HANDLING DJANGO IN A HIGHLY CONCURRENT & SCALE ENVIRONMENT

@Tarun_Garg2

What is this talk about 🤷🏽‍♀️

Theme #1

Django Admin Pagination

Django Admin Pagination

Django Admin

Django Admin Pagination

# {project}/orders/models.py
from django.db import models

class Order(models.Model):
    order_reference_id = models.CharField(max_length=50)


# {project}/users/admin.py
from django.contrib import admin
from .models import Order

admin.site.register(Order)

Django Admin Pagination

Django Admin Pagination

-- Count the total number of records - 2.43 seconds
SELECT COUNT(*) AS "__count"
  FROM "orders_order"

-- Get first page of orders - 2ms
SELECT "orders_order"."id",
       "orders_order"."reference_id"
  FROM "orders_order"
 ORDER BY "orders_order"."id" DESC
 LIMIT 100
# django/core/paginator.py

@cached_property
def count(self):
    try:
        return self.object_list.count()
    except (AttributeError, TypeError):
        return len(self.object_list)

Django Admin Pagination

# common/paginator.py

from django.core.paginator import Paginator

class DumbPaginator(Paginator):
    """
    Paginator that does not count the rows
    in the table.
    """
    @cached_property
    def count(self):
        return 9999999999

Attempt #1 - Dumb paginator

Django Admin Pagination

-- Get first page of orders - 2ms
SELECT "orders_order"."id",
       "orders_order"."reference_id"
  FROM "orders_order"
 ORDER BY "orders_order"."id" DESC
 LIMIT 100

Pros

Cons

• No more count queries & hence 

blazing fast admin.

• UX is kinda compromised because if even we have <1m rows it will show all the pages.

• Dumb & fast to implement.

• Dumb & we should be able to do better.

Django Admin Pagination

Attempt #2 - Best effort basis

Django Admin Pagination

@cached_property
def count(self):
  """
  Paginator that enforces a timeout on the count operation.
  If the operations times out, a dummy value is returned
  """

  with transaction.atomic(), connection.cursor() as cursor:
    # Prevent statement_timeout from affecting other trxn
    cursor.execute('SET LOCAL statement_timeout TO 300;')
      try:
        return super().count
      except OperationalError:
        return 99999999

Attempt #2 - Best effort basis

This is much better but the worst case time will still be ~300 ms & we've not removed dumbness entirely.

Django Admin Pagination

Attempt #3 - Best effort with approximation
                                                        * Postgres only

Django Admin Pagination

Attempt #3 - Best effort with approximation
                                                        * Postgres only

@cached_property
def count(self):
  query = self.object_list.query

  if not query.where and not is_env_local():
    cursor = connection.cursor()
    cursor.execute(
      "SELECT reltuples FROM pg_class WHERE relname = %s",
      [query.model._meta.db_table],
    )
    return int(cursor.fetchone()[0])

  with transaction.atomic(), connection.cursor() as cursor:
    # Prevent statement_timeout from affecting other trxn
    cursor.execute('SET LOCAL statement_timeout TO 300;')
    try:
      return super().count
    except OperationalError:
      return 99999999

Django Admin Pagination

Attempt #3 - Best effort with approximation

-- Get from pg catalogue - 2ms
SELECT reltuples FROM pg_class WHERE relname = "orders_order"

-- Get first page of orders - 2ms
SELECT "orders_order"."id",
       "orders_order"."reference_id"
  FROM "orders_order"
 ORDER BY "orders_order"."id" DESC
 LIMIT 100

Pros

Cons

• Good tradeoff between accuracy & latency.

• Does not work when we have filters applied(where clause).

• Might not be suitable for a) smaller tables & b) where analyse is run with lesser frequency.

• The probability of reaching to dumb value is reduced.

• Still dumb value problem.

Django Admin Pagination

Attempt #4 - Solving for 'where' clause from #3                                                                * Postgres only

Django Admin Pagination

Attempt #4 - Solving for 'where' clause from #3                                                                * Postgres only

@cached_property
def count(self):
  query = self.object_list.query

  if not query.where and not is_env_local():
    cursor = connection.cursor()
    cursor.execute(
      "SELECT reltuples FROM pg_class WHERE relname = %s",
      [query.model._meta.db_table],
    )
    return int(cursor.fetchone()[0])

  with transaction.atomic(), connection.cursor() as cursor:
    # Prevent statement_timeout from affecting other trxn
    cursor.execute('SET LOCAL statement_timeout TO 300;')
    try:
      return super().count
    except OperationalError:
      try:
        with connection.cursor() as cursor:
          cursor.execute("EXPLAIN(FORMAT JSON) {}".format(query))
          return cursor.fetchone()[0][0]["Plan"]["Plan Rows"]
        except Exception:
          return 99999999

Django Admin Pagination

Attempt #4 - Solving for 'where' clause from #3                                                                * Postgres only

-- Get from pg catalogue -- OR run some approximation queries..

-- Get first page of orders - 2ms
SELECT "orders_order"."id",
       "orders_order"."reference_id"
  FROM "orders_order"
 ORDER BY "orders_order"."id" DESC
 LIMIT 100

Pros

Cons

• Better tradeoff between accuracy & latency.

• Might not be suitable for tables where analyze is run with lesser frequency.

• The probability of reaching to dumb value is reduced the most.

• Getting count of rows using "Explain" is probabilistic & depends upon MCV list in pg.

Django Admin Pagination

Attempt #n 

Theme #2

Django Admin Text Search

Story Time

Django Admin Text Search

Story Time

Django Admin Text Search

class OrderAdmin(model.Admin):
  search_fields = (
    "reference_id",
  )

Story Time

Django Admin Text Search

class OrderAdmin(model.Admin):
  search_fields = (
    "reference_id", "user__username"
  )

Story Time

Django Admin Text Search

class OrderAdmin(model.Admin):
  search_fields = (
    "reference_id", "user__username", "user__first_name", "user__last_name"
  )

Story Time

Django Admin Text Search

class OrderAdmin(model.Admin):
  search_fields = (
    "reference_id", "user__username", "user__first_name", "user__last_name",
    "paymenr__reference_id"
  )

Django Admin Text Search

Select * from orders_order WHERE 
UPPER(reference_id::text) LIKE UPPER(<search_term>)
OR UPPER(payment.reference_id::text) LIKE UPPER(<search_term>)
OR UPPER(user.email::text) LIKE UPPER(<search_term>)
...and so on

Django Admin Text Search

Django Admin Text Search

Django Admin Text Search

class ReferenceIDFilter(InputFilter):
    parameter_name = 'reference_id'
    title = _('Order Reference ID')

    def queryset(self, request, queryset):
        if self.value() is not None:
            uid = self.value()

            return queryset.filter(reference_id__icontains=uid)

Theme #3

Caching Django Models 👀

Caching Django Models 👀

Cache SET - cache.set(key, django_model_object)

Cache GET - cache.get(key)

Caching Django Models 👀 

class Order(models.Model):
  reference_id = models.CharField(max_length=15)
  

cache.set(f"Order:{given_pk}", Order.objects.get(pk=given_pk))

cache.get(f"Order:{given_pk}")

Caching Django Models 👀 

class Order(models.Model):
  reference_id = models.CharField(max_length=15)
  order_amount = models.IntegerField()

cache.get(f"Order:{given_pk}")

Caching Django Models 👀

Set low enough TTL for cache keys

Delete all keys for changed model

Change key structure whenever model definition is updated

Caching Django Models 👀

Caching Django Models 👀

Theme #4 

Case of .save() in Django 👀

Case of .save() in Django 👀

How does .save() work?

class Record(models.Model):
  # id will be created automatically
  name = models.CharField(max_length=255)
  created_at = models.DateTimeField(auto_now_add=True)
  is_deleted = models.BooleanField(default=False)

>>> record = Record.objects.get(id=1)
>>> record.name = "new record name"
>>> record.save()
UPDATE "record"
SET    "name" = 'new record name',
       "created_at" = NOW(),
       "is_deleted" = FALSE
WHERE  "id" = 1

Case of .save() in Django 👀

In the case of 2 different updates done concurrently on the same object, the last write wins.

Case of .save() in Django 👀

Take lock on DB row when updating

Take distributed lock using Redis

Django update_fields to rescue

Case of .save() in Django 👀 

>>> record = Record.objects.get(id=1)
>>> record.name = "new record name"
>>> record.save(update_fields=["name"])
UPDATE "record"
SET    "name" = 'new record name'
WHERE  "id" = 1

Case of .save() in Django 👀

• The other 2 solutions also have their place but those are not the "best" for our problem.

• There are 3rd party libraries to do the job
automatically for you.

• You can create linters to check if the usage of .save() has update_fields in it.

Special Mentions

  • My colleagues & ex-colleagues at SquadStack
     
  • CitusData blog post around "Faster PostgreSQL Counting"
     
  • Haki Benita's blog posts about Django & Django Admin optimizations
     
  • Doordash blog posts about Django scaling issue
     
  • Excalidraw for all the beautiful graphics
     
  • Django community & volunteers

 🙏🏻 Open to
feedback or questions!

@Tarun_Garg2

tarungarg546

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