The sidecar component of Thanos gets deployed along with a Prometheus instance. This allows sidecar to optionally upload metrics to object storage and allow Queriers to query Prometheus data with common, efficient StoreAPI.
In details:
It implements Thanos’ Store API on top of Prometheus’ remote-read API. This allows Queriers to treat Prometheus servers as yet another source of time series data without directly talking to its APIs.
Optionally, the sidecar uploads TSDB blocks to an object storage bucket as Prometheus produces them every 2 hours. This allows Prometheus servers to be run with relatively low retention while their historic data is made durable and queryable via object storage.
NOTE: This still does NOT mean that Prometheus can be fully stateless, because if it crashes and restarts you will lose ~2 hours of metrics, so persistent disk for Prometheus is highly recommended. The closest to stateless you can get is using remote write (which Thanos experimentally supports, see this. Remote write has other risks and consequences, and still if crashed you loose in positive case seconds of metrics data, so persistent disk is recommended in all cases.
Optionally Thanos sidecar is able to watch Prometheus rules and configuration, decompress and substitute environment variables if needed and ping Prometheus to reload them. Read more about this in here
Prometheus servers connected to the Thanos cluster via the sidecar are subject to a few limitations and recommendations for safe operations:
The recommended Prometheus version is 2.2.1 or greater (including newest releases). This is due to Prometheus instability in previous versions as well as lack of flags endpoint.
(!) The Prometheus external_labels section of the Prometheus configuration file has unique labels in the overall Thanos system. Those external labels will be used by the sidecar and then Thanos in many places:
The --web.enable-lifecycle flag is enabled if you want to use sidecar reloading features (--reload.* flags).
If you choose to use the sidecar to also upload to object storage:
--storage.tsdb.min-block-duration and --storage.tsdb.max-block-duration must be set to equal values to disable local compaction on order to use Thanos sidecar upload, otherwise leave local compaction on if sidecar just exposes StoreAPI and your retention is normal. The default of 2h is recommended. Mentioned parameters set to equal values disable the internal Prometheus compaction, which is needed to avoid the uploaded data corruption when Thanos compactor does its job, this is critical for data consistency and should not be ignored if you plan to use Thanos compactor. Even though you set mentioned parameters equal, you might observe Prometheus internal metric prometheus_tsdb_compactions_total being incremented, don’t be confused by that: Prometheus writes initial head block to filesytem via its internal compaction mechanism, but if you have followed recommendations - data won’t be modified by Prometheus before the sidecar uploads it. Thanos sidecar will also check sanity of the flags set to Prometheus on the startup and log errors or warning if they have been configured improperly (#838).Thanos can watch changes in Prometheus configuration and refresh Prometheus configuration if --web.enable-lifecycle enabled.
You can configure watching for changes in directory via --reloader.rule-dir=DIR_NAME flag.
Thanos sidecar can watch --reloader.config-file=CONFIG_FILE configuration file, evaluate environment variables found in there, and produce generated config in --reloader.config-envsubst-file=OUT_CONFIG_FILE file.
$ prometheus \
--storage.tsdb.max-block-duration=2h \
--storage.tsdb.min-block-duration=2h \
--web.enable-lifecycle
$ thanos sidecar \
--tsdb.path "/path/to/prometheus/data/dir" \
--prometheus.url "http://localhost:9090" \
--objstore.config-file "bucket.yml"
The example content of bucket.yml:
type: GCS
config:
bucket: example-bucket
usage: thanos sidecar [<flags>]
sidecar for Prometheus server
Flags:
-h, --help Show context-sensitive help (also try
--help-long and --help-man).
--version Show application version.
--log.level=info Log filtering level.
--log.format=logfmt Log format to use.
--tracing.config-file=<file-path>
Path to YAML file with tracing configuration.
See format details:
https://thanos.io/tracing.md/#configuration
--tracing.config=<content>
Alternative to 'tracing.config-file' flag
(lower priority). Content of YAML file with
tracing configuration. See format details:
https://thanos.io/tracing.md/#configuration
--http-address="0.0.0.0:10902"
Listen host:port for HTTP endpoints.
--grpc-address="0.0.0.0:10901"
Listen ip:port address for gRPC endpoints
(StoreAPI). Make sure this address is routable
from other components.
--grpc-server-tls-cert="" TLS Certificate for gRPC server, leave blank to
disable TLS
--grpc-server-tls-key="" TLS Key for the gRPC server, leave blank to
disable TLS
--grpc-server-tls-client-ca=""
TLS CA to verify clients against. If no client
CA is specified, there is no client
verification on server side. (tls.NoClientCert)
--prometheus.url=http://localhost:9090
URL at which to reach Prometheus's API. For
better performance use local network.
--tsdb.path="./data" Data directory of TSDB.
--reloader.config-file="" Config file watched by the reloader.
--reloader.config-envsubst-file=""
Output file for environment variable
substituted config file.
--reloader.rule-dir=RELOADER.RULE-DIR ...
Rule directories for the reloader to refresh
(repeated field).
--objstore.config-file=<file-path>
Path to YAML file that contains object store
configuration. See format details:
https://thanos.io/storage.md/#configuration
--objstore.config=<content>
Alternative to 'objstore.config-file' flag
(lower priority). Content of YAML file that
contains object store configuration. See format
details:
https://thanos.io/storage.md/#configuration
--min-time=0000-01-01T00:00:00Z
Start of time range limit to serve. Thanos
sidecar will serve only metrics, which happened
later than this value. Option can be a constant
time in RFC3339 format or time duration
relative to current time, such as -1d or 2h45m.
Valid duration units are ms, s, m, h, d, w, y.