我正在尝试将谷歌云数据存储数据导出到谷歌云存储中的Avro文件，然后将这些文件加载到BigQuery中。

首先，我知道Big Query会加载数据存储备份。这有几个我想避免的缺点：

• 备份工具是闭源的
• 备份工具格式未记录。
• 数据流不能直接读取备份工具格式
• appengine的备份调度处于（显然是永久的）alpha。
• 可以在appengine中实现您自己的备份处理程序，但它被触发并忘记了。您不知道备份何时完成或文件名是什么。

澄清了这个实验的动机，这里是我的数据流管道，将数据导出为avro格式：

package com.example.dataflow;

import com.google.api.services.datastore.DatastoreV1;
import com.google.api.services.datastore.DatastoreV1.Entity;
import com.google.cloud.dataflow.sdk.Pipeline;
import com.google.cloud.dataflow.sdk.coders.AvroCoder;
import com.google.cloud.dataflow.sdk.io.AvroIO;
import com.google.cloud.dataflow.sdk.io.DatastoreIO;
import com.google.cloud.dataflow.sdk.io.Read;
import com.google.cloud.dataflow.sdk.options.DataflowPipelineOptions;
import com.google.cloud.dataflow.sdk.options.PipelineOptions;
import com.google.cloud.dataflow.sdk.options.PipelineOptionsFactory;
import com.google.cloud.dataflow.sdk.transforms.DoFn;
import com.google.cloud.dataflow.sdk.transforms.ParDo;
import org.apache.avro.Schema;
import org.apache.avro.file.DataFileReader;
import org.apache.avro.file.DataFileWriter;
import org.apache.avro.file.SeekableByteArrayInput;
import org.apache.avro.generic.GenericDatumReader;
import org.apache.avro.generic.GenericRecord;
import org.apache.avro.io.DatumReader;
import org.apache.avro.protobuf.ProtobufData;
import org.apache.avro.protobuf.ProtobufDatumWriter;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.ByteArrayOutputStream;

public class GCDSEntitiesToAvroSSCCEPipeline {

    private static final String GCS_TARGET_URI = "gs://myBucket/datastore/dummy";
    private static final String ENTITY_KIND = "Dummy";

    private static Schema getSchema() {
        return ProtobufData.get().getSchema(Entity.class);
    }

    private static final Logger LOG = LoggerFactory.getLogger(GCDSEntitiesToAvroSSCCEPipeline.class);
    public static void main(String[] args) {
        PipelineOptions options = PipelineOptionsFactory.fromArgs(args).withValidation().create();
        Pipeline p = Pipeline.create(options);

        DatastoreV1.Query.Builder q = DatastoreV1.Query.newBuilder()
                .addKind(DatastoreV1.KindExpression.newBuilder().setName(ENTITY_KIND));

        p.apply(Read.named("DatastoreQuery").from(DatastoreIO.source()
                .withDataset(options.as(DataflowPipelineOptions.class).getProject())
                .withQuery(q.build())))
            .apply(ParDo.named("ProtoBufToAvro").of(new ProtoBufToAvro()))
            .setCoder(AvroCoder.of(getSchema()))
            .apply(AvroIO.Write.named("WriteToAvro")
                    .to(GCS_TARGET_URI)
                    .withSchema(getSchema())
                    .withSuffix(".avro"));
        p.run();

    }

    private static class ProtoBufToAvro extends DoFn<Entity, GenericRecord> {
        private static final long serialVersionUID = 1L;

        @Override
        public void processElement(ProcessContext c) throws Exception {
            Schema schema = getSchema();
            ProtobufDatumWriter<Entity> pbWriter = new ProtobufDatumWriter<>(Entity.class);
            DataFileWriter<Entity> dataFileWriter = new DataFileWriter<>(pbWriter);
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            dataFileWriter.create(schema, bos);
            dataFileWriter.append(c.element());
            dataFileWriter.close();

            DatumReader<GenericRecord> datumReader = new GenericDatumReader<>(schema);
            DataFileReader<GenericRecord> dataFileReader = new DataFileReader<>(
                    new SeekableByteArrayInput(bos.toByteArray()), datumReader);

            c.output(dataFileReader.next());

        }
    }
}

管道运行正常，但是当我尝试将生成的Avro文件加载到大查询中时，我收到以下错误：

bq load --project_id=roodev001 --source_format=AVRO dummy.dummy_1 gs://roodev001.appspot.com/datastore/dummy-00000-of-00001.avro
Waiting on bqjob_r5c9b81a49572a53b_00000154951eb523_1 ... (0s) Current status: DONE   
BigQuery error in load operation: Error processing job 'roodev001:bqjob_r5c9b81a49572a53b_00000154951eb523_1': The Apache Avro library failed to parse file
gs://roodev001.appspot.com/datastore/dummy-00000-of-00001.avro.

但是，如果我使用avro工具加载生成的avro文件，一切都很好：

avro-tools tojson datastore-dummy-00000-of-00001.avro | head
log4j:WARN No appenders could be found for logger (org.apache.hadoop.metrics2.lib.MutableMetricsFactory).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
{"key":{"com.google.api.services.datastore.DatastoreV1$.Key":{"partition_id":{"com.google.api.services.datastore.DatastoreV1$.PartitionId":{"dataset_id":"s~roodev001","namespace":""}},"path_element":[{"kind":"Dummy","id":4503905778008064,"name":""}]}},"property":[{"name":"number","value":{"boolean_value":false,"integer_value":879,"double_value":0.0,"timestamp_microseconds_value":0,"key_value":null,"blob_key_value":"","string_value":"","blob_value":"","entity_value":null,"list_value":[],"meaning":0,"indexed":true}}]}
...

在运行Dataflow管道之前，我使用此代码用虚拟数据填充数据存储：

package com.example.datastore;

import com.google.gcloud.AuthCredentials;
import com.google.gcloud.datastore.*;

import java.io.IOException;

public static void main(String[] args) throws IOException {

    Datastore datastore = DatastoreOptions.builder()
            .projectId("myProjectId")
            .authCredentials(AuthCredentials.createApplicationDefaults())
            .build().service();

    KeyFactory dummyKeyFactory = datastore.newKeyFactory().kind("Dummy");


    Batch batch = datastore.newBatch();
    int batchCount = 0;
    for (int i = 0; i < 4000; i++){
        IncompleteKey key = dummyKeyFactory.newKey();
        System.out.println("adding entity " + i);
        batch.add(Entity.builder(key).set("number", i).build());
        batchCount++;
        if (batchCount > 99) {
            batch.submit();
            batch = datastore.newBatch();
            batchCount = 0;
        }
    }

    System.out.println("done");

}

那么为什么BigQuery拒绝我的avro文件呢？