Distributed Design Pattern: Request Waiting List [Capital Markets Use?Case]

Problem Statement:

In a distributed capital markets system, client requests like trade executions, settlement confirmations, and clearing operations must be replicated across multiple nodes for consistency and fault tolerance. Finalizing any operation requires confirmation from all nodes or a Majority Quorum.

For instance, when a trade is executed, multiple nodes (such as trading engines, clearing systems, and settlement systems) must confirm the transaction’s key details?—?such as the trade price, volume, and counterparty information. This multi-node confirmation ensures that the trade is valid and can proceed. These confirmations are critical to maintaining consistency across the distributed system, helping to prevent discrepancies that could result in financial risks, errors, or regulatory non-compliance.

However, asynchronous communication between nodes means responses arrive at different times, complicating the process of tracking requests and waiting for a quorum before proceeding with the operation.

Solution: Request Waiting List Distributed Design Pattern

The Request Waiting List pattern addresses the challenge of tracking client requests that require responses from multiple nodes asynchronously. It maintains a list of pending requests, with each request linked to a specific condition that must be met before the system responds to the client.

Definition:

The Request Waiting List pattern is a distributed system design pattern used to track client requests that require responses from multiple nodes asynchronously. It maintains a queue of pending requests, each associated with a condition or callback that triggers when the required criteria (such as a majority quorum of responses or a specific confirmation) are met. This ensures that operations are finalized consistently and efficiently, even in systems with asynchronous communication between nodes.

Application in Capital Markets Use?Case:

1. Asynchronous Communication in Trade Execution:

• When a client initiates a trade, the system replicates the trade details across multiple nodes (e.g., order matching systems, clearing systems, settlement systems).
• These nodes communicate asynchronously, meaning that responses confirming trade details or executing the order may arrive at different times.

2. Tracking with a Request Waiting?List:

• The system keeps a waiting list that maps each trade request to a unique identifier, such as a trade ID or correlation ID.
• Each request has an associated callback function that checks if the necessary conditions are fulfilled to proceed. For example, the callback may be triggered when confirmation is received from a specific node (such as the clearing system) or when a majority of nodes have confirmed the trade.

3. Majority Quorum for Trade Confirmation:

• The system waits until a majority quorum of confirmations is received (e.g., more than half of the relevant nodes) before proceeding with the next step, such as executing the trade or notifying the client of the outcome.

4. Callback and High-Water Mark in Settlement:

• In the case of settlement, the High-Water Mark represents the total number of confirmations required from settlement nodes before marking the transaction as complete.
• Once the necessary conditions?—?such as price match and volume confirmation?—?are satisfied, the callback is invoked. The system then informs the client that the trade has been successfully executed or settled.

This approach ensures that client requests in capital markets systems are efficiently tracked and processed, even in the face of asynchronous communication. By leveraging a waiting list and majority quorum, the system ensures that critical operations like trade execution and settlement are handled accurately and consistently.

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