Why Does My Flask GET Endpoint Return an Empty Queue Updated in a Separate Thread?

Below is an explanation addressing your specific issues:

---

1. Thread Safety of Python Queues

A Python Queue from the built‑in library is “thread‑safe” in that its put() and get() operations are internally synchronized. This means that you can safely call them from different threads without corrupting the data structure. However, thread safety does not automatically guarantee that the consumer sees every element as you might expect in your application logic.

---

2. Reading the Underlying Queue

In your GET route, you use:

PYTHONCopied
list(measurements.queue)

This accesses the internal deque of the Queue. While it may work most of the time, this isn’t part of the public API and is not protected by the same locks as put() and get(). In a multithreaded scenario you might see “empty” results because:

• Items could be removed by another thread before your GET request inspects them.
• The moment you read the underlying deque, another thread might modify it.

---

3. The Measurements Being Removed Too Early

In your TCP handler you call:

PYTHONCopied
measurements.get()
measurements.put({'value': value, 'timestamp': timestamp})

A couple of issues here:

• Removal: Every time you get a new measurement, you remove one element with measurements.get() before adding the new measurement. If your intention is to merely add a new measurement or maintain a circular buffer in a fixed-length queue, this removal might be discarding measurements that have not yet been seen by the GET endpoint.
• Undefined Variable: You are attempting to use value in your put() call, but based on your earlier code you decoded the incoming data into the variable dist. The undefined name value will cause an error. You should replace value with dist.

---

4. Recommendations to Fix Your Code

A. Use the Correct Variable Name

Replace:

PYTHONCopied
measurements.put({'value': value, 'timestamp': timestamp})

with:

PYTHONCopied
measurements.put({'value': dist, 'timestamp': timestamp})

B. Rethink the Removal Strategy

Only call measurements.get() when you need to remove an old measurement (for example, when the queue is full) rather than always removing an item on every TCP update. For example, you might do:

PYTHONCopied
if measurements.full():
    measurements.get()  # Remove the oldest measurement if at capacity.
measurements.put({'value': dist, 'timestamp': timestamp})

This way, you’ll retain measurements until you actually need to free up space.

C. Avoid Relying on measurements.queue in GET Route

Instead of directly accessing the internal deque, consider maintaining a secondary data structure (or use the Queue methods) if you need to inspect the measurements without removing them. If you are sure you want to inspect the current content, a careful approach might be:

PYTHONCopied
from threading import Lock

queue_lock = Lock()  # Create a dedicated lock for controlled access

@app.route('/measurements')
def measurements_api():
    current_time = time.time()
    with queue_lock:
        # Copy the relevant items to avoid simultaneous modifications.
        all_measurements = list(measurements.queue)
    recent_measurements = [m for m in all_measurements if current_time - m['timestamp'] <= 86400]
    return {'measurements': recent_measurements}

And then when updating the queue, wrap the operation in the same lock if needed. In many cases, simply calling put() and get() is acceptable because they are thread‑safe—but if you need to do non‑atomic operations (like iterating over the queue), extra care is warranted.

---

Summary

• The Queue is thread‑safe in that its put() and get() methods won’t corrupt its data when used from multiple threads.
• Accessing the internal .queue attribute is not fully safe under all circumstances, which might cause problems in your GET endpoint.
• Calling measurements.get() every time removes items, which may be why your GET route sees an empty queue.
• Finally, your code uses an undefined variable "value" instead of "dist". Correcting that will fix a runtime error.

Adjust your code as recommended above, and you should be able to see measurements as expected from your Flask GET endpoint.