在Golang实战开发的过程中，我们经常会遇到一些这样那样的问题，然后要卡好半天，等问题解决了才发现原来一些细节知识点还是没有掌握好。今天golang学习网就整理分享《大集合的 Firestore DeadlineExceeded 异常》，聊聊golang，希望可以帮助到正在努力赚钱的你。

问题内容

我正在尝试从 Google Firestore 中读取更大的集合以进行测试和归档。当我尝试从包含超过 6k 个文档的集合中获取所有文档时，我遇到了一些有趣的错误。

朴素的 Python 解决方案

我的第一次尝试是使用 Python google-cloud-firestore（版本 0.30.0）库。

source_client = firestore.Client()
source = source_client.collection(collection)
source_data = source.get()

counter = 0
for f in source_data:
    app.logger.info(f.id)
    counter += 1
    if counter % 100 == 0:
        app.logger.info('%s %d', datetime.now(), counter)

    app.logger.info('%s Finally read all %d documents', datetime.now(), counter)

这给出了以下输出：

INFO:flask.app:2018-11-08 09:49:03.923795 6400  
INFO:flask.app:2018-11-08 09:49:04.115410 6500  
... 
INFO:flask.app:2018-11-08 09:49:03.923795 6400
INFO:flask.app:2018-11-08 09:49:04.115410 6500
WARNING:flask.app:2018-11-08 09:49:04.128478 copy brocken by exception
Traceback (most recent call last):
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 2309, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 2295, in wsgi_app
    response = self.handle_exception(e)
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 1741, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 1815, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 1718, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 1813, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/carsten/projects/transfertool/firestore/transfertool/main.py", line 142, in transfer
    count_collection(source_collection)
  File "/home/carsten/projects/transfertool/firestore/transfertool/main.py", line 94, in count_collection
    for f in source_collection.offset(1000).get():
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/google/cloud/firestore_v1beta1/query.py", line 588, in get
    for index, response_pb in enumerate(response_iterator):
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/google/api_core/grpc_helpers.py", line 83, in next
    six.raise_from(exceptions.from_grpc_error(exc), exc)
  File "", line 3, in raise_from
    # Permission is hereby granted, free of charge, to any person obtaining a copy
google.api_core.exceptions.DeadlineExceeded: 504 Deadline Exceeded

这似乎是由配额引起的。即使我在这里看不到。它似乎是基于时间的，因为当我在元素之间以小睡眠运行时，我的吞吐量会降低，并且在大约 50 秒后会出现异常。

使用 Python 进行分页

对于这个问题，这个库中有一个分页部分。由于我的应用程序不应该关心我尝试传输什么样的数据，我无法使用该start_after接口，但仍然有一个偏移接口，我至少可以使用它分批读取。

for f in source_collection.offset(last_read_offset).get():

只要last_read_offset低于 1001，它就会给我正确的结果。如果我从 1000 的偏移量开始，我可以得到结果，直到我google.api_core.exceptions.DeadlineExceeded exception从上面得到。但是当我从更大的事情开始时，我得到：

Traceback (most recent call last):
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 2309, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 2295, in wsgi_app
    response = self.handle_exception(e)
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 1741, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 1815, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 1718, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 1813, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/carsten/projects/transfertool/firestore/transfertool/main.py", line 144, in transfer
    count_collection(source_collection)
  File "/home/carsten/projects/transfertool/firestore/transfertool/main.py", line 94, in count_collection
    for f in source_collection.offset(1001).get():
  File "/home/carsten/projects/transfertool/venv/lib/python3.6/site-packages/google/cloud/firestore_v1beta1/query.py", line 599, in get
    raise ValueError(msg)
ValueError: Unexpected server response. All responses other than the first must contain a document. The response at index 1 was
read_time {
  seconds: 1541668338
  nanos: 420813000
}
skipped_results: 1

查看库代码，后端似乎正在发送一条被解释为无效的消息。

通过 node.js 重试

好吧，也许我的代码或 Python 客户端库有问题。让我们尝试使用节点。

const admin = require('firebase-admin');
admin.initializeApp({
    credential: admin.credential.applicationDefault()
});

var db = admin.firestore();
admin.firestore().settings( { timestampsInSnapshots: true })
var counter = 0

console.log('Read collection')
db.collection(collection).get()
    .then(querySnapshot => {
        querySnapshot.forEach(documentSnapshot => {
            counter++;
        });
        console.log(counter)
    })
    .catch( error => {
        console.log(error)
});

即使超时更明显是 60 秒，它与 python 库的作用相同。

[2018-11-09T08:36:30.992Z] App listening on port 8080
[2018-11-09T08:36:30.993Z] Press Ctrl+C to quit.
[2018-11-09T08:36:37.390Z] Read collection
[2018-11-09T08:37:37.406Z] { Error: 4 DEADLINE_EXCEEDED: Deadline Exceeded
    at Object.exports.createStatusError (/home/carsten/projects/node_modules/grpc/src/common.js:87:15)
    at ClientReadableStream._emitStatusIfDone (/home/carsten/projects/node_modules/grpc/src/client.js:235:26)
    at ClientReadableStream._readsDone (/home/carsten/projects/node_modules/grpc/src/client.js:201:8)
    at /home/carsten/projects/node_modules/grpc/src/client_interceptors.js:679:15
  code: 4,
  metadata: Metadata { _internal_repr: {} },
  details: 'Deadline Exceeded' }

有没有人有类似的经历或很好的提示如何继续？

PS：exportDocument/importDocument接口是不够的，有时我们需要在读取后调整数据。而且我不知道 Firestore 将哪种格式导出到 Google Cloud Storage 或如何转换它。

编辑：golang

并且为了它，我尝试了golang api。

log.Println("Collecting data")
snapshotIter := client.Collection(collection.(string)).Documents(ctx)
defer snapshotIter.Stop()

if err != nil {
    log.Fatalln(err)
}

i := 0

for {
    _, err := snapshotIter.Next()

    if err == iterator.Done {
        break
    }
    if err != nil {
        log.Fatalln(err)
    }

    if i % 100 == 0{
        log.Println(i)
    }
    i++
}

log.Println("Done")

与预期的超时相同。

2018/11/12 15:01:20 Collecting data
2018/11/12 15:01:21 0
2018/11/12 15:01:21 100
2018/11/12 15:01:21 200
2018/11/12 15:01:21 300
2018/11/12 15:01:21 400
2018/11/12 15:01:22 500
2018/11/12 15:01:22 600
2018/11/12 15:01:22 700
....
2018/11/12 15:02:22 29800
2018/11/12 15:02:23 29900
2018/11/12 15:02:23 rpc error: code = DeadlineExceeded desc = The datastore operation timed out, or the data was temporarily unavailable.

但此外，偏移量也可以正常工作：

snapshotIter := client.Collection(collection.(string)).Offset(30000).Documents(ctx)

正确答案

在 firebase 支持团队的帮助下，我们发现 python 客户端 api 确实存在错误。在下一个版本中会有一个错误修复。很可能它将使 python 库能够按 documentid 排序，因此使用start_after().

在那之前，您有两种可能的解决方案：

1. 使用另一个字段进行排序和使用start_after()

2. 使用带有分页的 node.js 库，例如：

var db = admin.firestore();
admin.firestore().settings({ timestampsInSnapshots: true });
function readNextPage(lastReadDoc) {
  let query = db
    .collection(collection)
    .orderBy(admin.firestore.FieldPath.documentId())
    .limit(100);
}

终于介绍完啦！小伙伴们，这篇关于《大集合的 Firestore DeadlineExceeded 异常》的介绍应该让你收获多多了吧！欢迎大家收藏或分享给更多需要学习的朋友吧~golang学习网公众号也会发布Golang相关知识，快来关注吧！