Using PHP's memcache and memcached Both of them have addServer.
So I add 2 different memcache servers running on different hardware and even on different ports.
When I "set", "add" or "increment" -- which one will be choosen with memcache and which one with memcached? If one of the servers becomes unavailable, and then become available again, what happens?
And how do I turn on redunandancy/replication of data on both?
Say I want data to be stored both to server A and server B, and when one is unavailble, and then available again it gets back to whats on other server.
I think this can be done with memcache cluster, but how PHP's client libs memcache and memcached are handling that?
To get the desired effect some work has to be done on your part. I'd recommend using Membase instead, with replication turned on and persistence turned off to achieve the same effect. But first let me explain:
In both the Memcached and Memcache clients the hash of the key determines which memcache server stores the data. So which server stores the data is really dependent on the key.
Memcached allows the option to have consistent hashing (Memcached::DISTRIBUTION_CONSISTENT or Memcached::OPT_LIBKETAMA_COMPATIBLE) so that if a memache server goes down or a server is added, not all the data is reshuffled.
Not recommended but...
To achieve the desired effect (inefficiently), use consistent hashing and:
1.) Check if a server is down. Create a Memcache(d) object with only the server in question and store and retrieve a random piece of data with it. If it works, it's up.
2.) Take your working servers and add them to your server list when creating your Memcached object.
3.) Enjoy!
PS: As servers go up and down, your cache miss rates will too.
Related
I will like to know how memcached manage cache for php sessions i mean. I would like to design a php app that scale out and in each http-PHP server include a memcached layer for (db,app cache and session caching), but if memcached dont replicate de data when a user come to webserver1 dont see the same session in webserver2.
memcached1 and memcached2 need to be replicated to handle php sessions
thanks in advance.
regards.
While I agree there is no question here we could try to help the OP understand how memcache works.
When you use memcache which is an in-memory cache how you set it up is determined upon your current infrastructure.
For instance if you only have 1 web server you could install memcache on that same machine along with the database layer being on that machine as well. This works for increasing performance of the site because the site can get data from memcache (in memory) rather than from the database (on disk, and slower to read). Using it in this manner is good but as your site requires better performance or scalability you would probably start up a cluster of web servers behind a load balancer.
This is when things can get a bit tricky. You have all these machines and you are thinking that you need to have memcache on every machine so how do we replicate these instances? The simple answer is you don't. If you have multiple web servers the best method is to put memcache on it's own server (or cluster behind a load balancer), this way every web server is hitting the same IP address for the memcache server(s).
You do not need to worry about keeping anything in sync because the way memcache works is it creates a hash that specifies which server the key has been assigned to (when you have a cluster of memcache servers).
Based on this question it would appear that you would need to do one of the following:
1.) Read up on system architecture
2.) Hire someone to architect your systems layer.
My best suggestion would be to use a single server for your memcache instance and set the web servers to use that for memcache rather than trying to run memcache on each of the web servers.
Joseph.
I undertand your point, I already test the architecture with a separate memcached server (and redis too). My intentions is to "pack" the application server in a unit (docker) and the measure the load parameters to deploy a new instance, to scale out the infraestructure.
I found this.
https://www.digitalocean.com/community/tutorials/how-to-share-php-sessions-on-multiple-memcached-servers-on-ubuntu-14-04
thanks for your reply!
regards.
I have three servers for single domain
I'm using nginx as loadbalancer.
I want to share php sessions across these servers.
My application is heavily dependent on session. storing sessions in files is bad idea. i'm using memcached for this.
how exactly and efficiently should i configure memcached to read and write sessions and share between servers quickly.
or any other good alternative suggestion.
As I read your question, it looks like you installed Memcache on every server (and that's why you ask about "sharing between servers" ?).
What I would do is have a server where you ONLY have Memcached on it. Each web server would connect to your instance of Memcache. You can also have a pool of Memcache servers if needed and Memcache will take care of distributing your data and sessions correctly.
First, you may want to change the way sessions are handled in PHP (for each server) in order to read session's data in Memcache. Your php.ini file will need this:
[Session]
; Handler used to store/retrieve data.
session.save_handler = memcache[d] ; memcache or memcached
session.save_path = "127.0.0.1:11211"
See how Session Handlers work. Note that you can use memcache or memcached extension. They are not the same.
Here is the documentation for both extensions:
Memcache
Memcached
If you want more details about the right memcache to pick, I suggest you check this:
https://serverfault.com/questions/63383/memcache-vs-memcached
Note that storing sessions in Memcache can be problematic. If Memcache is stopped (for whatever reason) you will loose all data you have in it. You may want to consider storing your sessions in a database and also have them in Memcache to speedup the process.
You can build a custom Session Handler to do that and make sure it suits your needs. You can read more about The SessionHandler class.
Finally, if you are open to suggestion, I would also consider using Redis instead of Memcache as it offers more features and will enable you to reload data if shutdown correctly.
I have the following question regarding the memcached module in PHP:
Intro:
We're using the module to prevent the same queries from being sent to the Database server, on a site with 500+ users in every moment.
Sometimes (very rarely) the memcahed process defuncts and all active users start generating queries to the database, so everything stops working.
Question:
I know, that memcached supports multiple servers, but I want to know what happens when one of them dies? Is there some sort of balancer background or something, that can tell Ow! server 1 is dead. I'll send everything to server 2 until the server 1 goes back online. or the load is being sent equally to each one?
Possible sollutions:
I need to know this, because if it's not supported our sysadmin can set the current memcached server to be a load ballancer and to balance the load between several other servers.
Should I ask him to create the load-balancing manualy or is this feature supported by default and what are the risks for both methods?
Thank you!
You add multiple servers in your PHP script, not in Memcache's configuration.
When you use Memcached::addServers(), you can specify a weight for every server. In your case, you might set one Memcache server to be higher than the other and only have the second act as a failover.
Using Memcached::setOption(), you can set how often a connection should be retried and set the timeout. If you know your Memcache servers die a lot, it might be worth it to set this lower than the defaults, but it shouldn't be necessary.
Whats the best way to manage memcache servers in php? Im especially concerned about adding/removing servers, or having 1 fail.
As far as I understand, if you simply comment out addServer() of the server you want to take out, it will screw up the entire cache pool. Is that right?
Currently I'm resorting to this:
Enabled:
$memcache_obj->addServer('ip', port, true, 1, 1, 15, true);
Disabled
$memcache_obj->addServer('ip', port, true, 1, 1, -1, false);
Is this a good idea?
What about storing the memcache server list. I have several servers which all need to connect to the memcache pool. Doing any changes requires me to edit the config on all of these servers.
What about storing the server list in mysql?
Two ways (as far as I know) of doing it using php's Memcache library :
the method you used, which will enable transparent failover if memcache.allow_failover is enabled. While doing this, the keys you set when the server is disabled will need to be recreated when the server is put back into production.
change memcache.hash_strategy to consistent. This will make the hash strategy less influenced by the server pool.
Either way, disabling or removing a server from the pool will have an impact and some keys will be lost in the process.
More options are found in php's Memcached (notice the d) library (but of course that will mean changes in the code).
Take a look at your php.ini and the Memcache runtime configuration for your current setup.
As a side-note, storing the server list in mysql is a bad idea. Best way to have it hard-coded. Would be inefficient to do a database connection and query only to get a list of servers which will give you connectivity to your fast storage buffer. Caching should be a way to avoid connecting and querying the database.
Here's a little background, currently i have
3 web servers
one db server which also host memcache for php sessions for the 3 web servers.
I have the php configs on the 3 servers to point to the memcache server for sessions. It was working fine until alot of connections were being produced for reads etc, which then caused connection timeouts.
So I'm currently looking at clustering the memcache on each web server for sessions, my only concern is how to go about making sure that memcache on all the servers have the same information for sessions.
Someone guided me to http://github.com/trs21219/Memcached-Library because i am using codeigniter but how do i converge my php sessions onto this since memcache seems as a key-value store? Thanks in advance.
Has anyone checked out http://repcached.sourceforge.net/ and does it work?
I'm not sure you have the same expectations of memcache that its designers had.
First, however, memcache distribution works differently than you expect: there is no mechanism to replicate stored information. Each memcache instance is a simple key-value store, as you've noticed. The distribution is done by the client code which has a list of all configured memcache instances and does a hash of the key to direct it to one of the instances. It is possible for the client to store it everywhere and retrieve it locally, or for it to hash it multiple times for redundancy, but these are not straightforward exercises.
But the other issue is that memcache is designed for reasonably short-lived data that memcache is allowed to throw away at any time. This makes it really good for caching frequently accessed data that can be a little stale (say up to a few minutes old) but might be expensive to retrieve (such as almost a minute to generate from a query).
PHP sessions don't really qualify for this, in my experience. A database can easily support many thousands of PHP sessions with barely visible traffic, but you need a lot of memcache storage to support the same number: 50k per session and 5000 sessions means close to 256Mb, and then there is all the other data you want to put in there. Not enough storage and you get lots of unexplained logouts (as memcache discards session data when under memory pressure) and thus lots of annoyed users who have to keep logging in again.
We've found GREAT advantage applying MongoDB instead of MySQL for most things, including session handling. It's far faster, far smaller, far easier. We keep MySQL around for transactional needs, but everything else goes into Mongo now. We've relegated memcache to simply caching pages and other data that isn't critical if it's there or not, something like smarty does.
There is no need to use some 3rd party libraries to organize memcached "cluster".
http://ru.php.net/manual/en/memcached.addserver.php
Just use this function to add several servers into the pool and after that data will be stored and distributed over those servers. The server for storing/retrieving the data for the specific key will be selected according to consistent key distribution option.
So in this case you don't need to worry about "how to go about making sure that memcache on all the servers have the same information for sessions"