Go HTTP Client Pooling

Efficiently managing HTTP clients is crucial for applications that make frequent outbound HTTP requests. Recreating an http.Client for every request can be resource-intensive due to the overhead of establishing new connections, DNS lookups, and TLS handshakes. This challenge asks you to build a robust HTTP client pool in Go to reuse http.Client instances, thereby improving performance and reducing resource consumption.

Problem Description

You need to implement a package that provides an HTTPClientPool struct. This pool should manage a collection of http.Client instances. When a client is requested from the pool, it should return an available http.Client. If no clients are available, it should create a new one up to a specified maximum capacity. When a client is no longer needed, it should be returned to the pool.

Key Requirements:

• Pool Initialization: The pool should be initialized with a minimum and maximum number of HTTP clients.
• Client Acquisition: A GetClient method should be provided to retrieve an http.Client.
  • If an idle client is available, return it.
  • If no idle clients are available but the pool has not reached its maximum capacity, create and return a new http.Client.
  • If the pool has reached its maximum capacity and no idle clients are available, the GetClient call should block until a client becomes available.
• Client Release: A ReleaseClient method should be provided to return an http.Client back to the pool.
  • The released client should become available for reuse.
  • If the pool is over its minimum capacity, the released client can be discarded.
• Concurrency Safety: The pool must be safe for concurrent access from multiple goroutines.
• Client Configuration: The http.Client instances created by the pool should have configurable Timeout and Transport settings.

Expected Behavior: When GetClient is called:

1. Check if there are idle clients. If yes, return one.
2. If not, and current_clients < max_clients, create a new client, add it to the pool, and return it.
3. If current_clients == max_clients, block until ReleaseClient is called.

When ReleaseClient is called:

1. Add the client back to the pool of idle clients.
2. If the number of idle clients exceeds min_clients, discard an idle client.

Examples

Example 1: Basic Acquisition and Release

// Assume a pool is initialized with min=2, max=5
pool := NewHTTPClientPool(2, 5, &http.Transport{ /* ... */ }, 10*time.Second)

// Goroutine 1
client1, _ := pool.GetClient()
// ... use client1 for a request ...
pool.ReleaseClient(client1)

// Goroutine 2
client2, _ := pool.GetClient()
// ... use client2 for a request ...
pool.ReleaseClient(client2)

// At this point, both client1 and client2 should be available in the pool.

Explanation: Two clients are acquired and then released. They are now available for reuse.

Example 2: Reaching Maximum Capacity and Blocking

// Assume a pool is initialized with min=1, max=2
pool := NewHTTPClientPool(1, 2, &http.Transport{ /* ... */ }, 10*time.Second)

// Acquire first client
client1, _ := pool.GetClient() // Pool now has 1 client

// Acquire second client
client2, _ := pool.GetClient() // Pool now has 2 clients (max capacity reached)

// Goroutine 3 attempts to acquire a client. This call will block.
go func() {
    fmt.Println("Goroutine 3: Attempting to get client...")
    client3, _ := pool.GetClient()
    fmt.Println("Goroutine 3: Acquired client.")
    // ... use client3 ...
    pool.ReleaseClient(client3)
}()

time.Sleep(1 * time.Second) // Give Goroutine 3 time to block

fmt.Println("Main: Releasing client1...")
pool.ReleaseClient(client1) // This should unblock Goroutine 3

time.Sleep(1 * time.Second) // Allow Goroutine 3 to complete

Explanation: When the pool reaches its maximum capacity of 2 clients, subsequent GetClient calls will block. Releasing an existing client frees up a slot, allowing a blocked GetClient call to proceed.

Example 3: Discarding Excess Clients

// Assume a pool is initialized with min=1, max=3
pool := NewHTTPClientPool(1, 3, &http.Transport{ /* ... */ }, 10*time.Second)

// Acquire three clients
client1, _ := pool.GetClient()
client2, _ := pool.GetClient()
client3, _ := pool.GetClient()

// Release client1 (pool now has 2 idle clients: client1, client2)
pool.ReleaseClient(client1)

// Release client2 (pool now has 3 idle clients: client1, client2, client3)
pool.ReleaseClient(client2) // If current idle > min, one might be discarded.

// Release client3 (pool now has 4 idle clients: client1, client2, client3)
pool.ReleaseClient(client3) // This release should cause one client to be discarded to stay at or below max_clients effectively. The pool should now hold at most `max_clients` idle clients.

Explanation: After acquiring and releasing clients, if the number of idle clients exceeds the min_clients threshold, the pool should manage its size by discarding excess clients upon release, ensuring it doesn't grow indefinitely beyond the configured capacity. The implementation should adhere to maintaining at least min_clients if possible, and not exceeding max_clients idle clients.

Constraints

• The min_clients must be greater than or equal to 1.
• The max_clients must be greater than or equal to min_clients.
• The timeout for each http.Client must be configurable and applied to all clients created by the pool.
• The transport configuration for http.Client must be configurable and applied to all clients created by the pool.
• The pool should handle graceful shutdown, ensuring no clients are leaked if the application is terminating.
• Performance: Acquiring and releasing clients should take negligible time in the common case (when clients are available). Blocking GetClient calls should be responsive once a client is released.

Notes

• Consider using Go's built-in concurrency primitives like sync.Mutex, sync.Cond, or channels to manage the pool's state and blocking behavior.
• The http.Transport can be shared among clients if its configuration doesn't change after creation. However, for a pool, it's often better to create a new Transport for each http.Client to avoid potential race conditions if transport settings were to be modified externally, though for this challenge, sharing a pre-configured transport is acceptable if its settings are immutable. The Timeout is part of the http.Client itself, not the Transport.
• Think about how to signal when a client is no longer needed, especially in the context of long-running HTTP requests that might be cancelled. For this challenge, we will assume immediate release after use.
• A Close or Shutdown method on the pool might be necessary to clean up resources and close any active connections held by the clients in the pool.