Queue Services

When a client sends an email, that email may go through several email servers before arriving at its final destination. Each server is designed to take responsibility for delivering the message to the next one, retrying if necessary. Once a server has taken responsibility for a message, the connecting client (or server) may disconnect.

The queue service is responsible for making sure email messages are stored persistently somewhere (in case of catastrophic failure) and that delivery is tried and retried with due diligence.

Delivery Attempts

Delivery attempts for a queue are performed with the Relay object passed in to the Queue constructor. The delivery attempt will ultimately produce a Reply object indicating its success or failure. If delivery was successful, the queue will remove the message from persistent storage.

If delivery failed permanently, with a 5xx code or too many 4xx codes, a Bounce envelope is created from the original message, which is delivered back to the original message sender. The original message is removed from storage and not retried.

If delivery failed transiently, with a 4xx code (which usually includes connectivity issues), the message is left in storage and a new delivery attempt is scheduled in the future. The time between delivery attempts is managed by the backoff function passed in to the Queue constructor. If this backoff function returns None, the message is permanently failed.

Here is an example backoff function that makes 5 delivery attempts with an exponentially increasing backoff time:

def exponential_backoff(envelope, attempts):
    if attempts <= 5:
        return 12.0 * (5.0 ** attempts)
    return None

Persistent Storage

A storage mechanism should store the entirety of an Envelope object, such that it can be recreated on restart. Along with the envelope, queue services must also keep track of when a message’s next delivery attempt should be and how many attempts a message has undergone. In essence, a queue’s storage mechanism allows slimta to be stopped and restarted without losing state.


The DictStorage class is a simple storage mechanism that, by itself, does not provide on-disk persistence. By default, it creates two dicts in memory for queue data, but passing in shelve objects will allow basic persistence. Be aware, however, that shelve may not handle system or process failure and could leave corruption.

The DictStorage class is very useful for development and testing, but probably should be avoided for live systems.

Local Disk

In the fashion of traditional MTAs, diskstorage writes Envelope data and queue metadata directly to disk to configurable directories. This functionality relies on pyaio to asynchronously write and read files on disk, and as such it is only available on Linux.

To ensure file creation and modification is atomic, files are first written to a scratch directory and then os.rename() moves them to their final destination. For this reason, it is important that the scratch directory (tmp_dir argument in the constructor) reside on the same filesystem as the envelope and meta directories (env_dir and meta_dir arguments, respectively).

The files created in the envelope directory will be identified by a uuid4() hexadecimal string appended with the suffix .env. The files created in the meta directory will be identified by the same uuid string as its corresponding envelope file, but with the suffix .meta. The envelope and meta directories can be the same, but two DiskStorage should not share directories.

To use this storage:

$ pip install python-slimta[disk]

And to initialize a new DiskStorage:

from slimta.diskstorage import DiskStorage

queue_dir = '/var/spool/slimta/queue'
queue = DiskStorage(queue_dir, queue_dir)


Taking advantage of the advanced data structures and ease of use of the redis database, redisstorage simply creates a hash key for each queued message, containing its delivery metadata and a pickled version of the Envelope.

The keys created in redis will look like the following:

redis> KEYS *
1) "slimta:28195d3b0a5847f9853e5b0173c85151"
2) "slimta:5ebb94976cd94b418d6063a2ca4cbf8f"
3) "slimta:d33879cf66244472b983770ba762e07b"

Each key is a hash that will look something like:

redis> HGETALL slimta:d33879cf66244472b983770ba762e07b
1) "attempts"
2) "2"
3) "timestamp"
4) "1377121655"
5) "envelope"
6) "..."

On startup, the Queue will scan the keyspace (using the customizable prefix slimta:) and populate the queue with existing messages for delivery.

To use redis you must install:

$ pip install python-slimta[redis]

And to initialize a new RedisStorage:

from slimta.redisstorage import RedisStorage

store = RedisStorage('redis.example.com')

Cloud Storage

The slimta.cloudstorage module makes available connectors to the cloud service providers AWS. Envelope data and queue metadata are written to a cloud object store. Optionally, a reference to their location in the object store is then written to a cloud message queue, which can alert relayers in other processes of the availability of a new message in the object store.

To use AWS requires installation and configuration of the boto library:

$ pip install python-slimta[aws]

Using the boto library, we need to come up with references to a Bucket and optionally a Queue. Then, use them to create SimpleStorageService and SimpleQueueService objects:

from boto.s3.connection import S3Connection
s3_conn = S3Connection('1A2B3C4D5E', 'XXXXXXXXXXXX')
s3_bucket = s3_conn.get_bucket('slimta-queue')

import boto.sqs
sqs_conn = boto.sqs.connect_to_region('us-west-2',
sqs_queue = sqs_conn.create_queue('slimta-queue')

from slimta.cloudstorage.aws import SimpleStorageService, SimpleQueueService
s3 = SimpleStorageService(s3_bucket)
sqs = SimpleQueueService(sqs_queue)

Once you have these objects created for your cloud service, link them together into a queue storage driver using CloudStorage:

from slimta.cloudstorage import CloudStorage

storage = CloudStorage(cloud_files, cloud_queues)
# or...
storage = CloudStorage(s3, sqs)

This object can then be used anywhere a QueueStorage object is required.