How do I retry failed data transfers without creating duplicates?

To retry failed data transfers without creating duplicates, use an idempotent transfer design. The core idea is: if the same transfer is attempted more than once, only one copy should be applied.

Practical ways to do that

1. Use a unique transfer ID

   • Assign every transfer a stable, unique ID.
   • Before applying a transfer, check whether that ID has already been processed.
   • If yes, skip it; if no, process it and record it as completed.

2. Make writes idempotent

   • Prefer operations like:
     • Upserts (INSERT ... ON CONFLICT DO UPDATE)
     • Replace-by-key
     • Set state to final value
   • Avoid “blind append” unless duplicates are acceptable or deduplicated later.

3. Track transfer status

   • Store states such as:
     • pending
     • in_progress
     • succeeded
     • failed
   • On retry, only pick up records that are failed or pending.
   • Ensure only one worker can claim a transfer at a time.

4. Use a deduplication key

   • If the source doesn’t provide a transfer ID, create one from the content and context:
     • source system
     • source record ID
     • destination
     • timestamp or version
   • Store this key with a unique constraint.

5. Make retries safe with atomic updates

   • The “check then insert/update” step should be atomic.
   • Use database transactions or uniqueness constraints to prevent race conditions.

6. Handle partial failures carefully

   • If a transfer can fail halfway through, design it so rerunning it won’t double-apply side effects.
   • Example: write data first, then mark the transfer complete only after success.

Example pattern

• Table transfers:
  • transfer_id (unique)
  • status
  • payload_hash
  • attempt_count

Retry logic:

• Look up transfer_id
• If status = succeeded, do nothing
• If status = failed, retry processing
• When processing succeeds, update status to succeeded

Best practice summary

• Always identify each transfer uniquely
• Store completion state
• Use database uniqueness or idempotent operations
• Retry only unfinished transfers
• Ensure processing can be safely repeated

If you want, I can also show:

• a database schema example
• a Python/JavaScript retry pattern
• or a message-queue approach for exactly-once-ish processing.