MONITOR is a debugging command that streams back every command processed by the Valkey server. It can help in understanding what is happening to the database. This command can both be used via valkey-cli and via telnet.

The ability to see all the requests processed by the server is useful in order to spot bugs in an application both when using Valkey as a database and as a distributed caching system.

$ valkey-cli monitor
1339518083.107412 [0] "keys" "*"
1339518087.877697 [0] "dbsize"
1339518090.420270 [0] "set" "x" "6"
1339518096.506257 [0] "get" "x"
1339518099.363765 [0] "eval" "return'set','x','7')" "0"
1339518100.363799 [0 lua] "set" "x" "7"
1339518100.544926 [0] "del" "x"

Use SIGINT (Ctrl-C) to stop a MONITOR stream running via valkey-cli.

$ telnet localhost 6379
Connected to localhost.
Escape character is '^]'.
+1339518083.107412 [0] "keys" "*"
+1339518087.877697 [0] "dbsize"
+1339518090.420270 [0] "set" "x" "6"
+1339518096.506257 [0] "get" "x"
+1339518099.363765 [0] "del" "x"
+1339518100.544926 [0] "get" "x"
Connection closed by foreign host.

Manually issue the QUIT or RESET commands to stop a MONITOR stream running via telnet.

Commands not logged by MONITOR

Because of security concerns, no administrative commands are logged by MONITOR's output and sensitive data is redacted in the command AUTH.

Furthermore, the command QUIT is also not logged.

Cost of running MONITOR

Because MONITOR streams back all commands, its use comes at a cost. The following (totally unscientific) benchmark numbers illustrate what the cost of running MONITOR can be.

Benchmark result without MONITOR running:

$ src/valkey-benchmark -c 10 -n 100000 -q
PING_INLINE: 101936.80 requests per second
PING_BULK: 102880.66 requests per second
SET: 95419.85 requests per second
GET: 104275.29 requests per second
INCR: 93283.58 requests per second

Benchmark result with MONITOR running (valkey-cli monitor > /dev/null):

$ src/valkey-benchmark -c 10 -n 100000 -q
PING_INLINE: 58479.53 requests per second
PING_BULK: 59136.61 requests per second
SET: 41823.50 requests per second
GET: 45330.91 requests per second
INCR: 41771.09 requests per second

In this particular case, running a single MONITOR client can reduce the throughput by more than 50%. Running more MONITOR clients will reduce throughput even more.

Note that,

  • AUTH is excluded from the command's output.
  • RESET can be called to exit monitor mode.
  • AUTH, HELLO, EVAL, EVAL_RO, EVALSHA and EVALSHA_RO are included in the command's output.