[Feature][Java] Added retries to remote calls in MCP server

api/src/main/java/org/apache/flink/agents/api/RetryExecutor.java

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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF 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
+ *
+ *     http://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 org.apache.flink.agents.api;
+
+import java.net.ConnectException;
+import java.net.SocketTimeoutException;
+import java.util.Random;
+import java.util.concurrent.Callable;
+import java.util.function.Predicate;
+
+/**
+ * A reusable utility for executing operations with retry logic and binary exponential backoff.
+ *
+ * <p>By default, the following exceptions are considered retryable:
+ *
+ * <ul>
+ *   <li>{@link SocketTimeoutException}
+ *   <li>{@link ConnectException}
+ *   <li>Exceptions whose message contains "HTTP 503" or "HTTP 429"
+ *   <li>Exceptions whose message contains "Connection reset", "Connection refused", or "Connection
+ *       timed out"
+ * </ul>
+ *
+ * <p>A custom retryable predicate can be provided to override this behavior.
+ *
+ * <p>Example usage:
+ *
+ * <pre>{@code
+ * RetryExecutor executor = RetryExecutor.builder()
+ *     .maxRetries(3)
+ *     .initialBackoffMs(100)
+ *     .maxBackoffMs(10000)
+ *     .build();
+ *
+ * String result = executor.execute(() -> callRemoteService(), "callRemoteService");
+ * }</pre>
+ */
+public class RetryExecutor {
+
+    private static final Random RANDOM = new Random();
+
+    private static final int DEFAULT_MAX_RETRIES = 3;
+    private static final long DEFAULT_INITIAL_BACKOFF_MS = 100;
+    private static final long DEFAULT_MAX_BACKOFF_MS = 10000;
+
+    private final int maxRetries;
+    private final long initialBackoffMs;
+    private final long maxBackoffMs;
+    private final Predicate<Exception> retryablePredicate;
+
+    private RetryExecutor(
+            int maxRetries,
+            long initialBackoffMs,
+            long maxBackoffMs,
+            Predicate<Exception> retryablePredicate) {
+        this.maxRetries = maxRetries;
+        this.initialBackoffMs = initialBackoffMs;
+        this.maxBackoffMs = maxBackoffMs;
+        this.retryablePredicate =
+                retryablePredicate != null ? retryablePredicate : RetryExecutor::isRetryableDefault;
+    }
+
+    /** Creates a builder for {@link RetryExecutor}. */
+    public static Builder builder() {
+        return new Builder();
+    }
+
+    /** Creates a {@link RetryExecutor} with default settings. */
+    public static RetryExecutor withDefaults() {
+        return builder().build();
+    }
+
+    /**
+     * Execute an operation with retry logic.
+     *
+     * @param operation The operation to execute
+     * @param operationName Name of the operation for error messages
+     * @return The result of the operation
+     * @throws RuntimeException if all retries fail or a non-retryable exception occurs
+     */
+    public <T> T execute(Callable<T> operation, String operationName) {
+        int attempt = 0;
+        long window = initialBackoffMs;
+        Exception lastException = null;
+
+        while (attempt <= maxRetries) {
+            try {
+                return operation.call();
+            } catch (Exception e) {
+                lastException = e;
+                attempt++;
+
+                if (!retryablePredicate.test(e)) {
+                    throw new RuntimeException(
+                            String.format(
+                                    "Operation '%s' failed: %s", operationName, e.getMessage()),
+                            e);
+                }
+
+                if (attempt > maxRetries) {
+                    break;
+                }
+
+                // Binary Exponential Backoff: random wait from [0, window]
+                try {
+                    long sleepTime = (long) (RANDOM.nextDouble() * (window + 1));
+                    Thread.sleep(sleepTime);
+                } catch (InterruptedException ie) {
+                    Thread.currentThread().interrupt();
+                    throw new RuntimeException(
+                            "Interrupted while retrying operation: " + operationName, ie);
+                }
+
+                window = Math.min(window * 2, maxBackoffMs);
+            }
+        }
+
+        throw new RuntimeException(
+                String.format(
+                        "Operation '%s' failed after %d retries: %s",
+                        operationName, maxRetries, lastException.getMessage()),
+                lastException);
+    }
+
+    public int getMaxRetries() {
+        return maxRetries;
+    }
+
+    public long getInitialBackoffMs() {
+        return initialBackoffMs;
+    }
+
+    public long getMaxBackoffMs() {
+        return maxBackoffMs;
+    }
+
+    /**
+     * Default retryable check.
+     *
+     * @param e The exception to check
+     * @return true if the operation should be retried
+     */
+    static boolean isRetryableDefault(Exception e) {
+        if (e instanceof SocketTimeoutException || e instanceof ConnectException) {
+            return true;
+        }
+        String message = e.getMessage();
+        if (message != null) {
+            if (message.contains("HTTP 503") || message.contains("HTTP 429")) {
+                return true;
+            }
+            return message.contains("Connection reset")
+                    || message.contains("Connection refused")
+                    || message.contains("Connection timed out");
+        }
+        return false;
+    }
+
+    /** Builder for {@link RetryExecutor}. */
+    public static class Builder {
+        private int maxRetries = DEFAULT_MAX_RETRIES;
+        private long initialBackoffMs = DEFAULT_INITIAL_BACKOFF_MS;
+        private long maxBackoffMs = DEFAULT_MAX_BACKOFF_MS;
+        private Predicate<Exception> retryablePredicate;
+
+        public Builder maxRetries(int maxRetries) {
+            this.maxRetries = maxRetries;
+            return this;
+        }
+
+        public Builder initialBackoffMs(long initialBackoffMs) {
+            this.initialBackoffMs = initialBackoffMs;
+            return this;
+        }
+
+        public Builder maxBackoffMs(long maxBackoffMs) {
+            this.maxBackoffMs = maxBackoffMs;
+            return this;
+        }
+
+        public Builder retryablePredicate(Predicate<Exception> retryablePredicate) {
+            this.retryablePredicate = retryablePredicate;
+            return this;
+        }
+
+        public RetryExecutor build() {
+            return new RetryExecutor(
+                    maxRetries, initialBackoffMs, maxBackoffMs, retryablePredicate);
+        }
+    }
+}