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Choose the boundary between the callback and background work
Start by writing down the crawler’s actual contract. The title alone cannot determine the route, HTTP method, signature or authentication scheme, payload fields, stable crawl identifier, retry behavior, or required acknowledgment body. Obtain those details from the crawler documentation and implement them exactly.
| Pattern | What happens before acknowledgment | Best for | Trade-off |
|---|---|---|---|
| Synchronous persistence | Validate the request, write callback and job/result rows, commit, then respond | Small, bounded database work | The request remains open until MySQL commits |
| Queue-backed continuation | Validate and persist the receipt (or an inbox record), enqueue explicit task data, then respond | Parsing, enrichment, notifications, exports, and other slow work | Requires a durable queue, worker, monitoring, and failure handling |
An async Flask view is not a third option for durable processing. Async can permit concurrent I/O inside that request, but it does not increase the number of requests one worker handles and does not make spawned work survive the response. Flask’s project documentation states: “If you wish to use background tasks it is best to use a task queue to trigger background work, rather than spawn tasks in a view function.”
Prerequisites and project layout
- Python 3.10 or a version supported by your deployed Flask and Connector/Python releases.
- Flask, MySQL Connector/Python, and a queue client appropriate to your chosen worker system.
- A MySQL database account permitted to insert and update the callback tables.
- The crawler’s authentication, payload, acknowledgment, and retry specification.
callback_service/
app.py
worker.py
schema.sql
requirements.txt
Keep database credentials, signing secrets, and queue credentials in deployment configuration rather than source files. Never place them in request logs.
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Design an idempotent MySQL schema
Use a stable identifier supplied by the crawler (or a documented identifier you derive deterministically) as a uniqueness key. A retry after a lost response must not create a second result row. The example below stores the raw callback for auditability and keeps the current job state separately.
CREATE TABLE crawl_jobs (
crawl_id VARCHAR(191) NOT NULL,
status ENUM('received','queued','running','succeeded','failed') NOT NULL,
result_json JSON NULL,
last_error TEXT NULL,
updated_at TIMESTAMP(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6)
ON UPDATE CURRENT_TIMESTAMP(6),
PRIMARY KEY (crawl_id)
) ENGINE=InnoDB;
CREATE TABLE crawl_callbacks (
callback_id VARCHAR(191) NOT NULL,
crawl_id VARCHAR(191) NOT NULL,
payload_json JSON NOT NULL,
received_at TIMESTAMP(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6),
PRIMARY KEY (callback_id),
CONSTRAINT fk_callback_job FOREIGN KEY (crawl_id)
REFERENCES crawl_jobs (crawl_id)
) ENGINE=InnoDB;Choose retention periods for payloads and job history deliberately. If the crawler does not provide a callback identifier, resolve how duplicates are recognized before production; do not silently assume that a URL or timestamp is unique.
Implement the Flask receiver
Read and validate request data while Flask’s request context is active. The request object is a context-local proxy; Flask pushes it for request handling and pops it after response processing. Copy the validated primitive values you need into ordinary dictionaries before enqueuing work.
import json
import os
from flask import Flask, jsonify, request
import mysql.connector
from mysql.connector import Error, IntegrityError, pooling
app = Flask(__name__)
DB_CONFIG = {
"host": os.environ["MYSQL_HOST"],
"port": int(os.environ.get("MYSQL_PORT", "3306")),
"user": os.environ["MYSQL_USER"],
"password": os.environ["MYSQL_PASSWORD"],
"database": os.environ["MYSQL_DATABASE"],
}
# Pool size is an operational decision. It is fixed when the pool is created.
db_pool = pooling.MySQLConnectionPool(
pool_name="callback_pool",
pool_size=int(os.environ.get("MYSQL_POOL_SIZE", "5")),
**DB_CONFIG,
)
def validate_callback(data):
"""Adapt these checks to the crawler's documented contract."""
if not isinstance(data, dict):
raise ValueError("JSON object required")
callback_id = data.get("callback_id")
crawl_id = data.get("crawl_id")
if not isinstance(callback_id, str) or not callback_id:
raise ValueError("callback_id is required")
if not isinstance(crawl_id, str) or not crawl_id:
raise ValueError("crawl_id is required")
return callback_id, crawl_id, data
@app.post("/callbacks/crawler")
def crawler_callback():
# Authentication/signature verification must match the crawler protocol.
# Example placeholder: require a deployment-specific shared secret.
expected = os.environ.get("CRAWLER_CALLBACK_TOKEN")
if expected and request.headers.get("Authorization") != f"Bearer {expected}":
return jsonify(error="unauthorized"), 401
data = request.get_json(silent=True)
try:
callback_id, crawl_id, payload = validate_callback(data)
except ValueError as exc:
return jsonify(error=str(exc)), 400
conn = None
cursor = None
try:
conn = db_pool.get_connection()
cursor = conn.cursor()
cursor.execute(
"""INSERT INTO crawl_jobs (crawl_id, status)
VALUES (%s, 'received')
ON DUPLICATE KEY UPDATE crawl_id = VALUES(crawl_id)""",
(crawl_id,),
)
cursor.execute(
"""INSERT INTO crawl_callbacks
(callback_id, crawl_id, payload_json)
VALUES (%s, %s, %s)""",
(callback_id, crawl_id, json.dumps(payload)),
)
cursor.execute(
"UPDATE crawl_jobs SET status = 'queued' WHERE crawl_id = %s",
(crawl_id,),
)
conn.commit()
except IntegrityError:
if conn:
conn.rollback()
# A duplicate callback is an integration policy decision. This example
# treats the existing callback as already accepted.
return jsonify(status="accepted", duplicate=True), 200
except (Error, Exception):
if conn:
conn.rollback()
app.logger.exception("callback persistence failed")
# Return the status required by the crawler so it can retry if defined.
return jsonify(error="temporarily unavailable"), 503
finally:
if cursor:
cursor.close()
if conn:
conn.close() # returns a pooled connection to the pool
# Publish a serialized task only after the transaction has committed.
# enqueue_job({"callback_id": callback_id, "crawl_id": crawl_id})
return jsonify(status="accepted"), 200
if __name__ == "__main__":
app.run()
Use parameterized SQL, not string interpolation. Connector/Python has autocommit disabled by default, so the explicit commit() is what makes the related writes durable. Any exception path must call rollback() before returning or retrying. Closing a pooled connection returns it to the pool.
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Enqueue continued work safely
The commented enqueue_job call represents your selected durable queue (for example, a broker-backed worker system). Serialize only the values the worker needs, such as callback_id, crawl_id, and a version of the task schema. Do not pass Flask’s request proxy, a cursor, or an open connection.
Decide whether queue publication belongs inside the same atomic boundary as the database write. If your queue and database cannot participate in one transaction, an outbox table is safer: insert the callback, job state, and an undelivered outbox message in one MySQL transaction; a publisher worker sends outbox messages and marks them delivered. This prevents a process crash between commit() and queue publication from losing work.
Worker state transitions
- Load the task by its stable identifier.
- Atomically change
queuedtorunning; reject or safely ignore a task already completed. - Perform the slow operation without relying on Flask context.
- Write the result and set
succeeded, or record a bounded error and setfailed. - Acknowledge the queue message only after the state update commits.
Define retry count, visibility timeout, dead-letter handling, and whether operations are themselves idempotent for the queue you deploy. These values depend on crawler behavior and workload; there is no universal setting.
Connection pooling and capacity
Connector/Python’s pooling module creates a fixed-size pool. A requester obtains a connection, and closing it returns that connection for reuse. If all connections are checked out, acquisition raises PoolError. Size the pool against MySQL’s connection limit, web-worker count, queue-worker count, and expected transaction duration, then test the deployed Connector/Python version’s defaults.
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- Acquire as late as possible, immediately before the transaction.
- Keep transactions short; never perform crawling or network calls while holding a connection.
- Always close cursor and connection objects in
finally. - Return a retryable response or apply queue back-pressure when the pool is exhausted.
Security and operational controls
- Verify the crawler’s signature, token, or mTLS requirement before parsing trusted fields.
- Limit request size and reject malformed JSON.
- Use TLS at the edge and rotate secrets through deployment configuration.
- Log a correlation ID, crawl ID, state transition, duration, and outcome; redact authorization headers and sensitive payload fields.
- Monitor database errors, pool exhaustion, queue depth, worker age, failed jobs, and callback acknowledgment latency.
- Apply retention and deletion rules to raw payloads containing personal or proprietary data.
Troubleshooting common failures
The crawler keeps retrying
Check the crawler’s documented acknowledgment status and body, then inspect whether the response is sent before a successful commit. A timeout can cause a retry even when the database write succeeded; the uniqueness key should make that retry return your defined duplicate outcome.
Rows disappear after a successful response
Verify that conn.commit() runs on the success path. Connector/Python does not autocommit by default. Also confirm the tables use a transactional engine and that no later code rolls the transaction back.
Duplicate-key errors appear
This usually means the sender repeated a callback or your identifier mapping is not stable. Keep the uniqueness constraint, query the existing row, and implement the duplicate behavior required by the crawler rather than inserting a second result.
Pool acquisition raises PoolError
Connections are all busy or the pool is undersized for the deployed concurrency. Shorten transactions, close every connection on every path, and adjust pool size only within MySQL’s connection limits. Do not hide exhaustion with unbounded connection creation.
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Background work stops when Flask returns
That is expected for tasks spawned inside a view. Move the work to a durable queue and worker, or use an outbox publisher. Pass copied, serialized data—not request—to the worker.
Callbacks are accepted but no job runs
Inspect the gap between the database commit and queue publication. An outbox record, publisher retry, and alert on undelivered messages make this failure visible and recoverable.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Test the failure paths before deployment
- Send a valid callback twice and verify one callback row and one logical job.
- Send invalid JSON, missing identifiers, and an invalid signature.
- Force a database error and verify rollback plus the crawler-appropriate response.
- Kill the process after commit but before queue publication and verify your outbox or reconciliation process.
- Exhaust the pool under controlled load and confirm bounded, observable failure.
- Restart web and worker processes and confirm queued and running states recover according to your queue’s delivery semantics.
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FAQ
Should the callback endpoint return before the MySQL commit?
No when the acknowledgment means “accepted and persisted.” Commit the logically related writes first. If the contract only requires receipt, store an inbox record and make its durable handoff explicit.
Can I use Flask’s async syntax with a synchronous MySQL connector?
You can perform bounded I/O in an async view, but it still occupies the worker for the request cycle and does not replace a queue. Match the database driver and deployment model deliberately.
What identifier should be unique?
Prefer the crawler’s documented callback or event ID. If none exists, agree on deterministic semantics with the crawler provider before production; do not guess that a URL alone identifies one event.
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How long should callback payloads be retained?
Set retention from debugging, audit, privacy, and storage requirements. The crawler contract and your compliance obligations determine the period.
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