pindb

pindb is a master/slave router toolkit for Django. It provides database replica pinning, round-robin read from replicas, the use of unmanaged databases side by side with managed ones, and delegate routers for deciding among sets of replicated DBs.

TL;DR API

# Try to save without pinning:
foo = Model.objects.all()[0]

# UnpinnedWriteException raised under strict mode, or master pinning occurs under greedy mode.
foo.save()

# read only from the master, allow writes.
pindb.pin(alias)

# Initialize/end the container:
pindb.unpin_all()

with unpinned_replica(alias):
   ... # read from replicas despite pinning state
# or
queryset.using(pindb.get_replica('master-alias'))

with master(alias):
   ... # write to master despite pinning state

Jargon

Master refers to a writable DB. Replica (or slave) refers to a read-only DB whose data comes from master writes. One or more master/slave database sets can help scale reads and avoid lock contention on the master. Typically all reads go to replicas until a write occurs -- then all subsequent reads also go to the master to avoid inconsistent reads due to replication lag. Pinning is time-based and round-trips between web requests via cookies. See "design notes" below for more.

Installation

pip install pindb

TL;DR:

1. Set DATABASES_ROUTERS to use a pindb router
2. set PINDB_ENABLED to False under test
3. Define DB masters and replica sets.
4. populate DATABASES with pindb.populate_replicas.
5. Add PinDbMiddleware to your middleware.
6. Integrate with celery (if needed).
7. profile for places to explicitly side-step pinning.

More explicitly:

Add pindb.StrictPinDbRouter or pindb.GreedyPinDbRouter to DATABASE_ROUTERS.

StrictPinDbRouter requires that pinning be declared before a write is attempted. The advantage is that read replicas are used as much as possible. The disadvantage is that your code will need many declarations to explicitly allow and opt out of pinning.

GreedyPinDbRouter will pin to a master as soon as a write occurs. The advantage is that most of your code will just work. The disadvantage is that you will use the read replicas less than possible. You also might encounter more situations where the state of your DB changes behind your back: you might read from a lagged replica, then perform a write (which pins you to the master) based on that old information.

PINDB_ENABLED can be used to disable pindb under test. Each TEST_MIRROR'd alias gets its own connection (and hence transaction), which is problematic under Django's TestCase, where a master write will not be visible under the replica's connection.

pindb has an extensive test suite; disabling it under your own test suite is sane/recommended.

If you need to manage more than 1 master/replica set, add PINDB_DELEGATE_ROUTERS for pindb to defer to on DB set selection. This is just another Django database router which should return a master alias; pindb will then choose a master or replica as appropriate for the current pinning state of the returned master.

Define MASTER_DATABASES, same schema as DATABASES:

DATABASES = {
  "unmanaged": {
    # HOST0, etc.
  }
}

MASTER_DATABASES = {
  "default": {
    # HOST1, etc.
  },
  "some_other_master": {
    ...
  }
}

Define DATABASE_SETS, which overrides specific settings for replicas:

DATABASE_SETS = {
  "default": [{HOST:HOST1}, {HOST:HOST2}, ...],
  "some_other_master": [...] # zero or more replicas is fine.
}

Finalize DATABASES with pindb.populate_replicas:

DATABASES.update(populate_replicas(MASTER_DATABASES, DATABASE_SETS))

Add pindb.middleware.PinDbMiddleware to your MIDDLEWARE_CLASSES before any expected database access.

Optionally, throughout your codebase, if you intend to write, declare it as early as possible to avoid inconsistent reads off the related replicas:

pin("default")

That will cause all future reads to use the master.

To use under celery, hook celery.signals.task_postrun to call pindb.unpin_all:

import pindb
from celery.signals import task_postrun

def end_pinning(**kwargs):
  pindb.unpin_all()
task_postrun.connect(end_pinning)

Exceptions and avoiding them

Exceptions

PinDbConfigError may be caused by...

• Your settings not including MASTER_DATABASES and DATABASE_SETS
• Your MASTER_DATABASES not including a "default" and populate_replicas being called without passing unmanaged_default=True.
• Declaring an alias in MASTER_DATABASES which does not have a related DATABASE_SETS entry

UnpinnedWriteException may be caused by...

• Model.objects.create, Model.save, qs.update, or qs.delete without previously calling pindb.pin for the master

  Note that writes to unmanaged aliases (that is, ones unlisted in MASTER_DATABASES and related DATABASE_SETS) are allowed at any time.

Overriding pinning

If you wish to read from a replica despite having previously pinned the master, you can do so with...

with pindb.unpinned_replica(alias):
  # code which reads from replicas

If you wish to write to a master despite not having pinned to it, you can do so with...

with pindb.master(alias):
  # code which writes to the DB

Requirements and design notes

We have multiple separate masters (not necessarily sharded). Let's call a grouping of a master and its replicas a "DB set".

We would like to have read replicas of these masters, and we would like to read from replicas as much as possible and we would like all writes to go to the master of the set. But we would also like reads to be consistent to writers.

We would like this to be possible for web request cycles but also for units of work like tasks or shell scripts. So we call this unit of work the "pinning context".

Writes to a given master should continue to read from the master to avoid inconsistency in the replication lag window, so there will be an API for declaring that. Declaring (or otherwise preferring) that a set master is needed is "pinning" and the group of pins for all DB sets is called the "pinned set".

Code which plans on writing (or needs the very lastest data) should be able to declare that as early as possible to get a fully-consistent view from the master(s).

It should be a clear error if we've made a mistake in pinning (that is, writing after reading from a set). The issue here is that if we allow reads (not knowing that a write is coming) that gives us an inconsistency window. For example, a process reads from replica, gets a PK that has been deleted in master, writes to master, fails. Or gets a PK that's been mutated in master so that it shouldn't have been processed, etc.

Code which needs to write without pinning the whole container (e.g. a logging table) should be able to side-step the pinning.

We should be able to manage the DB sets in settings with minimal repetition, and it should compose well with multiple settings files.

Approach

We use a threadlocal to hold the pinned set.

The database router will then respect pinned set.

The DATABASES dict in settings is "final" in the sense that it isn't structured with any master/replica semantics. So we use an intermediate setting for defining sets:

MASTER_DATABASES = {
  'master-alias': { 'HOST':"a", ...normal settings },
   ...
}

DATABASE_SETS = {
  'master-alias': [{'HOST':'someotherhost',...},],
   # override some of the master settings
}

And replica config can be finalized...

DATABASES = DATABASES.update(populate_replicas(MASTER_DATABASES, DATABASE_SETS))

...resulting in something like...

DATABASES = {
  'master-alias': { 'HOST':"a", ...normal settings },
  'master-alias-1': { 'HOST':"someotherhost", ...merged settings,
                      TEST_MIRROR='master-alias' },
  ...}

If no master is named "default", then the master of your first DB set will also be aliased to "default". You should use django.utils.datastructures.SortedDict to make that stable.

If you have multiple database sets, you will also want to compose pinning with selection of the appropriate set. For this, there is one additional setting: DATABASE_ROUTER_DELEGATE. It has the same interface as a normal DATABASE_ROUTER, but db_for_read and db_for_write must return only master aliases. Then an appropriate master or replica will be chosen for that DB set.

More concretely, suppose you have 2 different masters, and each of them has a read slave. Your delegate router (as it existed before use of pindb) likely chooses which master based on app semantics. Keep doing that. Then pindb's router will select a read slave from the DB set whose master your existing (now delegate) router chose.

The strict router will throw an error if db_for_write is called without declaring that it's OK. The correct approach is to pin the DB you intend to do writes to before you read from a replica.

To explictly prefer a read replica despite pinning, use either...

with pindb.unpinned_replica('master-alias'):
       ...

...or the .using method of a queryset.

If you would like to explicitly use a replica, pindb.get_replica() will return a replica alias.

Pinning a set lasts the duration of a pinning context: once pinned, you should not unpin a DB. If you want to write to a DB without pinning the container, you can use queryset's .using method, which bypasses db_for_write. Careful with this axe.

To declare a pin...

pindb.pin('master-alias')

TODO: Use signed cookies if available (dj 1.4+) for web pinning context.

Coverage

To see coverage of pindb:

$ PYTHONPATH=.:$PYTHONPATH coverage run setup.py test $ coverage html

Example configuration

MASTER_DATABASES = {
    'default': {
        'NAME': 'db1',
        'ENGINE': DB_ENGINE,
        'USER': '...',
        'PASSWORD': '...',
        'HOST': '10.0.1.0',
        'PORT': 3306,
        'OPTIONS': DB_OPTIONS
    },
    'api': {
        'NAME': 'db2',
        'ENGINE': DB_ENGINE,
        'USER': '...',
        'PASSWORD': '...',
        'HOST': '10.0.2.0',
        'PORT': 3306,
        'OPTIONS': DB_OPTIONS
    },
}

DATABASE_SETS = {
    "default": [{'HOST': '10.0.1.1'},{'HOST': '10.0.1.2'}],
    "api": [{'HOST': '10.0.2.1'}]
}

DATABASES = {...}

DATABASES.update(pindb.populate_replicas(MASTER_DATABASES, DATABASE_SETS))


PINDB_DELEGATE_ROUTERS = ["myapp.router.Router"]
DATABASE_ROUTERS = ['pindb.GreedyPinDbRouter']

# default values which you can override:
PINDB_ENABLED = True
PINDB_PINNING_COOKIE = 'pindb_pinned_set'
PINDB_PINNING_SECONDS = 15