Rom - the Redis object mapper for Python

Copyright 2013-2018 Josiah Carlson

Released under the LGPL license version 2.1 and version 3 (you can choose which you'd like to be bound under).

Documentation

Updated documentation can be found: https://josiahcarlson.github.io/rom/

What

Rom is a package whose purpose is to offer active-record style data modeling within Redis from Python, similar to the semantics of Django ORM, SQLAlchemy, Google's Appengine datastore, and others.

Why

I was building a personal project, wanted to use Redis to store some of my data, but didn't want to hack it poorly. I looked at the existing Redis object mappers available in Python, but didn't like the features and functionality offered.

What is available

Data types:

• Strings (2.x: str/unicode, 3.3+: str), ints, floats, decimals, booleans
• datetime.datetime, datetime.date, datetime.time
• Json columns (for nested structures)
• OneToMany and ManyToOne columns (for model references)
• Non-rom ForeignModel reference support

Indexes:

• Numeric range fetches, searches, and ordering
• Full-word text search (find me entries with col X having words A and B)
• Prefix matching (can be used for prefix-based autocomplete)
• Suffix matching (can be used for suffix-based autocomplete)
• Pattern matching on string-based columns
• All indexing except Geo indexing is available when using Redis 2.6.0 and later
• Geo indexing available with Redis 3.2.0 and later

Other features:

• Per-thread entity cache (to minimize round-trips, easy saving of all entities)
• The ability to cache query results and get the key for any other use (see: Query.cached_result())

Getting started

1. Make sure you have Python 2.6, 2.7, or 3.3+ installed

2. Make sure that you have Andy McCurdy's Redis client library installed: https://github.com/andymccurdy/redis-py/ or https://pypi.python.org/pypi/redis

3. Make sure that you have the Python 2 and 3 compatibility library, 'six' installed: https://pypi.python.org/pypi/six

4. (optional) Make sure that you have the hiredis library installed for Python

5. Make sure that you have a Redis server installed and available remotely

6. Update the Redis connection settings for rom via rom.util.set_connection_settings() (other connection update options, including per-model connections, can be read about in the rom.util documentation):

   import redis
   from rom import util
   
   util.set_connection_settings(host='myhost', db=7)
   

Warning

If you forget to update the connection function, rom will attempt to connect to localhost:6379 .

7. Create a model:

   import rom
   
   # All models to be handled by rom must derived from rom.Model
   class User(rom.Model):
       email = rom.String(required=True, unique=True, suffix=True)
       salt = rom.String()
       hash = rom.String()
       created_at = rom.Float(default=time.time)
   

8. Create an instance of the model and save it:

   PASSES = 32768
   def gen_hash(password, salt=None):
       salt = salt or os.urandom(16)
       comp = salt + password
       out = sha256(comp).digest()
       for i in xrange(PASSES-1):
           out = sha256(out + comp).digest()
       return salt, out
   
   user = User(email='[email protected]')
   user.salt, user.hash = gen_hash(password)
   user.save()
   # session.commit() or session.flush() works too
   

9. Load and use the object later:

   user = User.get_by(email='[email protected]')
   at_gmail = User.query.endswith(email='@gmail.com').all()
   

Lua support

From version 0.25.0 and on, rom assumes that you are using Redis version 2.6 or later, which supports server-side Lua scripting. This allows for the support of multiple unique column constraints without annoying race conditions and retries. This also allows for the support of prefix, suffix, and pattern matching on certain column types.

If you are using a version of Redis prior to 2.6, you should upgrade Redis. If you are unable or unwilling to upgrade Redis, but you still wish to use rom, you should call rom._disable_lua_writes(), which will prevent you from using features that require Lua scripting support.

Expiring models/TTLs

There is a series of feature requests/bug reports/pull requests to add the ability for rom to automatically delete and/or expire entity data stored in Redis. This is a request that has been made (as of January 2016) 6 different times.

Long story short: rom stores a bunch of data in secondary structures to make querying fast. When a model "expires", that data doesn't get deleted. To delete that data, you have to run a cleanup function that literally has to scan over every entity in order to determine if the model had been expired. That is a huge waste and is the antithesis of good design.

Instead, if you create a new expire_at float column with index=True, the column can store when the entity is to expire. Then to expire the data, you can use: Model.query.filter(expire_at=(0, time.time())).limit(10) to (for example) get up to the 10 oldest entites that need to be expired.

Now, I know what you are thinking. You are thinking, "but I wish the data would just go away on its own." And I don't disagree. But for that to happen, Redis needs to grow Lua-script triggers, or you need to run a separate daemon to periodically clean up left-over data. But ... if you need to run a separate daemon to clean up left-over data by scanning all of your rom entities, wouldn't it just be better/faster in every way to keep an explicit column and do it efficiently? I think so, and you should too.