What happens when your application cannot open yet another connection to MySQL

Have you ever experienced a situation where one moment you can connect to the MySQL database and the next moment  you cannot, only to be able to connect again a second later? As you may know one cannot open infinite connections with MySQL. There’s a practical limit and more often than not it is imposed by the underlying operating system. If you’re getting:

…there’s a good chance you’re hitting such limit. What might be misleading in the information above is whom (which side) is preventing the connection from being established.

Understanding the problem at hand

Whenever a client uses the network to connect to a service running on a given port of a server this connection is established through the creation of a socket:

A socket is a tuple of 4 terms: the source and destination IPs and ports.

The destination port is obviously the one where the service is running on the server. For instance usually port 22 for SSH and port 3306 for MySQL. The source port is an arbitrary local network port on the client side, which should show in tools like netstat and ss:

Note: I’ve used netstat and ss alternately in this post but if you read the man page for netstat these days you’ll see a note that says: “This program is obsolete. Replacement for netstat is ss”. I was advised to give preference to ss over netstat on a busy server: besides being faster and providing a more rich set of information, ss puts less stress on the server as it talks directly to the kernel while what netstat does is to scan /dev.

The example above shows that server 192.168.0.100 has a TCP connection established with MySQL (3306) running on server 192.168.0.10 through local port 48681. The range of local ports that can be used for TCP and UDP traffic is found in /proc/sys/net/ipv4/ip_local_port_range:

Those values from above are the default ones found in many Linux distributions: 32768 denotes the first local port that can be used and 61000 indicates the last one, for a total of 28233 available ports. It may look like a lot but it truly depends on the nature of the local applications connecting to services in other servers through the network.

When it comes to connecting to database servers, many applications chose to open a new connection for a single request only, closing it right after the request is processed. Even though the connection is closed by the client (application) the local port it was using is not immediately released by the OS to be reused by another connection: it will sit in a TIME_WAIT state for (usually) 60 seconds – this value cannot be easily changed as it is hard coded in the kernel:

And contrary to what you may have heard or read, tunning /proc/sys/net/ipv4/tcp_fin_timeout is of no use here as it rules a different type of timeout and has no impact in releasing connections hanging in TIME_WAIT state. To better understand the role played by such state I suggest you read Vincent Bernat’s post, from which I reproduce:

There are two purposes for the TIME-WAIT state:

The most known one is to prevent delayed segments from one connection being accepted by a later connection relying on the same quadruplet (source address, source port, destination address, destination port) (…)

The other purpose is to ensure the remote end has closed the connection. When the last ACK is lost, the remote end stays in the LAST-ACK state. Without the TIME-WAIT state, a connection could be reopened while the remote end still thinks the previous connection is valid. (…)

The problem with this situation is that if you keep accumulating connections in TIME_WAIT state you’ll quickly saturate the available local ports. And if all ports are taken then any attempt for a new connection will result in an error similar to the one from above.

Reproducing the problem

It’s easy to verify this scenario, it suffices to decrease the local IP port range from the computer that’ll be starting the connections (usually the application server) to, for example, only half a dozen ports:

Then we proceed with opening 6 connections from the application server (192.168.0.100) to the database server (192.168.0.10):

Now, when we try to open a seventh connection to the database we’ll hit that error stated in the beginning of this post. What the error actually means is:

In fact, it should be complemented with “… because there’s no available local network ports left“.

And even if we close all 6 MySQL connections right away they’ll all move from ESTABLISHED to TIME_WAIT state and we’ll still need to wait for them to expire until we can open a new connection:

That’s where scalability problems happen if your application server keeps opening more connections than it can have old ones released in time. For example, considering the default port range from 32768 to 61000 and a TIME_WAIT of 60 seconds, in theory we can only open and close around 470 new network connections each second ((61000 – 32768 + 1)/60 = 470.55) before we saturate the available local network ports; that’s not much …

Possible Solutions

This is by no means an exhaustive list but here’s a few possible approaches to consider. If you have something to add about those or happen to known about any other please let me know by leaving a comment below.

Increasing port range

If the server initiating the connections is operating with the default port range you can start by increasing it somewhat. The first 1023 ports are said to be privileged, meaning only root can start an application listening to one of these initial ports. In the other extreme,  the highest port you can have assigned is 65535 (2^16-1). In practice, then, you can increase the port range to the maximum of 1024-65535, which would provide 64512 ports, allowing in theory around 1075 briefly connections being opened and closed per second:

To make this change permanent and survive a server reboot you need to add the following line to /etc/sysctl.conf:

You should however pay attention when stretching these values to the limits.

Adding extra IP addresses and listening to multiple ports

Something that wasn’t clear to me at first is that the port range limitation is applied per quadruplet (<source address>:<source port>, <destination address>:<destination port>). As such, if you have port range set from 60001 to 60006 in the client you should be able to open no more than 6 MySQL connections from the same <address>:<port> pair to the same <address>:<port> pair, as well as 6 SSH connections,  6 NC connections, etc:

Note in the list above there’s more than one connection established from the same source <address>:<port> pair, though binded to different destination <address>:<port> pairs. There’s even a 7th connection to MySQL, though to one running on a different server (192.168.0.11:3306).

So, another way of increasing the amount of concurrent connections is by adding an additional IP address to either the outgoing side (client) or to the incoming side (server) and making part of the connections happen through it. Likewise, even though you cannot make MySQL listen to multiple ports simultaneously you can configure your firewall to accept connections to other ports and redirect them to port 3306 (or any other you’re using). In the example below I configure iptables to redirect all connections destined to port 80 to port 3306:

Modifying the connection behavior of the application(s)

Expanding the number of possible quadruplets will certainly help in increasing the amount of possible concurrent connections but it won’t scale indefinitely. In the long term you may need to review your application configuration behavior and setup/re-engineer it in such a way to avoid it opening and closing many connections over the network too often – if that’s the problem after all. You may resort to some sort of connection pooling instead, but be sure to evaluate it well first.

Tweaking TCP parameter settings

Even though you cannot easily decrease the timeout for TIME_WAIT state there’s at least 3 different TCP parameters you can use to “bypass” this limitation. You should explore this options with caution though as these settings could affect the reliability of TCP connections.

tcp_tw_reuse

It is possible to force the kernel to reuse a connection hanging in TIME_WAIT state by setting /proc/sys/net/ipv4/tcp_tw_reuse to 1. What happens in practice is that you’ll keep seeing the closed connections hanging in TIME_WAIT until either they expire or a new connection is requested. In the later case, the connection will be “relived”.

Here’s an example from the previous scenario where I had limited the port range to only 6 ports. I enabled tcp_tw_reuse and opened 6 connections with MySQL, closing five of them soon afterwards:

The TIME_WAIT was still in countdown mode (both ss and netstat have option -o/–timers to display those) when I opened a new connection:

Note the new connection was established in lieu of the one hanging in TIME_WAIT state for longer (using local port 61003).

tcp_tw_recycle

When you enable /proc/sys/net/ipv4/tcp_tw_recycle closed connections will not show under TIME_WAIT anymore – they disappear from netstat altogether. But as soon as you open a new connection (within the 60 seconds mark) it will recycle one of those. But everyone writing about this alternative seems to advise against it’s use. Bottom line is: it’s preferable to reuse a connection than to recycle it.

tcp_max_tw_buckets

This parameter rules how many connections can remain in TIME_WAIT state concurrently: the kernel will
simply kill connections hanging in such state above that number. For example, continuing with the previous scenario where I had configured the client server with a port range composed of only 6 ports, if I set /proc/sys/net/ipv4/tcp_max_tw_buckets to 5, then open 6 concurrent connections with MySQL and then immediatelly close all 6 I’ll find only 5 of them hanging in the TIME_WAIT state – as with tcp_tw_recycle, one of then will simply disapear from netstat. This situation allows me to immediately open a new connection without needing to wait for a minute. However, I won’t be able to open a second one until one  of the other 5 connections in TIME_WAIT expire and freed the local port it was using. The secret here, then, is to find a compromise between the number of available network ports and the number of connections we allow to remain in TIME_WAIT state. The default value of this setting is 65536, which means by default the system allows all possible connections to go over the TIME_WAIT state when closed.

If “LAMP” server, use local socket

Finally, it’s important to mention that if your application is hosted in the same server as the database you may (should ?) open connections using MySQL’s socket file directly, without going over the network, and thus avoid this port range/TIME_WAIT problematic altogether. Interestingly, you can accomplish this in one of two ways: specifying the socket with the –socket option or using –host=localhost. As MySQL’s manual mentions:

On Unix, MySQL programs treat the host name localhost specially, in a way that is likely different from what you expect compared to other network-based programs. For connections to localhost, MySQL programs attempt to connect to the local server by using a Unix socket file.

Clarification

I prefer to overstate this problem in favor of avoiding any confusion: the error described in this post and all the situations surrounding it is to be found in the computer that starts the connections (usually the application server), and not in the server holding the services. Whenever a client closes the connection, while it shows hanging in TIME_WAIT state there won’t be any remaining trace of the connection showing in netstat in the server side. However, if for some reason it is the server that “closes the connection first, it gets the TIME-WAIT state while the client will consider the corresponding quadruplet free and hence may reuse it for a new connection.

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Comments (2)

  • michael morse Reply

    great post…lots of confusion/misinformation over the TIME_WAIT/FIN_WAIT(1 & 2) states and variables, what exactly what constitutes a unique pairing, and what server ends up hitting the limits (as clarified by your last paragraph) not really a db/memcached/anydestination server problem but the app servers connecting. Smaller and more app servers (and planning for less than 1000 new connections/s per box) is of course a more basic fix as well.

    December 8, 2014 at 1:44 pm
  • Wlad Reply

    So, what is the correct closing sequence for the socket,c w.r.t MySQL protocol.

    I assumed (read somewhere years ago) that if client sends COM_QUIT http://dev.mysql.com/doc/internals/en/com-quit.html, then the server should send OK packet back, and then the server should close the connection. Client should read OK, or perhaps read even more until recv() returns 0 or error, which means the connection is closed on the server side. Then the client closes the socket. Then there will be no TIME_WAIT sockets. Is this correct or not?

    (As a sidenote, this is how it is implemented in MariaDB JDBC. I tried rapid connection creation and close, and did not experienced TIME_WAIT, though IIRC all test were done with clent and server on the same box, which can produce skewed results)

    December 8, 2014 at 4:53 pm

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