Documentation: Writing persistent objects: Be careful about __eq__ and __hash__ about zodb

[tseaver]

SGTM

[jimfulton]

On Sat, Sep 10, 2016 at 1:08 PM, Jason Madden [email protected]
wrote:

In general, objects without custom eq and hash objects are going
to be friendlier on the DB and the cache (if you can get away with identity
semantics).

Suppose you have two classes:

import uuid
class WithHash(Persistent):
def init(self):
self.id = uuid.uuid()
def eq(self, other):
return self.id == other.id
def hash(self):
return hash(self.id)
class WithoutHash(Persistent):
def init(self):
self.id = uuid.uuid()

Now if you had some dictionaries using those classes as keys:

conn = db.open()
conn.root.with_hash_dict = {WithHash(): i for i in range(5000)}
conn.root.wout_hash_dict = {WithoutHash(): i for i in range(5000)}

Doing something like:

conn = db.open()len(conn.root.with_hash_dict)

is going to unghost 5000 WithHash objects, whereas

conn = db.open()len(conn.root.wout_hash_dict)

isn't going to unghost any objects (because unpickling a dictionary
re-hashes all the keys, and hash() looks up hash on the class not the
instance, so if the hash method doesn't access any attributes---like
the default one---nothing has to be unghosted) .

Even if your object cache is sized appropriately, large-ish dictionaries
can take a long time to unpickle when accessed for the first time in a
particular connection, adding lots of load to the DB and/or cache system;
the same happens when creating a dictionary in memory for the first time of
such persistent objects.

If you can accept identity semantics (and for persistent objects, you
surprisingly often can), it's better to avoid custom eq and hash
methods if you'll ever be creating dictionaries or sets of your persistent
objects.

This is a lesson we learned the hard way; coming from a Java background
almost all of our objects defined custom eq and hash methods, and
that was fine until we started to get a lot of objects, when it became a
performance burden. Now it turns out that many such of those objects don't
need these methods.

Worth adding to the docs?

Probably, but I'm not sure where. It's a bit obscure.

Maybe in "Other things you can do, but shouldn't".

Jim

Jim Fulton
http://jimfulton.info

[jamadden]

Maybe in "Other things you can do, but shouldn't".

As in, you can add custom __hash__ but you maybe shouldn't?

[jimfulton]

Yup.

It occurs to me that it might be nice to have mix-in classes (or maybe just one) that implements identity-based hash and comparison based on OIDs. In the past, I wanted PxBTrees, but maybe IdentityHashablePersistent and IdentyComparablePersistent (or maybe just the later and maybe with better names :))

[jamadden]

Ok, I'll write this up and submit a PR. (I think it could also use something about implementing comparable methods to be used in a BTree, at least a pointer.)

Those sound like pretty good mixin classes. What would do you do before the object is assigned an OID though? Use its id?

[jimfulton]

I would error. Using it's id would be a disaster.

[jamadden]

I had cause to remember about zope.keyreference today, which implements something remarkably similar

[jamadden]

Opened PR #118 for this.