TIL—Python has a built-in persistent key-value store

└─ 2018-06-10 • Reading time: ~2 minutes

Python is, in a lot of ways, a very rich language. After years of using it, I still regularly discover new parts of the ecosystem, even in the standard library. In particular, there are a few modules which are not very well-known, but can be very useful in some situations. Today I discovered dbm a persistent key/value store:

Quick start:

import dbm

with dbm.open('my_store', 'c') as db:
  db['key'] = 'value'
  print(db.keys()) # ['key']
  print(db['key']) # 'value'
  print('key' in db) # True

It behaves a lot like a dict except:

  • It persists its values on disk
  • You can only use str or bytes as key and values

The performance is also slower than a dictionary, but faster than sqlite3.

The benchmark consists in performing 10k writes and 10k random reads:

from random import random
import time

operations = 10000
writes = [str(i) for i in range(operations)]
reads = [str(int(random() * operations)) for _ in range(operations)]

# Create some records
for i in writes:
    db[i] = 'x'

# Read values in random order
for i in reads:
    x = db[i]

Here are the results:

  • dict – total: 0.002 seconds, 0.23398 μs/record x 1
  • dbm – total: 0.054 seconds, 5.35984 μs/record x 27
  • sqlite3 in-memory: total: 2.468 seconds, 246.84346 μs/record x 1234
  • sqlite3 file: total 42.407 seconds, 4240.69593 μs/record x 21207

Why is sqlite3 so slow? Well, the benchmark is probably not representative of the typical workload for sqlite (lots of individual insertions and selections). If we perform the same operations using executemany and one select of all the keys at once, we get:

  • :memory: – total: 0.038 seconds, 3.81415 μs/record x 19
  • file – total 0.071 seconds, 7.07369 μs/record x 35

It’s much better, but still not as fast as dbm (when we persist to a file). So, if you have a workload where keys and values are added and retrieved very often, are always str or bytes and need to be persisted on disk, dbm is a serious contender!

The code used to benchmark can be found here: dbm_bench.py

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