Version Stamps in NOSQL Databases
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This document discusses version stamps, which are fields that change each time underlying data is updated. They are used to check for data changes in NoSQL databases that lack transactions. Various methods for creating version stamps are described, including counters, GUIDs, hashes, and timestamps. The best approach may be a composite stamp using multiple methods to leverage their advantages and avoid single points of failure. Version stamps can enable consistency when reading and updating data.
Version Stamps in NOSQL Databases
- 1. Version Stamps Dr. Dipali Meher MCS, M.Phil,NET, Ph.D Assistant Professor Modern College of Arts Science and Commerce, Ganeshkhind, Pune 16 [email protected]
- 2. Agenda Business and System Transactions Version Stamps on Multiple Nodes
- 3. Collected by Dr. Dipali Meher Source: NoSQL Distilled: Introduction Lack of transaction in NoSQL database. But they have aggregates Basically transactions are useful tool for programmers to support consistency Aggregate oriented NoSQL DB support atomic updates with aggregates. Transactional needs are something to take into account when you decide what database to use. But transactions have limitations In transactional system user has to deal with updates with human intervention which may hold the transaction too long. Above is done using Version Stamps
- 4. Collected by Dr. Dipali Meher Source NoSQL Distilled by Business & System Transactions Business Transaction: A business transaction may be something like Browsing a product catalog Choosing a bottle of Milton at a good price, Filling in credit card information, and Confirming the order Locks are held for short period of time.
- 5. Collected by Dr. Dipali Meher Source NoSQL Distilled System Transactions System Transaction: applications only begin a system transaction at the end of the interaction with the user. Locks are held for short period of time.
- 6. Collected by Dr. Dipali Meher Source NoSQL Distilled As locks are held for long period of time in business transactions and short period of time in system transactions… so calculations and decisions may have been made based on data that’s changed. Queries should be fired according to changes in data. Example: price list may have updated the price of the Milton, updated the customer’s address, changing the shipping charges.
- 7. Collected by Dr. Dipali Meher Source NoSQL Distilled Version Stamp A field that changes every time the underlying data in the record changes. When you read the data you keep a note of the version stamp, so that when you write data you can check to see if the version has changed.
- 8. Collected by Dr. Dipali Meher Source NoSQL Distilled Example Version Stamps updating resources with HTTP using etag system Whenever you get a resource, the server responds with an etag in the header. etag(opaque string that indicates the version of the resource) If you then update that resource, you can use a conditional update by supplying the etag that you got from your last GET. If the resource has changed on the server, the etags won’t match and the server will refuse the update, returning a 412 (Precondition Failed) response.
- 9. Collected by Dr. Dipali Meher Source NoSQL Distilled by Ways of creating Version Stamps Counter GUID hash Timestamp You can use a counter always incrementing it when you update the resource. A large random number that’s guaranteed to be unique. hash of the contents of the resource timestamp of the last update
- 10. Collected by Dr. Dipali Meher Source NoSQL Distilled Ways of creating Version Stamps Counter You can use a counter always incrementing it when you update the resource. Counters are useful since they make it easy to tell if one version is more recent than another. They require the server to generate the counter value, They need a single master to ensure the counters aren’t duplicated.
- 11. Collected by Dr. Dipali Meher Source NoSQL Distilled Ways of creating Version Stamps GUID A large random number that’s guaranteed to be unique. These use some combination of dates, hardware information, and whatever other sources of randomness they can pick up. The nice thing about GUIDs is that they can be generated by anyone and you’ll never get a duplicate; A disadvantage is that they are large and can’t be compared directly for recentness.
- 12. Collected by Dr. Dipali Meher Source NoSQL Distilled Ways of creating Version Stamps Hash To make a hash of the contents of the resource. With a big enough hash key size, a content hash can be globally unique like a GUID Generated by anyone Advantage: deterministic any node will generate the same content hash for same resource data. Disadvantages: like GUIDs they can’t be directly compared for recentness, and they can be lengthy.
- 13. Collected by Dr. Dipali Meher Source NoSQL Distilled Ways of creating Version Stamps Timestamp To use the timestamp of the last update Like counters, they are reasonably short and can be directly compared for recentness. Advantage: doesnot need a single master, Multiple machines can generate timestamps( their clocks can be kep in synchronization way) Disadvantage :One node with a bad clock can cause all sorts of data corruptions. too granular you can get duplicates i.e. it’s no good using timestamps of a millisecond precision if you get many updates per millisecond.
- 14. Collected by Dr. Dipali Meher Source NoSQL Distilled blend the advantages of these different version stamp schemes by using more than one of them to create a composite stamp example CouchDB uses a combination of counter and content hash. version stamps are also useful for providing Session consistency