Is it necessary to open a SFTPClient per one thread in Paramiko with multi-threading?

Question

I want to download files from remote server using Paramiko with multithreading.

There are two solution came into my mind, but I'm not sure which is right (or better).

Solution 1: Assuming that the SFTPClient.get is thread safe (But I can't find any document mentioned that), a simple one would as:

from paramiko import SSHClient, AutoAddPolicy, SFTPClient
from concurrent.futures import ThreadPoolExecutor
from typing import List

client = SSHClient()
ciient.set_missing_host_key_policy(AutoAddPolicy())
client.connect( ... )
sftp = client.open_sftp()

files_to_download: List[str] = ...

with ThreadPoolExecutor(10) as pool:
    pool.map(lambda fn: sftp.get(fn, fn), files_to_download)

Solution 2: There are two questions in Solution 1

1. Is the Paramiko's API really thread-safe?
2. Is it efficient to download multi-files via a single connection?

So here is my second solution:

from paramiko import SSHClient, AutoAddPolicy, SFTPClient
from concurrent.futures import ThreadPoolExecutor
from threading import Lock, local
from typing import List

client = SSHClient()
ciient.set_missing_host_key_policy(AutoAddPolicy())
client.connect( ... )
thread_local = local()
thread_lock  = Lock()

files_to_download: List[str] = ...

def download(fn: str) -> None:
    """
    thread-safe and each thread has its own SFTPClient
    """
    if not hasattr(thread_local, 'sftp'):
        with thread_lock:
            thread_local.sftp = client.open_sftp()
    thread_local.sftp.get(fn, fn)

with ThreadPoolExecutor(10) as pool:
    pool.map(download, files_to_download)

Which solution is better?

Answer 1

Paramiko is not thread safe.

Using multiple threads over one connection might not give you the performance you hope for anyway. You would have to open a separate connection (SSHClient/SFTPClient) per thread.

With one connection, you would have better performance, only with scenarios like a transfer of large amount of small files. For that, see Slow upload of many small files with SFTP.