Guacamole is a Django model manager which uses in-memory caching to speed up some queries.
Consider the code:
class Currency(models.Model):
name = models.CharField(max_length=256)
class Security(models.Model):
name = models.CharField(max_length=256)
currency = models.ForeignKey(Currency)
With a view:
def view(request):
securities = Security.objects.filter(...)[0:50]
return render_to_response(...)
<ul>
{% for security in securities %}
<li>{{security.name}} (denominated in {{security.currency.name}})</li>
{% endfor %}
</ul>
When the view is rendered, there will be 51 SQL queries made - one to retrieve the list of Securities, and 50 more to retrieve each individual Currency. This can be slow - at 1ms of network latency to reach the database, this request will take a minimum of 51ms to return.
One solution is to use select_related(), which will condense this down to a single (large) query.
Another solution requires noticing two special properties of currencies - they are virtually immutable and there are very few of them (300-400, maybe?). Thus, it makes a great deal of sense to store them in RAM - they will rarely be out of sync with the database, and they will use up very little memory.
With Guacamole, the code could be modified as follows:
from guacamole import InMemoryCachingManager
class Currency(models.Model):
name = models.CharField(max_length=256)
objects = InMemoryCachingManager()
class Security(models.Model):
name = models.CharField(max_length=256)
currency = models.ForeignKey(Currency)
Whenever a currency foreign key is retrieved via a get method, it will be retrieved from an in-memory cache unless it is unavailable.
This is both faster than select_related and it is automatic. I.e., if I forget to use select_related in some particular view involving Securitys, it will not cause it to be slow.
The InMemoryCachingManager has three settings:
These are the fields of the object which the InMemoryCachingManager will use as cache keys. I.e., if we wanted this query to use the cache also:
Currency.objects.get(name="dollar")
Then you would need to modify the use of the InMemoryCachingManager to:
objects = InMemoryCachingManager(lookup_fields=['name'])
The maximum number of objects to be stored. The least recently used objects will be discarded first. 1024 by default.
How long objects should remain live. 15 minutes by default.