The Carbon Daemons

When we talk about “Carbon” we mean one or more of various daemons that make up the storage backend of a Graphite installation. In simple installations, there is typically only one daemon, carbon-cache.py. This document gives a brief overview of what each daemon does and how you can use them to build a more sophisticated storage backend.

All of the carbon daemons listen for time-series data and can accept it over a common set of protocols. However, they differ in what they do with the data once they receive it.

carbon-cache.py

carbon-cache.py accepts metrics over various protocols and writes them to disk as efficiently as possible. This requires caching metric values in RAM as they are received, and flushing them to disk on an interval using the underlying whisper library.

carbon-cache.py requires some basic configuration files to run:

carbon.conf
The [cache] section tells carbon-cache.py what ports (2003/2004/7002), protocols (newline delimited, pickle) and transports (TCP/UDP) to listen on.
storage-schemas.conf
Defines a retention policy for incoming metrics based on regex patterns. This policy is passed to whisper when the .wsp file is pre-allocated, and dictates how long data is stored for.

As the number of incoming metrics increases, one carbon-cache.py instance may not be enough to handle the I/O load. To scale out, simply run multiple carbon-cache.py instances (on one or more machines) behind a carbon-aggregator.py or carbon-relay.py.

Warning

If clients connecting to the carbon-cache.py are experiencing errors such as connection refused by the daemon, a common reason is a shortage of file descriptors.

In the console.log file, if you find presence of:

Could not accept new connection (EMFILE)

or

exceptions.IOError: [Errno 24] Too many open files: '/var/lib/graphite/whisper/systems/somehost/something.wsp'

the number of files carbon-cache.py can open will need to be increased. Many systems default to a max of 1024 file descriptors. A value of 8192 or more may be necessary depending on how many clients are simultaneously connecting to the carbon-cache.py daemon.

In Linux, the system-global file descriptor max can be set via sysctl. Per-process limits are set via ulimit. See documentation for your operating system distribution for details on how to set these values.

carbon-relay.py

carbon-relay.py serves two distinct purposes: replication and sharding.

When running with RELAY_METHOD = rules, a carbon-relay.py instance can run in place of a carbon-cache.py server and relay all incoming metrics to multiple backend carbon-cache.py‘s running on different ports or hosts.

In RELAY_METHOD = consistent-hashing mode, a DESTINATIONS setting defines a sharding strategy across multiple carbon-cache.py backends. The same consistent hashing list can be provided to the graphite webapp via CARBONLINK_HOSTS to spread reads across the multiple backends.

carbon-relay.py is configured via:

carbon.conf
The [relay] section defines listener host/ports and a RELAY_METHOD
relay-rules.conf
In RELAY_METHOD = rules, pattern/servers tuples define what servers metrics matching certain regex rules are forwarded to.

carbon-aggregator.py

carbon-aggregator.py can be run in front of carbon-cache.py to buffer metrics over time before reporting them into whisper. This is useful when granular reporting is not required, and can help reduce I/O load and whisper file sizes due to lower retention policies.

carbon-aggregator.py is configured via:

carbon.conf
The [aggregator] section defines listener and destination host/ports.
aggregation-rules.conf
Defines a time interval (in seconds) and aggregation function (sum or average) for incoming metrics matching a certain pattern. At the end of each interval, the values received are aggregated and published to carbon-cache.py as a single metric.
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