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Retrieves a document by its ID from an in-memory cache or loads it from persistent storage if not cached.

Purpose

This function implements a two-tier document retrieval system with thread-safe access. It first checks an in-memory cache (app_state['documents']) for the requested document. If not found in memory, it attempts to load the document from persistent storage using load_document_from_file(), caches it in memory for future access, and returns it. Returns None if the document is not found in either location. The function uses a lock to ensure thread-safe concurrent access to the document cache.

Source Code

def get_document(doc_id):
    """Get a document by ID"""
    with app_state['locks']['documents']:
        # Try memory first
        document = app_state['documents'].get(doc_id)
        if document:
            return document
        
        # If not in memory, try loading from file
        document = load_document_from_file(doc_id)
        if document:
            app_state['documents'][doc_id] = document
            return document
        
        return None

Parameters

Parameter Details

doc_id: A unique identifier for the document to retrieve. Expected to be a string or UUID that uniquely identifies a document in the system. This ID is used to look up the document in the cache and as a parameter to load_document_from_file() if the document needs to be loaded from storage.

Return Value

Returns a document object if found (type depends on the document structure used by the application, likely a dictionary or custom object containing document data). Returns None if the document with the specified doc_id is not found in either the in-memory cache or persistent storage.

Dependencies

• threading

Required Imports

from threading import Lock

Usage Example

import threading
from threading import Lock

# Initialize app_state
app_state = {
    'locks': {
        'documents': Lock()
    },
    'documents': {}
}

# Define load_document_from_file function (stub)
def load_document_from_file(doc_id):
    # Implementation to load from file system or database
    return {'id': doc_id, 'content': 'Document content'}

# Use get_document
doc_id = 'doc_12345'
document = get_document(doc_id)

if document:
    print(f"Document found: {document}")
else:
    print("Document not found")

Best Practices

• Ensure app_state is properly initialized before calling this function
• The lock mechanism prevents race conditions when multiple threads access documents simultaneously
• Documents are cached in memory after first load to improve performance on subsequent accesses
• Always check if the returned value is None before using the document
• Consider implementing cache eviction strategies if memory usage becomes a concern
• Ensure load_document_from_file() handles errors gracefully and returns None for missing documents
• The function assumes load_document_from_file() is thread-safe or handles its own locking

Tags

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