How should I practice Blocking Connection Pool Operation Trace?

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Blocking Connection Pool Operation Trace

Google● HardINTERNONSITE INTERVIEW

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Problem statement

Simulate the observable behavior of a blocking database connection pool over a finite operation trace.

The pool is initialized with maxSize independent connections numbered 0 through maxSize - 1. All connections begin idle in increasing-ID FIFO order. Each row operations[i] = [type, value] is processed in input order:

type = 0 is GET for the unique request ID value. If an idle connection exists, remove the FIFO head, mark it active, and emit [i, 0, value, connectionId]. Otherwise the call blocks in a FIFO waiter queue and emits nothing until a later valid release transfers a connection to it.
type = 1 is RELEASE for connection ID value. Emit [i, 1, value, success], where success is 1 only when that connection is currently active. A failed or repeated release changes nothing. After a successful release, transfer the connection directly to the oldest blocked GET, if one exists, and then emit that request's assignment event. Otherwise append the connection to the idle FIFO queue.
type = 2 is ACTIVE_COUNT; ignore value and emit [i, 2, activeCount, 0].

Events are returned in emission order. A GET still blocked after the final operation never completes and produces no event. Return the complete event list as an int[][].

Interview extensions

Describe a real thread-safe implementation using a semaphore, a blocking queue, and ownership tracking.
Explain how to prevent or recover leaked connections with bounded leases, health checks, and observability.

Function

simulateConnectionPool(maxSize: int, operations: int[][]) → int[][]

Examples

Example 1

maxSize = 2operations = [[0,101],[0,102],[2,0],[0,103],[1,0],[2,0],[1,0],[1,1],[2,0]]return = [[0,0,101,0],[1,0,102,1],[2,2,2,0],[4,1,0,1],[3,0,103,0],[5,2,2,0],[6,1,0,1],[7,1,1,1],[8,2,0,0]]

Requests 101 and 102 receive both connections. Request 103 blocks. Releasing connection 0 emits the release result and then completes request 103 with that same connection.

Example 2

maxSize = 1operations = [[0,7],[1,0],[1,0],[2,0]]return = [[0,0,7,0],[1,1,0,1],[2,1,0,0],[3,2,0,0]]

The first release succeeds. The repeated release is rejected and does not change the active count.

Example 3

maxSize = 1operations = [[0,10],[0,20],[0,30],[1,0],[1,0],[2,0]]return = [[0,0,10,0],[3,1,0,1],[1,0,20,0],[4,1,0,1],[2,0,30,0],[5,2,1,0]]

Requests 20 and 30 wait in FIFO order. Each successful release transfers connection 0 directly to the oldest waiter.

Example 4

maxSize = 1operations = [[0,1],[0,2],[2,0]]return = [[0,0,1,0],[2,2,1,0]]

Request 2 remains blocked when the trace ends, so it has no completion event.

Constraints

1 <= maxSize <= 100000
1 <= operations.length <= 200000
Each operations[i] contains exactly two integers.
0 <= type <= 2
Every GET request ID is positive and unique.
A RELEASE value may be any signed 32-bit integer; it succeeds only for a currently active connection ID.
The number of emitted events fits in memory.