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Decide what the callback endpoint owns
Before writing code, obtain the crawler’s actual callback 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. Treat those as integration inputs rather than defaults.
- Receiver: authenticate the sender, parse and validate the payload, and reject malformed data.
- Persistence: save the raw (or appropriately redacted) callback, result metadata, and job-state transition atomically.
- Continuation: enqueue serialized data for work that is too slow for the request.
- Operations: correlate logs, expose state transitions, and alert on database or queue failures.
Keep credentials in deployment configuration, never in source code or ordinary logs. Decide how long callback payloads and job records are retained, which queue and worker runtime you will operate, and what delivery guarantees your crawler requires.
Why an async Flask view is not a durable worker
Flask is a WSGI application. Its documentation explains that one worker handles one request/response cycle. An async view can perform concurrent I/O during that cycle, but it does not increase the number of requests that worker handles at once and does not turn unfinished work into a durable job.
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The Flask project specifically advises: “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.” A task started with asyncio.create_task() in a normal view can be cancelled when the view’s event loop ends or disappear when the process restarts.
Choose the execution pattern
Persist before acknowledging
For bounded validation and database work, perform both in the request, commit, and then return the crawler’s documented acknowledgment. The caller’s success response now means the data was durably written, but the request remains open for the database operation.
Queue continued work
If parsing, enrichment, exports, or other processing can be slow, commit the callback and an initial state such as queued, then submit explicit serialized task data to a durable queue. A separate worker moves the record through running, succeeded, or failed. The queue and database failure behavior must be designed together: a database commit can succeed while queue submission fails, so record that condition and provide a reconciliation or retry path rather than silently losing the job.
| Pattern | Acknowledgment latency | Restart durability | Operational cost |
|---|---|---|---|
| Validate and write in Flask | Includes database time | Persisted after commit | Small; request must handle database failures |
| Commit then queue a worker task | Shorter after the commit | Depends on durable queue and state records | Higher; operate queue, worker, retries, and reconciliation |
MySQL schema for callbacks and jobs
Use a stable identifier supplied by the crawler (for example, a callback or crawl ID) and enforce uniqueness. Confirm that the identifier is truly stable across retries before relying on it for deduplication.
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CREATE TABLE crawl_jobs (
id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
crawler_job_id VARCHAR(191) NOT NULL,
status ENUM('received','queued','running','succeeded','failed') NOT NULL,
result_json JSON NULL,
last_error TEXT NULL,
created_at TIMESTAMP(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6),
updated_at TIMESTAMP(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6)
ON UPDATE CURRENT_TIMESTAMP(6),
PRIMARY KEY (id),
UNIQUE KEY uq_crawler_job (crawler_job_id)
) ENGINE=InnoDB;
CREATE TABLE crawl_callbacks (
id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
crawler_job_id VARCHAR(191) NOT NULL,
callback_id VARCHAR(191) NOT NULL,
payload_json JSON NOT NULL,
received_at TIMESTAMP(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6),
PRIMARY KEY (id),
UNIQUE KEY uq_callback (callback_id),
KEY ix_callback_job (crawler_job_id),
CONSTRAINT fk_callback_job FOREIGN KEY (crawler_job_id)
REFERENCES crawl_jobs (crawler_job_id)
) ENGINE=InnoDB;If the crawler has no callback ID, use another documented, stable event key; do not manufacture an identifier from mutable payload text without considering collisions. Decide whether duplicate callbacks should return the same accepted outcome, update the existing result, or be recorded as a separate event.
Install dependencies and configure the pool
python -m venv .venv
. .venv/bin/activate
pip install Flask mysql-connector-python
export MYSQL_HOST=127.0.0.1
export MYSQL_PORT=3306
export MYSQL_DATABASE=crawling
export MYSQL_USER=crawler_app
export MYSQL_PASSWORD='use-a-secret-manager'
export CALLBACK_TOKEN='replace-with-your-crawler-secret'Connector/Python has autocommit disabled by default, so successful related writes require an explicit commit(). Roll back on exceptions. Its pooling module creates a fixed-size pool; requesting a connection when every slot is in use raises PoolError. Size the pool against your deployment’s concurrency and MySQL connection limits, and handle exhaustion deliberately.
Complete Flask receiver
import json
import os
from flask import Flask, jsonify, request
from mysql.connector import pooling, Error
app = Flask(__name__)
pool = pooling.MySQLConnectionPool(
pool_name="crawler_pool",
pool_size=int(os.getenv("MYSQL_POOL_SIZE", "5")),
pool_reset_session=True,
host=os.environ["MYSQL_HOST"],
port=int(os.getenv("MYSQL_PORT", "3306")),
database=os.environ["MYSQL_DATABASE"],
user=os.environ["MYSQL_USER"],
password=os.environ["MYSQL_PASSWORD"],
)
def valid_token(req):
# Replace this with the crawler's documented signature or auth protocol.
expected = os.environ["CALLBACK_TOKEN"]
return req.headers.get("Authorization") == f"Bearer {expected}"
@app.post("/callbacks/crawl")
def crawl_callback():
if not valid_token(request):
return jsonify(error="unauthorized"), 401
data = request.get_json(silent=True)
if not isinstance(data, dict):
return jsonify(error="JSON object required"), 400
crawler_job_id = data.get("job_id")
callback_id = data.get("callback_id")
if not isinstance(crawler_job_id, str) or not crawler_job_id:
return jsonify(error="job_id is required"), 400
if not isinstance(callback_id, str) or not callback_id:
return jsonify(error="callback_id is required"), 400
conn = None
try:
conn = pool.get_connection()
cur = conn.cursor()
cur.execute(
"INSERT INTO crawl_jobs (crawler_job_id, status) "
"VALUES (%s, 'received') "
"ON DUPLICATE KEY UPDATE updated_at = CURRENT_TIMESTAMP(6)",
(crawler_job_id,),
)
cur.execute(
"INSERT INTO crawl_callbacks "
"(crawler_job_id, callback_id, payload_json) VALUES (%s, %s, %s)",
(crawler_job_id, callback_id, json.dumps(data)),
)
cur.execute(
"UPDATE crawl_jobs SET result_json=%s, status='queued' "
"WHERE crawler_job_id=%s",
(json.dumps(data.get("result")), crawler_job_id),
)
conn.commit()
except Error as exc:
if conn is not None:
conn.rollback()
# Log a correlation ID and the database error without secrets or payloads.
return jsonify(error="persistence_failed"), 503
finally:
if conn is not None:
conn.close() # returns a pooled connection
# Enqueue explicit data here, after commit, if continued work is required.
# queue.publish({"crawler_job_id": crawler_job_id, "callback_id": callback_id})
return jsonify(accepted=True, callback_id=callback_id), 202
if __name__ == "__main__":
app.run()
The field names and 202 response above are examples only. Match them to the crawler’s documented protocol. If the sender expects a different status or body, use that contract. For a callback that is already present, catch the duplicate-key condition and return your defined idempotent outcome instead of inserting a second result. Do not acknowledge a callback before the transaction commits.
Keep deferred work outside Flask
At enqueue time, copy primitive values such as IDs, not Flask’s request proxy. Flask pushes the request context for handling and pops it after response processing; teardown callbacks run even when an exception escapes. A worker cannot safely access that context later.
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# worker.py (queue library is deployment-specific)
def process(task):
job_id = task["crawler_job_id"]
# Open its own database connection, mark running, do bounded work,
# then commit succeeded or failed and record a safe error message.
# Make the operation idempotent: a redelivered task must not duplicate output.
pass
Use queue acknowledgments and retry limits appropriate to the selected queue. Persist state transitions so a process restart leaves enough information to resume or reconcile. Add alerts for repeatedly failed jobs, queue backlog, database connection-pool exhaustion, and callbacks stuck in an intermediate state.
Connection management and transaction rules
One connection per operation
Opening a connection for each callback is simple and can be adequate at low volume, but connection setup adds overhead and can pressure MySQL under bursts.
Use a fixed pool
A pool reuses connections and makes capacity explicit. Always close the acquired connection in a finally block, including validation, SQL, and queue-error paths. A pool does not remove the database limit: when exhausted, fail or retry according to a bounded policy and return an acknowledgment that causes the crawler to retry only if its contract defines that behavior.
Security, validation, and observability
- Require the crawler’s documented authentication or signature check before parsing sensitive fields.
- Use parameterized SQL; never concatenate callback values into statements.
- Apply payload-size limits and schema validation suitable for the crawler.
- Redact credentials, authorization headers, and unnecessary personal data from logs.
- Include a correlation ID in logs and state records, and record receive, commit, enqueue, start, success, and failure times.
- Use TLS at the public edge and restrict database credentials to the required schema operations.
Common failures and fixes
The crawler times out and sends duplicates
Measure the time spent validating and committing, then verify the crawler’s retry policy. Keep the transaction short and enforce the callback uniqueness key. Return the documented acknowledgment for an already-processed callback.
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Rows are missing after a successful response
Check that conn.commit() runs after both writes. Connector/Python does not commit automatically by default. Ensure no exception path returns success before rollback or commit.
PoolError appears under load
All pool slots are busy. Close every acquired connection, inspect transaction duration, and size the fixed pool within the MySQL server’s connection budget. Do not hide exhaustion with unbounded retries.
Worker code raises “working outside of request context”
The worker retained Flask’s request proxy. Extract and validate the needed values in the route and pass a serialized task object instead.
Callbacks remain queued forever
Inspect queue publication, worker health, and the database state transition. Record queue-publication failures after the database commit and run a reconciliation process that republishes eligible records.
Do these 3 things before closing this tab:
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Consent dialogs, newsletter overlays, chat widgets, bot checks, and failed page loads are capture concerns rather than Flask transaction concerns. Keep the callback receiver independent of the browser automation provider so either side can be replaced.
Or skip the browser setup
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For a direct call, see the ScreenshotNeo API documentation:
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Verification checklist
- Confirm the crawler’s authentication, schema, stable identifier, retry rules, and acknowledgment.
- Send an invalid-authentication request and verify it is rejected without a database write.
- Send a valid callback and verify the callback row and job transition commit together.
- Replay the same callback and verify the defined idempotent result.
- Force a SQL error and confirm rollback plus a retry-compatible response.
- Exhaust the pool in a staging load test and verify bounded handling and alerts.
- Stop the worker after enqueueing and confirm durable queue recovery and state reconciliation.
- Inspect logs to ensure secrets and sensitive payload fields are absent.
Frequently Asked Questions
Should the callback endpoint return 200 or 202?
Use the status and response body required by the crawler’s documented callback contract; the example’s 202 is not universal.
Can I use Flask’s development server in production?
No. Deploy Flask behind a production WSGI server and an appropriate TLS-capable edge; the receiver/worker separation remains the same.
How long should callback payloads be retained?
Choose retention from your debugging, audit, privacy, and storage requirements; the crawler contract and deployment determine the correct period.
Quick Recap
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