Cortex Architecture

Cortex consists of multiple horizontally scalable microservices. Each microservice uses the most appropriate technique for horizontal scaling; most are stateless and can handle requests for any users while some (namely the ingesters) are semi-stateful and depend on consistent hashing. This document provides a basic overview of Cortex’s architecture.

Cortex Architecture

The role of Prometheus

Prometheus instances scrape samples from various targets and then push them to Cortex (using Prometheus’ remote write API). That remote write API emits batched Snappy-compressed Protocol Buffer messages inside the body of an HTTP PUT request.

Cortex requires that each HTTP request bear a header specifying a tenant ID for the request. Request authentication and authorization are handled by an external reverse proxy.

Incoming samples (writes from Prometheus) are handled by the distributor while incoming reads (PromQL queries) are handled by the querier or optionally by the query frontend.

Storage

Cortex currently supports two storage engines to store and query the time series:

  • Chunks (default)
  • Blocks

The two engines mostly share the same Cortex architecture with few differences outlined in the rest of the document.

Chunks storage (default)

The chunks storage stores each single time series into a separate object called Chunk. Each Chunk contains the samples for a given period (defaults to 12 hours). Chunks are then indexed by time range and labels, in order to provide a fast lookup across many (over millions) Chunks.

For this reason, the chunks storage consists of:

Internally, the access to the chunks storage relies on a unified interface called “chunks store”. Unlike other Cortex components, the chunk store is not a separate service, but rather a library embedded in the services that need to access the long-term storage: ingester, querier and ruler.

The chunk and index format are versioned, this allows Cortex operators to upgrade the cluster to take advantage of new features and improvements. This strategy enables changes in the storage format without requiring any downtime or complex procedures to rewrite the stored data. A set of schemas are used to map the version while reading and writing time series belonging to a specific period of time.

The current schema recommendation is the v9 schema for most use cases and v10 schema if you expect to have very high cardinality metrics (v11 is still experimental). For more information about the schema, please check out the Schema documentation.

Blocks storage

The blocks storage is based on Prometheus TSDB: it stores each tenant’s time series into their own TSDB which write out their series to a on-disk Block (defaults to 2h block range periods). Each Block is composed by few files storing the chunks and the block index.

The TSDB chunk files contain the samples for multiple series. The series inside the Chunks are then indexed by a per-block index, which indexes metric names and labels to time series in the chunk files.

The blocks storage doesn’t require a dedicated storage backend for the index. The only requirement is an object store for the Block files, which can be:

For more information, please check out the Blocks storage documentation.

Services

Cortex has a service-based architecture, in which the overall system is split up into a variety of components that perform a specific task. These components run separately and in parallel. Cortex can alternatively run in a single process mode, where all components are executed within a single process. The single process mode is particularly handy for local testing and development.

Cortex is, for the most part, a shared-nothing system. Each layer of the system can run multiple instances of each component and they don’t coordinate or communicate with each other within that layer.

The Cortex services are:

Distributor

The distributor service is responsible for handling incoming samples from Prometheus. It’s the first stop in the write path for series samples. Once the distributor receives samples from Prometheus, each sample is validated for correctness and to ensure that it is within the configured tenant limits, falling back to default ones in case limits have not been overridden for the specific tenant. Valid samples are then split into batches and sent to multiple ingesters in parallel.

The validation done by the distributor includes:

  • The metric labels name are formally correct
  • The configured max number of labels per metric is respected
  • The configured max length of a label name and value is respected
  • The timestamp is not older/newer than the configured min/max time range

Distributors are stateless and can be scaled up and down as needed.

High Availability Tracker

The distributor features a High Availability (HA) Tracker. When enabled, the distributor deduplicates incoming samples from redundant Prometheus servers. This allows you to have multiple HA replicas of the same Prometheus servers, writing the same series to Cortex and then deduplicate these series in the Cortex distributor.

The HA Tracker deduplicates incoming samples based on a cluster and replica label. The cluster label uniquely identifies the cluster of redundant Prometheus servers for a given tenant, while the replica label uniquely identifies the replica within the Prometheus cluster. Incoming samples are considered duplicated (and thus dropped) if received by any replica which is not the current primary within a cluster.

The HA Tracker requires a key-value (KV) store to coordinate which replica is currently elected. The distributor will only accept samples from the current leader. Samples with one or no labels (of the replica and cluster) are accepted by default and never deduplicated.

The supported KV stores for the HA tracker are:

Note: Memberlist is not supported. Memberlist-based KV store propagates updates using gossip, which is very slow for HA purposes: result is that different distributors may see different Prometheus server as elected HA replica, which is definitely not desirable.

For more information, please refer to config for sending HA pairs data to Cortex in the documentation.

Hashing

Distributors use consistent hashing, in conjunction with a configurable replication factor, to determine which ingester instance(s) should receive a given series.

Cortex supports two hashing strategies:

  1. Hash the metric name and tenant ID (default)
  2. Hash the metric name, labels and tenant ID (enabled with -distributor.shard-by-all-labels=true)

The trade-off associated with the latter is that writes are more balanced across ingesters but each query needs to talk to any ingester since a metric could be spread across multiple ingesters given different label sets.

The hash ring

A hash ring (stored in a key-value store) is used to achieve consistent hashing for the series sharding and replication across the ingesters. All ingesters register themselves into the hash ring with a set of tokens they own; each token is a random unsigned 32-bit number. Each incoming series is hashed in the distributor and then pushed to the ingester owning the tokens range for the series hash number plus N-1 subsequent ingesters in the ring, where N is the replication factor.

To do the hash lookup, distributors find the smallest appropriate token whose value is larger than the hash of the series. When the replication factor is larger than 1, the next subsequent tokens (clockwise in the ring) that belong to different ingesters will also be included in the result.

The effect of this hash set up is that each token that an ingester owns is responsible for a range of hashes. If there are three tokens with values 0, 25, and 50, then a hash of 3 would be given to the ingester that owns the token 25; the ingester owning token 25 is responsible for the hash range of 1-25.

The supported KV stores for the hash ring are:

Quorum consistency

Since all distributors share access to the same hash ring, write requests can be sent to any distributor and you can setup a stateless load balancer in front of it.

To ensure consistent query results, Cortex uses Dynamo-style quorum consistency on reads and writes. This means that the distributor will wait for a positive response of at least one half plus one of the ingesters to send the sample to before successfully responding to the Prometheus write request.

Load balancing across distributors

We recommend randomly load balancing write requests across distributor instances. For example, if you’re running Cortex in a Kubernetes cluster, you could run the distributors as a Kubernetes Service.

Ingester

The ingester service is responsible for writing incoming series to a long-term storage backend on the write path and returning in-memory series samples for queries on the read path.

Incoming series are not immediately written to the storage but kept in memory and periodically flushed to the storage (by default, 12 hours for the chunks storage and 2 hours for the blocks storage). For this reason, the queriers may need to fetch samples both from ingesters and long-term storage while executing a query on the read path.

Ingesters contain a lifecycler which manages the lifecycle of an ingester and stores the ingester state in the hash ring. Each ingester could be in one of the following states:

  • PENDING
    The ingester has just started. While in this state, the ingester doesn’t receive neither write and read requests, and could be waiting for time series data transfer from another ingester if running the chunks storage and the hand-over is enabled.
  • JOINING
    The ingester is starting up and joining the ring. While in this state the ingester doesn’t receive neither write and read requests. The ingester will join the ring using tokens received by a leaving ingester as part of the hand-over process (if enabled), otherwise it could load tokens from disk (if -ingester.tokens-file-path is configured) or generate a set of new random ones. Finally, the ingester optionally observes the ring for tokens conflicts and then, once any conflict is resolved, will move to ACTIVE state.
  • ACTIVE
    The ingester is up and running. While in this state the ingester can receive both write and read requests.
  • LEAVING
    The ingester is shutting down and leaving the ring. While in this state the ingester doesn’t receive write requests, while it could receive read requests.
  • UNHEALTHY
    The ingester has failed to heartbeat to the ring’s KV Store. While in this state, distributors skip the ingester while building the replication set for incoming series and the ingester does not receive write or read requests.

The ingester states are interally used for different purposes, including the series hand-over process supported by the chunks storage. For more information about it, please check out the Ingester hand-over documentation.

Ingesters are semi-stateful.

Ingesters failure and data loss

If an ingester process crashes or exits abruptly, all the in-memory series that have not yet been flushed to the long-term storage will be lost. There are two main ways to mitigate this failure mode:

  1. Replication
  2. Write-ahead log (WAL)

The replication is used to hold multiple (typically 3) replicas of each time series in the ingesters. If the Cortex cluster looses an ingester, the in-memory series hold by the lost ingester are also replicated at least to another ingester. In the event of a single ingester failure, no time series samples will be lost while, in the event of multiple ingesters failure, time series may be potentially lost if failure affects all the ingesters holding the replicas of a specific time series.

The write-ahead log (WAL) is used to write to a persistent disk all incoming series samples until they’re flushed to the long-term storage. In the event of an ingester failure, a subsequent process restart will replay the WAL and recover the in-memory series samples.

Contrary to the sole replication and given the persistent disk data is not lost, in the event of multiple ingesters failure each ingester will recover the in-memory series samples from WAL upon subsequent restart. The replication is still recommended in order to ensure no temporary failures on the read path in the event of a single ingester failure.

The WAL for the chunks storage is disabled by default, while it’s always enabled for the blocks storage.

Ingesters write de-amplification

Ingesters store recently received samples in-memory in order to perform write de-amplification. If the ingesters would immediately write received samples to the long-term storage, the system would be very difficult to scale due to the very high pressure on the storage. For this reason, the ingesters batch and compress samples in-memory and periodically flush them out to the storage.

Write de-amplification is the main source of Cortex’s low total cost of ownership (TCO).

Querier

The querier service handles queries using the PromQL query language.

Queriers fetch series samples both from the ingesters and long-term storage: the ingesters hold the in-memory series which have not yet been flushed to the long-term storage. Because of the replication factor, it is possible that the querier may receive duplicated samples; to resolve this, for a given time series the querier internally deduplicates samples with the same exact timestamp.

Queriers are stateless and can be scaled up and down as needed.

Query frontend

The query frontend is an optional service providing the querier’s API endpoints and can be used to accelerate the read path. When the query frontend is in place, incoming query requests should be directed to the query frontend instead of the queriers. The querier service will be still required within the cluster, in order to execute the actual queries.

The query frontend internally performs some query adjustments and holds queries in an internal queue. In this setup, queriers act as workers which pull jobs from the queue, execute them, and return them to the query-frontend for aggregation. Queriers need to be configured with the query frontend address (via the -querier.frontend-address CLI flag) in order to allow them to connect to the query frontends.

Query frontends are stateless. However, due to how the internal queue works, it’s recommended to run a few query frontend replicas to reap the benefit of fair scheduling. Two replicas should suffice in most cases.

Queueing

The query frontend queuing mechanism is used to:

  • Ensure that large queries, that could cause an out-of-memory (OOM) error in the querier, will be retried on failure. This allows administrators to under-provision memory for queries, or optimistically run more small queries in parallel, which helps to reduce the TCO.
  • Prevent multiple large requests from being convoyed on a single querier by distributing them across all queriers using a first-in/first-out queue (FIFO).
  • Prevent a single tenant from denial-of-service-ing (DOSing) other tenants by fairly scheduling queries between tenants.

Splitting

The query frontend splits multi-day queries into multiple single-day queries, executing these queries in parallel on downstream queriers and stitching the results back together again. This prevents large (multi-day) queries from causing out of memory issues in a single querier and helps to execute them faster.

Caching

The query frontend supports caching query results and reuses them on subsequent queries. If the cached results are incomplete, the query frontend calculates the required subqueries and executes them in parallel on downstream queriers. The query frontend can optionally align queries with their step parameter to improve the cacheability of the query results. The result cache is compatible with any cortex caching backend (currently memcached, redis, and an in-memory cache).

Ruler

The ruler is an optional service executing PromQL queries for recording rules and alerts. The ruler requires a database storing the recording rules and alerts for each tenant.

Ruler is semi-stateful and can be scaled horizontally. Running rules internally have state, as well as the ring the rulers initiate. However, if the rulers all fail and restart, Prometheus alert rules have a feature where an alert is restored and returned to a firing state if it would have been active in its for period. However, there would be gaps in the series generated by the recording rules.

Alertmanager

The alertmanager is an optional service responsible for accepting alert notifications from the ruler, deduplicating and grouping them, and routing them to the correct notification channel, such as email, PagerDuty or OpsGenie.

The Cortex alertmanager is built on top of the Prometheus Alertmanager, adding multi-tenancy support. Like the ruler, the alertmanager requires a database storing the per-tenant configuration.

Alertmanager is semi-stateful. The Alertmanager persists information about silences and active alerts to its disk. If all of the alertmanager nodes failed simultaneously there would be a loss of data.

Configs API

The configs API is an optional service managing the configuration of Rulers and Alertmanagers. It provides APIs to get/set/update the ruler and alertmanager configurations and store them into backend. Current supported backend are PostgreSQL and in-memory.

Configs API is stateless.