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Consolidates notes on "extra" data
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Before this change, notes about state, particularly the "one instance
per plugin" rule were not consolidated on the method that can cause
problems `addExtra`. This migrates notes together with additional
warnings that when we talk about this being used for propagation
plugins, we also mean not for ad-hoc usage, as that can lead to
excessive overhead.

See #1421

Signed-off-by: Adrian Cole <[email protected]>
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Adrian Cole committed Mar 12, 2024
1 parent d8dc495 commit 33780e0
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Showing 3 changed files with 45 additions and 14 deletions.
4 changes: 2 additions & 2 deletions brave/src/main/java/brave/baggage/BaggageFields.java
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@@ -1,5 +1,5 @@
/*
* Copyright 2013-2020 The OpenZipkin Authors
* Copyright 2013-2024 The OpenZipkin Authors
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
* in compliance with the License. You may obtain a copy of the License at
Expand All @@ -25,7 +25,7 @@
* <h3>Built-in fields</h3>
* The following are fields that dispatch to methods on the {@link TraceContext}. They are available
* regardless of {@link BaggagePropagation}. None will return in lookups such as {@link
* BaggageField#getAll(TraceContext)} or {@link BaggageField#getByName(TraceContext, String)}
* BaggageField#getAllValues(TraceContext)} or {@link BaggageField#getByName(TraceContext, String)}.
*
* <p><ol>
* <li>{@link #TRACE_ID}</li>
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42 changes: 31 additions & 11 deletions brave/src/main/java/brave/propagation/TraceContext.java
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Expand Up @@ -14,9 +14,12 @@
package brave.propagation;

import brave.Span;
import brave.baggage.BaggageFields;
import brave.baggage.BaggagePropagation;
import brave.internal.InternalPropagation;
import brave.internal.Nullable;
import brave.internal.Platform;
import brave.internal.baggage.ExtraBaggageContext;
import java.lang.ref.WeakReference;
import java.util.Collections;
import java.util.List;
Expand Down Expand Up @@ -182,27 +185,26 @@ public boolean shared() {
}

/**
* Returns a list of additional data propagated through this trace.
* Used internally by propagation plugins to search for their instance of state regardless of if
* it is in a {@linkplain TraceContext} or {@linkplain TraceContextOrSamplingFlags}.
*
* <p>The contents are intentionally opaque, deferring to {@linkplain Propagation} to define. An
* example implementation could be storing a class containing a correlation value, which is
* extracted from incoming requests and injected as-is onto outgoing requests.
*
* <p>Implementations are responsible for scoping any data stored here. This can be performed
* when {@link Propagation.Factory#decorate(TraceContext)} is called.
* <p>Tools that work only on {@linkplain TraceContext} should use {@link #findExtra(Class)}
* instead.
*
* @see #findExtra(Class)
* @see TraceContextOrSamplingFlags#extra()
* @see Builder#addExtra(Object) for notes on extra values.
* @since 4.9
*/
public List<Object> extra() {
return extraList;
}

/**
* Returns a {@linkplain #extra() propagated state} of the given type if present or null if not.
* Used internally by propagation plugins to search for their instance of state.
*
* <p>Note: It is the responsibility of {@link Propagation.Factory#decorate(TraceContext)}
* to consolidate elements. If it doesn't, there could be multiple instances of a given type and
* this can break logic.
* @see #extra()
* @see Builder#addExtra(Object) for notes on extra values.
*/
@Nullable public <T> T findExtra(Class<T> type) {
return findExtra(type, extraList);
Expand Down Expand Up @@ -372,6 +374,24 @@ public Builder clearExtra() {
}

/**
* This is an advanced function used for {@link Propagation} plugins, such as
* {@link BaggagePropagation}, to add an internal object to hold state when it isn't already in
* {@linkplain #extra()}.
*
* <p>The "extra" parameter is intentionally opaque, and should be an internal type defined by
* a {@linkplain Propagation} plugin. An example implementation could be storing a class
* containing a correlation value, which is extracted from incoming requests and injected as-is
* onto outgoing requests. A real world use is {@link BaggageFields}, which holds all baggage
* fields to propagate in one instance.
*
* <p>Note: It is the responsibility of {@link Propagation.Factory#decorate(TraceContext)}
* to consolidate elements (by searching for an existing instance of their state with {@link
* #findExtra(Class)} or {@link #extra()}). If it doesn't, there could be multiple instances of
* a given type and this can break logic and add overhead. Decoration is also when
* implementations define their scope. For example, a plugin that only propagates a field
* without changing will be constant for the whole request. A plugin that allows changes of
* state need careful coding, such as done in {@link BaggagePropagation}.
*
* @see #extra()
* @since 5.12
*/
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Expand Up @@ -14,6 +14,7 @@
package brave.propagation;

import brave.Tracer;
import brave.baggage.BaggagePropagation;
import brave.internal.InternalPropagation;
import brave.internal.Nullable;
import brave.propagation.TraceContext.Extractor;
Expand Down Expand Up @@ -245,9 +246,10 @@ public TraceContextOrSamplingFlags sampled(boolean sampled) {
* #context()} is {@code null}.
*
* @see TraceContext#extra()
* @see Builder#addExtra(Object) for notes on extra values.
* @since 4.9
*/
public final List<Object> extra() {
public List<Object> extra() {
return extraList;
}

Expand Down Expand Up @@ -309,7 +311,16 @@ public Builder sampledLocal() {
}

/**
* This is an advanced function used for {@link Propagation} plugins, such as
* {@link BaggagePropagation}, to add an internal object to hold state before extracting a
* remote request.
*
* <p>Implications of data are the same as {@link TraceContext.Builder#addExtra(Object)}. The
* main difference here is that {@link Extractor#extract(Object)} may not result in a trace
* context. For example, baggage fields can exist without an incoming trace.
*
* @see TraceContextOrSamplingFlags#extra()
* @see TraceContext.Builder#addExtra(Object)
* @since 4.9
*/
public Builder addExtra(Object extra) {
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