Switch to a different protocol, optionally authenticating and setting the connection's name, or provide a contextual client report.
Redis version 6 and above supports two protocols: the old protocol, RESP2, and a new one introduced with Redis 6, RESP3. RESP3 has certain advantages since when the connection is in this mode, Redis is able to reply with more semantical replies: for instance, HGETALL will return a map type, so a client library implementation no longer requires to know in advance to translate the array into a hash before returning it to the caller. For a full coverage of RESP3, please check this repository.
In Redis 6 connections start in RESP2 mode, so clients implementing RESP2 do not need to updated or changed. There are no short term plans to drop support for RESP2, although future version may default to RESP3.
HELLO always replies with a list of current server and connection properties, such as: versions, modules loaded, client ID, replication role and so forth. When called without any arguments in Redis 6.2 and its default use of RESP2 protocol, the reply looks like this:
> HELLO 1) "server" 2) "redis" 3) "version" 4) "255.255.255" 5) "proto" 6) (integer) 2 7) "id" 8) (integer) 5 9) "mode" 10) "standalone" 11) "role" 12) "master" 13) "modules" 14) (empty array)
Clients that want to handshake using the RESP3 mode need to call the HELLO
command and specify the value "3" as the
protover argument, like so:
> HELLO 3 1# "server" => "redis" 2# "version" => "6.0.0" 3# "proto" => (integer) 3 4# "id" => (integer) 10 5# "mode" => "standalone" 6# "role" => "master" 7# "modules" => (empty array)
Because HELLO replies with useful information, and given that
optional or can be set to "2", client library authors may consider using this
command instead of the canonical PING when setting up the connection.
When called with the optional
protover argument, this command switches the
protocol to the specified version and also accepts the following options:
AUTH <username> <password>: directly authenticate the connection in addition to switching to the specified protocol version. This makes calling AUTH before HELLO unnecessary when setting up a new connection. Note that the
usernamecan be set to "default" to authenticate against a server that does not use ACLs, but rather the simpler
requirepassmechanism of Redis prior to version 6.
SETNAME <clientname>: this is the equivalent of calling CLIENT SETNAME.
Array reply: a list of server properties. The reply is a map instead of an array when RESP3 is selected. The command returns an error if the
protover requested does not exist.
protovermade optional; when called without arguments the command reports the current connection's context.