Method Signatures

• Constructor: ConnectionPool(int capacity)
  • capacity is always between 1 and 5000.
  
  
• Acquire: int acquireConnection(String requestId)
  • requestId is always non-empty and unique. It contains only characters from [a-z] and [0-9].
  • acquireConnection will never be called with a duplicate requestId.
  • If a connection is available, assign it to the request and return its connectionId.
  • If multiple connections are available, assign the connection with the lowest index.
  • If all connections are blocked, enqueue the request and return -1. The request will be assigned a connection later when one becomes free.
  
  
• Release: boolean releaseConnection(String requestId)
  • Return true if requestId is valid and currently holds a connection, and that connection is released.
  • Otherwise, return false.
  • As soon as a connection is released, it is immediately assigned to the oldest request waiting in the internal queue (if any).
  
  
• Expire: void expireRequest(String requestId)
  • If requestId is present in the internal request queue, remove it. No connection will ever be assigned to it.
  • If the request is not in the queue or is currently holding a connection, do nothing.
  
  
• Query: List<String> getRequestsWithConnection()
  • Return a list of all requests that are currently holding a connection, along with the connection id.
  • Each item must be formatted as: "requestId-connectionId" (e.g. "request12-4").
  • The list must be sorted in lexicographically ascending order by requestId.

Examples

Example 1: Acquire when connections are free

ConnectionPool pool = new ConnectionPool(3);   // connections: 0,1,2

pool.acquireConnection("a1");  // returns 0
pool.acquireConnection("a2");  // returns 1
pool.acquireConnection("a3");  // returns 2

pool.getRequestsWithConnection();  // ["a1-0", "a2-1", "a3-2"]
        

Example 2: Queue requests when pool is full, then assign on release (FIFO)

ConnectionPool pool = new ConnectionPool(2);   // connections: 0,1

pool.acquireConnection("a1");  // returns 0
pool.acquireConnection("a2");  // returns 1

pool.acquireConnection("b1");  // returns -1 (queued)
pool.acquireConnection("b2");  // returns -1 (queued)

pool.releaseConnection("a1");  // returns true
// connection 0 is immediately assigned to oldest queued request "b1"

pool.getRequestsWithConnection();  // ["a2-1", "b1-0"]
        

Example 3: Expire a queued request

ConnectionPool pool = new ConnectionPool(1);   // connection: 0

pool.acquireConnection("a1");  // returns 0
pool.acquireConnection("b1");  // returns -1 (queued)
pool.acquireConnection("b2");  // returns -1 (queued)

pool.expireRequest("b1");      // b1 removed from queue

pool.releaseConnection("a1");  // returns true
// connection 0 is assigned to oldest remaining queued request "b2"

pool.getRequestsWithConnection();  // ["b2-0"]
        

Example 4: Invalid release

ConnectionPool pool = new ConnectionPool(1);

pool.releaseConnection("x1");       // false (unknown request)
pool.acquireConnection("a1");       // 0
pool.releaseConnection("a1");       // true
pool.releaseConnection("a1");       // false (already released, not holding now)