如何使用 Python 读取大型 Firestore 集合而不会遇到 503 超时错误

Question

Cloud Firestore 有一个默认的 60 秒限制来读取集合。当集合中文档过多时，连接会超时并抛出 503 错误。

如何处理？我开发了一个解决方案——

Answer 1

解决这个问题的方法是使用分页查询，将数据分成批次或“页面”，递归处理。然而Firestore documentation 并没有为此提供完整的解决方案。所以在阅读了this post 之后，我决定编写一个“准备使用”的函数，如下所示。它将 Firestore 集合提取到 CSV 文件中。您可以修改 CSV 编写器部分以适合您使用 Firestore 数据的目的。

我还在this GitHub repo 中分享了使用此功能的演示。如果你想进一步开发它，欢迎你 fork 它。

# Demo of extracting Firestore data into CSV file using a paginated algorithm (can prevent 503 timeout error for large dataset)

import firebase_admin
from firebase_admin import credentials
from firebase_admin import firestore
from datetime import datetime
import csv

def firestore_to_csv_paginated(db, coll_to_read, fields_to_read, csv_filename='extract.csv', max_docs_to_read=-1, write_headers=True):
    """ Extract Firestore collection data and save in CSV file
    Args:
        db: Firestore database object
        coll_to_read: name of collection to read from in Unicode format (like u'CollectionName')
        fields_to_read: fields to read (like ['FIELD1', 'FIELD2']). Will be used as CSV headers if write_headers=True
        csv_filename: CSV filename to save
        max_docs_to_read: max # of documents to read. Default to -1 to read all
        write_headers: also write headers into CSV file
    """

    # Check input parameters
    if (str(type(db)) != "<class 'google.cloud.firestore_v1.client.Client'>") or (type(coll_to_read) is not str) or not (isinstance(fields_to_read, list) or isinstance(fields_to_read, tuple) or isinstance(fields_to_read, set)):
        print(f'??? {datetime.now().strftime("%Y-%m-%d %H:%M:%S")} firestore_to_csv() - Unexpected parameters: \n\tdb = {db} \n\tcoll_to_read = {coll_to_read} \n\tfields_to_read = {fields_to_read}')
        return

    # Read Firestore collection and write CSV file in a paginated algorithm
    page_size = 1000   # Preset page size (max # of rows per batch to fetch/write at a time). Adjust in your case to avoid timeout in default 60s
    total_count = 0
    coll_ref = db.collection(coll_to_read)
    docs = []
    cursor = None
    try:
        # Open CSV file and write header if required
        print(f'>>> {datetime.now().strftime("%Y-%m-%d %H:%M:%S")} firestore_to_csv() - Started processing collection {coll_to_read}...')
        with open(csv_filename, 'w', newline='', encoding='utf-8') as csvfile:
            writer = csv.DictWriter(csvfile, fieldnames=fields_to_read, extrasaction='ignore', restval='Null')
            if write_headers: 
                writer.writeheader()
                print(f'<<< {datetime.now().strftime("%Y-%m-%d %H:%M:%S")} firestore_to_csv() - Finished writing CSV headers: {str(fields_to_read)} \n---')
        
            # Append each page of data fetched into CSV file
            while True:
                docs.clear()    # Clear page
                count = 0       # Reset page counter

                if cursor:      # Stream next page starting from cursor
                    docs = [snapshot for snapshot in coll_ref.limit(page_size).order_by('__name__').start_after(cursor).stream()]
                else:           # Stream first page if cursor not defined yet
                    docs = [snapshot for snapshot in coll_ref.limit(page_size).order_by('__name__').stream()]
            
                print(f'>>> {datetime.now().strftime("%Y-%m-%d %H:%M:%S")} firestore_to_csv() - Started writing CSV row {total_count+1}...')    # +1 as total_count starts at 0
                for doc in docs:
                    doc_dict = doc.to_dict()
                    
                    # Process columns (e.g. add an id column)
                    doc_dict['FIRESTORE_ID'] = doc.id   # Capture doc id itself. Comment out if not used
                    
                    # Process rows (e.g. convert all date columns to local time). Comment out if not used
                    for header in doc_dict.keys():
                        if (header.find('DATE') >= 0) and (doc_dict[header] is not None) and (type(doc_dict[header]) is not str):
                            try:
                                doc_dict[header] = doc_dict[header].astimezone()
                            except Exception as e_time_conv:
                                print(f'??? {datetime.now().strftime("%Y-%m-%d %H:%M:%S")} firestore_to_csv() - Exception in converting timestamp of {doc.id} in {doc_dict[header]}', e_time_conv)

                    # Write rows but skip certain rows. Comment out "if" and unindent "write" and "count" lines if not used
                    if ('TO_SKIP' not in doc_dict.keys()) or (('TO_SKIP' in doc_dict.keys()) and (doc_dict['TO_SKIP'] is not None) and (doc_dict['TO_SKIP'] != 'VALUE_TO_SKIP')):
                        writer.writerow(doc_dict)
                        count += 1

                # Check if finished writing last page or exceeded max limit
                total_count += count                # Increment total_count
                if len(docs) < page_size:           # Break out of while loop after fetching/writing last page (not a full page)
                    break
                else:
                    if (max_docs_to_read >= 0) and (total_count >= max_docs_to_read):
                        break                       # Break out of while loop after preset max limit exceeded
                    else:
                        cursor = docs[page_size-1]  # Move cursor to end of current page
                        continue                    # Continue to process next page

    except Exception as e_read_write:
        print(f'??? {datetime.now().strftime("%Y-%m-%d %H:%M:%S")} firestore_to_csv() - Exception in reading Firestore collection / writing CSV file:', e_read_write)
    else:
        print(f'<<< {datetime.now().strftime("%Y-%m-%d %H:%M:%S")} firestore_to_csv() - Finished writing CSV file with {total_count} rows of data \n---')