The delay is not constant, in the absolute majority of cases, connecting is almost instant, but occasionally, connecting to a server takes a few seconds.
We can't even reproduce it reliably, we can see in our server logs that sometimes, maybe a few times a day, maybe once every few days, a cassandra server will be slow in accepting connections, and after a little while everything will be ok again. It's not a network saturation error, it's not a CPU saturation error. Not even GC pauses.
Has anyone else noticed something similar? Or is this simply a result of us running a tight connection pool which recycles connections every few hours and only waits a few seconds for a connection before timing out?
Have you ironed out non-cassandra potential causes ?
On 2012-06-14, at 10:38 AM, Henrik Schröder wrote:
> Hi everyone,
> We have problem with our Cassandra cluster, and that is that sometimes it takes several seconds to open a new Thrift connection to the server. We've had this issue when we ran on windows, and we have this issue now that we run on Ubuntu. We've had it with our old networking setup, and we have it with our new networking setup where we're running it over a dedicated gigabit network. Normally estabishing a new connection is instant, but once in a while it seems like it's not accepting any new connections until three seconds have passed.
> We're of course running a connection-pooling client which mitigates this, since once a connection is established, it's rock solid.
> We tried switching the rpc_server_type to hsha, but that seems to have made the problem worse, we're seeing more connection timeouts because of this.
> For what it's woth, we're running Cassandra version 1.0.10 on Ubuntu, and our connection pool is configured to abort a connection attempt after two seconds, and each connection lives for six hours and then it's recycled. Under current load we do about 500 writes/s and 100 reads/s, we have 20 clients, but each has a very small connection pool of maybe up to 5 simultaneous connections against each Cassandra server. We see these connection issues maybe once a day, but always at random intervals.
> We've tried to get more information through Datastax Opscenter, the JMX console, and our own application monitoring and logging, but we can't see anything out of the ordinary. Sometimes, seemingly by random, it's just really slow to connect. We're all out of ideas. Does anyone here have suggestions on where to look and what to do next?
3 seconds constantly sounds it could be a timeout/retry somewhere. Do you contact cassandra via a hostname or IP address ? If via hostname, iron out DNS.
Either way, I'd fire up tcpdump, both on both the client and the server, and observe the TCP handshake. Specifically see if the SYN packet is sent and received, whether the SYN-ACK is sent back right away and received, and final ACK.
If that looks good, then TCP-wise you're in good shape and the problem is in a higher layer (thrift). If not, see where the delay/drop/retry happens. If it's in the first packet, it may be a networking/routing issue. If in the second, it may me capacity at the server (investigate with lsof/netstat/JMX), etc..