Skip to main content

5.1 Data points in Redis

There are several Redis metrics that can be viewed through redis-cli.

Redis INFO command

Running the INFO command provides many of the metrics available in a single view.

127.0.0.1:6379> INFO
# Server
redis_version:6.0.1
redis_git_sha1:00000000
redis_git_dirty:0
redis_build_id:e02d1d807e41d65
redis_mode:standalone
os:Linux 4.19.121-linuxkit x86_64

There are several sections that can be pulled individually. For example, if you wanted to just get the CLIENTS section you can pass that section as an argument to the INFO command.

127.0.0.1:6379> INFO CLIENTS
# Clients
connected_clients:1
client_recent_max_input_buffer:2
client_recent_max_output_buffer:0
blocked_clients:0
tracking_clients:0
clients_in_timeout_table:0

Sections

Server: the current Redis server info.

Metrics of note:

  • redis_version
  • process_id
  • config_file
  • uptime_in_seconds
  • uptime_in_days

Clients: available data on clients connected or failed connections.

Metrics of note:

  • connected_clients
  • blocked_clients

Memory: memory usage and stats

Metrics of note:

  • used_memory
  • mem_fragmentation_ratio

Persistence: RDB or AOF metrics

Metrics of note:

  • rdb_last_save_time
  • rdb_changes_since_last_save
  • aof_rewrite_in_progress

Stats: some general statistics

Metrics of note:

  • keyspace_hits
  • keyspace_misses
  • expired_keys
  • evicted_keys
  • instantaneous_ops_per_sec

Replication: replication data including primary/replica identifiers and offsets

Metrics of note:

  • master_link_down_since
  • connected_slaves
  • master_last_io_seconds_ago

CPU: compute consumption stats

Metrics of note:

  • used_cpu_sys
  • used_cpu_user

Modules: data from any loaded modules

Metrics of note (per module):

  • ver
  • options

Cluster: whether cluster is enabled

Metric of note:

  • cluster_enabled

Keyspace: keys and expiration data

Metrics of note (per db):

  • keys
  • expires
  • avg_ttl

The output can be read from the results or piped into a file.

127.0.0.1:6379> redis-cli INFO STATS > redis-info-stats

This could be done at intervals and consumed by a local or third party monitoring service.

Some of the data returned by INFO are going to be static.  For example the Redis version which won't change until an update is made.  Other data is dynamic, for keyspace_hits ÷ keyspace_misses. The latter could be taken to compute a hit ratio and observed as a long term metric. The replication section field master_link_down_since could be a metric to connect an alert.

Some examples of possible alerts that could be setup for a given metric:

MetricExample Alert
uptime_in_seconds< 300 seconds: to ensure the server is staying up
connected_clients< minimum number of expected application connections
master_link_down_since> 30 seconds: replication should be operational
rdb_last_save_time> maximum acceptable interval without taking a snapshot
NOTE: This is not an exhaustive list, but just to give you an idea of how the metrics in INFO could be used.

Latency and stats data via redis-cli options

The redis-cli client has some built-in options that allow you to pull some real-time latency and stats data.

Note: these are not available as commands from Redis but as options in redis-cli.

Latency options:

Continuously sample latency:

$ redis-cli --latency
min: 1, max: 17, avg: 4.03 (927 samples)

The raw or csv output flag can be added:

$ redis-cli --latency --csv
1,4,1.94,78

In order to sample for longer than one second you can use latency-history which has a default interval of 15 seconds but can be specified using the -i param.

$ redis-cli --latency-history -i 60
min: 1, max: 30, avg: 4.84 (328 samples)

This can also be combined with the csv or raw output format flag.

$ redis-cli --latency-history -i 60 --csv
13,13,13.00,1
5,13,9.00,2
3,13,7.00,3
3,13,6.00,4
3,13,5.60,5
2,13,5.00,6
2,13,5.43,7
2,13,5.62,8
2,13,5.22,9
2,13,5.00,10
1,13,4.64,11

Both of these could be piped to a file as well.

The latency-dist option shows latency as a spectrum. The default interval is one second but can be changed using the -i param.

Latency Distribution diagramStats option:

Get rolling stats from the server using the stat flag.

$ redis-cli --stat
------- data ------ --------------------- load -------------------- - child -
keys mem clients blocked requests connections
4 9.98M 51 0 8168035 (+0) 4132
4 9.98M 51 0 8181542 (+13507) 4132
4 9.98M 51 0 8196100 (+14558) 4132
4 9.98M 51 0 8209794 (+13694) 4132
4 9.98M 51 0 8223420 (+13626) 4132
4 9.98M 51 0 8236624 (+13204) 4132
4 9.98M 51 0 8251376 (+14752) 4132
4 9.98M 51 0 8263417 (+12041) 4182
4 9.98M 51 0 8276781 (+13364) 4182
4 9.90M 51 0 8289693 (+12912) 4182

Memory stats

Redis includes a MEMORY command that includes a subcommand to get stats.

127.0.0.1:6379> memory stats
1) "peak.allocated"
2) (integer) 11912984
3) "total.allocated"
4) (integer) 8379168
5) "startup.allocated"
6) (integer) 5292168
7) "replication.backlog"
8) (integer) 0
9) "clients.slaves"
10) (integer) 0
11) "clients.normal"
12) (integer) 16986
13) "aof.buffer"
14) (integer) 0

These values are available in the INFO MEMORY command as well, but here they are returned in a typical Redis RESP Array reply.

There is also a LATENCY DOCTOR subcommand with an analysis report of the current memory metrics.

Latency Monitoring

As we know Redis is fast and as a result is often used in very extreme scenarios where low latency is a must. Redis has a feature called Latency Monitoring which allows you to dig into possible latency issues. Latency monitoring is composed of the following conceptual parts:

  • Latency hooks that sample different latency sensitive code paths.
  • Time series recording of latency spikes split by different events.
  • A reporting engine to fetch raw data from the time series.
  • Analysis engine to provide human readable reports and hints according to the measurements.

By default this feature is disabled because most of the time it is not needed. In order to enable it you can update the threshold time in milliseconds that you want to monitor in your Redis configuration. Events that take longer than the threshold will be logged as latency spikes. The threshold configuration should be set accordingly if the requirement is to identify all events blocking the server for a time of 10 milliseconds or more.

latency-monitor-threshold 10

If the debugging session is intended to be temporary the threshold can be set via redis-cli.

127.0.0.1:6379> CONFIG SET latency-monitor-threshold 10

To disable the latency framework the threshold should be set back to 0.

127.0.0.1:6379> CONFIG SET latency-monitor-threshold 0

The latency data can be viewed using the LATENCY command with it's subcommands:

  • LATENCY LATEST - latest samples for all events
  • LATENCY HISTORY - latest time series for a given event
  • LATENCY RESET - resets the time series data
  • LATENCY GRAPH - renders an ASCII-art graph
  • LATENCY DOCTOR - analysis report

In order to make use of these commands you need to make yourself familiar with the different events that the latency monitoring framework is tracking. (taken from https://redis.io/topics/latency-monitor" )

EventDescription
commandregular commands
fast-commandO(1) and O(log N) commands
forkthe fork(2) system call
comrdb-unlink-temp-filethe unlink(2) system call
aof-writewriting to the AOF - a catchall event fsync(2) system calls
aof-fsync-alwaysthe fsync(2) system call when invoked by the appendfsync allways policy
aof-write-pending-fsyncthe fsync(2) system call when there are pending writes
aof-write-active-childthe fsync(2) system call when performed by a child process
aof-write-alonethe fsync(2) system call when performed by the main process
aof-fstatthe fstat(2) system call
aof-renamethe rename(2) system call for renaming the temporary file after completing BGREWRITEAOF
aof-rewrite-diff-writewriting the differences accumulated while performing BGREWRITEAOF
active-defrag-cyclethe active defragmentation cycle
expire-cyclethe expiration cycle
eviction-cyclethe eviction cycle
eviction-deldeletes during the eviction cycle

For example, you can use the LATENCY LATEST subcommand and you may see some data like this:

127.0.0.1:6379> latency latest
1) 1) "command"
2) (integer) 1616372606
3) (integer) 600
4) (integer) 600
2) 1) "fast-command"
2) (integer) 1616372434
3) (integer) 12
4) (integer) 12

The results of this command provide the timestamp, latency and max latency for this event. Utilizing the events table above I can see we had latency spikes for a regular command with the latest and max latency of 600 MS while a O(1) or O(log N) command had a latency spike of 12 MS.

Some of the latency commands require a specific event be passed.

127.0.0.1:6379> latency graph command
command - high 600 ms, low 100 ms (all time high 600 ms)
--------------------------------------------------------------------------------
_##
o|||
o||||
_#|||||

3222184
05308ss
sssss

While the cost of enabling latency monitoring is near zero and memory requirements are very small it will raise your baseline memory usage so if you are getting the required performance out of Redis there is no need to leave this enabled.

Monitoring Tools

There are many open source monitoring tools and services to visualize your Redis metrics - some of which also provide alerting capabilities.

One example of this is the Redis Data Source for Grafana. It is a Grafana plug-in that allows users to connect to the Redis database and build dashboards to easily observe Redis data. It provides an out-of-the-box predefined dashboard but also lets you build customized dashboards tuned to your specific needs.