Transactions pymongo

This repo contains some basic demo scripts used to illustrate the implementation of transactions with MongoDB in Python.

The monitor.py script repeatedly calculates the sum of balances across all account documents in four collections, while the transfer_loop.py script transfers random amounts between random accounts across collections.

If the transfers are ACID compliant then the monitor will report a consistent total balance across the entire system (as no money is entering/leaving the system, only internal transfers are taking place).

However, if the transfers are not ACID compliant then the monitor may report a balance which varies - this is because the monitor may either:

• See data at a point in time when funds have left one account and not yet arrived in another (i.e. the monitor can see inside of a transfer which is not adhering to the principle of ISOLATION) OR
• It may read the balances for different collections at different moments in a way which is interleaves with complete transactions (i.e. the monitor is not reading from a data source exhibiting the principle of CONSISTENCY)

By default, neither the monitor nor the transfer loop will use transactions and consequently the system will not exhibit either ISOLATION or CONSISTENCY principles.

The argument txns can be passed to either of the monitor of transfer_loop scripts in order to alter this behaviour. Specifically:

• Providing the txns argument to the monitor.py script ensures CONSISTENT reads
• Providing the txns argument to the transfer_loop.py script ensures ISOLATED write transactions

Usage

1. Configure your connection string in ./settings.py
2. Adjust any other config in the settings as you wish
3. Run initialise.py to create the DB/Collections/Data
4. Run monitor.py to start the monitor
5. Run transfer_loop.py to start the transfer loop

The monitor.py and transfer_loop.py scripts both take an option argument 'txns' which indicates the use of transactions (in order to guarantee consistency/isolation behaviours).

python3 initialise.py
python3 monitor.py txns
python3 transfer_loop.py txns