I'm trying to figure out how different instances of mongos server work together.
If I have 1 configserver and some shards, for example four, each of them composed by only one node (a master of course), and have four mongos server... do the mongos server communicate between them? Is it possible that one mongos redirect its load to another mongos?
When you have multiple mongos instances, they do not automatically load-balance between each other. They don't even know about each others existence.
The MongoDB drivers for most programming languages allow to specify multiple mongos instances when creating a connection. In that case the driver will usually ping all of them and connect to the one with the lowest latency. This will usually be the one which is closest geographically. When all have the same network distance, the one which is least busy right now will usually respond first. The driver will then stay connected to that one mongos, unless the program explicitely reconnects or the mongos can no longer be reached (in that case the driver will usually automatically pick another one from the initial list).
That means using multiple mongos instances is normally only a valid method for scaling when you have a large number of low-load clients, not one high-load client. When you want your one high-load client to make use of many mongos instances, you need to implement this yourself by creating a separate connection to each mongos instance and implement your own mechanism to distribute queries among them.
Short answer
As of MongoDB 2.4, the mongos servers only provide a routing service to direct read/write queries to the appropriate shard(s). The mongos servers discover the configuration for your sharded cluster via the config servers. You can find out more details in the MongoDB documentation: Sharded Cluster Query Routing.
Longer scoop
I'm trying to figure out how different instances of mongos server work togheter.
The mongos servers do not currently talk directly to each other. They do coordinate activity some activity via your config servers:
reading the sharded cluster metadata
initiating a balancing round (any mongos can start a balancing round, but only one round can be active at a time)
If I have 1 configserver
You should always have 3 config servers in production. If you somehow lose or corrupt your config server, you will have to combine your data and re-shard your database(s). The sharded cluster metadata saved on the config servers is the definitive source for what sharded data ranges should live on each shard.
some shards, for example four, each of them composed by only one node (a master of course)
Ideally each shard should be backed by a replica set if you want optimal uptime. Replica sets provide for auto-failover and can be very useful for administrative purposes (for example, taking backups or adding indexes offline).
Is it possible that one mongos redirect its load to another mongos?
No, the mongos do not perform any load balancing. The typical recommendation is to deploy one mongos per app server.
From an application/driver point of view you can specify multiple mongos in your connect string for failover purposes. The application drivers will generally connect to the nearest available mongos (by network ping time), and attempt to reconnect to in the event the current mongos connection fails.
Related
In a regular sharded replica cluster, it consists of 10 mongos, 5 config servers and 10 shards. I use mongo client to connect to multiple mongos instances.
I have two questions.
The first question: What is load policy in this situation? Is it round-robin scheduing?
The second one: What if one onf the mongos instances is down, what is the move that mongoclient would take? Will it still connect to this mongos instance or drop this one from the list.
Please help with these.thanks
The mongos servers provide a routing service to direct read/write queries to the appropriate shard(s).
You are specifying multiple mongos's to connect to the MongoDB sharded cluster. An available mongos will be used to connect to the server.
The first question: What is load policy in this situation? Is it
round-robin scheduling?
The client will connect to the server with an available mongos. There is no "load policy" and there is no round-robin scheduling. You use multiple mongos's for high availability.
See: Number of mongos and Distribution
The second one: What if one of the mongos instances is down, what is
the move that mongoclient would take? Will it still connect to this
mongos instance or drop this one from the list.
If a mongos is down, the client will connect to the server using another available mongos from the list (you have more than one mongos to connect with).
Technically is it supported to begin with just one shard for a shard cluster? So we can be ready for adding additional one(s) at anytime, at the same time save the cost of additional shard(s) before we really need it(them)?
To go further, is it possible to have a shard running on one single instance, instead of having to be based off of a 3 instance replica set?
From here, sharding is:
A database architecture that partitions data by key ranges and
distributes the data among two or more database instances.
A shard will be either a replica set or a standalone mongod instance. It is possible for you to use a single machine by using different ports to establish distinct communication endpoints for the config, mongod and mongos processes on the single machine. Also, yes, you may add a shard at a later time when you need to expand.
However, the point of providing sharding is to support horizontal scaling. Additionally, the point of sharded clustering is to provide failover and redundancy support. By using a single shard on a single server, you are losing the benefits of scaling and certainly failover.
The recommended production architecture includes:
Three config servers on separate machines for each sharded cluster.
Two or more replica sets as shards.
One or more query routers (mongos); typically, one mongos instance per application server.
Peruse the Sharded Cluster Requirements section in the documentation to get a feel for whether or not your environment needs sharding and sharded clusters since there is complexity in establishing such an architecture.
I am aware that mongodb has a master-slave architecture.
Therefore, I was thinking that the master would be the single point of failure in mongoDB since it takes care of all the requests and sends it to the slave nodes. However, when the master fails, then a new master is reelected from the slaves. Therefore I need some clarification on where the single point of failure lies.
Does mongoDB have a single point of failure? Is it in the master node?
Thanks,
MongoDB can be set up in a way that there is no single point of failure (at least none specific to MongoDB).
When you set up replication as suggested (which includes primary, secondary and an arbiter on a 3rd server), the secondary will take the role of the primary when it goes down. Keep in mind that this only works when the applications know both the primary and the secondary (how to make it aware depends on the driver).
When you have a sharded cluster, the mongo router process (mongos) and the config servers becomes additional possible points of failure, but you can also set up reduntant routers and config servers. To send the clients to another mongos server when theirs goes down, you need a 3rd party load-balancing solution.
For a proper production MongoDB setup with clustering, MongoDB Inc. suggests:
At least 2 mongos routers
Exactly 3 config servers
3 servers per shard (primary, secondary and arbiter), where the arbiters do not necessarily need dedicated servers and can share hardware with the routers, config servers, members of a different replica-set or app servers.
All in the title : do we need 2 mongos per shard in MongoDB ? I am not sure to understand exactly what mongos are for and if my website will communicate with them or if it is something internal to MongoDB.
If you have cluster set up (with shards, not to be confused with replica set), then you have to have mongos instances deployed. It's a router process. It knows which data resides where. Application talks to mongos, it routes the request to corresponding shard. Talking to shards directly is strongly discouraged.
You must have at least one mongos process. You can have more, they have small resource footprint. I usually deploy one mongos per application server.
A mongos is basically nothing more than a router which gathers a configuration of your cluster from config servers, caches that config, and uses it to route targeted and scatter and gather operations within a cluster of shards. It can also be used for aggregation as such if aggregation queries are common in your app the mongos can take some CPU and memory, however, for the most part they have no weight and can run on the smallest server.
You do not require 2 mongos, the number depends upon the operations being sent through that router. You can in theory do with one, however, that isn't very redundant and cerates a single point of failure, 2 makes that less possible.
The rule of thumb is to have the 'mongos' process running on each of your application servers. This keeps your application talking to localhost which is fast and your mongos processes scale with your app.
Say we have 2 distinct mongo clusters (sharded), is it possible to configure one mongos process to talk to two different clusters? It would be awesome to abstract away the fact that the databases lived in different places.
Or would you have to launch two different mongos processes on different ports? If this IS possible I still worry that it might be dangerous having two different mongos processes fighting for resources.
Or something completely different? Ideas?
Each mongos belongs to one, only one, cluster (defined by the config db servers). The mongos processes don't use much resources; you can run multiple on a machine.
You can have more than one sharded db/collection per cluster.