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. As an installation grows, the carbon-relay.py and carbon-aggregator.py daemons can be introduced to distribute metrics load and perform custom aggregations, respectively.

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. This document gives a brief overview of what each daemon does and how you can use them to build a more sophisticated storage backend.


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. It also provides a query service for in-memory metric datapoints, used by the Graphite webapp to retrieve “hot data”.

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

The [cache] section tells carbon-cache.py what ports (2003/2004/7002), protocols (newline delimited, pickle) and transports (TCP/UDP) to listen on.
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.


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)


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 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:

The [relay] section defines listener host/ports and a RELAY_METHOD
With RELAY_METHOD = rules set, pattern/servers tuples in this file define which metrics matching certain regex rules are forwarded to which hosts.


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:

The [aggregator] section defines listener and destination host/ports.
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.


carbon-aggregator-cache.py combines both carbon-aggregator.py and carbon-cache.py. This is useful to reduce the resource and administration overhead of running both daemons.

carbon-aggregator-cache.py is configured via:

The [aggregator-cache] section defines listener and destination host/ports.
See carbon-relay.py section.
See carbon-aggregator.py section.

© 2008–2012 Chris Davis
© 2011–2016 The Graphite Project
Licensed under the Apache License, Version 2.0.