Async

fsspec supports asynchronous operations on certain implementations. This allows for concurrent calls within bulk operations such as cat (fetch the contents of many files at once) even from normal code, and for the direct use of fsspec in async code without blocking. Async implementations derive from the class fsspec.async.AsyncFileSystem. The class attribute async_impl can be used to test whether an implementation is async of not.

AsyncFileSystem contains async def coroutine versions of the methods of AbstractFileSystem. By convention, these methods are prefixed with “_” to indicate that they are not to called directly in normal code, only when you know what you are doing. In most cases, the code is identical or slightly modified by replacing sync calls with await calls to async functions.

The only async implementation built into fsspec is HTTPFileSystem.

Synchronous API

The methods of AbstractFileSystem are available and can be called from normal code. They call and wait on the corresponding async function. The work is carried out in a separate threads, so if there are many fsspec operations in flight at once, launched from many threads, they will still all be processed on the same IO-dedicated thread.

Most users should not be aware that their code is running async.

Note that the sync functions are wrapped using sync_wrapper, which copies the docstrings from AbstractFileSystem, unless they are explicitly given in the implementation.

Example:

fs = fsspec.filesystem("http")
out = fs.cat([url1, url2, url3])  # fetches data concurrently

Coroutine batching

The various methods which create many coroutines to be passed to the event loop for processing may be batched: submitting a certain number in one go and waiting for them to complete before launching more. This is important to work around local open-file limits (which can be <~100) and not to swamp the heap.

fsspec.asyn._run_coros_in_chunks controls this process, but from the user’s point of view, there are three ways to affect it. In increasing order or precedence:

  • the global variables fsspec.asyn._DEFAULT_BATCH_SIZE and fsspec.asyn._NOFILES_DEFAULT_BATCH_SIZE (for calls involving local files or not, respectively)

  • config keys “gather_batch_size” and “nofiles_gather_batch_size”

  • the batch_size keyword, accepted by the batch methods of an async filesystem.

Using from Async

File system instances can be created with asynchronous=True. This implies that the instantiation is happening within a coroutine, so the various async method can be called directly with await, as is normal in async code.

Note that, because __init__ is a blocking function, any creation of asynchronous resources will be deferred. You will normally need to explicitly await a coroutine to create them. Since garbage collection also happens in blocking code, you may wish to explicitly await resource destructors too. Example:

async def work_coroutine():
    fs = fsspec.filesystem("http", asynchronous=True)
    session = await fs.set_session()  # creates client
    out = await fs._cat([url1, url2, url3])  # fetches data concurrently
    await session.close()  # explicit destructor

asyncio.run(work_coroutine())

Bring your own loop

For the non-asynchronous case, fsspec will normally create an asyncio event loop on a specific thread. However, the calling application may prefer IO processes to run on a loop that is already around and running (in another thread). The loop needs to be asyncio compliant, but does not necessarily need to be an ayncio.events.AbstractEventLoop. Example:

loop = ...  # however a loop was made, running on another thread
fs = fsspec.filesystem("http", loop=loop)
out = fs.cat([url1, url2, url3])  # fetches data concurrently

Implementing new backends

Async file systems should derive from AsyncFileSystem, and implement the async def _* coroutines there. These functions will either have sync versions automatically generated is the name is in the async_methods list, or can be directly created using sync_wrapper.

class MyFileSystem(AsyncFileSystem):

    async def _my_method(self):
        ...

    my_method = sync_wrapper(_my_method)

These functions must not call methods or functions which themselves are synced, but should instead await other coroutines. Calling methods which do not require sync, such as _strip_protocol is fine.

Note that __init__, cannot be async, so it might need to allocate async resources using the sync function, but only if asynchronous=False. If it is True, you probably need to require the caller to await a coroutine that creates those resources. Similarly, any destructor (e.g., __del__) will run from normal code, and possibly after the loop has stopped/closed.

To call sync, you will need to pass the associated event loop, which will be available as the attribute .loop.

fsspec.asyn.AsyncFileSystem(*args, **kwargs)

Async file operations, default implementations

fsspec.asyn.sync(loop, func, *args[, timeout])

Make loop run coroutine until it returns.

fsspec.asyn.sync_wrapper(func[, obj])

Given a function, make so can be called in async or blocking contexts

fsspec.asyn.get_loop()

Create or return the default fsspec IO loop

class fsspec.asyn.AsyncFileSystem(*args, **kwargs)[source]

Async file operations, default implementations

Passes bulk operations to asyncio.gather for concurrent operation.

Implementations that have concurrent batch operations and/or async methods should inherit from this class instead of AbstractFileSystem. Docstrings are copied from the un-underscored method in AbstractFileSystem, if not given.

fsspec.asyn.sync(loop, func, *args, timeout=None, **kwargs)[source]

Make loop run coroutine until it returns. Runs in other thread

fsspec.asyn.sync_wrapper(func, obj=None)[source]

Given a function, make so can be called in async or blocking contexts

Leave obj=None if defining within a class. Pass the instance if attaching as an attribute of the instance.

fsspec.asyn.get_loop()[source]

Create or return the default fsspec IO loop

The loop will be running on a separate thread.

fsspec.asyn.fsspec_loop()[source]

Temporarily switch the current event loop to the fsspec’s own loop, and then revert it back after the context gets terminated.