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The Case of the Mysterious Memory Consumption

This is the story of a vexing bug I solved at a previous job which taught me a valuable debugging lesson. The application in question was an HTTP API whose primary function was to act as a proxy for data stored in S3. It used Flask and Boto to achieve this. The core of it was something like this:

@app.route("/data/<id>")  # `app` is a Flask application object
def download(id):
    # authenticate user,
    # figure out what bucket data is stored in, bookkeeping, etc.
    conn = boto.connect_s3(
        # s3 connection details
    buck = conn.get_bucket(bucket_where_id_is_stored)
    headers = {}
    if request.headers.get("Range"):
        headers["Range"] = request.headers["Range"]
        code = 206
        code = 200
    obj = buck.get_key(id, headers=headers)
    return Response(obj, code)

This of course elides huge amounts of detail but it gets the big picture across. We get a request to download some data, we connect to S3 and get the correct Boto Key, and then return a streaming Flask Response that iterates over the contents of that key. One peculiarity of this system compared to typical uses of S3 was that the objects being streamed were often quite large (think tens to hundreds of gigabytes). An additional detail relevant to the story is that, as illustrated above, the API supported using the HTTP Range header to stream only a portion of a requested object.

At some point we noticed memory usage on the boxes that ran this code sky-rocketing, to the point that they would frequently exhaust all available memory, crash, and need to be restarted.

The only thing that had changed recently was that we had started testing a command line download client written against this API, which used Range requests to download small chunks of a single object in parallel. We figured that had to have something to do with it, and indeed we initially couldn’t reproduce the bug except via our parallel Range request-ing client. I spent a long while on a wild goose chase through the Range handling code in the API before we got our first real clue.

By judicious insertion of calls to pdb (my go-to Python debugging technique), we were eventually able to pinpoint a single request made by the download client that triggered the bug in the API. When starting to download an object, the client would, before making any Range requests, first connect to the server and make an unadorned GET in order to gather some information (e.g. file name, size) from the response headers. It would then close the connection before streaming any of the response body (this was effectively a HEAD request). It was this preliminary request that triggered the bug—Range support had been a red herring.

Moreover, the bug was triggered only after the request had completed. If we delayed the client’s closing of the connection by inserting a pdb call, the memory usage on the server would remain constant until we broke out of pdb and allowed the connection to be closed. This was very frustrating because it seemed to imply that our code couldn’t be at fault1, and it left us with no obvious place to insert pdb calls in the API code to debug further.

When debuggers fail me, I usually reach for strace2, and this time was no different. I straced the API server process then made requests to it, disconnecting each request before it completed. This revealed lots of recvfrom calls to a single file descriptor happening after the client closed its connection. I used lsof to examine the server process’s open file descriptors and sure enough the one it was receiving data from was its connection to S3. It appeared that the API was continuing to read data from S3 into memory (to the point of exhausting servers with multiple gigabytes of memory) after the requests completed.

To review what we know so far:

  1. We are streaming data from S3 via an intervening webserver that uses Flask and Boto.
  2. When a client disconnects from the server prematurely (i.e. without actually reading the whole request body), the server continues to read (excessive amounts of) data from S3 into memory.

Before we move on to reveal what was actually going on, I’ll give you a chance to try and figure it out for yourself! I’ve put together a repo that reproduces the problem that you can try out on your own machine if you like.

I’ve also made a short, poorly-produced demonstration video:

One shell in the video is running top tracking the demo Flask webserver, which streams data from a local moto S3 server. In the other shell I make some requests using curl to download a 1GB test object so you can observe their effects on memory usage by the server. Note that downloading the entire test object does not impact the server’s memory usage, but downloading only the first 10 bytes and then closing the connection (via piping the output of curl to head -c 10) causes memory usage to balloon to approximately 1GB.

If you want to try to figure it out for yourself, go ahead and download that demo repo and get debugging before reading on (spoilers ahead!).

At this point it looked likely the the problem was in one of the libraries we were using. It took me a long time to accept this since Flask and Boto are both very widely deployed, battle-tested tools, but eventually I started digging through the relevant parts of their source code.

We were using the Flask Response object to wrap a Boto Key, so I figured looking for anything suspicious in the code for those two classes was the way to go.

The Flask Response class is a straightforward subclass wrapper of the Werkzeug3 Response class. There’s quite a bit in that file, but eventually I honed in on the .close method, reasoning that this might be called when the client disconnects:

def close(self):
    """Close the wrapped response if possible.  You can also use the object
    in a with statement which will automatically close it.
    .. versionadded:: 0.9
       Can now be used in a with statement.
    if hasattr(self.response, 'close'):
    for func in self._on_close:

What stood out to me here is self.response.close(): if self.response has a .close method, the Response will call it when it gets .closed. Here self.response is simply the first argument to the Response constructor4: the iterator whose data we are going to stream to the client. In our case this is a Boto Key, so let’s look at that code next, focusing on the .close method:

def close(self, fast=False):
    Close this key.
    :type fast: bool
    :param fast: True if you want the connection to be closed without first
    reading the content. This should only be used in cases where subsequent
    calls don't need to return the content from the open HTTP connection.
    Note: As explained at,
    callers must read the whole response before sending a new request to the
    server. Calling Key.close(fast=True) and making a subsequent request to
    the server will work because boto will get an httplib exception and
    close/reopen the connection.
    if self.resp and not fast:
    self.resp = None
    self.mode = None
    self.closed = True

After reading this, understanding finally dawned on me. self.resp is the httplib response object that Boto uses to read the Keys data from S3. When .close is called on the Key, will also be called by default. This is analogous to calling .read on a Python file object: all of the unread data will be pulled into memory. Calling .close on the Flask Response (which presumably happens when the client disconnects) causes this to happen, triggering a memory explosion when the Key is a large object. I’m not sure why this is the default behavior, altough the docstring suggests that it’s so the key can be reopened later and the data read without making another HTTP request.

This theory also explains why a client disconnecting without reading the entire object first is what triggers the bug. If all the data from self.resp has already been read (in order to write it out to the client) when is called, nothing bad will happen. If, however, only a small amount of data has been read from a large key, the results of will be disastrous.

This theory predicts that we should be able to make the bug disappear by monkey-patching boto.Key so that its .close method always behaves as though fast=True was passed:

def fast_close(self, fast=True):
    self.resp = None
    self.mode = None
    self.closed = True

from boto.s3.key import Key
Key.close = fast_close

Indeed this makes the problem disappear5—you can confirm this for yourself by adding the above code to in the demo repo.

So what did we learn from all this? There are several really interesting things about this bug.

The first is that there’s no one party on which we can pin the blame. The problem arises from reasonable assumptions on everyone’s part. When I wrote the API code, I assumed could pass any old iterator to the Response constructor, as the docs suggest. Flask/Werkzeug assume that calling .close on the response iterator is (a) a good idea and (b) not too “expensive” in any sense of the word. In most cases these are reasonable assumptions. Consider a file object: calling .close on it is a good idea, since it avoids a resource leak, and is not very costly. This assumption breaks down in the case of a Boto Key, however, since Boto assumes that the caller of a Keys close method has read the documentation, which clearly states that calling close results in the rest of the Keys data being read (a potentially very expensive operation).

All of these implicit assumptions being violated illustrate one of the weaknesses of the “duck-typing” style of interface-driven programming popular in Python. A Boto Key quacks like a duck”, or in this case “closes like a file object”, but the fact that it can quack/close tells us nothing about what it really means or costs to perform that operation. More sophisticated approaches (Java/Go interfaces, typeclasses, etc.) don’t really solve this problem either, but at least they give you a place to document constraints on the behavior of implementations so it’s clear when an implementation is out of line. In our case, if Python had a place where the “file-like object” interface was declared and it was documented there that the close method should have O(1) space complexity, then the boto.Key implementation would very clearly be at fault. As it stands though, “file-like object” is just a loose set of conventions that that community agrees upon (or in this case, doesn’t).

It also occurs to me that it’s possible this bug exists in codebases whose maintainers aren’t even aware of it. Since S3 is primarily used for “small” objects that fit in memory, this bug could easily go completely unnoticed, wasting memory and CPU time whenever a request is close prematurely. The only reason it was so obvious here is due to the large size of the Keys being streamed.

Lastly and most importantly, this bug illustrates that even if you use proven, high quality libraries, you will eventually run into problems that require you to dive into and debug their code. This doesn’t mean that you should renounce libraries or only agree to use them after vetting every line of them—after all, modern software systems are largely about standing on the shoulders of giants. In most projects you’ll only write a tiny fraction of the lines of code you use6. It does mean, however, that you should own your chosen libraries as a part of your complete system, and be willing analyze, debug, and fix library code just as you would your own when it causes problems. So choose libraries with care and don’t be afraid to dive into their code when the need arises—you might just learn something.

  1. In some sense, this turned out to be true—read on. 

  2. strace is also a great debugging tool, you can read all about it on Julia’s blog

  3. Werkzeug is the underlying WSGI library that Flask uses under the hood. 

  4. This is set in the BaseResponse constructor

  5. A less hacky solution would involve wrapping Key in a class with a smarter .close method. 

  6. Think about a web application written using Django, which runs in Python, is deployed using Apache, on top of Linux, etc.