System requirements

Functional:

1. Lock Acquisition: The system should allow clients to acquire locks on shared resources.
2. Lock Release: Clients must be able to release locks they have acquired.
3. Lock Renewal: The service should support renewing locks to prevent expiration while still in use.
4. Fairness: The system should guarantee fairness (e.g., FIFO order) in lock acquisition.
5. Deadlock Prevention: The system should have mechanisms to prevent deadlocks.
6. Timeouts: Clients should be able to specify a timeout for lock acquisition attempts.
7. Monitoring and Alerts: Ability to monitor locks and alert when issues occur.

Non-Functional:

1. Availability: The system should be highly available, with minimal downtime.
2. Scalability: It should be scalable to handle increasing numbers of clients and resources.
3. Latency: The locking operations (acquire and release) should have low latency.
4. Consistency: The lock state must be consistent across the distributed system.
5. Performance: The system must handle a specific number of requests per second (e.g., 10,000 requests per second).
6. Security: Ensure secure access to the locking mechanism, preventing unauthorized use.
7. Fault Tolerance: The system should be resilient to failures in nodes.

Considerations for Capacity Estimation:

1. Number of Clients: Estimate the number of clients that will attempt to acquire locks. For example, let's say we expect around 5,000 clients.
2. Lock Acquisition Rate: Determine how often clients will acquire locks. For example, if each client attempts to acquire a lock every second, that results in 5,000 lock acquisition requests per second.
3. Lock Lifetime: Estimate how long the locks are generally held. If a lock is held for an average of 2 seconds, this impacts how many locks need to be managed concurrently.
4. System Throughput: You should aim for a throughput that can handle peak loads. For example, if you want to support a peak of 10,000 requests per second, you'll need adequate resources to manage that capacity.
5. Replication Factor: If you're designing for high availability, consider a replication factor (e.g., serving across three nodes) which can influence your system design and capacity.

Example Calculation:

• Clients: 5,000 clients
• Lock Acquisition Rate: 5,000 attempts/second
• Lock Lifetime: 2 seconds
• Peak Load: 10,000 attempts/second
• Replication Factor: 3 nodes
• The system should effectively handle:
• Concurrent Locks: ( 5,000 \text{ clients} \times 2 \text{ seconds} = 10,000 \text{ concurrent locks} ).
• Request Load: With a peak of 10,000 requests per second, this indicates how many locks the system should be ready to manage concurrently, likely necessitating a scaling strategy.

API design

Core APIs

1. AcquireLock (resource, timeout=None, options=None):
   1. Acquires a lock on the specified resource.
   2. timeout (optional): Specifies the maximum time to wait for the lock.
   3. options (optional): Additional options, such as lease duration, priority, or specific lock type (e.g., exclusive, shared).
2. ReleaseLock (resource):
   1. Releases the lock on the specified resource.
3. RenewLock (resource):
   1. Renews the lease on the existing lock on the resource, preventing it from expiring.

Additional APIs

1. CheckLockStatus(resource):
   1. Checks the status of the lock on the specified resource (e.g., acquired, available, expired).
2. ListLocks(client_id=None):
   1. Lists all locks held by the specified client_id or all locks in the system.
3. SetLockOptions(resource, options):
   1. Updates the options for the existing lock on the resource.

Implementing an Asynchronous Mechanism for Lock Acquisition

• We introduce an optional asynchronous mechanism for clients to request locks without waiting for immediate responses. This can reduce contention during heavy loads.
• Example Header: Clients can set Prefer:async in the request header for POST call for acquiring locks
• Response can then include a 202 Accepted status while the actual acquisition is processed in the background.

Database design

For the above entities, we will use the following database

1. Locks, Clients:

• Database Choice: PostgreSQL (SQL)
• Reasoning: Given the need for strong consistency and transactional integrity, PostgreSQL is suitable for managing lock metadata as it handles concurrent transactions effectively and ensures ACID properties. This is crucial for avoiding issues such as double locking or stale lock states.

2. Lock Requests

• Database Choice: MongoDB (No-SQL)
• Reasoning: MongoDB is ideal for lock requests due to its flexibility in storing varying data structures. Lock request data can vary in size and format (e.g., timestamps, results, client IDs), and MongoDB's document-oriented model allows for efficient querying and indexing of these requests.

Sharding, partitioning, and scaling are important considerations when designing a distributed locking system, especially when aiming for high availability and performance. Let’s break down each concept:

Sharding

• For the Locks table, sharding could be based on the resource_name. All locks associated with a particular resource could be stored in a specific shard, allowing queries related to that resource to be faster.

Partitioning

• Using partitioning within PostgreSQL, you could create partitions for the Locks table based on acquired_at timestamps. For instance, locks acquired in the last month could go into one partition, and older locks could go into others.

Scaling

In the case of your distributed locking system:

• Vertical Scaling: If you find that the PostgreSQL instance handling locks and clients is reaching resource limits, you might upgrade to a more powerful server (faster CPU, more RAM).
• Horizontal Scaling: Use a replica set for PostgreSQL for read scaling, where read requests can be shared among read replicas.

High-level design

A high-level design for a distributed locking system involves outlining the key components, their interactions, and the overall architecture. Here’s how we can structure the design:

Key Components

1. Client Applications: Clients requesting locks on shared resources are the points of interaction with the locking system.
2. Lock Management Service: Core service that handles all lock-related operations, including acquisition, release, renewal, and monitoring for deadlocks.
3. Databases: Different databases for different entities:
   1. PostgreSQL for Locks and Clients
   2. MongoDB for Lock_Requests
4. Load Balancer: Distributes incoming requests among multiple instances of the Lock Management Service to ensure high availability and efficient use of resources.
5. Monitoring and Alerts: A component that tracks the health and performance of the system, sending alerts for operations like lock timeouts or unusual request spikes.
6. Caching Layer: In in-memory cache (like Redis) that can store frequently accessed lock states to reduce latency for lock acquisition requests.

Request flows

Let's create a sequence diagram to illustrate the scenario where User 1 and User 2 attempt to acquire a lock on the same resource simultaneously. This will show the interactions between the clients, the Lock Management Service, and the database.

Sequence Diagram Description

In this sequence:

1. Both users send a lock acquisition request to the Load Balancer.
2. The Load Balancer forwards their requests to the Lock Management Service.
3. The Lock Management Service checks the current lock status in the PostgreSQL Lock Database.
4. If the lock is available, it grants it to one of the users, updates the database, and issues the lock to that user.
5. The other user receives a notification that the lock is unavailable.

Here’s the sequence diagram representing this interaction:

Explanation of the Diagram:

1. User1 and User2: Both users initiate requests to acquire a lock on the same resource, which arrive at the Load Balancer.
2. Load Balancer: It directs each request to the Lock Management Service.
3. Lock Management Service: It queries the Lock Database to check the lock's status and processes the requests sequentially or concurrently, depending on the implementation.
4. Lock Status Check: The Lock Management Service checks whether the lock is currently available.
5. Responses: User1 is granted the lock if available; User2 is notified that the lock cannot currently be acquired.

Detailed component design

Let's dive deeper into the detailed design of specific components within the distributed locking system. We'll look closer at the following components:

1. Lock Management Service
2. Load Balancer
3. Databases (PostgreSQL and MongoDB)
4. Caching Layer (Redis)

Lock Management Service

Responsibilities:

• Handle lock acquisition, release, and renewal requests.
• Monitor for potential deadlocks.
• Ensure fairness in lock acquisition (e.g., FIFO order).
• Interact with databases to store and retrieve lock and client information.

Key Functions:

• acquire_lock : Checks lock availability, acquires it if available, updates the database, and caches the lock state.
• release_lock : Validates the request by the client and releases the lock, updating the database.
• renew_lock : Extends the expiration time of the lock for the given resource.
• check_for_deadlocks: Monitors active locks and clients to detect and resolve any possible deadlocks.

Load Balancer

Responsibilities:

• Distribute incoming requests to multiple instances of the Lock Management Service to ensure no single instance is overwhelmed.
• Provide failover capabilities to reroute traffic in case an instance goes down.

Key Features:

• Session Persistence: Ensure users remain connected to the same service instance for related requests.
• Health Monitoring: Regularly check the health of instances and route traffic only to healthy instances.
• Scalability: Allow the addition of new service instances during traffic spikes.

Databases

a. PostgreSQL for Locks and Clients

• Schema Definition:
  • LOCKS Table:
    • id (Primary Key)
    • resource_name
    • owner_id
    • acquired_at
    • expiration_at
    • status
  • CLIENTS Table:
    • id (Primary Key)
    • name
    • created_at

Transactions: Use transactional support for lock acquisition and release operations to ensure correctness and consistency throughout the processes. Utilize PostgreSQL’s ACID properties to prevent anomalies.

b. MongoDB for Lock Requests

• Schema Definition:
  • LOCK_REQUESTS Collection:
  • id (Primary Key)
  • client_id (Foreign Key)
  • lock_id (Foreign Key)
  • requested_at
  • result (e.g., "granted", "denied")

Indexes: Create indexes on client_id and lock_id to facilitate efficient querying and tracking of request statuses.

Caching Layer (Redis)

Responsibilities:

Store the current state of active locks for fast access and minimal latency.

Cache lock acquisition results temporarily to improve performance for repeat requests.

Key Functions:

cache_lock_state : Store the lock state in Redis, where lock_state indicates whether it's "locked" or "available".

get_lock_state: Retrieve the current state of the lock.

expire_lock_state: Automatically expire the lock state in the cache after a specified timeout duration to prevent stale data.

Trade offs/Tech choices

When evaluating the distributed locking system design, it's crucial to recognize the trade-offs that arise in balancing performance, consistency, and complexity. Here are three key trade-offs relevant to this design:

1. Consistency vs. Availability (CAP Theorem)

• Description: The system must adhere to the principles laid out in the CAP theorem — it can be consistent (CP) or available (AP), but not both during network partitions.
• Trade-off: By prioritizing strong consistency through PostgreSQL for locks and clients, the system may experience potential availability issues during high loads or network partitions since transactions can block client requests for locks while maintaining ACID properties. Conversely, if we emphasized availability (using a NoSQL database for all data), we might introduce scenarios where clients receive stale or inconsistent data about locks.

2. Complexity vs. Flexibility

• Description: The design leverages multiple types of databases (SQL and NoSQL) to meet diverse requirements.
• Trade-off: While using PostgreSQL for consistency and MongoDB for flexibility helps cater to various data types, it introduces architectural complexity. Developers must manage multiple database connections, handle different query languages, and ensure data synchronization between databases. This complexity may inhibit rapid development and increase the overall maintenance burden.

3. Performance vs. Latency

• Description: The system employs a caching layer (Redis) to reduce latency for lock state access.
• Trade-off: Using Redis improves performance for frequent read operations on lock states; however, it introduces the complexity of cache invalidation. There is a potential lag in consistency between the cached data and the actual database state. If locks are modified directly in the database without updating the cache, clients may receive incorrect lock states leading to incorrect behavior (e.g., multiple clients acquiring the same lock).

Failure scenarios/bottlenecks

Understanding failure scenarios and potential bottlenecks is crucial for improving the resilience and reliability of the distributed locking system. Here are two key failure scenarios:

Database Failures

Scenario Description:

• Imagine a situation where the PostgreSQL database (either the one managing locks or clients) becomes unavailable due to factors such as hardware failure, software bugs, or network issues.

Impact:

• If the Lock Management Service cannot access the database, it will be unable to check the status of locks or update lock information. Consequently, clients may experience timeouts or errors when trying to acquire locks, leading to increased latency, user frustration, and possible application downtime.
• Moreover, if the Lock Management Service still allows lock acquisition despite the inability to update the database, this could lead to inconsistencies and data corruption, where multiple clients might believe they hold the same lock.

Mitigation Strategies:

• Implement database redundancy and failover mechanisms, such as active-passive or active-active replication, to ensure high availability and automatic switching during failures.
• Regularly back up the databases and employ strategies for graceful degradation, allowing for read operations on cached states, if immediate correctness is not critical.

Network Partitioning

Scenario Description:

• In a distributed system, there can be scenarios where network partitions occur due to issues like network hardware failures or configuration errors, isolating certain nodes from the rest of the network.

Impact:

• During such network partitions, different parts of the distributed locking system may become unable to communicate with each other. Some clients might successfully acquire locks through one segment of the system while others may be denied access through another segment.
• Depending on the locking algorithm and implementation, this can lead to scenarios where “split-brain” situations arise, where two different parts of the system believe they own the same lock, causing inconsistency and potential deadlocks once the network is restored and the partitions merge.

Mitigation Strategies:

• Employ consensus algorithms (like Raft or Paxos) to ensure that any acquired locks or changes to lock states require majority consensus from nodes, thus preventing split-brain situations.
• Use partition detection mechanisms and design fallback behaviors, such as temporarily denying lock acquisition requests during suspected partitions, until a clear network state is re-established.

Future improvements

Future improvements to the distributed locking system can focus on enhancing performance, scalability, robustness, and ease of maintenance. Here are several potential improvements:

1. Dynamic Scaling

• Description: Implement auto-scaling mechanisms for the Lock Management Service based on traffic patterns and load.
• Benefit: This would enable the system to handle sudden spikes in requests efficiently without manual intervention, ensuring consistent performance during peak times.

2. Improved Locking Algorithms

• Description: Explore advanced locking algorithms such as ticket locks or quorum-based locking to enhance the lock acquisition process.
• Benefit: These algorithms can reduce conflicts and improve the overall throughput of lock acquisitions, especially in highly concurrent environments.

3. Enhanced Monitoring and Observability

• Description: Implement comprehensive monitoring solutions (e.g., distributed tracing and logging) to track performance, system health, and the behavior of locks in real time.
• Benefit: Through better observability, troubleshooting and optimization efforts can be more effectively prioritized, leading to improved reliability and quicker resolution of issues.

4. Optimizing Cache Usage

• Description: Introduce smarter cache invalidation strategies and data expiration policies to keep the Redis cache synchronized with the primary databases.
• Benefit: Improved cache management can reduce latency and ensure that clients are always interacting with the most up-to-date lock states, thus enhancing consistency.