add semaphore concurrency limit example

examples/limit_concurrency.py

@@ -0,0 +1,36 @@
+from sanic import Sanic
+from sanic.response import json
+
+import asyncio
+import aiohttp
+
+app = Sanic(__name__)
+
+sem = None
+
+def init(sanic, loop):
+    global sem
+    CONCURRENCY_PER_WORKER = 4
+    sem = asyncio.Semaphore(CONCURRENCY_PER_WORKER, loop=loop)
+
+async def bounded_fetch(session, url):
+    """
+    Use session object to perform 'get' request on url
+    """
+    async with sem, session.get(url) as response:
+        return await response.json()
+
+
+@app.route("/")
+async def test(request):
+    """
+    Download and serve example JSON
+    """
+    url = "https://api.github.com/repos/channelcat/sanic"
+
+    async with aiohttp.ClientSession() as session:
+        response = await bounded_fetch(session, url)
+        return json(response)
+
+
+app.run(host="0.0.0.0", port=8000, workers=2, before_start=init)