About Promscale

Promscale is an open source observability backend for metrics and traces powered by SQL.

It's built on the robust and high-performance foundation of PostgreSQL and TimescaleDB. It has native support for Prometheus metrics and OpenTelemetry traces as well as many other formats like StatsD, Jaeger and Zipkin through the OpenTelemetry Collector and is 100% PromQL compliant. It's full SQL capabilities enable developers to correlate metrics, traces and also business data to derive new valuable insights not possible when data is siloed in different systems. It easily integrates with Grafana and Jaeger for visualizing metrics and traces.

Built on top of PostgreSQL and TimescaleDB it inherits rock-solid reliability, native compression up to 90%, continuous aggregates and the operational maturity of a system that is run on millions of instances worldwide.

For the Promscale source code, see our GitHub repository.

If you have any questions, join the #promscale channel on the TimescaleDB Community Slack.

Architecture

Promscale includes two components:

Promscale Connector: a stateless service that provides the ingest interfaces for observability data, processes that data and stores it in TimescaleDB. It also provides an interface to query the data with PromQL. The Promscale Connector automatically sets up the data structures in TimescaleDB to store the data and handles changes in those data structures if required for upgrading to newer versions of Promscale.

TimescaleDB: the Postgres-based database where all the observability data is stored. It offers a full SQL interface for querying the data as well as advanced capabilities like analytical functions, columnar compression and continuous aggregates. TimescaleDB offers a lot of flexibility to also store business and other types of data that you can then use to correlate with observability data.

The Promscale Connector ingests Prometheus metrics, metadata and OpenMetrics exemplars using the Prometheus remote_write interface. It also ingests OpenTelemetry traces using the OpenTelemetry protocol (OTLP). It can also ingest metrics and traces in other formats using the OpenTelemetry Collector to process and send them over the Prometheus remote_write interface and the OpenTelemetry protocol. For example, you can use the OpenTelemetry Collector to ingest Jaeger traces and StatsD metrics into Promscale.

For Prometheus metrics, the Promscale Connector exposes Prometheus API endpoints for running PromQL queries and reading metadata. This allows you to connect tools that support the Prometheus API, such as Grafana, directly to Promscale for querying. It's also possible to send queries to Prometheus and have Prometheus read data from Promscale using the Promscale Connector on the remote_read interface.

For OpenTelemetry traces, there is a Jaeger storage plugin that implements the interface for querying and retrieving traces. This allows you to visualize traces stored in Promscale in Jaeger as well as Grafana by configuring a Jaeger data source. In this case, Grafana queries Jaeger, which then queries Promscale.

You can also query metrics and traces in Promscale using SQL which allows you to use many different visualization tools that integrate with PostgreSQL. For example, Grafana supports querying data in Promscale using SQL out of the box through the PostgreSQL data source.

Promscale PostgreSQL extension

Promscale has a dependency on the Promscale PostgreSQL extension, which contains support functions to improve the performance of Promscale. While Promscale is able to run without the additional extension installed, adding this extension gets better performance from Promscale.

Promscale schema for metric data

To achieve high ingestion, query performance, and optimal storage the Promscale schema writes the data in the most optimal format for storage and querying in TimescaleDB. Promscale translates data from the Prometheus data model into a relational schema that is optimized for TimescaleDB.

The basic schema uses a normalized design where time-series data is stored in compressed hypertables. These tables have a foreign key to series tables that are stored as regular PostgreSQL tables, and each series consists of a unique set of labels.

For more information about compression, see the compression section. For more information about hypertables, see the hypertables section.

Metrics storage schema

Each metric is stored in a separate hypertable. In particular, the schema decouples individual metrics, allowing for the collection of metrics with vastly different cardinalities and retention periods. At the same time, Promscale exposes simple, user-friendly views so that you do not have to understand this optimized schema.

The latest chunk is decompressed to serve as a high-speed query cache. Older chunks are stored as compressed chunks. We configure compression with the segment_by column set to the series_id and the order_by column set to time DESC. These settings control how data is split into blocks of compressed data. Each block can be accessed and decompressed independently.

These settings mean that a block of compressed data is always associated with a single series_id and that the data is sorted by time before being split into blocks. This means each block is associated with a fairly narrow time range. As a result, in compressed form, access by series_id and time range are optimized.

For example, the hypertables for each metric use the following schema, using cpu_usage as an example metric:

The cpu_usage table schema:

CREATE TABLE cpu_usage (
    time        TIMESTAMPTZ,
    value   DOUBLE PRECISION,
    series_id   BIGINT,
)
CREATE INDEX ON cpu_usage (series_id, time) INCLUDE (value)
Column   |           Type           | Modifiers
-----------+--------------------------+-----------
time      | TIMESTAMPTZ              |
value     | DOUBLE PRECISION         |
series_id | BIGINT                   |

In this example, series_id is a foreign key to the series table described in the next section.

Series storage schema

Conceptually, each row in the series table stores a set of key-value pairs. In Prometheus, a series like this is represented as a one-level JSON string, such as { "key1":"value1", "key2":"value2" }. But the strings representing keys and values are often long and repeating. So, to save space, we store a series as an array of integer foreign keys to a normalized labels table.

The definition of these two tables is:

CREATE TABLE _prom_catalog.series (
    id serial,
    metric_id int,
    labels int[],
    UNIQUE(labels) INCLUDE (id)
);
CREATE INDEX series_labels_id ON _prom_catalog.series USING GIN (labels);

CREATE TABLE _prom_catalog.label (
    id serial,
    key TEXT,
    value text,
    PRIMARY KEY (id) INCLUDE (key, value),
    UNIQUE (key, value) INCLUDE (id)
);

Promscale views

You interact with Prometheus data in Promscale through views. These views are automatically created and are used to interact with metrics and labels.

Each metric and label has its own view. You can see a list of all metrics by querying the view named metric. Similarly, you can see a list of all labels by querying the view named label. These views are found in the prom_info schema.

Querying the metric view returns all metrics collected by Prometheus:

SELECT *
FROM prom_info.metric;

Here is one row of a sample output for the query shown earlier:

id                | 16
metric_name       | process_cpu_seconds_total
table_name        | process_cpu_seconds_total
retention_period  | 90 days
chunk_interval    | 08:01:06.824386
label_keys        | {__name__,instance,job}
size              | 824 kB
compression_ratio | 71.60883280757097791800
total_chunks      | 11
compressed_chunks | 10

Each row in the metric view contains fields with the metric id, as well as information about the metric, such as its name, table name, retention period, compression status, chunk interval etc.

Promscale maintains isolation between metrics. This allows you to set retention periods, downsampling, and compression settings on a per metric basis, giving you more control over your metrics data.

Querying the label view returns all labels associated with metrics collected by Prometheus:

SELECT *
FROM prom_info.label;

Here is one row of a sample output for the query shown earlier:

key               | collector
value_column_name | collector
id_column_name    | collector_id
values            | {arp,bcache,bonding,btrfs,conntrack,cpu,cpufreq,diskstats,edac,entropy,filefd,filesystem,hwmon,infiniband,ipvs,loadavg,mdadm,meminfo,netclass,netdev,netstat,nfs,nfsd,powersupplyclass,pressure,rapl,schedstat,sockstat,softnet,stat,textfile,thermal_zone,time,timex,udp_queues,uname,vmstat,xfs,zfs}
num_values        | 39

Each label row contains information about a particular label, such as the label key, the label's value column name, the label's ID column name, the list of all values taken by the label,and the total number of values for that label.

For examples of querying a specific metric view, see Query data in Promscale.

Found an issue on this page?

Report an issue!

Keywords

Related Content