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COMMAND

Syntax​

COMMAND

Time complexity: O(N) where N is the total number of Dragonfly commands

ACL categories: @slow, @connection

Return an array with details about every Dragonfly command.

The COMMAND command is introspective. Its reply describes all commands that the server can process. Clients can call it to obtain the server's runtime capabilities during the handshake.

Subcommands​

Unsupported in Dragonfly:

  • COMMAND DOCS: Not implemented. The server returns an error.

Return​

Array reply: a nested list of command details. The order of commands in the array is random.

The reply is an array with an element per command. Each element that describes a Dragonfly command is represented as a fixed-length array by itself. The exact number of elements in the command's array depends on the server's version.

  1. Name
  2. Arity
  3. Flags
  4. First key
  5. Last key
  6. Step
  7. ACL categories

Name​

This is the command's name. Note that command names are case-insensitive.

Arity​

Arity is the number of arguments a command expects. It follows a simple pattern:

  • A positive integer means a fixed number of arguments.
  • A negative integer means a minimal number of arguments.

Command arity always includes the command's name itself (and the subcommand when applicable). For example:

  • GET's arity is 2 since the command only accepts one argument and always has the format GET key.
  • MGET's arity is -2 since the command accepts at least one argument but possibly multiple ones: MGET key [key ...].

Flags​

Command flags are returned as an array. It can contain the following simple strings (status reply):

  • admin: The command is an administrative command.
  • blocking: The command may block the requesting client.
  • denyoom: The command is rejected if the server's memory usage is too high (see the maxmemory configuration directive).
  • fast: The command operates in constant or log(N) time.
  • loading: The command is allowed while the database is loading.
  • noscript: The command can't be called from scripts.
  • readonly: The command doesn't modify data.
  • write: The command may modify data.

First key​

The position of the command's first key name argument. For most commands, the first key's position is 1. Position 0 is always the command name itself.

Last key​

The position of the command's last key name argument. Commands usually accept one, two, or multiple number of keys.

  • Commands that accept a single key have both the first key and the last key set to 1.
  • Commands that accept two key name arguments (e.g., SMOVE) have this value set to the position of their second key.
  • Multi-key commands that accept an arbitrary number of keys (e.g., MSET) have the value -1.

Step​

The step, or increment, between the first key and the position of the next key. Consider the following two examples:

dragonfly> COMMAND
# ...
1) MSET
2) (integer) -3
3) 1) write
   2) denyoom
   3) interleaved-keys
   4) custom-journal
4) (integer) 1
5) (integer) -1
6) (integer) 2
7) 1) @WRITE
   2) @STRING
   3) @SLOW
# ...
dragonfly> COMMAND
# ...
1) MGET
2) (integer) -2
3) 1) readonly
   2) fast
   3) idempotent
4) (integer) 1
5) (integer) -1
6) (integer) 1
7) 1) @READ
   2) @STRING
   3) @FAST
# ...

The step count allows us to find keys' positions.

  • For example, the MSET command has a syntax of MSET key1 val1 [key2] [val2] [key3] [val3]..., so the keys are at every other position, which has a step value of 2.
  • For the MGET command with a syntax of MGET key1 [key2] [key3]..., it uses a step value of 1.

ACL categories​

Command ACL categories are returned as an array. For instance, the MSET command shown above is categorized as @write, @string, and @slow from the ACL perspective. Learn more about ACL rules to control your data store access.

Examples​

The following is COMMAND's output for the GET command:

dragonfly> COMMAND
# ...
1) GET         # Name
2) (integer) 2 # Arity
3) 1) readonly # Flags (array)
   2) fast
4) (integer) 1 # First key
5) (integer) 1 # Last key
6) (integer) 1 # Step
7) 1) @READ    # ACL categories (array)
   2) @STRING
   3) @FAST
# ...

Tips​

  • Command names are case-insensitive.
  • Hidden commands (with the HIDDEN flag) are not returned.
  • Use COMMAND HELP to discover supported subcommands.