Distributed System Implementation

Oct 13, 2025 by wbrum.org 23 views

As a service provider, I need a service to persist the last known count, so that users don't lose track of their counts after the service is restarted.

Problem Statement

When a service is restarted, any data that was stored in memory is lost. This can be a problem for services that need to keep track of a counter, such as a user's score in a game or a counter for a task that needs to be completed. If the service is restarted, the counter will be reset to zero, and the user will lose their progress.

Solution Overview

To solve this problem, we need to find a way to persist the counter across restarts. This means that we need to store the counter in a way that it will not be lost when the service is restarted. There are several ways to do this, including:

Using a database to store the counter
Using a file to store the counter
Using a cache to store the counter
Using a distributed system to store the counter

Details and Assumptions

The service is a single instance, and it will be restarted periodically.
The service needs to persist the counter across restarts.
The service does not need to persist any other data.
The service will be running on a single machine.

Acceptance Criteria

The service will persist the counter across restarts.
The service will not lose any data when it is restarted.
The service will be able to retrieve the last known count when it is restarted.

Technical Requirements

The service will use a database to store the counter.
The service will use a file to store the counter.
The service will use a cache to store the counter.
The service will use a distributed system to store the counter.

Design Considerations

The service will need to be designed to handle restarts.
The service will need to be designed to handle failures.
The service will need to be designed to handle concurrent access.

Implementation

To implement the solution, we will use a database to store the counter. We will use a file to store the counter. We will use a cache to store the counter. We will use a distributed system to store the counter.

Database Implementation

To implement the database solution, we will use a relational database management system (RDBMS) such as MySQL or PostgreSQL. We will create a table to store the counter, and we will use a primary key to identify the counter.

CREATE TABLE counter (
  id INT PRIMARY KEY,
  value INT
);

We will then use a stored procedure to update the counter when it is incremented.

CREATE PROCEDURE increment_counter()
BEGIN
  UPDATE counter SET value = value + 1;
END;

We will then use a stored procedure to retrieve the last known count.

CREATE PROCEDURE get_last_known_count()
BEGIN
  SELECT value FROM counter;
END;

File Implementation

To implement the file solution, we will use a file to store the counter. We will use a file system such as the file system on a Unix-like system or the file system on a Windows system.

#include <stdio.h>

int main() {
  FILE *file = fopen("counter.txt", "r+");
  if (file == NULL) {
    file = fopen("counter.txt", "w+");
  }
  int value = 0;
  fscanf(file, "%d", &value);
  value++;
  fprintf(file, "%d", value);
  fclose(file);
  return 0;
}

Cache Implementation

To implement the cache solution, we will use a cache such as Redis or Memcached. We will use a key to store the counter, and we will use a TTL (time to live) to specify how long the counter will be stored in the cache.

#include <redis/redis.h>

int main() {
  redisContext *c = redisConnect("localhost", 6379);
  if (c == NULL) {
    return 1;
  }
  redisCommand(c, "SET counter 0 EX 3600");
  redisCommand(c, "INCR counter");
  redisCommand(c, "GET counter");
  redisFree(c);
  return 0;
}

Distributed System Implementation

To implement the distributed system solution, we will use a distributed system such as Apache ZooKeeper or etcd. We will use a key to store the counter, and we will use a TTL to specify how long the counter will be stored in the distributed system.

#include <zookeeper/zookeeper.h>

int main() {
  zookeeper *zk = zookeeper_init("localhost:2181", NULL, 10000, 10000, 0, NULL);
  if (zk == NULL) {
    return 1;
  }
  char *value = zoo_get(zk, "/counter", 0, NULL);
  if (value == NULL) {
    value = "0";
  }
  int count = atoi(value);
  count++;
  zoo_set(zk, "/counter", &count, strlen(value), 0);
  zoo_close(zk);
  return 0;
}

Testing

To test the solution, we will use a testing framework such as JUnit or PyUnit. We will write test cases to test the solution, and we will use a mocking framework such as Mockito or PyMock to mock out any dependencies.

import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class CounterTest {
  @Test
  public void testIncrementCounter() {
    Counter counter = new Counter();
    counter.increment();
    assertEquals(1, counter.getValue());
  }
}

Conclusion

Q: What is the problem of persisting a counter across restarts?

A: The problem of persisting a counter across restarts is that when a service is restarted, any data that was stored in memory is lost. This can be a problem for services that need to keep track of a counter, such as a user's score in a game or a counter for a task that needs to be completed.

Q: Why is it important to persist a counter across restarts?

A: It is important to persist a counter across restarts because it ensures that the counter is not lost when the service is restarted. This is important for services that need to keep track of a counter, as it allows the service to continue where it left off.

Q: What are some solutions to the problem of persisting a counter across restarts?

A: Some solutions to the problem of persisting a counter across restarts include:

Using a database to store the counter
Using a file to store the counter
Using a cache to store the counter
Using a distributed system to store the counter

Q: What are the advantages and disadvantages of using a database to store a counter?

A: The advantages of using a database to store a counter include:

Robustness: Databases are designed to handle failures and restarts, making them a good choice for storing counters.
Scalability: Databases can handle large amounts of data and can be scaled up or down as needed.
Security: Databases provide a high level of security, making them a good choice for storing sensitive data.

The disadvantages of using a database to store a counter include:

Complexity: Databases can be complex to set up and manage.
Cost: Databases can be expensive to purchase and maintain.

Q: What are the advantages and disadvantages of using a file to store a counter?

A: The advantages of using a file to store a counter include:

Simplicity: Files are easy to set up and manage.
Cost: Files are free to use.

The disadvantages of using a file to store a counter include:

Robustness: Files are not designed to handle failures and restarts, making them a less reliable choice for storing counters.
Scalability: Files can become large and unwieldy, making them a less scalable choice for storing counters.

Q: What are the advantages and disadvantages of using a cache to store a counter?

A: The advantages of using a cache to store a counter include:

Speed: Caches are designed to provide fast access to data, making them a good choice for storing counters.
Scalability: Caches can be scaled up or down as needed.

The disadvantages of using a cache to store a counter include:

Robustness: Caches are not designed to handle failures and restarts, making them a less reliable choice for storing counters.
Security: Caches do not provide a high level of security, making them a less secure choice for storing sensitive data.

Q: What are the advantages and disadvantages of using a distributed system to store a counter?

A: The advantages of using a distributed system to store a counter include:

Robustness: Distributed systems are designed to handle failures and restarts, making them a good choice for storing counters.
Scalability: Distributed systems can handle large amounts of data and can be scaled up or down as needed.
Security: Distributed systems provide a high level of security, making them a good choice for storing sensitive data.

The disadvantages of using a distributed system to store a counter include:

Complexity: Distributed systems can be complex to set up and manage.
Cost: Distributed systems can be expensive to purchase and maintain.

Q: How do I choose the best solution for persisting a counter across restarts?

A: To choose the best solution for persisting a counter across restarts, you should consider the following factors:

Robustness: Choose a solution that is designed to handle failures and restarts.
Scalability: Choose a solution that can handle large amounts of data and can be scaled up or down as needed.
Security: Choose a solution that provides a high level of security.
Cost: Choose a solution that fits within your budget.
Complexity: Choose a solution that is easy to set up and manage.

By considering these factors, you can choose the best solution for persisting a counter across restarts.