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backend-fetcher.md

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Open Fetcher

Fetcher is a concept in SkyWalking backend. It uses pulling mode rather than receiver, which read the data from the target systems. This mode is typically in some metrics SDKs, such as Prometheus.

Prometheus Fetcher

prometheus-fetcher:
  selector: ${SW_PROMETHEUS_FETCHER:default}
  default:
    active: ${SW_PROMETHEUS_FETCHER_ACTIVE:false}

Configuration file

Prometheus fetcher is configured via a configuration file. The configuration file defines everything related to fetching services and their instances, as well as which rule files to load.

OAP can load the configuration at bootstrap. If the new configuration is not well-formed, OAP fails to start up. The files are located at $CLASSPATH/fetcher-prom-rules.

The file is written in YAML format, defined by the scheme described below. Brackets indicate that a parameter is optional.

A full example can be found here

Generic placeholders are defined as follows:

  • <duration>: a duration This will parse a textual representation of a duration. The formats accepted are based on the ISO-8601 duration format PnDTnHnMn.nS with days considered to be exactly 24 hours.
  • <labelname>: a string matching the regular expression [a-zA-Z_][a-zA-Z0-9_]*
  • <labelvalue>: a string of unicode characters
  • <host>: a valid string consisting of a hostname or IP followed by an optional port number
  • <path>: a valid URL path
  • <string>: a regular string
# How frequently to fetch targets.
fetcherInterval: <duration> 
# Per-fetch timeout when fetching this target.
fetcherTimeout: <duration>
# The HTTP resource path on which to fetch metrics from targets.
metricsPath: <path>
#Statically configured targets.
staticConfig:
  # The targets specified by the static config.
  targets:
    [ - <host> ]
  # Labels assigned to all metrics fetched from the targets.
  labels:
    [ <labelname>: <labelvalue> ... ]
# Metrics rule allow you to recompute queries.
metricsRules:
   [ - <metric_rules> ]

<metric_rules>

# The name of rule, which combinates with a prefix 'meter_' as the index/table name in storage.
name: <string>
# Scope should be one of SERVICE, INSTANCE and ENDPOINT.
scope: <string>
# The transformation operation from prometheus metrics to skywalking ones. 
operation: <operation>
# The prometheus sources of the transformation operation.
sources:
  # The prometheus metric family name 
  <string>:
    # Function for counter, one of INCREASE, RATE, and IRATE.
    [counterFunction: <string> ]
    # The range of a counterFunction.
    [range: <duration>]
    # The percentile rank of percentile operation
    [percentiles: [<rank>,...]]
    # Relabel prometheus labels to skywalking dimensions.
    relabel:
      service: [<labelname>, ...]
      [instance: [<labelname>, ...]]
      [endpoint: [<labelname>, ...]]

The available operations are avg, avgHistogram and avgHistogramPercentile. The avg and avgXXX mean to average the raw fetched metrics or high rate metrics into low rate metrics. The process is the extension of skywalking downsampling, that adds the procedure from raw data to minute rate.

When you specify avgHistogram and avgHistogramPercentile, the source should be the type of histogram. A counterFunction is also needed due to the bucket, sum and count of histogram are counters.

Kafka Fetcher

Kafka Fetcher pulls messages from Kafka Broker(s) what is the Agent delivered. Check the agent documentation about the details. Typically Tracing Segments, Service/Instance properties, JVM Metrics, and Meter system data are supported. Kafka Fetcher can work with gRPC/HTTP Receivers at the same time for adopting different transport protocols.

Kafka Fetcher is disabled in default, and we configure as following to enable.

kafka-fetcher:
  selector: ${SW_KAFKA_FETCHER:default}
  default:
    bootstrapServers: ${SW_KAFKA_FETCHER_SERVERS:localhost:9092}

skywalking-segments, skywalking-metrics, skywalking-profile, skywalking-managements and skywalking-meters topics are required by kafka-fetcher. If they do not exist, Kafka Fetcher will create them in default. Also, you can create them by yourself before the OAP server started.

When using the OAP server automatical creation mechanism, you could modify the number of partitions and replications of the topics through the following configurations:

kafka-fetcher:
  selector: ${SW_KAFKA_FETCHER:default}
  default:
    bootstrapServers: ${SW_KAFKA_FETCHER_SERVERS:localhost:9092}
    partitions: ${SW_KAFKA_FETCHER_PARTITIONS:3}
    replicationFactor: ${SW_KAFKA_FETCHER_PARTITIONS_FACTOR:2}
    enableMeterSystem: ${SW_KAFKA_FETCHER_ENABLE_METER_SYSTEM:false}
    isSharding: ${SW_KAFKA_FETCHER_IS_SHARDING:false}
    consumePartitions: ${SW_KAFKA_FETCHER_CONSUME_PARTITIONS:""}

In cluster mode, all topics have the same number of partitions. Then we have to set "isSharding" to "true" and assign the partitions to consume for OAP server. The OAP server can use commas to separate multiple partitions.

Kafka Fetcher allows to configure all the Kafka producers listed here in property kafkaConsumerConfig. Such as:

kafka-fetcher:
  selector: ${SW_KAFKA_FETCHER:default}
  default:
    bootstrapServers: ${SW_KAFKA_FETCHER_SERVERS:localhost:9092}
    partitions: ${SW_KAFKA_FETCHER_PARTITIONS:3}
    replicationFactor: ${SW_KAFKA_FETCHER_PARTITIONS_FACTOR:2}
    enableMeterSystem: ${SW_KAFKA_FETCHER_ENABLE_METER_SYSTEM:false}
    isSharding: ${SW_KAFKA_FETCHER_IS_SHARDING:true}
    consumePartitions: ${SW_KAFKA_FETCHER_CONSUME_PARTITIONS:1,3,5}
    kafkaConsumerConfig:
      enable.auto.commit: true
      ...