Varnish 2.0 bonus feature #1: backend->max_connections

        backend default {
                .host = "127.0.0.1";
                .port = "9080";
                .max_connections = 100;
        }

will limit the simultaneous TCP connections to this backend to 100.

Stats counter backend_busy == "Backend connections too many" tells
how often we have hit this limit for a backend.



git-svn-id: http://www.varnish-cache.org/svn/trunk@3135 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishd/cache.h

@@ -407,7 +407,6 @@ void VBE_Poll(void);
 
 /* cache_backend_cfg.c */
 void VBE_Init(void);
-void VBE_DropRef(struct backend *);
 struct backend *VBE_AddBackend(struct cli *cli, const struct vrt_backend *vb);
 
 /* cache_backend_poll.c */

bin/varnishd/cache_backend.c

@@ -240,6 +240,7 @@ bes_conn_try(const struct sess *sp, struct backend *bp)
 
 	LOCK(&bp->mtx);
 	bp->refcount++;
+	bp->n_conn++;		/* It mostly works */
 	UNLOCK(&bp->mtx);
 
 	s = -1;
@@ -256,6 +257,7 @@ bes_conn_try(const struct sess *sp, struct backend *bp)
 
 	if (s < 0) {
 		LOCK(&bp->mtx);
+		bp->n_conn--;		
 		bp->refcount--;		/* Only keep ref on success */
 		UNLOCK(&bp->mtx);
 	}
@@ -318,6 +320,11 @@ VBE_GetVbe(struct sess *sp, struct backend *bp)
 		return (NULL);
 	}
 
+	if (bp->max_conn > 0 && bp->n_conn >= bp->max_conn) {
+		VSL_stats->backend_busy++;
+		return (NULL);
+	}
+
 	vc = VBE_NewConn();
 	assert(vc->fd == -1);
 	AZ(vc->backend);
@@ -347,7 +354,7 @@ VBE_ClosedFd(struct sess *sp)
 
 	WSL(sp->wrk, SLT_BackendClose, sp->vbe->fd, "%s", bp->vcl_name);
 	TCP_close(&sp->vbe->fd);
-	VBE_DropRef(bp);
+	VBE_DropRefConn(bp);
 	sp->vbe->backend = NULL;
 	VBE_ReleaseConn(sp->vbe);
 	sp->vbe = NULL;

bin/varnishd/cache_backend.h

@@ -113,6 +113,8 @@ struct backend {
 	struct sockaddr		*ipv6;
 	socklen_t		ipv6len;
 
+	unsigned		max_conn;
+	unsigned		n_conn;
 	VTAILQ_HEAD(, vbe_conn)	connlist;
 
 	struct vbp_target	*probe;
@@ -125,6 +127,8 @@ struct vbe_conn *VBE_GetVbe(struct sess *sp, struct backend *bp);
 
 /* cache_backend_cfg.c */
 extern MTX VBE_mtx;
+void VBE_DropRefConn(struct backend *);
+void VBE_DropRef(struct backend *);
 void VBE_DropRefLocked(struct backend *b);
 
 /* cache_backend_poll.c */

bin/varnishd/cache_backend_cfg.c

@@ -132,6 +132,18 @@ VBE_DropRef(struct backend *b)
 	VBE_DropRefLocked(b);
 }
 
+void
+VBE_DropRefConn(struct backend *b)
+{
+
+	CHECK_OBJ_NOTNULL(b, BACKEND_MAGIC);
+
+	LOCK(&b->mtx);
+	assert(b->n_conn > 0);
+	b->n_conn--;
+	VBE_DropRefLocked(b);
+}
+
 /*--------------------------------------------------------------------*/
 
 static void
@@ -210,6 +222,7 @@ VBE_AddBackend(struct cli *cli, const struct vrt_backend *vb)
 	REPLACE(b->hosthdr, vb->hosthdr);
 
 	b->connect_timeout = vb->connect_timeout;
+	b->max_conn = vb->max_connections;
 
 	/*
 	 * Copy over the sockaddrs
@@ -252,10 +265,9 @@ cli_debug_backend(struct cli *cli, const char * const *av, void *priv)
 	ASSERT_CLI();
 	VTAILQ_FOREACH(b, &backends, list) {
 		CHECK_OBJ_NOTNULL(b, BACKEND_MAGIC);
-		cli_out(cli, "%p %s %d\n",
-		    b,
-		    b->vcl_name,
-		    b->refcount);
+		cli_out(cli, "%p %s %d %d/%d\n",
+		    b, b->vcl_name, b->refcount,
+		    b->n_conn, b->max_conn);
 	}
 }
 

bin/varnishtest/tests/v00012.vtc

+# $Id$
+
+test "Check backend connection limit"
+
+server s1 {
+	rxreq
+	sema r1 sync 2
+	sema r2 sync 2
+	txresp
+} -start
+
+varnish v1 -vcl {
+
+	backend default {
+		.host = "127.0.0.1";
+		.port = "9080";
+		.max_connections = 1;
+	}
+	sub vcl_recv {
+		pass;
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+} -start
+
+
+client c2 {
+	sema r1 sync 2
+	txreq
+	rxresp
+	expect resp.status == 503
+} -run
+
+varnish v1 -cli "debug.backend"
+
+sema r2 sync 2
+client c1 -wait
+
+varnish v1 -expect backend_busy == 1

include/stat_field.h

@@ -38,6 +38,7 @@ MAC_STAT(cache_miss,		uint64_t, 'a', "Cache misses")
 
 MAC_STAT(backend_conn,		uint64_t, 'a', "Backend connections success")
 MAC_STAT(backend_unhealthy,	uint64_t, 'a', "Backend connections not attempted")
+MAC_STAT(backend_busy,		uint64_t, 'a', "Backend connections too many")
 MAC_STAT(backend_fail,		uint64_t, 'a', "Backend connections failures")
 MAC_STAT(backend_reuse,		uint64_t, 'a', "Backend connections reuses")
 MAC_STAT(backend_recycle,	uint64_t, 'a', "Backend connections recycles")

include/vrt.h

@@ -69,6 +69,7 @@ struct vrt_backend {
 	const unsigned char		*ipv6_sockaddr;
 
 	double				connect_timeout;
+	unsigned			max_connections;
 	struct vrt_backend_probe 	probe;
 };
 

lib/libvcl/vcc_backend.c

@@ -476,13 +476,16 @@ vcc_ParseHostDef(struct tokenlist *tl, int *nbh, const struct token *name, const
 	const char *ep;
 	struct fld_spec *fs;
 	struct vsb *vsb;
+	unsigned u;
 
 	fs = vcc_FldSpec(tl,
 	    "!host",
 	    "?port",
 	    "?host_header",
 	    "?connect_timeout",
-	    "?probe", NULL);
+	    "?probe",
+	    "?max_connections",
+	    NULL);
 	t_first = tl->t;
 
 	ExpectErr(tl, '{');
@@ -546,6 +549,13 @@ vcc_ParseHostDef(struct tokenlist *tl, int *nbh, const struct token *name, const
 			Fb(tl, 0, ",\n");
 			ExpectErr(tl, ';');
 			vcc_NextToken(tl);
+		} else if (vcc_IdIs(t_field, "max_connections")) {
+			u = vcc_UintVal(tl);
+			vcc_NextToken(tl);
+			ERRCHK(tl);
+			ExpectErr(tl, ';');
+			vcc_NextToken(tl);
+			Fb(tl, 0, "\t.max_connections = %u,\n", u);
 		} else if (vcc_IdIs(t_field, "probe")) {
 			vcc_ParseProbe(tl);
 			ERRCHK(tl);

lib/libvcl/vcc_fixed_token.c

@@ -349,6 +349,7 @@ vcl_output_lang_h(struct vsb *sb)
 	vsb_cat(sb, "	const unsigned char		*ipv6_sockaddr;\n");
 	vsb_cat(sb, "\n");
 	vsb_cat(sb, "	double				connect_timeout;\n");
+	vsb_cat(sb, "	unsigned			max_connections;\n");
 	vsb_cat(sb, "	struct vrt_backend_probe 	probe;\n");
 	vsb_cat(sb, "};\n");
 	vsb_cat(sb, "\n");