Relatively DB newbie here.
So I'm facing a recurring problem that multiple processes attempts Read-Modify-Write operations to the same DB instance, be it MongoDB, Redis, or SQL.
In Redis, one solution is to leverage the atomicity of the Redis Lua scripting to guarantee atomicity, but may result moving a considerable amount of application logic onto Redis. (whether good or bad?)
In SQL, it seems there are atomic stored procedures that achieves similar results, but also risking moving too much application logic into the DB itself (whether good or bad?)
MongoDB doesn't even really have a concept of internal scripting (the javascript solution seems to be deprecated)
Then in the general sense, as implied above, it might be good (?) to keep the application logic outside of the data store to achieve maximum application logic distribution and scalability across multiple nodes of services.
But making application logic distributed across multiple processes (nodes) and have them concurrently access the shared data store warrants the read-modify-write cycle to be guarded from possible race conditions.
So my questions are:
for Redis or SQL, should I abuse the provided atomic scripting support to totally avoid any possible race, but putting more and more application logic into the data store, or
is the read-modify-write model more common for the majority of the DB concurrency access, and if yes, are there some "standard" guidelines about how to synchronize the concurrent read-modify-write from multiple processes?
Thank!
Nosql databases are not ACID compliant. These are distributed nosql databases. Example - mongodb, redis, cassandra etc These nosql databases satisfy either CP or AP sections of CAP theorem. ACID compliant databases like RDBMS satisfy AC section of CAP theorem.
The usecase for nosql databases are either heavy read , heavy write nit both. Its mostly related to performance i.e speed , high availability.
Hope i am clear
Related
I am new to this field of studies and I want to know if I understood everything right.
In Memory database management systems (like Redis and memcached) are for just keeping data as in cache to access quicker. I mean everything is done manually: you read from database then write in cache and next time read from cache. The only task that redis/Memcached do, is they remove records after some time and manage task queues? Am I right?
That may be the primary use case for redis/memcached, but not for the broader category of in-memory database systems.
Many (most, for my company) use cases don't involve another DBMS; the in-memory database system (IMDS) is the only database management system.
Further, there are IMDS that resemble persistent DBMS (HANA, TimesTen, VoltDB, eXtremeDB and others). They can be embedded or client/server, SQL or NoSQL, offer high availability, clustering, sharding and much more.
Many articles claim that relational databases cannot be scaled and NOSQL is better at it but do not explain why. Scalability is often projected as an advantage of NOSQL. What is the problem with scaling relational databases? What makes NOSQL databases superior to relational databases in the aspect of scalability?
Both SQL and NOSQL databases can scale. However, NOSQL databases have some simplified functionality that can improve scalability.
For instance, SQL databases generally enforce a set of properties called ACID properties. These ensure the consistency of the data over time and the ability implement an entire transaction "all at once".
However, when running in a multi-processor environment, there is overhead to strictly maintaining the ACID properties. Basically, the data needs to look the same from any processor at the same time.
NOSQL databases often implement "ACID-lite". For instance, they offer "eventual-consistency". This means that for a few seconds or minutes, a query might return different values depending on which processor(s) process it. And, this is fine for many applications.
This truly depends on the requirement of the enterprise in long run and volume of the data expected. The other key factor is the requirement in terms of do we need OLTP kind of scenario only and reporting is less which means implementing ACID scenario. No SQL is usually best for the scenario where reporting is vital as compare to SQL. As both carry its own Mertis but ideally its hybrid model to take adavntage of both usually works better where you have scalability and better transaction control on SQL DB's and high performance rreporting using NO SQL DB which allow all level of freedom such as graph DB, Key value pair. There are lot of intresting comparision are available evne for specific DB you want to evaluate.
Puneet
So I've been trying hard to figure out if NoSQL is really bringing that much value outside of auto-sharding and handling UNSTRUCTURED data.
Assuming I can fit my STRUCTURED data on a single machine OR have an effective 'auto-sharding' feature for SQL, what advantages do any NoSQL options offer? I've determined the following:
Document-based (MongoDB, Couchbase, etc) - Outside of it's 'auto-sharding' capabilities, I'm having a hard time understanding where the benefit is. Linked objects are quite similar to SQL joins, while Embedded objects significantly bloat doc size and causes a challenge regarding to replication (a comment could belong to both a post AND a user, and therefore the data would be redundant). Also, loss of ACID and transactions are a big disadvantage.
Key-value based (Redis, Memcached, etc) - Serves a different use case, ideal for caching but not complex queries
Columnar (Cassandra, HBase, etc ) - Seems that the big advantage here is more how the data is stored on disk, and mostly useful for aggregations rather than general use
Graph (Neo4j, OrientDB, etc) - The most intriguing, the use of both edges and nodes makes for an interesting value-proposition, but mostly useful for highly complex relational data rather than general use.
I can see the advantages of Key-value, Columnar and Graph DBs for specific use cases (Caching, social network relationship mapping, aggregations), but can't see any reason to use something like MongoDB for STRUCTURED data outside of it's 'auto-sharding' capabilities.
If SQL has a similar 'auto-sharding' ability, would SQL be a no-brainer for structured data? Seems to me it would be, but I would like the communities opinion...
NOTE: This is in regards to a typical CRUD application like a Social Network, E-Commerce site, CMS etc.
If you're starting off on a single server, then many advantages of NoSQL go out the window. The biggest advantages to the most popular NoSQL are high availability with less down time. Eventual consistency requirements can lead to performance improvements as well. It really depends on your needs.
Document-based - If your data fits well into a handful of small buckets of data, then a document oriented database. For example, on a classifieds site we have Users, Accounts and Listings as the core data. The bulk of search and display operations are against the Listings alone. With the legacy database we have to do nearly 40 join operations to get the data for a single listing. With NoSQL it's a single query. With NoSQL we can also create indexes against nested data, again with results queried without Joins. In this case, we're actually mirroring data from SQL to MongoDB for purposes of search and display (there are other reasons), with a longer-term migration strategy being worked on now. ElasticSearch, RethinkDB and others are great databases as well. RethinkDB actually takes a very conservative approach to the data, and ElasticSearch's out of the box indexing is second to none.
Key-value store - Caching is an excellent use case here, when you are running a medium to high volume website where data is mostly read, a good caching strategy alone can get you 4-5 times the users handled by a single server. Key-value stores (RocksDB, LevelDB, Redis, etc) are also very good options for Graph data, as individual mapping can be held with subject-predicate-target values which can be very fast for graphing options over the top.
Columnar - Cassandra in particular can be used to distribute significant amounts of load for even single-value lookups. Cassandra's scaling is very linear to the number of servers in use. Great for heavy read and write scenarios. I find this less valuable for live searches, but very good when you have a VERY high load and need to distribute. It takes a lot more planning, and may well not fit your needs. You can tweak settings to suite your CAP needs, and even handle distribution to multiple data centers in the box. NOTE: Most applications do emphatically NOT need this level of use. ElasticSearch may be a better fit in most scenarios you would consider HBase/Hadoop or Cassandra for.
Graph - I'm not as familiar with graph databases, so can't comment here (beyond using a key-value store as underlying option).
Given that you then comment on MongoDB specifically vs SQL ... even if both auto-shard. PostgreSQL in particular has made a lot of strides in terms of getting unstrictured data usable (JSON/JSONB types) not to mention the power you can get from something like PLV8, it's probably the most suited to handling the types of loads you might throw at a document store with the advantages of NoSQL. Where it happens to fall down is that replication, sharding and failover are bolted on solutions not really in the box.
For small to medium loads sharding really isn't the best approach. Most scenarios are mostly read so having a replica-set where you have additional read nodes is usually better when you have 3-5 servers. MongoDB is great in this scenario, the master node is automagically elected, and failover is pretty fast. The only weirdness I've seen is when Azure went down in late 2014, and only one of the servers came up first, the other two were almost 40 minutes later. With replication any given read request can be handled in whole by a single server. Your data structures become simpler, and your chances of data loss are reduced.
Again in my own example above, for a mediums sized classifieds site, the vast majority of data belongs to a single collection... it is searched against, and displayed from that collection. With this use case a document store works much better than structured/normalized data. The way the objects are stored are much closer to their representation in the application. There's less of a cognitive disconnect and it simply works.
The fact is that SQL JOIN operations kill performance, especially when aggregating data across those joins. For a single query for a single user it's fine, even with a dozen of them. When you get to dozens of joins with thousands of simultaneous users, it starts to fall apart. At this point you have several choices...
Caching - caching is always a great approach, and the less often your data changes, the better the approach. This can be anything from a set of memcache/redis instances to using something like MongoDB, RethinkDB or ElasticSearch to hold composite records. The challenge here comes down to updating or invalidating your cached data.
Migrating - migrating your data to a data store that better represents your needs can be a good idea as well. If you need to handle massive writes, or very massive read scenarios no SQL database can keep up. You could NEVER handle the likes of Facebook or Twitter on SQL.
Something in between - As you need to scale it depends on what you are doing and where your pain points are as to what will be the best solution for a given situation. Many developers and administrators fear having data broken up into multiple places, but this is often the best answer. Does your analytical data really need to be in the same place as your core operational data? For that matter do your logins need to be tightly coupled? Are you doing a lot of correlated queries? It really depends.
Personal Opinions Ahead
For me, I like the safety net that SQL provides. Having it as the central store for core data it's my first choice. I tend to treat RDBMS's as dumb storage, I don't like being tied to a given platform. I feel that many people try to over-normalize their data. Often I will add an XML or JSON field to a table so additional pieces of data can be stored without bloating the scheme, specifically if it's unlikely to ever be queried... I'll then have properties in my objects in the application code that store in those fields. A good example may be a payment... if you are currently using one system, or multiple systems (one for CC along with Paypal, Google, Amazon etc) then the details of the transaction really don't affect your records, why create 5+ tables to store this detailed data. You can even use JSON for primary storage and have computed columns derived and persisted from that JSON for broader query capability and indexing where needed. Databases like postgresql and mysql (iirc) offer direct indexing against JSON data as well.
When data is a natural fit for a document store, I say go for it... if the vast majority of your queries are for something that fits better to a single record or collection, denormalize away. Having this as a mirror to your primary data is great.
For write-heavy data you want multiple systems in play... It depends heavily on your needs here... Do you need fast hot-query performance? Go with ElasticSearch. Do you need absolute massive horizontal scale, HBase or Cassandra.
The key take away here is not to be afraid to mix it up... there really isn't a one size fits all. As an aside, I feel that if PostgreSQL comes up with a good in the box (for the open-source version) solution for even just replication and automated fail-over they're in a much better position than most at that point.
I didn't really get into, but feel I should mention that there are a number of SaaS solutions and other providers that offer hybrid SQL systems. You can develop against MySQL/MariaDB locally and deploy to a system with SQL on top of a distributed storage cluster. I still feel that HBase or ElasticSearch are better for logging and analitical data, but the SQL on top solutions are also compelling.
More: http://www.mongodb.com/nosql-explained
Schema-less storage (or schema-free). Ability to modify the storage (basically add new fields to records) without having to modify the storage 'declared' schema. RDBMSs require the explicit declaration of said 'fields' and require explicit modifications to the schema before a new 'field' is saved. A schema-free storage engine allows for fast application changes, just modify the app code to save the extra fields, or rename the fields, or drop fields and be done.
Traditional RDBMS folk consider the schema-free a disadvantage because they argue that on the long run one needs to query the storage and handling the heterogeneous records (some have some fields, some have other fields) makes it difficult to handle. But for a start-up the schema-free is overwhelmingly alluring, as fast iteration and time-to-market is all that matter (and often rightly so).
You asked us to assume that either the data can fit on a single machine, OR your database has an effective auto-sharding feature.
Going with the assumption that your SQL data has an auto-sharding feature, that means you're talking about running a cluster. Any time you're running a cluster of machines you have to worry about fault-tolerance.
For example, let's say you're using the simplest approach of sharding your data by application function, and are storing all of your user account data on server A and your product catalog on server B.
Is it acceptable to your business if server A goes down and none of your users can login?
Is it acceptable to your business if server B goes down and no one can buy things?
If not, you need to worry about setting up data replication and high-availability failover. Doable, but not pleasant or easy for SQL databases. Other types of sharding strategies (key, lookup service, etc) have the same challenges.
Many NoSQL databases will automatically handle replication and failovers. Some will do it out of the box, with very little configuration. That's a huge benefit from an operational point of view.
Full disclosure: I'm an engineer at FoundationDB, a NoSQL database that automatically handles sharding, replication, and fail-over with very little configuration. It also has a SQL layer so you you don't have to give up structured data.
I am asking this in the context of NoSQL - which achieves scalability and performance without being expensive.
So, if I needed to achieve massively parallel distributed computing across databases ...
What are the various methodologies available today (within the RDBMS paradigm) to achieve distributed computing with high-scalability?
Does database clustering & mirroring contribute in any way towards distributed computing?
I guess you are asking about scalability of RDBMS databases. Talking about NoSQL databases based on ( amazon dynamo, BigTable ) are a whole another topic. I am talking about HBase, Cassandra etc. There are also commerical products like Oracle Coherence thats more like a distributed cache and key value store , to put it crudely.
going back to rdbms,
Sharding
to scale RDBMS one can do cusstom sharding. Sharding is a technique where you have multiple table is possibly multiple hosts. And then you decide in a certain fashion to assign certain rows to certain tables. For example you can say that rows 1-1M goes to table1, 1M-2M goes to table2 etc. But, this is a difficult process from an administration point of view. A lot of large scale websites scale by relying on sharding. Other techniques worth mentioning are partioning and mysql federation and mysql cluster.
MPP databases
Then there are databases are there very RDBMS which does distribution and scaling for you. Terradata is the most successful of these companies. I believe they used postgres core code at some point. A significant number of fortune 500 companies and a lot of the airlines use Terradata. But, its ridiculously expensive. There are newer companies like greenplum, vertica, netezza.
Unless you're a very big company with extreme scalability requirements, you can horizontally and ACID scale up your DB by building a cluster of identical RDBMS instances and synchronizing them with JTA transactions.
Take a look to this Java/JDBC based article the JEPLayer framework is used but you can use straight JDBC and JTA code.
Within the RDBMS paradigm: Sharding.
Outside the RDBMS paradigm: Key-value stores.
My pick: (I come from an RDBMS background) Key-value stores of the tabluar type - HBase.
Within the RDBMS paradigm, sharding will not get you far.
Use the RDBMS paradigm to design your model, to get your project up and running.
Use tabular key-value stores to SCALE OUT.
Sharding:
A good way to think about sharding is to see it as user-account-oriented
DB design.
The all schema entities touched by a user-account are kept on one host.
The assignment of user to host happens when the user creates an account.
The least loaded host gets that user.
When that user signs on after account creation, he gets connected
to the host that has his data.
Each host has a set of user accounts.
The problem with this approach is that if the host gets hosed,
a fraction of users will be blacked out.
The solution to this is have a replicated standby host that
becomes the primary when the primary host encounters problems.
Also, it's a fairly rigid setup for processes where the design does
not change dramatically.
From the user standpoint, I've noticed that web sites
with a sharded DB backend are not as quick to "turn on a dime"
to create different business models on their platform.
Contrast this with web sites that have truly distributed
key-value stores. These businesses can host any range of
services. Their platform is just that - a platform.
It's not relational and it does have an API interface,
but it just seems to work.
I am interested in implementing an architecture that has two databases one for read operations and the other for writes. I have never implemented something like this and have always built single database, highly normalised systems so I am not quite sure where to begin. I have a few parts to this question.
1. What would be a good resource to find out more about this architecture?
2. Is it just a question of replicating between two identical schemas, or would your schemas differ depending on the operations, would normalisation vary too?
3. How do you insure that data written to one database is immediately available for reading from the second?
Any further help, tips, resources would be appreciated. Thanks.
EDIT
After some research I have found this article which I found very informative for those interested..
http://www.codefutures.com/database-sharding/
I found this highscalability article very informative
I'm not a specialist but the read/write master database and read-only slaves pattern is a "common" pattern, especially for big applications doing mostly read accesses or data warehouses:
it allows to scale (you add more read-only slaves if required)
it allows to tune the databases differently (for either efficient reads or efficient writes)
What would be a good resource to find out more about this architecture?
There are good resources available on the Internet. For example:
Highscalability.com has good examples (e.g. Wikimedia architecture, the master-slave category,...)
Handling Data in Mega Scale Systems (starting from slide 29)
MySQL Scale-Out approach for better performance and scalability as a key factor for Wikipedia’s growth
Chapter 24. High Availability and Load Balancing in PostgreSQL documentation
Chapter 16. Replication in MySQL documentation
http://www.google.com/search?q=read%2Fwrite+master+database+and+read-only+slaves
Is it just a question of replicating between two identical schemas, or would your schemas differ depending on the operations, would normalisation vary too?
I'm not sure - I'm eager to read answers from experts - but I think the schemas are identical in traditional replication scenari (the tuning may be different though). Maybe people are doing more exotic things but I wonder if they rely on database replication in that case, it sounds more like "real-time ETL".
How do you insure that data written to one database is immediately available for reading from the second?
I guess you would need synchronous replication for that (which is of course slower than asynchronous). While some databases do support this mode, not all do AFAIK. But have a look at this answer or this one for SQL Server.
You might look up data warehouses.
These serve as 'normalized for reporting' type databases, while you can keep a normalized OLTP style instance for the data maintenance.
I don't think the idea of 'immediate' equivalence will be a reality. There will be some delay while the new data and changes are migrated in to the other system. The schedule and scope will be your big decisions here.
In regards to questions 2:
It really depends on what you are trying to achieve by having two databases. If it is for performance reasons (which i suspect it may be) i would suggest you look into denormalizing the read-only database as needed for performance. If performance isn't an issue then I wouldn't mess with the read-only schema.
I've worked on similar systems where there would be a read/write database that was only lightly used by administrative users. That database would then be replicated to the read only database during a nightly process.
Question 3:
How immediate are we talking here? Less than a second? 10 seconds? Minutes?