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Thanos

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What is it?

Thanos is a CNCF (Sandbox) Prometheus-compatible system with global view queries and long-term storage of metrics. See also the project's homepage at https://thanos.io. It is used at the Foundation to enhance the Prometheus deployment, the Thanos interface is available to SRE/NDA at https://thanos.wikimedia.org

Thanos is composed of orthogonal components, as of Aug 2021 at the Foundation the following components are deployed:

Sidecar
co-located with existing Prometheus server and implements the "Store API", meaning the Prometheus local storage is available to other Thanos components for querying. Additionally the sidecar can upload local time series blocks (i.e. generated by Prometheus’ tsdb library) to object storage for long term retention.
Querier
receives Prometheus queries and reaches out to all configured Store APIs, merging the results as needed. Querier is fully stateless and scales horizontally.
Store
exposes the "Store API", but instead of talking to the local Prometheus storage it talks to a remote object storage. The data found in the object storage bucket resembles what’s written on the local disk by Prometheus, i.e. each directory represents one block of time series data spanning a certain time period.
Compactor
takes care of the same process Prometheus does on local disk but on the object storage blocks, namely joining one or more blocks together for efficient querying and space savings.
Rule
reaches out to all Thanos components and runs periodic queries, both for recording and alerting rules. Alerts that require a "global view" are handled by this component. See also https://wikitech.wikimedia.org/wiki/Alertmanager#Local_and_global_alerts for more information on alerts.

The following diagram illustrates the logical view of Thanos operations (the "data flow") and their protocols:

File:Thanos logical view.svg

Use cases

Global view

Thanos query enables the so called "global view" for metric queries. In other words metric queries are sent out to Prometheis in all sites and the results are merged and deduplicated as needed. Thanos is aware of Prometheus HA pairs (replicas) and thus is able to "fill the gaps" for missing data (e.g. as a result of maintenance).

Thanos query is available internally in production at http://thanos-query.discovery.wmnet and in Grafana as the "thanos" datasource. Metrics results will have additional labels, site and prometheus to identify which site and which Prometheus instance the metrics are coming from. For example the query count by (site) (node_boot_time_seconds) will result in aggregated host counts:

{site="eqiad"}  831
{site="codfw"}  667
{site="esams"}   29
{site="ulsfo"}   24
{site="eqsin"}   24

Similarly, this query returns how many targets each Prometheus instance is currently scraping across all sites: count by (prometheus) (up)

{prometheus="k8s"}	     386
{prometheus="k8s-staging"}    39
{prometheus="ops" }         9547
{prometheus="analytics"}     241
{prometheus="services"}	     103

Internally each Prometheus instance also exports a replica label (a, b) for Thanos to pick up while deduplicating results. Deduplication can be turned off at query time and in the examples above and will result in doubled counts.

Long-term storage and downsampling

Each Thanos sidecar uploads blocks to object storage, in the Foundation's deployment we are using Openstack Swift with the S3 compatibility API. The Swift cluster is independent of Swift for media-storage (ms-* hosts) and available at https://thanos-swift.discovery.wmnet. Data is replicated (without encryption) four times spanning codfw and eqiad (multi-region in Swift parlance) thus making the service fully multi-site.

Metric data uploaded to object storage is retained raw (i.e. unsampled) and periodically downsampled at 5m and 1h resolutions for fast results over long periods of time. See also more information about downsampling at https://thanos.io/components/compact.md/#downsampling-resolution-and-retention

Deployment

The current (Jun 2020) Thanos deployment in Production is illustrated by the following diagram:

File:Thanos deployment view.svg

Web interfaces

The query interface of Thanos is available (SSO-authenticated) at https://thanos.wikimedia.org and can be used to run queries for exploration purposes. The same queries can then be used e.g. within Grafana or in alerts.

The underlying block storage viewer is exposed at https://thanos.wikimedia.org/bucket/ and allows inspection of what blocks are stored and their time range. Occasionally useful to investigate problems with Thanos compactor.

Ports in use

Each Thanos component has two ports listening: gRPC for inter-component communication and HTTP to expose Prometheus metrics. Aside from the metrics use case the only HTTP port in use by external systems is for Thanos query (10902, proxied on port 80 by Apache).

Component HTTP gRPC
Query 10902 10901
Compact 12902 N/A
Store 11902 11901
Sidecar Prometheus port + 10000 Prometheus port + 20000
Bucket web 15902 N/A
Rule 17902 17901

Porting dashboards to Thanos

This section outlines strategies to port existing Grafana dashboards to Thanos. See also bug T256954 for the tracking task.

single "namespace"
these dashboard display a "thing" which is uniquely named across sites, for example host overview dashboard. In this case it is sufficient to default the datasource to "thanos". The key here being the fact that there's no ambiguity for hostnames across sites.
multiple sites in the same panel
these dashboard use multiple datasources mixed in the same panel. the migration to Thanos is straightforward: use thanos as the single datasource and run a single query: the results can be aggregated by site label as needed.
overlapping names across sites
the dashboard displays a "thing" deployed with the same name across sites. For example clusters are uniquely identified by their name plus site. In these cases there is usually a datasource variable to select the site-local Prometheus. Especially when first migrating a dashboard it is important to be able to go back to this model and bypass Thanos. Such "backwards compatibility" is achieved by the following steps:
  1. Introduce a template variable $site with datasource thanos and query label_values(node_boot_time_seconds, site)
  2. Change the existing datasource variable instance name filter to include thanos as well. The thanos datasource will be the default for the dashboard.
  3. Change the remaining query-based variables to include a site=~"$site" selector, as needed. For example the cluster variable is based on the label_values(node_boot_time_seconds{site=~"$site"}, cluster) query. Ditto for instance variable, the query is label_values(node_boot_time_seconds{cluster="$cluster",site=~"$site"}, instance).
  4. Add the same site=~"$site" selector to all queries in panels that need it. Without the selector the panel queries will return data for all sites otherwise.

The intended usage is to select the site via site variable and no longer via datasource (which must default to thanos). If for some reason querying through thanos doesn't work, it is possible to temporarily fallback to querying Prometheus directly:

  1. Input .* as the site
  2. Select the desired site-local Prometheus from the datasource dropdown.

Operations

Pool / depool a site

The read path is served by thanos-query.discovery.wmnet and the write path by thanos-swift.discovery.wmnet. Each can be pooled/depooled individually and both are generally active/active.

thanos-swift

The actual depool is a confctl away:

 confctl --object-type discovery select 'dnsdisc=thanos-swift,name=SITE' set/pooled=true

The Thanos sidecar maintains persistent connections to object storage. If draining such connections is desired then a roll-restart of thanos-sidecar is sufficient:

 cumin -b1 'R:thanos::sidecar' 'systemctl restart thanos-sidecar@*'

thanos-query

There are generally no persistent connections (i.e. Grafana) to the thanos-query endpoint, thus the following is sufficient to (de)pool:

 confctl --object-type discovery select 'dnsdisc=thanos-query,name=SITE' set/pooled=true

Alerts

More than one Thanos compact running

Thanos compact has not run / is halted

The compactor has ran into some trouble while compacting (e.g. out of disk space) and has halted compaction. The thanos-compact service logs will have more detailed information about the failure. See also thanos-compact dashboard. The compactor runs as a singleton, the active host is defined by profile::thanos::compact_host in Hiera.

Thanos <component> has disappeared from Prometheus discovery

Thanos <component> has high percentage of failures

Thanos <component> has high latency

Thanos sidecar cannot connect to Prometheus

Thanos sidecar is unhealthy

Thanos query has high gRPC client errors

The thanos-query service is experiencing errors, consulting its logs will reveal the detailed error cause.

Thanos sidecar is failing to upload blocks

The Thanos sidecar component has failed to upload blocks to object storage. The alert contains which Prometheus instance, host and site the error is occurring in. Next action is inspecting Thanos sidecar logs (unit thanos-sidecar@INSTANCE, one per Prometheus instance) for the upload error. The alert will clear once the affected thanos sidecar unit is restarted.