blog: Add quick post on FLM

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+---
+date: 2022-02-20
+title: "Half-deallocated flows"
+linkTitle: "Flows vs connections/sockets (2)"
+author: Dimitri Staessens
+---
+
+A few weeks back I wrote a post about Ouroboros flows vs TCP
+connections, and how "half-closed connections" should be handled in
+the Ouroboros architecture. This was very basic functionality that was
+sorely missing. You can refresh your memory on that
+[post](/blog/2021/12/29/behaviour-of-ouroboros-flows-vs-udp-sockets-and-tcp-connections/sockets/)
+if needed.
+
+Today I wrapped up an initial implementation without whistles and
+bells (fixed timeout at 120s), and I'll share a bit with you how it
+works.
+
+The modified oecho application looks as follows (decluttered). On the
+server side, we have:
+
+```C
+       while (true) {
+                fd = flow_accept(NULL, NULL);
+
+                printf("New flow.\n");
+
+                count = flow_read(fd, &buf, BUF_SIZE);
+
+                printf("Message from client is %.*s.\n", (int) count, buf);
+
+                flow_write(fd, buf, count);
+
+                flow_dealloc(fd);
+        }
+
+        return 0;
+```
+And on the client side, we have:
+
+```C
+        char *  message = "Client says hi!";
+        qosspec_t qs = qos_raw;
+
+        fd = flow_alloc("oecho", &qs, NULL);
+
+        flow_write(fd, message, strlen(message) + 1);
+
+        count = flow_read(fd, buf, BUF_SIZE);
+
+        printf("Server replied with %.*s\n", (int) count, buf);
+
+        /* The server has deallocated the flow, this read should fail. */
+        count = flow_read(fd, buf, BUF_SIZE);
+        if (count < 0) {
+                printf("Failed to read packet: %zd.\n", count);
+                flow_dealloc(fd);
+                return -1;
+        }
+
+        flow_dealloc(fd);
+
+```
+
+Previously, the second flow_read would hang forever, (unless a timeout
+was set on the read operation using fccntl, which we didn't do).
+
+Now the IPCP will detect the peer as gone, and mark the flow as DOWN
+to the application.
+
+```
+[dstaesse@heteropoda website]$ oecho
+Server replied with Client says hi!
+Failed to read packet: -1005.
+```
+
+We can see this in a simple test case over the
+loopback adapter:
+
+```
+feb 20 18:50:06 heteropoda irmd[70364]: irmd: Flow on flow_id 13 allocated.
+feb 20 18:50:06 heteropoda irmd[70364]: irmd: Flow on flow_id 12 allocated.
+feb 20 18:50:06 heteropoda irmd[70364]: irmd: Partial deallocation of flow_id 13 by process 70597.
+feb 20 18:50:06 heteropoda irmd[70364]: irmd: Completed deallocation of flow_id 13 by process 70534.
+feb 20 18:50:06 heteropoda irmd[70364]: irmd: New instance (70597) of oecho added.
+feb 20 18:50:06 heteropoda irmd[70364]: irmd: This process accepts flows for:
+feb 20 18:50:06 heteropoda irmd[70364]: irmd:         oecho
+feb 20 18:52:13 heteropoda ipcpd-unicast[70405]: flow-allocator: Flow 66 down: Unresponsive peer.
+feb 20 18:52:13 heteropoda irmd[70364]: irmd: Partial deallocation of flow_id 12 by process 70598.
+feb 20 18:52:13 heteropoda irmd[70364]: irmd: Completed deallocation of flow_id 12 by process 70405.
+feb 20 18:52:13 heteropoda irmd[70364]: irmd: Dead process removed: 70598.
+```
+
+In the first 2 lines, the flow between the oecho client and server is
+allocated, creating a flow endpoint 13 at the server side, and flow
+endpoint id 12 at the client side. Then the server calls flow_dealloc
+and the flow is deallocated (lines 3 and 4). The server re-enters its
+accept loop, and it's ready for new incoming flow requests (lines
+5-7). About 2 minutes later, the flow liveness mechanism in the flow
+allocator at the client side detects that the remote is gone, and
+flags the flow as DOWN (line 8).  After that, the client's read call
+terminates and the client calls dealloc, after which the flow is
+deallocated (lines 9-10) and the client exits (last line).
+
+Note that works independent of the QoS of the flow. I'll add a
+configurable timeout soon, and it will work at any scale, from seconds
+to years. I thought seconds should be small enough, but if anyone
+makes a good case for timing out flows at sub-second timescales, I'll
+happily enable it.
+
+Stay curious,
+
+Dimitri
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