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# Batch Operations
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Efficient batch request processing to combine multiple API calls into a single HTTP request, reducing network overhead and improving performance.
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## Core Imports
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```java
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import com.google.api.client.googleapis.batch.BatchRequest;
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import com.google.api.client.googleapis.batch.BatchCallback;
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import com.google.api.client.googleapis.batch.json.JsonBatchCallback;
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import com.google.api.client.googleapis.batch.BatchUnparsedResponse;
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import com.google.api.client.googleapis.batch.HttpRequestContent;
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```
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## Batch Request Management
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### BatchRequest
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Container for multiple HTTP requests to be executed as a single batch operation.
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```java { .api }
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public final class BatchRequest {
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    public BatchRequest(HttpTransport transport, HttpRequestInitializer httpRequestInitializer);
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    public <T, E> BatchRequest queue(HttpRequest request, Class<T> dataClass, Class<E> errorClass, 
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        BatchCallback<T, E> callback) throws IOException;
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    public <T, E> BatchRequest queue(AbstractGoogleClientRequest<T> request, Class<T> dataClass, 
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        Class<E> errorClass, BatchCallback<T, E> callback) throws IOException;
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    public void execute() throws IOException;
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    public int size();
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    public BatchRequest setSleeper(Sleeper sleeper);
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    public Sleeper getSleeper();
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}
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```
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**Usage Example:**
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```java
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import com.google.api.client.googleapis.batch.BatchRequest;
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import com.google.api.client.googleapis.batch.BatchCallback;
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import com.google.api.client.http.HttpHeaders;
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// Create batch request
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BatchRequest batch = new BatchRequest(transport, credential);
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// Define callback for handling responses
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BatchCallback<MyResource, GoogleJsonError> callback = new BatchCallback<MyResource, GoogleJsonError>() {
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    @Override
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    public void onSuccess(MyResource resource, HttpHeaders responseHeaders) throws IOException {
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        System.out.println("Successfully processed: " + resource.getId());
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    }
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    @Override
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    public void onFailure(GoogleJsonError error, HttpHeaders responseHeaders) throws IOException {
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        System.err.println("Error: " + error.getMessage());
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    }
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};
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// Queue multiple requests
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for (String itemId : itemIds) {
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    MyApiRequest request = myApiClient.items().get(itemId);
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    batch.queue(request, MyResource.class, GoogleJsonError.class, callback);
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}
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// Execute all requests as a single batch
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batch.execute();
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```
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## Batch Callbacks
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### BatchCallback
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Interface for handling batch request responses.
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```java { .api }
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public interface BatchCallback<T, E> {
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    void onSuccess(T t, HttpHeaders responseHeaders) throws IOException;
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    void onFailure(E e, HttpHeaders responseHeaders) throws IOException;
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}
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```
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### JsonBatchCallback
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JSON-specific batch callback implementation.
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```java { .api }
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public abstract class JsonBatchCallback<T> implements BatchCallback<T, GoogleJsonError> {
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    public abstract void onSuccess(T t, HttpHeaders responseHeaders) throws IOException;
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    public abstract void onFailure(GoogleJsonError e, HttpHeaders responseHeaders) throws IOException;
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}
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```
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**Usage Example:**
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```java
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import com.google.api.client.googleapis.batch.json.JsonBatchCallback;
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import com.google.api.client.googleapis.json.GoogleJsonError;
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// Use JsonBatchCallback for simplified JSON error handling
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JsonBatchCallback<MyResource> jsonCallback = new JsonBatchCallback<MyResource>() {
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    @Override
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    public void onSuccess(MyResource resource, HttpHeaders responseHeaders) throws IOException {
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        System.out.println("Resource created: " + resource.getName());
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        System.out.println("ETag: " + responseHeaders.getETag());
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    }
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    @Override
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    public void onFailure(GoogleJsonError error, HttpHeaders responseHeaders) throws IOException {
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        System.err.println("Request failed with code: " + error.getCode());
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        System.err.println("Error message: " + error.getMessage());
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        if (error.getErrors() != null) {
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            for (GoogleJsonError.ErrorInfo errorInfo : error.getErrors()) {
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                System.err.println("  - " + errorInfo.getReason() + ": " + errorInfo.getMessage());
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            }
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        }
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    }
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};
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// Queue request with JSON callback
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batch.queue(createRequest, MyResource.class, GoogleJsonError.class, jsonCallback);
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```
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## Advanced Batch Operations
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### Batch Response Handling
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```java
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// Custom callback with detailed response handling
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BatchCallback<MyResource, GoogleJsonError> detailedCallback = new BatchCallback<MyResource, GoogleJsonError>() {
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    @Override
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    public void onSuccess(MyResource resource, HttpHeaders responseHeaders) throws IOException {
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        // Process successful response
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        String requestId = responseHeaders.getFirst("X-Request-Id");
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        long rateLimitRemaining = Long.parseLong(responseHeaders.getFirst("X-RateLimit-Remaining"));
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        System.out.println("Request ID: " + requestId);
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        System.out.println("Rate limit remaining: " + rateLimitRemaining);
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        // Handle the resource data
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        processResource(resource);
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    }
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    @Override
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    public void onFailure(GoogleJsonError error, HttpHeaders responseHeaders) throws IOException {
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        // Handle different types of errors
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        switch (error.getCode()) {
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            case 404:
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                System.err.println("Resource not found: " + error.getMessage());
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                break;
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            case 403:
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                System.err.println("Permission denied: " + error.getMessage());
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                break;
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            case 429:
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                System.err.println("Rate limit exceeded, retry after: " + 
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                    responseHeaders.getRetryAfter());
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                break;
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            default:
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                System.err.println("Unexpected error: " + error.getMessage());
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        }
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    }
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};
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```
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### Batch Size Management
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```java
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import java.util.List;
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import java.util.ArrayList;
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// Process large number of requests in manageable batches
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public void processBatchesInChunks(List<String> itemIds, int batchSize) throws IOException {
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    List<List<String>> chunks = partition(itemIds, batchSize);
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    for (List<String> chunk : chunks) {
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        BatchRequest batch = new BatchRequest(transport, credential);
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        for (String itemId : chunk) {
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            MyApiRequest request = myApiClient.items().get(itemId);
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            batch.queue(request, MyResource.class, GoogleJsonError.class, callback);
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        }
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        System.out.println("Executing batch of " + batch.size() + " requests");
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        batch.execute();
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        // Optional delay between batches to respect rate limits
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        Thread.sleep(1000);
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    }
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}
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private <T> List<List<T>> partition(List<T> list, int size) {
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    List<List<T>> chunks = new ArrayList<>();
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    for (int i = 0; i < list.size(); i += size) {
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        chunks.add(list.subList(i, Math.min(i + size, list.size())));
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    }
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    return chunks;
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}
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```
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## Internal Batch Components
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### BatchUnparsedResponse
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Represents an unparsed response within a batch operation.
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```java { .api }
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public class BatchUnparsedResponse {
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    public String getContentId();
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    public int getStatusCode();
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    public String getReasonPhrase();
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    public HttpHeaders getHeaders();
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    public InputStream getContent();
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}
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```
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### HttpRequestContent
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Represents HTTP request content for batch operations.
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```java { .api }
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public class HttpRequestContent implements HttpContent {
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    public HttpRequestContent(HttpRequest request);
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    public long getLength() throws IOException;
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    public String getType();
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    public boolean retrySupported();
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    public void writeTo(OutputStream out) throws IOException;
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}
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```
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## Error Handling in Batches
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### Individual Request Failures
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```java
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// Callback that tracks both successes and failures
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public class BatchTrackingCallback implements BatchCallback<MyResource, GoogleJsonError> {
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    private final AtomicInteger successCount = new AtomicInteger(0);
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    private final AtomicInteger failureCount = new AtomicInteger(0);
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    private final List<String> errors = new ArrayList<>();
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    @Override
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    public void onSuccess(MyResource resource, HttpHeaders responseHeaders) throws IOException {
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        successCount.incrementAndGet();
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        // Process successful resource
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    }
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    @Override
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    public void onFailure(GoogleJsonError error, HttpHeaders responseHeaders) throws IOException {
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        failureCount.incrementAndGet();
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        errors.add("Error " + error.getCode() + ": " + error.getMessage());
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    }
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    public void printSummary() {
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        System.out.println("Batch completed: " + successCount.get() + " successes, " + 
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                          failureCount.get() + " failures");
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        if (!errors.isEmpty()) {
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            System.out.println("Errors:");
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            errors.forEach(System.out::println);
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        }
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    }
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}
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```
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### Retry Logic for Failed Batches
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```java
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public void executeWithRetry(BatchRequest batch, int maxRetries) throws IOException {
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    int attempt = 0;
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    while (attempt < maxRetries) {
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        try {
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            batch.execute();
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            return; // Success
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        } catch (IOException e) {
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            attempt++;
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            if (attempt >= maxRetries) {
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                throw e; // Final attempt failed
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            }
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            // Exponential backoff
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            try {
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                Thread.sleep(Math.min(1000 * (1L << attempt), 30000)); // Max 30 seconds
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            } catch (InterruptedException ie) {
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                Thread.currentThread().interrupt();
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                throw new IOException("Interrupted during retry", ie);
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            }
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        }
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    }
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}
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```
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## Best Practices
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### Optimal Batch Sizes
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```java
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// Recommended batch sizes for different operations
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public static final int OPTIMAL_READ_BATCH_SIZE = 100;
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public static final int OPTIMAL_WRITE_BATCH_SIZE = 50;
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public static final int MAX_BATCH_SIZE = 1000; // Google API limit
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// Adjust batch size based on operation type
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public BatchRequest createOptimalBatch(String operationType) {
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    BatchRequest batch = new BatchRequest(transport, credential);
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    // Configure batch based on operation type
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    switch (operationType) {
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        case "read":
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            // Larger batches for read operations
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            return batch;
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        case "write":
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            // Smaller batches for write operations to handle errors better
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            return batch;
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        default:
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            return batch;
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    }
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}
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```
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## Types
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### HttpTransport
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HTTP transport interface for network communication.
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### HttpRequestInitializer  
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Interface for initializing HTTP requests.
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### HttpRequest
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HTTP request representation.
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### HttpHeaders
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HTTP headers container.
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### AbstractGoogleClientRequest
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Base class for Google API client requests.
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### GoogleJsonError
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Google JSON error response representation.
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### InputStream
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Java input stream for reading response content.
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### OutputStream
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Java output stream for writing request content.
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### IOException
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Exception for I/O operations.
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### Sleeper
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Interface for controlling sleep behavior during batch operations.
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### Class<T>
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Java class type parameter for response type specification.
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### AtomicInteger
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Thread-safe integer for counting operations in concurrent scenarios.