我在两个Amazon EC2实例(版本1.8.0)和大约5个应用程序服务器上有一个实时Couchbase集群,每个服务器运行
PHP,并在其上运行moxi客户端.偶尔,Moxi将在尝试访问数据时返回SERVER_ERROR.这种情况平均每隔几分钟发生一次.群集每秒处理大约500次操作.

检查moxi日志(启用了-vvv)后,我在获得SERVER_ERROR时会注意到以下情况：

2013-07-16 03:07:22: (cproxy.c.2680) downstream_timeout
2013-07-16 03:07:22: (cproxy.c.1911) 56: could not forward upstream to downstream
2013-07-16 03:07:22: (cproxy.c.2004) 56: upstream_error: SERVER_ERROR proxy downstream timeout^M

我尝试将moxi configs中的下游超时从5000增加到25000,但这根本没有帮助.错误仍然经常发生.

有人可以建议我发现问题的原因吗？或者如果有可能是罪魁祸首？

最佳答案

SERVER_ERROR proxy downstream timeout

In this error response, moxi reached a timeout while waiting for a
downstream server to respond to a request. That is, moxi did not see
any explicit errors such as a connection going down, but the response
is just taking too long. The downstream connection will be also closed
by moxi rather than putting the downstream connection back into a
connection pool. The default downstream_timeout configuration is 5000
(milliseconds).

非常直接的错误,但它可能是由一些可能的事情引起的.

尝试从moxi获取“stats proxy”的输出：

echo stats proxy | nc HOST 11211

显然你已经发现你关注这些设置：

STAT 11211:default:pstd_stats:tot_downstream_timeout xxxx
STAT 11211:default:pstd_stats:tot_wait_queue_timeout nnnnn

你所说的你的downstream_timeout应该显示为5000

还要看看：

STAT 11211:default:pstd_stats:tot_downstream_conn_queue_timeout 0

来自网址：

http://www.couchbase.com/docs/moxi-manual-1.8/moxi-dataflow.html

几乎完美地了解了莫西的运作方式.

To understand some of the configurable command-line flags in moxi
(concurrency, downstream_max, downstream_conn_max, downstream_timeout,
wait_queue_timeout, etc), it can be helpful to follow a request
through moxi…

The normal flow of data for moxi is as follows:

A client connects

A client creates a connection (an upstream conn) to moxi.
moxi’s -c command-line parameter ultimately controls the limits on
the maximum number of connections.

In this -c parameter, moxi inherits the same behavior as memcached,
and will stop accept()’ing client connections until
existing connections are closed. When the count of existing
connections drops below the -c defined level, moxi will accept() more
client connections.

The client makes a request, which goes on the wait queue

Next, the client makes a request — such as simple single-key
command (like set, add, append, or a single-key get).

At this point, moxi places the upstream conn onto the tail of a wait
queue. moxi’s wait_queue_timeout parameter controls how long an
upstream conn should stay on the wait queue before moxi times it out
and responds to the client with a SERVER_ERROR response.

The concurrency parameter

Next, there’s a configurable max limit to how many upstream conn
requests moxi will process concurrently off the head of the wait
queue. This configurable limit is called concurrency. (This formerly
used to be known, perhaps confusingly, as downstream_max. For
backwards compatibility, concurrency and downstream_max configuration
flags are treated as synonyms.)

The concurrency configuration is per-thread and per-bucket. That
is, the moxi process-level concurrency is actually concurrency X
num-worker-threads X num-buckets.

The default concurrency configuration value is 1024. This means moxi
will concurrently process 1024 upstream connection requests from
the head of the wait queue. (There are more queues in moxi, however,
before moxi actually forwards a request. This is discussed in later
sections.)

Taking the concurrency value of 1024 as an example, if you have 4
worker threads (the default, controlled by moxi’s -t parameter) and 1
bucket (what most folks start out with, such as the “default” bucket),
you’ll have a limit of 1024 x 4 x 1 or 4096 concurrently processed
client requests in that single moxi process.

The rationale behind the concurrency increase to 1024 for moxi’s
configuration (it used to be much lower) is due to the evolving design
of moxi. Originally, moxi only had the wait queue as its only internal
queue. As more, later-stage queues were added during moxi’s history,
we found that getting requests off the wait queue sooner and onto the
later stage queues was a better approach. We’ll discuss these
later-stage queues below.

Next, let’s discuss how client requests are matched to downstream connections.

Key hashing

The concurrently processed client requests (taken from the head
of the wait queue) now need to be matched up with downstream connections
to the Couchbase server. If the client’s request comes with a key
(like a SET, DELETE, ADD, INCR, single-key GET), the request’s key is
hashed to find the right downstream server “host:port:bucket” info.
For example, something like — “memcache1:11211:default”. If the
client’s request was a broadcast-style command (like FLUSH_ALL, or a
multi-key GET), moxi knows the downstream connections that it needs to
acquire.

The downstream conn pool

Next, there’s a lookup using those host:port:bucket identifiers into
a downstream conn pool in order to acquire or reserve the
appropriate downstream conns. There’s a downstream conn pool per
thread. Each downstream conn pool is just a hashmap keyed by
host:port:bucket with hash values of a linked-list of available
downstream conns. The max length of any downstream conn linked list is
controlled by moxi’s downstream_conn_max configuration parameter.

The downstream_conn_max parameter

By default the downstream_conn_max value is 4. A value of 0 means no limit.

So, if you’ve set downstream_conn_max of 4, have 4 worker threads,
and have 1 bucket, you should see moxi create a maximum of 4
X 4 X 1 or 16 connections to any Couchbase server.

Connecting to a downstream server

If there isn’t a downstream conn available, and the
downstream_conn_max wasn’t reached, moxi creates a downstream conn as
needed by doing a connect() and SASL auth as needed.

The connect_timeout and auth_timeout parameters

The connect() and SASL auth have their own configurable timeout
parameters, called connect_timeout and auth_timeout, and these
are in milliseconds. The default value for connect_timeout is 400
milliseconds, and the auth_timeout default is 100 milliseconds.

The downstream conn queue

If downstream_conn_max is reached, then the request must wait until a
downstream conn becomes available; the request therefore is
placed on a per-thread, per-host:port:bucket queue, which is called a
downstream conn queue. As downstream conns are released back into the
downstream conn pool, they will be assigned to any requests that are
waiting on the downstream conn queue.

The downstream_conn_queue_timeout parameter

There is another configurable timeout, downstream_conn_queue_timeout,
that defines how long a request should
stay on the downstream conn queue in milliseconds before timing out.
By default, the downstream_conn_queue_timeout is 200 milliseconds. A
value of 0 indicates no timeout.

A downstream connection is reserved

Finally, at this point, downstream conn’s are matched up for the
client’s request. If you’ve configured moxi to track timing histogram
statistics, moxi will now get the official start time of the request.
moxi now starts asynchronously sending request message bytes to the
downstream conn and asynchronously awaits responses.

To turn on timing histogram statistics, use the “time_stats=1”
configuration flag. By default, time_stats is 0 or off.

The downstream_timeout parameter

Next, if you’ve configured a downstream_timeout, moxi starts a timer
for the request where moxi can limit the time it will spend
processing a request at this point. If the timer fires, moxi will
return a “SERVER_ERROR proxy downstream timeout” back to the client.

The downstream_timeout default value is 5000 milliseconds. If moxi sees
this time elapse, it will close any downstream connections that
were assigned to the request. Due to this simple behavior of closing
downstream connections on timeout, having a very short
downstream_timeout is not recommended. This will help avoid repetitive
connection creation, timeout, closing and reconnecting. On an
overloaded cluster, you may want to increase downstream_timeout so
that moxi does not constantly attempt to time out downstream
connections on an already overloaded cluster, or by creating even more
new connections when servers are already trying to process requests on
old, closed connections. If you see your servers greatly spiking, you
should consider making this adjustment.

Responses are received

When all responses are received from the downstream servers for a request (or the
downstream conn had an error), moxi asynchronously
sends those responses to the client’s upstream conn. If you’ve
configured moxi to track timing histogram statistics, moxi now tracks
the official end time of the request. The downstream conn is now
released back to the per-thread downstream conn pool, and another
waiting client request (if any) is taken off the downstream conn queue
and assigned to use that downstream conn.

Backoff/Blacklisting

At step 6, there’s a case where a connect() attempt might fail. Moxi
can be configured to count up the number of connect() failures for a
downstream server, and will also track the time of the last failing
connect() attempt.

With the connect() failure counting, moxi can be configured to
blacklist a server if too many connect() failures are seen, which is
defined by the connect_max_errors configuration parameter. When more
than connect_max_errors number of connect() failures are seen, moxi
can be configured to temporarily stop making connect() attempts to
that server (or backoff) for a configured amount of time. The backoff
time is defined via the connect_retry_interval configuration, in
milliseconds.

The default for connect_max_errors is 5 and the connect_retry_interval
is 30000 millisecods, that is, 30 seconds.

If you use connect_max_errors parameter, it should be set greater than
the downstream_conn_max configuration parameter.