - keys stop after numkeys count arguments
  • EVALSHA - keys stop after numkeys count arguments

Also see 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. BRPOPLPUSH, SMOVE, 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.

Step Count

1) 1) "mset"
   2) (integer) -3
   3) 1) write
      2) denyoom
   4) (integer) 1
   5) (integer) -1
   6) (integer) 2

1) 1) "mget"
   2) (integer) -2
   3) 1) readonly
   4) (integer) 1
   5) (integer) -1
   6) (integer) 1

Key step count allows us to find key positions in commands like 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 with MGET above where the step value is just 1.

@return

@array-reply: nested list of command details. Commands are returned in random order.

@examples

COMMAND
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