django-lambda-tasks 0.3.0

Run async tasks in a lambda function

Django Lambda Tasks

A Django library for offloading work to AWS Lambda outside of the request-response cycle. Tasks are defined with a decorator, enqueued to SQS on transaction commit, and executed by a Lambda handler that AWS invokes with SQS message batches. Task results, status, and metadata are persisted in the Django database.

Platform note: Unix-based systems only (Linux, macOS). Timeout enforcement relies on SIGALRM.

---

Installation

pip install django-lambda-tasks

Add to INSTALLED_APPS:

INSTALLED_APPS = [
    ...
    "lambda_tasks",
]

Run migrations:

python manage.py migrate lambda_tasks

---

Quick Start

1. Define a task

Create a tasks.py in your Django app and decorate any function with @lambda_task. All task arguments must be keyword-only:

# myapp/tasks.py
from lambda_tasks.decorators import lambda_task

@lambda_task
def send_welcome_email(*, user_id: int, template: str) -> str:
    # ... send the email
    return "sent"

2. Enqueue from a view

Call .execute_on_commit() to enqueue the task. It will be dispatched to SQS after the current database transaction commits:

# myapp/views.py
from myapp.tasks import send_welcome_email

def register(request):
    user = User.objects.create(...)
    send_welcome_email.execute_on_commit(user_id=user.id, template="welcome")
    return HttpResponse("Registered")

3. Configure AWS

See AWS Lambda and SQS setup for queue and Lambda configuration.

---

Example app

A runnable Django project is included in the example/ directory. It uses LAMBDA_TASKS_EAGER = True so no AWS infrastructure is needed.

cd example
uv run python manage.py migrate
uv run python manage.py createsuperuser
uv run python manage.py runserver

Then visit http://127.0.0.1:8000/trigger/?name=Alice to trigger a task and http://127.0.0.1:8000/admin/ to inspect the resulting TaskRecord.

See example/README.md for full details.

---

Configuration

All settings are read from your Django settings module and prefixed with LAMBDA_TASKS_.

Queue settings

Setting	Type	Description
LAMBDA_TASKS_QUEUES	dict[str, str]	Map of queue name → SQS queue URL. Must include a "default" key.

LAMBDA_TASKS_QUEUES = {
    "default": "https://sqs.us-east-1.amazonaws.com/123456789/default",
    "high_memory": "https://sqs.us-east-1.amazonaws.com/123456789/high-memory",
}

LAMBDA_TASKS_QUEUES must be set and must include a "default" key, otherwise ImproperlyConfigured is raised on first use.

Timeout settings

Setting	Type	Default	Description
LAMBDA_TASKS_DEFAULT_SOFT_TIMEOUT	int	270	Seconds before SoftTimeLimitExceeded is raised inside the task.
LAMBDA_TASKS_DEFAULT_HARD_TIMEOUT	int	300	Seconds before the task is forcibly terminated.

Both values must be between 1 and 900 seconds (the AWS Lambda maximum runtime is 15 minutes). soft_timeout must always be strictly less than hard_timeout.

LAMBDA_TASKS_DEFAULT_SOFT_TIMEOUT = 240
LAMBDA_TASKS_DEFAULT_HARD_TIMEOUT = 270

Retry settings

Setting	Type	Default	Description
LAMBDA_TASKS_MAX_RETRIES	int	2880	Maximum retry attempts before MaxRetriesExceededError is raised (default is 60 × 24 × 2).

Singleton settings

Setting	Type	Default	Description
LAMBDA_TASKS_SINGLETON_CACHE	str	"default"	Django cache backend used for singleton task locks.

Environment and secrets

Setting	Type	Description
LAMBDA_TASKS_ENVIRONMENT_SECRETS_MANAGER_ARN	env var	Secrets Manager reference (<arn>:<version-stage>:<version-id>) to load as environment variables at Lambda cold start.
LAMBDA_TASKS_SECRET_*	env var(s)	Secrets Manager references resolved into env vars at Lambda cold start. The unprefixed name becomes the target env var.

These are environment variables set on the Lambda function, not Django settings. See Loading environment variables from Secrets Manager and Resolving individual secrets from AWS Secrets Manager for full details.

Eager execution (development / testing)

Setting	Type	Default	Description
LAMBDA_TASKS_EAGER	bool	False	When True, tasks run synchronously in-process instead of being sent to SQS.

# settings/local.py
LAMBDA_TASKS_EAGER = True

With eager mode enabled, .execute_on_commit() executes the task immediately without touching SQS. Useful for local development and test suites where you don't want to mock AWS infrastructure.

Note: Timeouts are not enforced in eager mode. soft_timeout and hard_timeout values are accepted and stored but TimeoutContext becomes a no-op — it checks LAMBDA_TASKS_EAGER internally and skips SIGALRM setup. This is intentional: SIGALRM-based timeouts require a Lambda/Unix worker process, not a Django dev server thread.

---

Decorator options

@lambda_task(
    delay=0,               # seconds — SQS DelaySeconds before message becomes visible
    soft_timeout=60,       # seconds — overrides global default for this task
    hard_timeout=90,       # seconds — overrides global default for this task
    queue="default",       # named queue from LAMBDA_TASKS_QUEUES
    ignore_errors=(),      # exception types treated as non-fatal
    retry_on=(),           # exception types that trigger automatic retry
    retry_delay=0,         # base retry delay in seconds (jitter always added, capped at 900)
    singleton=False,       # prevent concurrent execution via Redis lock
)
def my_task(*, arg: str) -> None:
    ...

Parameter	Type	Default	Description
delay	int	0	Seconds to delay the SQS message before it becomes visible to consumers (max 900).
soft_timeout	int | None	None (uses global default)	Per-task soft timeout in seconds (max 900).
hard_timeout	int | None	None (uses global default)	Per-task hard timeout in seconds (max 900).
queue	str	"default"	Named queue to route this task to.
ignore_errors	tuple[type[BaseException], ...]	()	Exception types to treat as non-fatal (see Ignored exceptions).
retry_on	tuple[type[BaseException], ...]	()	Exception types that trigger an automatic retry (see Automatic retries).
retry_delay	int	0	Base delay in seconds when enqueuing a retry. Jitter (1–5s) is always added; result capped at 900. Requires retry_on to be non-empty.
singleton	bool	False	Prevent concurrent execution via a Redis lock (see Singleton tasks).

---

Serializing a task invocation

serialize() builds and validates a task invocation the same way execute_on_commit() does, but returns the payload as a plain dict instead of enqueuing it. Useful when you need to inspect, store, or forward the message before deciding to send it.

payload = send_welcome_email.serialize(user_id=42, template="welcome")
# {
#   "message": {
#     "task_name": "myapp.tasks.send_welcome_email",
#     "kwargs": {"user_id": 42, "template": "welcome"}
#   },
#   "delay": 0,
#   "queue": "default"
# }

The returned dict matches the SQSLambdaTask schema (message, delay, queue). The delay value comes from the decorator's delay parameter. To reconstruct and enqueue it later:

from lambda_tasks.models import SQSLambdaTask

task = SQSLambdaTask.model_validate(payload)
task.execute_on_commit()

---

Timeouts

Tasks support a two-phase timeout mechanism:

1. Soft timeout — SoftTimeLimitExceeded is raised inside the running task, giving it a chance to catch the exception and clean up gracefully.
2. Hard timeout — if the task is still running after the hard timeout, it is forcibly terminated and the TaskRecord is marked FAILED.

from lambda_tasks.decorators import lambda_task
from lambda_tasks.timeouts import SoftTimeLimitExceeded

@lambda_task(soft_timeout=25, hard_timeout=30)
def long_running_task(*, item_id: int) -> None:
    try:
        do_expensive_work(item_id)
    except SoftTimeLimitExceeded:
        # clean up before the hard timeout kills the process
        cleanup(item_id)
        raise

Timeout resolution order (first non-None value wins):

1. Per-task decorator default (soft_timeout / hard_timeout on @lambda_task)
2. Global settings (LAMBDA_TASKS_DEFAULT_SOFT_TIMEOUT / LAMBDA_TASKS_DEFAULT_HARD_TIMEOUT)

---

Named queues

Route tasks to queues backed by Lambda functions with different hardware profiles (e.g. more memory or CPU):

# settings.py
LAMBDA_TASKS_QUEUES = {
    "default": "https://sqs.us-east-1.amazonaws.com/123456789/default",
    "high_memory": "https://sqs.us-east-1.amazonaws.com/123456789/high-memory",
}

@lambda_task(queue="high_memory")
def process_large_file(*, file_id: int) -> None:
    ...

---

Task results

Every task invocation creates a TaskRecord row in the database. You can query it via the Django ORM:

from lambda_tasks.models import TaskRecord

# All recent tasks
TaskRecord.objects.all()

# Filter by status
TaskRecord.objects.filter(status=TaskRecord.TaskStatus.FAILED)

# Look up a specific invocation
TaskRecord.objects.get(pk="<uuid>")

TaskRecord fields

Field	Type	Description
id	UUID	Primary key — set to the SQS messageId for deduplication.
task_name	str	Fully-qualified function name (e.g. myapp.tasks.send_welcome_email).
kwargs	dict	Serialized task arguments.
n_retries	int	Number of retries attempted so far (starts at 0).
status	str	One of RUNNING, SUCCESS, FAILED, RETRYING.
start_time	datetime | None	When the worker began executing the task.
end_time	datetime | None	When the task completed or failed.
result	any | None	Return value of the task on success. None when the task raised an ignored exception.
traceback	str | None	Full traceback string on failure, or on success when an ignored exception was raised. None on clean success.

---

Logging

Import task_logger to emit log records that are automatically prefixed with the active message_id. This makes it straightforward to filter all logs for a specific task invocation in CloudWatch Logs Insights.

from lambda_tasks.logging import task_logger
from lambda_tasks.decorators import lambda_task

@lambda_task(soft_timeout=60, hard_timeout=90)
def send_welcome_email(*, user_id: int, template: str) -> str:
    task_logger.info("sending email to user %s", user_id)
    # → "[abc-123] sending email to user 42"
    return "sent"

task_logger is a LoggerAdapter wrapping the lambda_tasks.task logger. SQSLambdaTaskMessage.execute_immediately() sets the message_id before each task runs and clears it afterwards — you don't need to manage it yourself.

The Lambda handler configures the lambda_tasks logger hierarchy to INFO at cold start so that task_logger lines appear in CloudWatch. You can control the level by configuring the lambda_tasks logger in your Django LOGGING setting (e.g. set it to DEBUG or WARNING). If your LOGGING dictConfig sets a root logger with a handler (as most Django projects do), lambda_tasks will inherit from it via propagation.

Using your own logging.getLogger(__name__) is fine too; those records just won't carry the message_id prefix.

To filter by invocation in CloudWatch Logs Insights:

fields @timestamp, @message
| filter @message like "[your-invocation-id]"
| sort @timestamp asc

---

Ignored exceptions

Pass a tuple of exception types to ignore_errors on @lambda_task. If the task raises an instance of any listed type (or a subclass), the executor treats it as a non-fatal outcome:

• TaskRecord.status is set to SUCCESS
• The exception traceback is saved to TaskRecord.traceback for observability
• Task-side ORM writes inside the transaction.atomic() block are still rolled back
• The TaskRecord update is committed outside the atomic block

Exceptions not in ignore_errors continue to produce FAILED with a rollback. The default (()) preserves existing behaviour.

from lambda_tasks.decorators import lambda_task

class RecordNotFound(Exception):
    pass

@lambda_task(ignore_errors=(RecordNotFound,))
def sync_user(*, user_id: int) -> None:
    user = fetch_user(user_id)   # raises RecordNotFound if already deleted
    update_profile(user)

If sync_user raises RecordNotFound, the TaskRecord will have status=SUCCESS and the traceback recorded in traceback. Any ORM writes made before the exception are rolled back.

ignore_errors is validated at decoration time — passing a non-exception type raises TypeError immediately. It is stored on LambdaTaskWrapper and read by the executor at execution time; it is never serialised into the SQS message.

---

Automatic retries

Pass a tuple of exception types to retry_on on @lambda_task. If the task raises an instance of any listed type (or a subclass), the executor re-enqueues the task with an incremented retry counter:

• TaskRecord.status is set to RETRYING and the traceback is recorded
• The retry is a new invocation — the current record is terminal at RETRYING
• Retries continue until n_retries reaches LAMBDA_TASKS_MAX_RETRIES (default 2880), at which point MaxRetriesExceededError is raised and the record is saved as FAILED
• ignore_errors is checked first — a type in both ignore_errors and retry_on is treated as ignored (SUCCESS), not retried
• retry_on and ignore_errors must not overlap; overlapping raises TypeError at decoration time

from lambda_tasks.decorators import lambda_task

@lambda_task(retry_on=(RateLimitError, ConnectionError), retry_delay=30)
def sync_data(*, record_id: int) -> None:
    # RateLimitError or ConnectionError → RETRYING + re-enqueued
    # anything else → FAILED
    ...

When a retry is enqueued, the delay is set to min(retry_delay + round(random.uniform(1, 5)), 900) seconds. The jitter is always added to spread out competing retries. The result is capped at 900 (the SQS DelaySeconds maximum).

retry_on is validated at decoration time — passing a non-exception type raises TypeError immediately. It is stored on LambdaTaskWrapper and read by the executor at execution time; it is never serialised into the SQS message.

---

Singleton tasks

Pass singleton=True on @lambda_task to prevent concurrent execution of the same task. When enabled, the executor acquires a Redis lock via Django's cache framework before running the task function. The lock wraps the entire transaction.atomic() block.

from lambda_tasks.decorators import lambda_task

@lambda_task(singleton=True)
def sync_inventory(*, warehouse_id: int) -> None:
    # Only one instance runs at a time
    ...

• Lock key format: lambda_tasks.singleton_lock.{task_name}
• The lock is acquired with blocking_timeout=0 (fail immediately if held) and timeout=hard_timeout (auto-expire if the worker crashes)
• If the lock cannot be acquired (LockError), the task is retried via the existing retry mechanism — TaskRecord is set to RETRYING, the traceback is recorded, and the task is re-enqueued with n_retries + 1
• If n_retries reaches LAMBDA_TASKS_MAX_RETRIES, MaxRetriesExceededError is raised and the record is saved as FAILED
• The cache backend used for locks is controlled by LAMBDA_TASKS_SINGLETON_CACHE (default "default")

LockError is retried automatically for singleton tasks — do not include it in retry_on (doing so raises TypeError at decoration time). You may include LockError in ignore_errors if you want lock contention to be treated as a non-fatal outcome instead of triggering a retry.

singleton is stored on LambdaTaskWrapper and read by the executor at execution time; it is never serialised into the SQS message.

---

Atomicity

Each task runs inside a django.db.transaction.atomic block. If the task raises an unhandled exception, all ORM writes made inside the task are rolled back. The TaskRecord failure update is always written outside the atomic block so it survives the rollback.

When an exception matches ignore_errors, the same rollback applies to task-side writes — the atomic block still exits via exception. Only the TaskRecord update (written outside the block) is committed, recording the SUCCESS outcome and traceback.

---

Error handling

Scenario	Exception	When
Task function has positional parameters	TypeError	At decoration time
soft_timeout >= hard_timeout	ValueError	At decoration time
Timeout value exceeds 900 seconds	ValueError	At decoration time or settings load
Unknown queue name	ImproperlyConfigured	At .execute_on_commit()
LAMBDA_TASKS_QUEUES not set	ImproperlyConfigured	On first settings access
LAMBDA_TASKS_QUEUES missing "default"	ImproperlyConfigured	On first settings access
SQS send_message failure	boto3 exception (propagated)	At .execute_on_commit()

---

AWS Lambda and SQS setup

SQS queue configuration

Each queue in LAMBDA_TASKS_QUEUES should be a standard SQS queue (not FIFO) with the following recommended settings:

• Visibility timeout — set this higher than your Lambda function's hard_timeout. If the Lambda times out before SQS receives a deletion confirmation, the message becomes visible again and will be redelivered.
• Dead letter queue (DLQ) — configure a DLQ to capture messages that cannot be processed. Without one, messages that exhaust retries are silently deleted.
• maxReceiveCount — controls how many times a message is retried before being moved to the DLQ. A value of 1 is recommended: task failures are recorded as FAILED TaskRecord entries and are not retried by design, so the only messages that reach the DLQ are pre-execution failures (malformed message body, import errors, misconfiguration). These are unlikely to succeed on automatic retry and are better handled by fixing the underlying issue and manually redriving from the DLQ.

Lambda trigger

Configure the SQS queue as an event source trigger for your Lambda function. AWS will invoke the handler with batches of messages and use the batchItemFailures response to determine which messages to return to the queue.

Only pre-execution failures (e.g. a malformed message or an import error) are reported as batchItemFailures. Task logic failures are caught by SQSLambdaTaskMessage.execute_immediately(), recorded in TaskRecord, and treated as successful from SQS's perspective — they will not be redelivered.

Lambda handler

Point your Lambda function's handler at:

lambda_tasks.handler.handler

The handler performs cold-start initialisation on the first invocation (not at module import time) to avoid Lambda init-duration timeouts. The sequence is: temporary log handler attached → resolve_environment() → resolve_secrets_into_env() → log handler removed → conditional django.setup(). A module-level sentinel ensures this runs only once; subsequent warm invocations skip it entirely.

Ensure the Lambda execution environment has DJANGO_SETTINGS_MODULE set and that all task modules are importable (i.e. your application code is on the Python path).

Environment Variable	Required	Description
DJANGO_SETTINGS_MODULE	Yes	Django settings module path (e.g. myapp.settings.production).
LAMBDA_TASKS_ENVIRONMENT_SECRETS_MANAGER_ARN	No	Secrets Manager reference (<arn>:<version-stage>:<version-id>) to load as environment variables at cold start (runs first).
LAMBDA_TASKS_SECRET_*	No	Secrets Manager references resolved into individual env vars at cold start (runs second, after environment loading).

Loading environment variables from Secrets Manager

The Lambda handler supports loading environment variables from an AWS Secrets Manager secret at cold start. This lets you manage environment configuration centrally in Secrets Manager without baking values into the Lambda deployment package.

Set the LAMBDA_TASKS_ENVIRONMENT_SECRETS_MANAGER_ARN environment variable to a full reference including version stage and version ID:

LAMBDA_TASKS_ENVIRONMENT_SECRETS_MANAGER_ARN=arn:aws:secretsmanager:eu-west-1:123456789012:secret:myapp/prod/environment:AWSCURRENT:v1

The format is <arn>:<version-stage>:<version-id> (9 colon-separated segments — the ARN is 7 segments, plus version-stage and version-id). Both suffix fields must be non-empty.

The secret value must be a flat JSON object where all keys and values are strings:

{
  "DATABASE_URL": "postgres://user:pass@host:5432/db",
  "REDIS_URL": "redis://host:6379/0",
  "DJANGO_SETTINGS_MODULE": "myapp.settings.production"
}

At cold start (on the first handler invocation), before resolve_secrets_into_env() and django.setup(), the handler calls resolve_environment() which:

1. Checks for the LAMBDA_TASKS_ENVIRONMENT_SECRETS_MANAGER_ARN env var — if not set, does nothing
2. Parses and validates the reference format (9 segments, non-empty version-stage and version-id)
3. Fetches the secret via secretsmanager.get_secret_value(SecretId=..., VersionStage=..., VersionId=...)
4. Validates the secret value is a flat JSON object (all values must be strings, no empty keys)
5. Sets each key-value pair in os.environ — existing env vars are overridden
6. Caches the result via a module-level sentinel — subsequent calls are free no-ops

Because environment loading runs first, the secret can provide DJANGO_SETTINGS_MODULE itself, and individual secrets loaded by resolve_secrets_into_env() can reference environment-loaded values.

Validation errors

The following raise ValueError at cold start, preventing the Lambda container from starting:

• Reference format is invalid (wrong segment count, empty version-stage or version-id)
• Secret value is not valid JSON
• JSON is not a flat object (contains non-string values) — error message lists the offending keys
• JSON contains an empty string key

AWS errors (secret not found, permission denied) propagate as boto3 exceptions and crash the container at cold start.

Resolving individual secrets from AWS Secrets Manager

The Lambda handler supports loading individual secret values from AWS Secrets Manager into the environment before Django starts. This lets your Django settings file read from os.environ as normal while keeping secrets out of plaintext environment variables.

Set any env var with the prefix LAMBDA_TASKS_SECRET_ to a full Secrets Manager dynamic reference. The unprefixed name becomes the target env var:

LAMBDA_TASKS_SECRET_DATABASE_URL=arn:aws:secretsmanager:eu-west-1:123456789012:secret:myapp/prod:DATABASE_URL:AWSCURRENT:v1

At cold start (on the first handler invocation), after resolve_environment() and before django.setup(), the handler calls resolve_secrets_into_env() which:

1. Scans all env vars for the LAMBDA_TASKS_SECRET_ prefix
2. Validates every reference — malformed references raise immediately so the container fails to start rather than misconfiguring Django silently
3. Groups references by (ARN, version-stage, version-id) and makes one GetSecretValue call per unique combination
4. Extracts the named JSON key from the secret and writes it into os.environ
5. Caches fetched secrets in-process — warm invocations pay no extra cost

Reference format

Every value must follow the full dynamic reference syntax:

<arn>:<json-key>:<version-stage>:<version-id>

All four fields are required and must be non-empty. The secret value must be a JSON object; json-key names the field to extract.

# arn (7 segments) : json-key : version-stage : version-id
arn:aws:secretsmanager:eu-west-1:123456789012:secret:myapp/prod:DATABASE_URL:AWSCURRENT:v1

Multiple env vars can reference different keys from the same secret — only one GetSecretValue call is made for that (ARN, version-stage, version-id) combination:

LAMBDA_TASKS_SECRET_DATABASE_URL=arn:...:myapp/prod:DATABASE_URL:AWSCURRENT:v1
LAMBDA_TASKS_SECRET_SECRET_KEY=arn:...:myapp/prod:SECRET_KEY:AWSCURRENT:v1

Validation errors

The following all raise ValueError at cold start, preventing the Lambda container from starting with a misconfigured environment:

• Wrong number of colon-separated segments (must be exactly 10)
• Empty json-key, version-stage, or version-id
• Both LAMBDA_TASKS_SECRET_FOO and FOO are set — use one or the other
• The named JSON key does not exist in the fetched secret
• The secret value is not valid JSON

---

Built-in tasks

cleanup_task_records

A maintenance task that deletes TaskRecord rows older than a given number of days. This is the equivalent of Celery's backend cleanup — without periodic pruning, the TaskRecord table will grow indefinitely.

from lambda_tasks.tasks import cleanup_task_records

# Delete records older than 7 days (default)
cleanup_task_records.execute_on_commit()

# Delete records older than 30 days
cleanup_task_records.execute_on_commit(retention_days=30)

The task deletes all records whose start_time is strictly before now() - retention_days, regardless of status. It returns the number of deleted rows.

Schedule it however suits your infrastructure — an EventBridge rule triggering a Lambda, a Django management command in a cron job, or a call from another task. The library does not impose a scheduling mechanism.

---

Direct (synchronous) invocation

You can call a decorated task directly like a normal function — useful in tests or management commands where you want synchronous execution without going through SQS:

result = send_welcome_email(user_id=1, template="welcome")

This bypasses the queue entirely and runs the function in the current process. No TaskRecord is created, no transaction.atomic() block is used, and no timeout enforcement applies — it behaves exactly like calling the underlying function directly. Kwargs are still validated against the task's type annotations via Pydantic.