Metrics instrumentation guide

This guide describes how to develop Service Ping metrics using metrics instrumentation.

For a video tutorial, see the Adding Service Ping metric via instrumentation class.

Nomenclature

• Instrumentation class:

  • Inherits one of the metric classes: DatabaseMetric, NumbersMetric or GenericMetric.
  • Implements the logic that calculates the value for a Service Ping metric.

• Metric definition The Service Data metric YAML definition.

• Hardening: Hardening a method is the process that ensures the method fails safe, returning a fallback value like -1.

How it works

A metric definition has the instrumentation_class field, which can be set to a class.

The defined instrumentation class should inherit one of the existing metric classes: DatabaseMetric, NumbersMetric or GenericMetric.

The current convention is that a single instrumentation class corresponds to a single metric.

Using an instrumentation class ensures that metrics can fail safe individually, without breaking the entire process of Service Ping generation.

Database metrics

NOTE: Whenever possible we recommend using internal event tracking instead of database metrics. Database metrics can create unnecessary load on the database of bigger GitLab instances and potential optimisations can affect instance performance.

You can use database metrics to track data kept in the database, for example, a count of issues that exist on a given instance.

• operation: Operations for the given relation, one of count, distinct_count, sum, and average.
• relation: Assigns lambda that returns the ActiveRecord::Relation for the objects we want to perform the operation. The assigned lambda can accept up to one parameter. The parameter is hashed and stored under the options key in the metric definition.
• start: Specifies the start value of the batch counting, by default is relation.minimum(:id).
• finish: Specifies the end value of the batch counting, by default is relation.maximum(:id).
• cache_start_and_finish_as: Specifies the cache key for start and finish values and sets up caching them. Use this call when start and finish are expensive queries that should be reused between different metric calculations.
• available?: Specifies whether the metric should be reported. The default is true.
• timestamp_column: Optionally specifies timestamp column for metric used to filter records for time constrained metrics. The default is created_at.

Example of a merge request that adds a database metric.

Optimization recommendations and examples

Any single query for a Service Ping metric must stay below the 1 second execution time with cold caches.

• Use specialized indexes. For examples, see these merge requests:
  • Example 1
  • Example 2
• Use defined start and finish. These values can be memoized and reused, as in this example merge request.
• Avoid joins and unnecessary complexity in your queries. See this example merge request as an example.
• Set a custom batch_size for distinct_count, as in this example merge request.

Database metric Examples

Count Example

module Gitlab
  module Usage
    module Metrics
      module Instrumentations
        class CountIssuesMetric < DatabaseMetric
          operation :count

          relation ->(options) { Issue.where(confidential: options[:confidential]) }
        end
      end
    end
  end
end

Batch counters Example

module Gitlab
  module Usage
    module Metrics
      module Instrumentations
        class CountIssuesMetric < DatabaseMetric
          operation :count

          start { Issue.minimum(:id) }
          finish { Issue.maximum(:id) }

          relation { Issue }
        end
      end
    end
  end
end

Distinct batch counters Example

# frozen_string_literal: true

module Gitlab
  module Usage
    module Metrics
      module Instrumentations
        class CountUsersAssociatingMilestonesToReleasesMetric < DatabaseMetric
          operation :distinct_count, column: :author_id

          relation { Release.with_milestones }

          start { Release.minimum(:author_id) }
          finish { Release.maximum(:author_id) }
        end
      end
    end
  end
end

Sum Example

# frozen_string_literal: true

module Gitlab
  module Usage
    module Metrics
      module Instrumentations
        class JiraImportsTotalImportedIssuesCountMetric < DatabaseMetric
          operation :sum, column: :imported_issues_count

          relation { JiraImportState.finished }
        end
      end
    end
  end
end

Average Example

# frozen_string_literal: true

module Gitlab
  module Usage
    module Metrics
      module Instrumentations
        class CountIssuesWeightAverageMetric < DatabaseMetric
          operation :average, column: :weight

          relation { Issue }
        end
      end
    end
  end
end

Estimated batch counters

Estimated batch counter functionality handles ActiveRecord::StatementInvalid errors when used through the provided estimate_batch_distinct_count method. Errors return a value of -1.

WARNING: This functionality estimates a distinct count of a specific ActiveRecord_Relation in a given column, which uses the HyperLogLog algorithm. As the HyperLogLog algorithm is probabilistic, the results always include error. The highest encountered error rate is 4.9%.

When correctly used, the estimate_batch_distinct_count method enables efficient counting over columns that contain non-unique values, which cannot be assured by other counters.

estimate_batch_distinct_count method

Method:

estimate_batch_distinct_count(relation, column = nil, batch_size: nil, start: nil, finish: nil)

The method includes the following arguments:

• relation: The ActiveRecord_Relation to perform the count.
• column: The column to perform the distinct count. The default is the primary key.
• batch_size: From Gitlab::Database::PostgresHll::BatchDistinctCounter::DEFAULT_BATCH_SIZE. Default value: 10,000.
• start: The custom start of the batch count, to avoid complex minimum calculations.
• finish: The custom end of the batch count to avoid complex maximum calculations.

The method includes the following prerequisites:

• The supplied relation must include the primary key defined as the numeric column. For example: id bigint NOT NULL.

• The estimate_batch_distinct_count can handle a joined relation. To use its ability to count non-unique columns, the joined relation must not have a one-to-many relationship, such as has_many :boards.

• Both start and finish arguments should always represent primary key relationship values, even if the estimated count refers to another column, for example:

    estimate_batch_distinct_count(::Note, :author_id, start: ::Note.minimum(:id), finish: ::Note.maximum(:id))

Examples:

1. Simple execution of estimated batch counter, with only relation provided, returned value represents estimated number of unique values in id column (which is the primary key) of Project relation:

     estimate_batch_distinct_count(::Project)

2. Execution of estimated batch counter, where provided relation has applied additional filter (.where(time_period)), number of unique values estimated in custom column (:author_id), and parameters: start and finish together apply boundaries that defines range of provided relation to analyze:

     estimate_batch_distinct_count(::Note.with_suggestions.where(time_period), :author_id, start: ::Note.minimum(:id), finish: ::Note.maximum(:id))

Numbers metrics

• operation: Operations for the given data block. Currently we only support add operation.
• data: a block which contains an array of numbers.
• available?: Specifies whether the metric should be reported. The default is true.

# frozen_string_literal: true

module Gitlab
  module Usage
    module Metrics
      module Instrumentations
          class IssuesBoardsCountMetric < NumbersMetric
            operation :add

            data do |time_frame|
              [
                 CountIssuesMetric.new(time_frame: time_frame).value,
                 CountBoardsMetric.new(time_frame: time_frame).value
              ]
            end
          end
        end
      end
    end
  end
end

You must also include the instrumentation class name in the YAML setup.

time_frame: 28d
instrumentation_class: IssuesBoardsCountMetric

Generic metrics

You can use generic metrics for other metrics, for example, an instance's database version.

• value: Specifies the value of the metric.
• available?: Specifies whether the metric should be reported. The default is true.

Example of a merge request that adds a generic metric.

module Gitlab
  module Usage
    module Metrics
      module Instrumentations
        class UuidMetric < GenericMetric
          value do
            Gitlab::CurrentSettings.uuid
          end
        end
      end
    end
  end
end

Prometheus metrics

This instrumentation class lets you handle Prometheus queries by passing a Prometheus client object as an argument to the value block. Any Prometheus error handling should be done in the block itself.

• value: Specifies the value of the metric. A Prometheus client object is passed as the first argument.
• available?: Specifies whether the metric should be reported. The default is true.

Example of a merge request that adds a Prometheus metric.

module Gitlab
  module Usage
    module Metrics
      module Instrumentations
        class GitalyApdexMetric < PrometheusMetric
          value do |client|
            result = client.query('avg_over_time(gitlab_usage_ping:gitaly_apdex:ratio_avg_over_time_5m[1w])').first

            break FALLBACK unless result

            result['value'].last.to_f
          end
        end
      end
    end
  end
end

Create a new metric instrumentation class

The generator takes the class name as an argument and the following options:

• --type=TYPE Required. Indicates the metric type. It must be one of: database, generic, redis, numbers.
• --operation Required for database & numbers type.
  • For database it must be one of: count, distinct_count, estimate_batch_distinct_count, sum, average.
  • For numbers it must be: add.
• --ee Indicates if the metric is for EE.

rails generate gitlab:usage_metric CountIssues --type database --operation distinct_count
        create lib/gitlab/usage/metrics/instrumentations/count_issues_metric.rb
        create spec/lib/gitlab/usage/metrics/instrumentations/count_issues_metric_spec.rb

Migrate Service Ping metrics to instrumentation classes

This guide describes how to migrate a Service Ping metric from lib/gitlab/usage_data.rb or ee/lib/ee/gitlab/usage_data.rb to instrumentation classes.

1. Choose the metric type:

• Database metric
• Numbers metric
• Generic metric

1. Determine the location of instrumentation class: either under ee or outside ee.

2. Generate the instrumentation class file.

3. Fill the instrumentation class body:

   • Add code logic for the metric. This might be similar to the metric implementation in usage_data.rb.
   • Add tests for the individual metric spec/lib/gitlab/usage/metrics/instrumentations/.
   • Add tests for Service Ping.

4. Generate the metric definition file.

5. Remove the code from lib/gitlab/usage_data.rb or ee/lib/ee/gitlab/usage_data.rb.

6. Remove the tests from spec/lib/gitlab/usage_data.rb or ee/spec/lib/ee/gitlab/usage_data.rb.

Troubleshoot metrics

Sometimes metrics fail for reasons that are not immediately clear. The failures can be related to performance issues or other problems. The following pairing session video gives you an example of an investigation in to a real-world failing metric.