Is there any advantage to using multiple collections within a database, when multiple databases each with a single collection would accomplish the same thing? From what I can gather, using multiple databases reduces lock contention because locks are per-database, so I wonder why you'd ever want to put more than one collection in a single database.
The only downside I've found mentioned is that there's some overhead (~200MB) per database, and that with a large number of databases, OS file handles can become a limitation, but I imagine that if you have enough collections/databases for those to be issues, you've got too many databases. These overheads are bearable in my case; I'd like to know if there's something else I should know about.
EDIT: In my situation there are currently 30 collections spread across 8 databases. I'm asking this question because I think it may be better to make this 30 collections across 30 databases. There's no real reason for the current structure; it was set up by a team who don't know much about databases so picked arbitrarily. It's now used frequently enough for lock contention to be a factor (profiling shows some operations spending >1 second waiting for locks). We'll scale horizontally too, I just saw this as a potential low-hanging fruit since it just means using a different database name for some operations, instead of a different collection name.
Apologies if this has been asked before; the only similar questions I've found have been about whether to use e.g. "a collection per user" which isn't quite the same thing. In my case I have heterogeneous documents which I definitely do want stored in different collections, I'd just like to know whether to store those collections in the same database or not.
may be duplicate of this: creating a different database for each collection in MongoDB 2.2
in my solution I created own database for each large and highload collection, for rest collections I create another common database. Mongodb implements locks on a per-database basis for most read and write operations: http://docs.mongodb.org/manual/faq/concurrency/ But locks in mongoDb not so nasty as in SQL.
This solution increase productivity of mongodb for me.
Related
I'm new to NoSQL and I'm trying to figure out the best way to model my database. I'll be using ArangoDB in the project but I think this question also stands if using MongoDB.
The database will store 12 categories of products. Each category is expected to hold hundreds or thousands of products. Products will also be added / removed constantly.
There will be a number of common fields across all products, but each category will also have unique fields / different restrictions to data.
Keep in mind that there are instances where I'd need to query all the categories at the same time, for example to search a product across all categories, and other instances where I'll only need to query one category.
Should I create one single collection "Product" and use a field to indicate the category, or create a seperate collection for each category?
I've read many questions related to this idea (1 collection vs many) but I haven't been able to reach a conclusion, other than "it dependes".
So my question is: In this specific use case which option would be most optimal, multiple collections vs single collection + sharding, in terms of performance and speed ?
Any help would be appreciated.
As you mentioned, you need to play with your data and use-case. You will have better picture.
Some decisions required as below.
Decide the number of documents you will have in near future. If you will have 1m documents in an year, then try with at least 3m data
Decide the number of indices required.
Decide the number of writes, reads per second.
Decide the size of documents per category.
Decide the query pattern.
Some inputs based on the requirements
If you have more writes with more indices, then single monolithic collection will be slower as multiple indices needs to be updated.
As you have different set of fields per category, you could try with multiple collections.
There is $unionWith to combine data from multiple collections. But do check the performance it purely depends on the above decisions. Note this open issue also.
If you decide to go with monolithic collection, defer the sharding. Implement this once you found that queries are slower.
If you have more writes on the same document, writes will be executed sequentially. It will slow down your read also.
Think of reclaiming the disk space when more data is cleared from the collections. Multiple collections do good here.
The point which forces me to suggest monolithic collections is that I'd need to query all the categories at the same time. You may need to add more categories, but combining all of them in single response would not be better in terms of performance.
As you don't really have a join use case like in RDBMS, you can go with single monolithic collection from model point of view. I doubt you could have a join key.
If any of my points are incorrect, please let me know.
To SQL or to NoSQL?
I think that before you implement this in NoSQL, you should ask yourself why you are doing that. I quite like NoSQL but some data is definitely a better fit to that model than others.
The data you are describing is a classic case for a relational SQL DB. That's fine if it's a hobby project and you want to try NoSQL, but if this is for a production environment or client, you are likely making the situation more difficult for them.
Relational or non-relational?
You mention common fields across all products. If you wish to update these fields and have those updates reflected in all products, then you have relational data.
Background
It may be worth reading Sarah Mei 2013 article about this. Skip to the section "How MongoDB Stores Data" and read from there. Warning: the article is called "Why You Should Never Use MongoDB" and is (perhaps intentionally) somewhat biased against Mongo, so it's important to read this through the correct lens. The message you should get from this article is that MongoDB is not a good fit for every data type.
Two strategies for handling relational data in Mongo:
every time you update one of these common fields, update every product's document with the new common field data. This is generally only ok if you have few updates or few documents, but not both.
use references and do joins.
In Mongo, joins typically happen code-side (multiple db calls)
In Arango (and in other graph dbs, as well as some key-value stores), the joins happen db-side (single db call)
Decisions
These are important factors to consider when deciding which DB to use and how to model your data
I've used MongoDB, ArangoDB and Neo4j.
Mongo definitely has the best tooling and it's easy to find help, but I don't believe it's good fit in this case
Arango is quite pleasant to work with, but doesn't yet have the adoption that it deserves
I wouldn't recommend Neo4j to anyone looking for a NoSQL solution, as its nodes and relations only support flat properties (no nesting, so not real documents)
It may also be worth considering MariaDB or Postgres
I am designing a system with MongoDb (64 bit version) to handle a large amount of users (around 100,000) and each user will have large amounts of data (around 1 million records).
What is the best strategy of design?
Dump all records in single collection
Have a collection for each user
Have a database for each user.
Many Thanks,
So you're looking at somewhere in the region of 100 billion records (1 million records * 100,000 users).
The preferred way to deal with large amounts of data is to create a sharded cluster that splits the data out over several servers that are presented as single logical unit via the mongo client.
Therefore the answer to your question is put all your records in a single sharded collection.
The number of shards required and configuration of the cluster is related to the size of the data and other factors such as the quantity and distribution of reads and writes. The answers to those questions are probably very specific to your unique situation, so I won't attempt to guess them.
I'd probably start by deciding how many shards you have the time and machines available to set up and testing the system on a cluster of that many machines. Based on the performance of that, you can decide whether you need more or fewer shards in your cluster
So you are looking for 100,000,000 detail records overall for 100K users?
What many people don't seem to understand is that MongoDB is good at horizontal scaling. Horizontal scaling is normally classed as scaling huge single collections of data across many (many) servers in a huge cluster.
So already if you use a single collection for common data (i.e. one collection called user and one called detail) you are suiting MongoDBs core purpose and build.
MongoDB, as mentioned, by others is not so good at scaling vertically across many collections. It has a nssize limit to begin with and even though 12K initial collections is estimated in reality due to index size you can have as little as 5K collections in your database.
So a collection per user is not feasible at all. It would be using MongoDB against its core principles.
Having a database per user involves the same problems, maybe more, as having singular collections per user.
I have never encountered some one not being able to scale MongoDB to the billions or even close to the 100s of billions (or maybe beyond) on a optimised set-up, however, I do not see why it cannot; after all Facebook is able to make MySQL scale into the 100s of billions per user (across 32K+ shards) for them and the sharding concept is similar between the two databases.
So the theory and possibility of doing this is there. It is all about choosing the right schema and shard concept and key (and severs and network etc etc etc etc).
If you were to witness problems you could go for splitting archive collections, or deleted items away from the main collection but I think that is overkill, instead you want to make sure that MongoDB knows where each segment of your huge dataset is at any given point in time on the master and ensure that this data is always hot, that way queries that don't do a global and scatter OP should be quite fast.
About a collection on each users:
By default configuration, MongoDB is limited to 12k collections. You can increase the size of this with --nssize but it's not unlimited.
And you have to count index into this 12k. (check "namespaces" concept on mongo documentation).
About a database for each user:
For a model point of view, that's very curious.
For technical, there is no limit on mongo, but you probably have a limit with file descriptor (limit from you OS/settings).
So as #Rohit says, the two last are not good.
Maybe you should explain more about your case.
Maybe you can cut users into different collections (ex: one for each first letter of name etc., or for each service of the company...).
And, of course use sharding.
Edit: maybe MongoDb is not the best database for your use case.
Do many mongodb collections have a big impact on mongodb performance, memory and capacity? I am designing an api with mvc pattern, and a collection is being created for each model. I question the way I am doing now.
MongoDB with the WirdeTiger engine supports an unlimited number of collections. So you are not going to run into any hard technical limitations.
When you wonder if something should be in one collection or in multiple collections, these are some of the considerations you need to keep in mind:
More collections = more maintenance work. Sharding is configured on the collection level. So having a large number of collections will make shard configuration a lot more work. You also need to set up indexes for each collection separately, but this is quite easy to automatize, because createIndex on an index which already exists does nothing.
The MongoDB API is designed in a way that every database query operates on one collection at a time. That means when you need to search for a document in n different collections, you need to perform n queries. When you need to aggregate data stored in multiple collections, you run into even more problems. So any data which is queried together should be stored together in the same collection.
Creating one collection for each class in your model is usually a god rule of thumb, but it is not a golden hammer solution. There are situations where you want to embed object in their parent-object documents instead of putting them into a separate collection. There are also cases where you want to put all objects with the same base-class in the same collection to benefit from MongoDB's ability to handle heterogeneous collections. But that goes beyond the scope of this question.
Why don't you use this and test your application ?
https://docs.mongodb.com/manual/tutorial/evaluate-operation-performance/
By the way your question is not completely clear... is more like a "discussion" rather than question. And you're asking others to evaluate your work instead of searching the web the rigth approach.
I'm new to mongodb and I'm facing a dilemma regarding my DB Schema design:
Should I create one single collection or put my data into several collections (we could call these categories I suppose).
Now I know many such questions have been asked, but I believe my case is different for 2 reasons:
If I go for many collections, I'll have to create about 120 and that's it. This won't grow in the future.
I know I'll never need to query or insert into multiple collections. I will always have to query only one, since a document in collection X is not related to any document stored in the other collections. Documents may hold references to other parts of the DB though (like userId etc).
So my question is: could the 120 collections improve query performance? Is this a useful optimization in my case?
Or should I just go for single collection + sharding?
Each collection is expected hold millions of documents. If use only one, it will store billions of docs.
Thanks in advance!
------- Edit:
Thanks for the great answers.
In fact the 120 collections is only a self made limit, it's not really optimal:
The data in the collections is related to web publishers. There could be millions of these (any web site can join).
I guess the ideal situation would be if I could create a collection for each publisher (to hold their data only). But obviously, this is not possible due to mongo limitations.
So I came up with the idea of a fixed number of collections to at least distribute the data somehow. Like: collection "A_XX" would hold XX Platform related data for publishers whose names start with "A".. etc. We'll only support a few of these platforms, so 120 collections should be more than enough.
On another website someone suggested using many databases instead of many collections. But this means overhead and then I would have to use / manage many different connections.
What do you think about this? Is there a better solution?
Sorry for not being specific enough in my original question.
Thanks in advance
Single Sharded Collection
The edited version of the question makes the actual requirement clearer: you have a collection that can potentially grow very large and you want an approach to partition the data. The artificial collection limit is your own planned partitioning scheme.
In that case, I think you would be best off using a single collection and taking advantage of MongoDB's auto-sharding feature to distribute the data and workload to multiple servers as required. Multiple collections is still a valid approach, but unnecessarily complicates your application code & deployment versus leveraging core MongoDB features. Assuming you choose a good shard key, your data will be automatically balanced across your shards.
You can do not have to shard immediately; you can defer the decision until you see your workload actually requiring more write scale (but knowing the option is there when you need it). You have other options before deciding to shard as well, such as upgrading your servers (disks and memory in particular) to better support your workload. Conversely, you don't want to wait until your system is crushed by workload before sharding so you definitely need to monitor the growth. I would suggest using the free MongoDB Monitoring Service (MMS) provided by 10gen.
On another website someone suggested using many databases instead of many collections. But this means overhead and then I would have to use / manage many different connections.
Multiple databases will add significantly more administrative overhead, and would likely be overkill and possibly detrimental for your use case. Storage is allocated at the database level, so 120 databases would be consuming much more space than a single database with 120 collections.
Fixed number of collections (original answer)
If you can plan for a fixed number of collections (120 as per your original question description), I think it makes more sense to take this approach rather than using a monolithic collection.
NOTE: the design considerations below still apply, but since the question was updated to clarify that multiple collections are an attempted partitioning scheme, sharding a single collection would be a much more straightforward approach.
The motivations for using separate collections would be:
Your documents for a single large collection will likely have to include some indication of the collection subtype, which may need to be added to multiple indexes and could significantly increase index sizes. With separate collections the subtype is already implicit in the collection namespace.
Sharding is enabled at the collection level. A single large collection only gives you an "all or nothing" approach, whereas individual collections allow you to control which subset(s) of data need to be sharded and choose more appropriate shard keys.
You can use the compact to command to defragment individual collections. Note: compact is a blocking operation, so the normal recommendation for a HA production environment would be to deploy a replica set and use rolling maintenance (i.e. compact the secondaries first, then step down and compact the primary).
MongoDB 2.4 (and 2.2) currently have database-level write lock granularity. In practice this has not proven a problem for the vast majority of use cases, however multiple collections would allow you to more easily move high activity collections into separate databases if needed.
Further to the previous point .. if you have your data in separate collections, these will be able to take advantage of future improvements in collection-level locking (see SERVER-1240 in the MongoDB Jira issue tracker).
The main problem here is that you will gain very little performance in the current MongoDB versions if you separate out collections into the same database. To get any sort of extra performance over a single collection setup you would need to move the collections out into separate databases, then you will have operational overhead for judging what database you should query etc.
So yes, you could go for 120 collections easily however, you won't really gain anything currently due to: https://jira.mongodb.org/browse/SERVER-1240 not being implemented (anytime soon).
Housing billions of documents in a single collection isn't too bad. I presume that even if you was to house this in separate collections it probably would not be on a single server either, just like sharding a single collection, so any speed reduction due to multi server setup will also not matter in this case.
In my personal opinion, using a single collection is easier on everything.
I am building a site with users who have discussions and write blogs and plan to use MongoDB as the database for the site. Which architecture option would be more efficient and allow for easier data flow between them:
One Database with a Blogs Collection, a Discussions Collection, and a User Activity Collection? Each collection would be sharded as appropriate.
A Blogs Database, a Discussions Database, and a User Activity Database? Each database would be broken into collections and sha rded as appropriate.
It won't make a big difference whether you put everything into a single database or into multiple databases until you find you need to do something that's handled on the database level, for example access control, or placing database files on separate physical devices (to reduce I/O contention).
In addition, currently locking granularity is on the database level so if you happen to have a very large number of small writes having them go to different databases will mean that they will not be contending for the same lock. Since you anticipate sharding you can also place each database on a different shard which may allow you to defer actually needing to shard any particular collection as each shard would only be handling the traffic for that database's collection(s).
I would say if you are in doubt go ahead and put them in separate databases, it's unlikely to hurt and it may help.
Mongo will work, but getting familiar with it may take time depending on your experience.
If you use MySQL (or another SQL db) you may have an easier time. You should probably just create separate tables for your blogs, discussions, and activity, rather than multiple databases.
Another factor to consider is the size of your databases. An SQL database is fine for most applications, even fairly large ones. MongoDB (and other NoSQL db's) are great for scaling big data.
Hope this helps!