Coding Question 45 minutesMedium

CRM Billing Synchronizer

Build an accurate, efficient sync across two changing systems. The CRM is the source of truth.

Question

Implement the synchronization function in sync.py. Read every customer, contract, and file from the CRM, then create or update the corresponding records in Billing. The CRM is the source of truth, and the grader will run your function twice with CRM changes between runs.

Function parameters

crm: A CrmApi object used to list and fetch CRM customers, contracts, and files.
billing: A BillingApi object used to find, create, and update Billing records and upload files.

Inputs and API responses

CRM

Entity	Fields
Customer	id, name, city
Contract	id, customer_id, name, close_date
File	id, contract_id, file_name

These are Python dictionaries returned by the mock API, not JSON strings or direct database rows.
A Customer has many Contracts.
A Contract has many Files.
file_name can be None.

crm.list_customers(limit=10, offset=0)
→ [{"id": "crm_cust_1", "name": "Customer 1", "city": "Austin"}, ...]

crm.list_contracts(limit=10, offset=0)
→ [{"id": "crm_cont_1", "customer_id": "crm_cust_1", "name": "Contract 1", "file_ids": ["crm_file_1"]}, ...]

crm.get_file("crm_file_1")
→ {"id": "crm_file_1", "contract_id": "crm_cont_1", "file_name": "file_1.pdf"}

Billing

Entity	Fields
Customer	id, external_id, name, city
Contract	id, external_id, customer_id, file_name

Each named CRM File becomes one Billing Contract.

Critical mapping

Billing Customer.external_id = CRM Customer ID
Billing Contract.external_id = CRM File ID, not Contract ID

Requirements

Edit sync.py; it is the only writable file.
Paginate all list endpoints; maximum limit is 10.
Create and update Billing customers from CRM.
Each CRM file with a file_name becomes exactly one Billing contract.
Skip files whose file_name is None; they may become eligible on a later run.
Create the Billing contract and upload its file as separate API calls.
Treat find-method results as lists.
Handle changes and retries on the second run without creating duplicate customers or contracts.
Accuracy matters most; API efficiency breaks ties.

Editable

Test output

Run the challenge to test initial and incremental sync.