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/**
 * Manages custom metrics. Names of custom metrics registers are always stored with the prefix "custom.".
 * If provided registry name is "a.b.regname", it is automatically converted to "custom.a.b.regname".
 * If the name is "custom.a.b.regname", it is used as is.  
 */
public interface IgniteCustomMetrics {
    /**
     * Registers metric in registry {@code registryName}. Adds new metric registry if absent. 
     *
     * @return Previous metric if it already exists with the same name. {@code metric} otherwise. 
     */
    <T extends Metric> T metric(String registryName, String metricName, T metric);

    /** Removes metric from registry {@code registryName}. */
    void removeMetric(String registryName, String metricName);

    /** @return Metric registry named '{@code registryName}'. */
    ReadOnlyMetricRegistry registry(String registryName);

    /** Removes registry named '{@code registryName}'. */
    void removeRegistry(String registryName);


    // Essential metrics. Int, Long and Double should be enough.

    /**
     * Registers a {@link LongMetric} in registry {@code registryName}. Adds new metric registry if absent.
     *
     * @return Previous long value setter if it already exists with the same name. A new one otherwise.
     */
    LongConsumer longMetric(String registryName, String metricName, @Nullable String description);

    /**
     * Registers a {@link DoubleMetric} in registry {@code registryName}. Adds new metric registry if absent.
     *
     * @return Previous double value setter if it already exists with the same name. A new one otherwise.
     */
    DoubleConsumer doubleMetric(String registryName, String metricName, @Nullable String description);

    /**
     * Registers a {@link IntMetric} in registry {@code registryName}. Adds new metric registry if absent.
     *
     * @return Previous int value setter if it already exists with the same name. A new one otherwise.
     */
    IntConsumer booleanMetric(String registryName, String metricName, @Nullable String description);



    // Might be useful the gauges.

    /**
     * Registers a {@link LongMetric} metric in registry {@code registryName} with long value supplier {@code valueSupplier}. 
     * Adds new metric registry if absent.
     *
     * @return {@code True} if metric was added. {@code False} if another meric already exists under the same name.  
     */
    boolean longMetric(String registryName, String metricName, LongSupplier valueSupplier, @Nullable String description);

    /**
     * Registers a {@link DoubleMetric} metric in registry {@code registryName} with double value supplier {@code valueSupplier}. 
     * Adds new metric registry if absent.
     *
     * @return {@code True} if metric was added. {@code False} if another meric already exists under the same name.  
     */
    boolean doubleMetric(String registryName, String metricName, DoubleSupplier valueSupplier, @Nullable String description);

    /**
     * Registers a {@link IntMetric} metric in registry {@code registryName} with int value supplier {@code valueSupplier}. 
     * Adds new metric registry if absent.
     *
     * @return {@code True} if metric was added. {@code False} if another meric already exists under the same name.  
     */
    boolean intMetric(String registryName, String metricName, BooleanSupplier valueSupplier, @Nullable String description);
    // Additionnaly, we might expose an object metrics API.
    /**
     * Registers a {@link ObjectMetric} metric in registry {@code registryName} with object value supplier {@code valueSupplier}. 
     * Adds new metric registry if absent.
     *
     * @return {@code True} if metric was added. {@code False} if another meric already exists under the same name.  
     */
    <T> boolean objectMetric(String registryName, String metricName, Class<T> valueType, Supplier<T> valueSupplier, @Nullable String description);
    /**
    * Registers a {@link ObjectMetric} in registry {@code registryName} with object value supplier {@code valueSupplier}. 
    * Adds new metric registry if absent.
    *
    * @return New or previously registered object setter.
    */
    <T> Consumer<T> objectMetric(String registryName, String metricName, Class<T> valueType, Supplier<T> valueSupplier, @Nullable String description);
}

Risks and Assumptions

• Custom metrics can affect the performance.
• There could be race conditions on metric registrations.
• We do not provide varuous useful metric implementatons.
• Custom metrics aren't stored and require re-registration after node restart. At least at the first phase.

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