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

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:

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.

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

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

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.

carbon-aggregator-cache.py

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:

carbon.conf

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

relay-rules.conf

See carbon-relay.py section.

aggregation-rules.conf

See carbon-aggregator.py section.