Introduce RetryLimiter

core/src/main/java/com/linecorp/armeria/client/retry/ConcurrencyBasedRetryLimiter.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2025 LY Corporation
+ *
+ * LY Corporation licenses this file to you 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:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package com.linecorp.armeria.client.retry;
+
+import static com.google.common.base.Preconditions.checkArgument;
+
+import java.util.concurrent.atomic.AtomicLong;
+
+import com.google.common.base.MoreObjects;
+
+import com.linecorp.armeria.client.ClientRequestContext;
+
+final class ConcurrencyBasedRetryLimiter implements RetryLimiter {
+
+    private final long maxRequests;
+    private final AtomicLong activeRequests = new AtomicLong();
+
+    ConcurrencyBasedRetryLimiter(long maxRequests) {
+        checkArgument(maxRequests > 0, "threshold must be positive: %s.", maxRequests);
+        this.maxRequests = maxRequests;
+    }
+
+    @Override
+    public boolean shouldRetry(ClientRequestContext ctx) {
+        final long cnt = activeRequests.incrementAndGet();
+        if (cnt > maxRequests) {
+            activeRequests.decrementAndGet();
+            return false;
+        }
+        ctx.log().whenComplete().thenRun(activeRequests::decrementAndGet);
+        return true;
+    }
+
+    @Override
+    public void handleDecision(ClientRequestContext ctx, RetryDecision decision) {
+    }
+
+    @Override
+    public String toString() {
+        return MoreObjects.toStringHelper(this)
+                          .add("activeRequests", activeRequests)
+                          .add("maxRequests", maxRequests)
+                          .toString();
+    }
+}

core/src/main/java/com/linecorp/armeria/client/retry/RetryConfig.java

@@ -78,6 +78,7 @@ static <T extends Response> RetryConfigBuilder<T> builder0(
     private final int maxTotalAttempts;
     private final long responseTimeoutMillisForEachAttempt;
     private final int maxContentLength;
+    private final RetryLimiter retryLimiter;
 
     @Nullable
     private final RetryRule retryRule;
@@ -88,27 +89,30 @@ static <T extends Response> RetryConfigBuilder<T> builder0(
     @Nullable
     private RetryRuleWithContent<T> fromRetryRule;
 
-    RetryConfig(RetryRule retryRule, int maxTotalAttempts, long responseTimeoutMillisForEachAttempt) {
+    RetryConfig(RetryRule retryRule, int maxTotalAttempts, long responseTimeoutMillisForEachAttempt,
+                RetryLimiter retryLimiter) {
         this(requireNonNull(retryRule, "retryRule"), null,
-                maxTotalAttempts, responseTimeoutMillisForEachAttempt, 0);
+                maxTotalAttempts, responseTimeoutMillisForEachAttempt, 0, retryLimiter);
         checkArguments(maxTotalAttempts, responseTimeoutMillisForEachAttempt);
     }
 
     RetryConfig(
             RetryRuleWithContent<T> retryRuleWithContent,
             int maxContentLength,
             int maxTotalAttempts,
-            long responseTimeoutMillisForEachAttempt) {
+            long responseTimeoutMillisForEachAttempt,
+            RetryLimiter retryLimiter) {
         this(null, requireNonNull(retryRuleWithContent, "retryRuleWithContent"),
-                maxTotalAttempts, responseTimeoutMillisForEachAttempt, maxContentLength);
+                maxTotalAttempts, responseTimeoutMillisForEachAttempt, maxContentLength, retryLimiter);
     }
 
     private RetryConfig(
             @Nullable RetryRule retryRule,
             @Nullable RetryRuleWithContent<T> retryRuleWithContent,
             int maxTotalAttempts,
             long responseTimeoutMillisForEachAttempt,
-            int maxContentLength) {
+            int maxContentLength, RetryLimiter retryLimiter) {
+        this.retryLimiter = new RetryLimiters.CatchingRetryLimiter(retryLimiter);
         checkArguments(maxTotalAttempts, responseTimeoutMillisForEachAttempt);
         this.retryRule = retryRule;
         this.retryRuleWithContent = retryRuleWithContent;
@@ -147,7 +151,8 @@ public RetryConfigBuilder<T> toBuilder() {
         }
         return builder
                 .maxTotalAttempts(maxTotalAttempts)
-                .responseTimeoutMillisForEachAttempt(responseTimeoutMillisForEachAttempt);
+                .responseTimeoutMillisForEachAttempt(responseTimeoutMillisForEachAttempt)
+                .retryLimiter(retryLimiter);
     }
 
     /**
@@ -197,6 +202,13 @@ public boolean needsContentInRule() {
         return retryRuleWithContent != null;
     }
 
+    /**
+     * Returns the configured {@link RetryLimiter}.
+     */
+    public RetryLimiter retryLimiter() {
+        return retryLimiter;
+    }
+
     /**
      * Returns whether the associated {@link RetryRule} or {@link RetryRuleWithContent} requires
      * response trailers.

core/src/main/java/com/linecorp/armeria/client/retry/RetryConfigBuilder.java

@@ -24,6 +24,7 @@
 import com.google.common.base.MoreObjects;
 import com.google.common.base.MoreObjects.ToStringHelper;
 
+import com.linecorp.armeria.client.retry.RetryLimiters.AlwaysRetryLimiter;
 import com.linecorp.armeria.common.Flags;
 import com.linecorp.armeria.common.Response;
 import com.linecorp.armeria.common.annotation.Nullable;
@@ -42,6 +43,7 @@ public final class RetryConfigBuilder<T extends Response> {
     private final RetryRule retryRule;
     @Nullable
     private final RetryRuleWithContent<T> retryRuleWithContent;
+    private RetryLimiter retryLimiter = AlwaysRetryLimiter.INSTANCE;
 
     /**
      * Creates a {@link RetryConfigBuilder} with this {@link RetryRule}.
@@ -111,19 +113,30 @@ public RetryConfigBuilder<T> responseTimeoutForEachAttempt(Duration responseTime
         return this;
     }
 
+    /**
+     * Sets a {@link RetryLimiter} which may limit retry requests.
+     * @see RetryLimiter
+     */
+    public RetryConfigBuilder<T> retryLimiter(RetryLimiter retryLimiter) {
+        this.retryLimiter = requireNonNull(retryLimiter, "retryLimiter");
+        return this;
+    }
+
     /**
      * Returns a newly-created {@link RetryConfig} from this {@link RetryConfigBuilder}'s values.
      */
     public RetryConfig<T> build() {
         if (retryRule != null) {
-            return new RetryConfig<>(retryRule, maxTotalAttempts, responseTimeoutMillisForEachAttempt);
+            return new RetryConfig<>(retryRule, maxTotalAttempts, responseTimeoutMillisForEachAttempt,
+                                     retryLimiter);
         }
         assert retryRuleWithContent != null;
         return new RetryConfig<>(
                 retryRuleWithContent,
                 maxContentLength,
                 maxTotalAttempts,
-                responseTimeoutMillisForEachAttempt);
+                responseTimeoutMillisForEachAttempt,
+                retryLimiter);
     }
 
     @Override
@@ -139,6 +152,7 @@ ToStringHelper toStringHelper() {
                 .add("retryRuleWithContent", retryRuleWithContent)
                 .add("maxTotalAttempts", maxTotalAttempts)
                 .add("responseTimeoutMillisForEachAttempt", responseTimeoutMillisForEachAttempt)
-                .add("maxContentLength", maxContentLength);
+                .add("maxContentLength", maxContentLength)
+                .add("retryLimiter", retryLimiter);
     }
 }

core/src/main/java/com/linecorp/armeria/client/retry/RetryDecision.java

@@ -18,6 +18,9 @@
 
 import static java.util.Objects.requireNonNull;
 
+import com.google.common.base.MoreObjects;
+import com.google.common.base.MoreObjects.ToStringHelper;
+
 import com.linecorp.armeria.common.annotation.Nullable;
 
 /**
@@ -26,27 +29,44 @@
  */
 public final class RetryDecision {
 
-    private static final RetryDecision NO_RETRY = new RetryDecision(null);
-    private static final RetryDecision NEXT = new RetryDecision(null);
-    static final RetryDecision DEFAULT = new RetryDecision(Backoff.ofDefault());
+    private static final RetryDecision NO_RETRY = new RetryDecision(null, -1);
+    private static final RetryDecision NEXT = new RetryDecision(null, 0);
+    static final RetryDecision DEFAULT = new RetryDecision(Backoff.ofDefault(), 1);
 
     /**
      * Returns a {@link RetryDecision} that retries with the specified {@link Backoff}.
+     * The permits will be {@code 1} by default.
      */
     public static RetryDecision retry(Backoff backoff) {
-        if (backoff == Backoff.ofDefault()) {
+        return retry(backoff, 1);
+    }
+
+    /**
+     * Returns a {@link RetryDecision} that retries with the specified {@link Backoff}.
+     */
+    @SuppressWarnings("FloatingPointEquality")
+    public static RetryDecision retry(Backoff backoff, double permits) {
+        if (backoff == DEFAULT.backoff() && permits == DEFAULT.permits()) {
             return DEFAULT;
         }
-        return new RetryDecision(requireNonNull(backoff, "backoff"));
+        return new RetryDecision(requireNonNull(backoff, "backoff"), permits);
     }
 
     /**
      * Returns a {@link RetryDecision} that never retries.
+     * The permits will be {@code -1} by default.
      */
     public static RetryDecision noRetry() {
         return NO_RETRY;
     }
 
+    /**
+     * Returns a {@link RetryDecision} that never retries.
+     */
+    public static RetryDecision noRetry(double permits) {
+        return new RetryDecision(null, permits);
+    }
+
     /**
      * Returns a {@link RetryDecision} that skips the current {@link RetryRule} and
      * tries to retry with the next {@link RetryRule}.
@@ -57,24 +77,41 @@ public static RetryDecision next() {
 
     @Nullable
     private final Backoff backoff;
+    private final double permits;
 
-    private RetryDecision(@Nullable Backoff backoff) {
+    private RetryDecision(@Nullable Backoff backoff, double permits) {
         this.backoff = backoff;
+        this.permits = permits;
     }
 
     @Nullable
     Backoff backoff() {
         return backoff;
     }
 
+    /**
+     * The number of permits associated with this {@link RetryDecision}.
+     * This may be used by {@link RetryLimiter} to determine whether retry requests should
+     * be limited or not. The semantics of whether or how the returned value affects {@link RetryLimiter}
+     * depends on what type of {@link RetryLimiter} is used.
+     */
+    public double permits() {
+        return permits;
+    }
+
     @Override
     public String toString() {
-        if (this == NO_RETRY) {
-            return "RetryDecision(NO_RETRY)";
-        } else if (this == NEXT) {
-            return "RetryDecision(NEXT)";
+        final ToStringHelper stringHelper = MoreObjects.toStringHelper(this);
+        if (this == NEXT) {
+            stringHelper.add("type", "NEXT");
+        } else if (backoff != null) {
+            stringHelper.add("type", "RETRY");
         } else {
-            return "RetryDecision(RETRY(" + backoff + "))";
+            stringHelper.add("type", "NO_RETRY");
         }
+        return stringHelper.omitNullValues()
+                           .add("backoff", backoff)
+                           .add("permits", permits)
+                           .toString();
     }
 }

core/src/main/java/com/linecorp/armeria/client/retry/RetryLimitedException.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2025 LY Corporation
+ *
+ * LY Corporation licenses this file to you 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:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package com.linecorp.armeria.client.retry;
+
+/**
+ * An exception thrown when a retry is limited by a {@link RetryLimiter}.
+ */
+public final class RetryLimitedException extends RuntimeException {
+
+    private static final long serialVersionUID = 7203512016805562689L;
+
+    private static final RetryLimitedException INSTANCE = new RetryLimitedException();
+
+    /**
+     * Returns an instance of {@link RetryLimitedException}.
+     */
+    public static RetryLimitedException of() {
+        return INSTANCE;
+    }
+
+    private RetryLimitedException() {}
+
+    @Override
+    public Throwable fillInStackTrace() {
+        return this;
+    }
+}