loop with backup servers and proxy_next_upstream http_404

[Yasar Semih Alev]

Hi,

I added backup upstream server and proxy_next_upstream http_404 in conf if master upstream servers send 404 Nginx is trying from backup upstream. If backup upstream send 404, Nginx is starting loop on backup upstream and nonstop request same file.

I searched the problem and changed these codes in ngx_http_upstream.c and problem solved but I don't know exactly maybe wrong changes.

< if (ft_type == NGX_HTTP_UPSTREAM_FT_HTTP_404) {
< state = NGX_PEER_NEXT;
< } else {
< state = NGX_PEER_FAILED;
< }
---

state = NGX_PEER_FAILED;

Thanks.

Kind Regards

Semih Alev

[Maxim Dounin]

Yes, thank you, it's known problem. Current implementation of backup servers doesn't work well with "proxy_next_upstream http_404".

[Maxim Dounin]

In [4622/nginx]:

Upstream: smooth weighted round-robin balancing.

For edge case weights like { 5, 1, 1 } we now produce { a, a, b, a, c, a, a }
sequence instead of { c, b, a, a, a, a, a } produced previously.

Algorithm is as follows: on each peer selection we increase current_weight
of each eligible peer by its weight, select peer with greatest current_weight
and reduce its current_weight by total number of weight points distributed
among peers.

In case of { 5, 1, 1 } weights this gives the following sequence of
current_weight's:

a b c
0 0 0 (initial state)

5 1 1 (a selected)

-2 1 1

3 2 2 (a selected)

-4 2 2

1 3 3 (b selected)
1 -4 3

6 -3 4 (a selected)

-1 -3 4

4 -2 5 (c selected)
4 -2 -2

9 -1 -1 (a selected)
2 -1 -1

7 0 0 (a selected)
0 0 0

To preserve weight reduction in case of failures the effective_weight
variable was introduced, which usually matches peer's weight, but is
reduced temporarily on peer failures.

This change also fixes loop with backup servers and proxy_next_upstream
http_404 (ticket #47), and skipping alive upstreams in some cases if there
are multiple dead ones (ticket #64).

[Maxim Dounin]

Fix committed, thanks.

[sync]

In [4668/nginx]:

Merge of r4622, r4623: balancing changes.

*) Upstream: smooth weighted round-robin balancing.

For edge case weights like { 5, 1, 1 } we now produce { a, a, b, a, c, a, a }
sequence instead of { c, b, a, a, a, a, a } produced previously.

Algorithm is as follows: on each peer selection we increase current_weight
of each eligible peer by its weight, select peer with greatest current_weight
and reduce its current_weight by total number of weight points distributed
among peers.

In case of { 5, 1, 1 } weights this gives the following sequence of
current_weight's:

a b c
0 0 0 (initial state)

5 1 1 (a selected)

-2 1 1

3 2 2 (a selected)

-4 2 2

1 3 3 (b selected)
1 -4 3

6 -3 4 (a selected)

-1 -3 4

4 -2 5 (c selected)
4 -2 -2

9 -1 -1 (a selected)
2 -1 -1

7 0 0 (a selected)
0 0 0

To preserve weight reduction in case of failures the effective_weight
variable was introduced, which usually matches peer's weight, but is
reduced temporarily on peer failures.

This change also fixes loop with backup servers and proxy_next_upstream
http_404 (ticket #47), and skipping alive upstreams in some cases if there
are multiple dead ones (ticket #64).

*) Upstream: fixed ip_hash rebalancing with the "down" flag.

Due to weight being set to 0 for down peers, order of peers after sorting
wasn't the same as without the "down" flag (with down peers at the end),
resulting in client rebalancing for clients on other servers. The only
rebalancing which should happen after adding "down" to a server is one
for clients on the server.

The problem was introduced in r1377 (which fixed endless loop by setting
weight to 0 for down servers). The loop is no longer possible with new
smooth algorithm, so preserving original weight is safe.