Blocks storage

The blocks storage is a Cortex storage engine based on Prometheus TSDB, which only requires an object store (eg. AWS S3, Google GCS, …) as backend storage.

For more information, please refer to the Cortex blocks storage documentation.


A chunk is an object containing compressed timestamp-value pairs.

A single chunk contains timestamp-value pairs for several series.


Churn is the frequency at which series become idle.

A series become idle once it’s not exported anymore by the monitored targets. Typically, series become idle when the monitored target itself disappear (eg. the process or node gets terminated).


Series flushing is the operation run by ingesters to offload time series from memory and store them in the long-term storage.

HA Tracker

The HA Tracker is a feature of Cortex distributor which is used to deduplicate received series coming from two (or more) Prometheus servers configured in HA pairs.

For more information, please refer to the guide “Config for sending HA Pairs data to Cortex”.

Hash ring

The hash ring is a distributed data structure used by Cortex for sharding, replication and service discovery. The hash ring data structure gets shared across Cortex replicas via gossip or a key-value store.

For more information, please refer to the Architecture documentation.


See Tenant.


See Hash ring.


A sample is a single timestamped value in a time series.

For example, given the series node_cpu_seconds_total{instance="",mode="system"} its stream of values (samples) could be:

# Display format: <value> @<timestamp>
11775 @1603812134
11790 @1603812149
11805 @1603812164
11819 @1603812179
11834 @1603812194


In the Prometheus ecosystem, a series (or time series) is a single stream of timestamped values belonging to the same metric, with the same set of label key-value pairs.

For example, given a single metric node_cpu_seconds_total you may have multiple series, each one uniquely identified by the combination of metric name and unique label key-value pairs:



A tenant (also called “user” or “org”) is the owner of a set of series written to and queried from Cortex. Cortex multi-tenancy support allows you to isolate series belonging to different tenants. For example, if you have two tenants team-A and team-B, team-A series will be isolated from team-B, and each team will be able to query only their own series.

For more information, please refer to:

Time series

See Series.


See Tenant.


The Write-Ahead Log (WAL) is an append only log stored on disk used by ingesters to recover their in-memory state after the process gets restarted, either after a clear shutdown or an abruptly termination.

For more information, please refer to Ingesters with WAL.